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pase which are all present in this protein fraction. The presence of choline is not due to the action of phospholipase D, but rather to further metabolism of phosphocholine and glycerophosphocholine by other lysosomal acid hydrolases. This analysis is confirmed by the fact that the lipid products (Table I) consist only of lysophosphatidylcholine and diglyceride. (Monoglyceride was also produced by the action of a lysosomal lipase.) The absence of phosphatidic acid in the lipid products shown in Table I further emphasizes the absence of phospholipase D in lysosomes. Summary The activity of a phospholipase C or phospholipase D may be assessed by measuring the radioactivity or phosphate released into the aqueous phase of a lipid extract. However, in crude enzyme fractions, this type of analysis may not be possible due to formation of water-soluble metabolites by other enzymatic reactions, as demonstrated here with a crude lysosomal enzyme fraction. In such instances, analysis of both water-soluble and lipid-soluble metabolites, at various times of incubation, may still provide clear identification of phospholipases C or D, even when a variety of lipases and other hydrolases are present.
[12] P r e p a r a t i o n of Alkyl E t h e r a n d Vinyl E t h e r Substrates for Phospholipases
By FRITZ
PALTAUF a n d ALBIN HERMETTER
Introduction Glycerophospholipids containing an alkyl or 1'-alkenyl ether I group in position 1 of sn-glycerol are widely distributed in human and animal cell membranes and in some microorganisms.2 Acyl ester and phospho ester bonds of ionic ether lipids are cleaved by the same phospholipases that hydrolyze the corresponding bonds in diacylglycerophospholipids, although the rates of reaction catalyzed by the same phospholipase may differ considerably depending on the phospholipid subclass (diacyl-, alkyl-
i 1-O-l'-Alkenylacylglycerophospholipids are commonly termed plasmalogens. 2 See respective chapters in "Ether Lipids: Chemistry and Biology" (F. Snyder, ed.). Academic Press, New York and London, 1972.
METHODS IN ENZYMOLOGY, VOL. 197
Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.
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acyl-, or l'-alkenylacylglycerophospholipid) serving as the substrate, s Both the l'-alkenyl and alkyl ether bonds resist phospholipases, and specific enzymes exist that are involved in their degradation: Whereas the vinyl ether bond of plasmalogens is rather labile in that it is readily cleaved by acid hydrolysis, the ether bond in alkylglycerolipids is stable under acidic and alkaline conditions. Therefore, synthetic alkyl ether analogs of phospholipids with one or two nonhydrolyzable ether bonds have been used successfully for the assay of phospholipases under conditions where the presence of other acyl ester hydrolases would interfere if 1,2-diacylglycerophospholipids were used as the substrates. 3 For example, 1,2dialkylglycerophospholipids are potential substrates for phospholipases C or D, but cannot be hydrolyzed by phospholipases A and B or by lipases. 1-O-Alkylglycerolipids resist the action of phospholipases Al and of l(3)regiospecific lipases. It is the intent of this chapter to describe procedures for the isolation from natural sources, the semisynthesis, and the total chemical synthesis of selected alkyl ether and 1'-alkenyl ether glycerophospholipids that can serve as substrates for phospholipases A 2, C, or D, or for phosphatidate phosphohydrolase (EC 3.1.3.4, phosphatidate phosphatase). The choice of procedures reflects the experience of the present authors and, because of space limitation, is restricted to the preparation of representative phospholipid classes and species. In addition, variations in standard procedures are reported that facilitate the synthesis of radioactively labeled phospholipids for use in phospholipase assays. These adaptations can also be used for syntheses involving other expensive substrates. Laboratory directions for the synthesis of phospholipids containing ether bonds have been provided previously, 5,6 and the pertinent literature has been reviewed. 7 Semisynthetic Procedures for Preparation of 1-O-Alkylacyl- and 1-O- 1 '-Alkenylacylglycerophospholipids Synthesis starting from natural ether lipid substrates results in products which are heterogeneous with respect to the alkyl or l'-alkenyl chain composition, but it is presently the only practical route for the preparation s F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Mangold and F. Paltauf, eds.), p. 211. Academic Press, New York, 1983. 4 F. Snyder, T.-C. Lee, and R. L. Wykle, in "The Enzymes of Biological Membranes" (A. N. Martonosi, ed.), Vol. 2, p. 1. Plenum, New York, 1985. 5 A. F. Rosenthal, this series, Vol. 35, p. 429. 6 A. Hermetter and F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Marigold and F. Paltauf, eds.), p. 390. Academic Press, New York, 1983. 7 F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Mangold and F. Paltauf, eds.), p. 49. Academic Press, New York, 1983.
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of vinyl ether glycerophospholipids (plasmalogens). Total chemical syntheses of plasmalogens have been reported by several groups 7 but to our knowledge have not been adopted by the "users" of this phospholipid subclass. A convenient source from which to isolate choline or ethanolamine plasmalogens are beef heart choline or ethanolamine phospholipids, which contain approximately 50% plasmalogens. The diacyl subclass can be selectively removed either by very mild alkaline hydrolysis8 or by treatment with a regiospecific lipase, such as Rhizopus lipase, 9 which attacks only the acyl ester bond in the sn-1 position of diacylglycerophospholipids and converts them to lysophospholipids. The plasmalogens thus obtained are accompanied by small quantities (-5%) of the alkylacyl subclasses. By mild alkaline hydrolysis of choline glycerophospholipids, the diacyl subclass is completely hydrolyzed, whereas plasmalogens as well as alkylacylglycerophosphocholines are converted to the respective lyso derivatives, which can be separated into molecular species by reversed-phase high-performance liquid chromatography1°(HPLC). The 1'-alkenyl groups in beef heart choline plasmalogen are mainly C16:0(62%), C18:0(11%), and C~8:~ (7%), with the remaining 20% containing C~7:0, C15:0, and C~4:0.11 Acylation of the lysoplasmalogens yields choline plasmalogens with a defined acyl chain composition. Catalytic hydrogenation of the vinyl ether double bond converts plasmalogens or lysoplasmalogens to alkyacylglycerophosphocholines and alkylglycerophosphocholines, respectively. Acylation of the latter, for example, with acetic acid or fatty acid anhydrides, provides a straightforward method for synthesizing platelet-activating factor or alkylacylglycerophosphocholines of defined acyl chain composition. Modification of the head group, for instance, replacement of choline with ethanolamine, is achieved by phospholipase D-catalyzed transphosphatidylation. 12'~3By this procedure primary aliphatic alcohols or polyols with a chain length not exceeding 6 carbon atoms can be introduced. General Procedures and Reagents Anhydrous solvents required for syntheses are prepared by chromatography of commercially available solvents on basic alumina (0.063-0.2 mm, activity I from Merck, Darmstadt, FRG). Medium-pressure liquid 8 0 . Renkonen, Acta. Chem. Scand. 17, 634 (1963). 9 F. Paltauf, Lipids 13, 165 (1978). 10 M. H. Creer and R. W. Gross, J. Chromatogr. 338, 61 (1985). u H. H. O. Schmid and T. Takahashi, Biochim. Biophys. Acta 164, 141 (1968). 12 p. Comfurius and R. F. A. Zwaal, Biochirn. Biophys. Acta 488, 36 (1977). 13 H. Eibl and S. Kovatchev, this series, Vol. 72, p. 632.
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chromatography TM(MPLC) is carded out on columns packed with silica gel 60 (0.04-0.06 mm; Merck). Purity of products is checked by thin-layer chromatography (TLC) on plates precoated with 0.2 mm silica gel 60 using the following developing solvents: solvent A, light petroleum (bp 40°-60°)-diethyl ether-acetic acid (80: 20: 2, by volume); solvent B, chloroform-methanol-25% ammonia (65 : 35 : 5, by volume); solvent C, chloroform-acetone-methanol-acetic acid-water (50 : 20 : 10 : 10 : 5, by volume); solvent D, chloroform-methanol-water (65 : 25 : 4, by volume). Lipids are detected on TLC plates by exposure to iodine vapor, by charring after spraying with 50% H 2 8 0 4 , by spraying with molybdic acid reagent ~5 (for phospholipids), by heating after spraying with ninhydrin (0.2% in methanol; for phospholipids containing amino groups), or by spraying with 2,4-dinitrophenylhydrazine (0.4% in 2 N HC1; for plasmalogens) or with Dragendorff's reagent ~6(for choline-containing phospholipids). Phospholipids are stored either in the dry state at - 2 0 ° or, if they contain polyunsaturated fatty acids, in toluene-ethanol (2 : 3, v/v) under argon at - 20°. The vinyl ether linkage is sensitive to acid-catalyzed hydrolysis; therefore, all operations with plasmalogens should be carried out at neutral or slightly alkaline pH.
Isolation of Phospholipidsfrom Beef Heart Beef hearts obtained immediately after slaughter are freed of pericardial fat, and the tissue is homogenized in the presence of chloroform-methanol (2: 1, v/v; 10 liters solvent/kg tissue). ~7After standing for 10 hr at 4° the mixture is filtered, and the residue is reextracted with chloroform-methanol (2: 1, v/v; 3 liters/kg tissue). The extracts are combined and kept in the cold for 1-2 hr for the separation of water and organic phases. The upper water phase is removed, and the lower phase is washed with 0.2 volumes of water. After evaporation of the organic phase to dryness, the residue (~45 g) is dissolved in a 6-fold (v/w) volume of chloroform. From this solution phospholipids are precipitated by the addition of a 10-fold volume of acetone and incubation for 3 hr at 4 °. The solvent is decanted and the precipitate isolated by centrifugation at 4 °. Then total phospholipids (30 g) are dissolved in 150 ml chloroform, and the resulting solution is filtered through a thin (0.5 cm) layer of silica gel and applied to a column (5.5 × 45 cm) for MPLC separation. The 14 H. Loibner and G. Seidl, Chromatographia 12, 600 (1979). 15 j. C. Dittmer and R. L. Lester, J. LipM Res. 5, 126 (1964). 16 H. Wagner, L. H6rhammer, and P. Wolff, Biochem. Z. 334, 175 (1961). 17 Chloroform and methanol are toxic; therefore, all operations with these solvents must be carried out under a hood or in an efficiently ventilated laboratory.
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phospholipids are eluted in 500-ml fractions using the following solvents: 4 liters chloroform-methanol (9:1, v/v), 4 liters chloroform-methanol (7 : 3, v/v), and 26 liters chloroform-methanol (1 : 1, v/v). Cardiolipin (3.3 g, fractions 7-16), ethanolamine glycerophospholipids (10.7 g, fractions 27-36), and choline glycerophospholipids (7.1 g, fractions 43-66) are obtained. Fractions are analyzed by TLC with solvent D.
Treatment of Choline and Ethanolamine Glycerophospholipids with Lipase from Rhizopus arrhizus Identical procedures 9 can be followed for the treatment of either choline or ethanolamine glycerophospholipids with lipase. The phospholipid (100 mg) is dispersed in 20 ml of buffer [40 mM Tris-HCl, pH 7.6, 0.1 M NaC1, 3 mg/ml sodium deoxycholate, 4.5 mg/ml bovine serum albumin (Calbiochem, San Diego, CA), 5 mM CaC12] by shaking for 10 min, followed by sonication for 20 sec at 00-5 °. Then lipase (1 mg, 600 IU) from Rhizopus arrhizus (Boehringer, Mannheim, FRG) is added, and the mixture is incubated with stirring at 25° for 2 hr. After incubation the lipids are extracted with I00 ml chloroform-methanol (2 : 1, v/v), and the lower organic phase is recovered and evaporated under reduced pressure. Lipids can be isolated by silica gel chromatography on small columns according to the procedure described for the isolation of l'-alkenyloleoylglycerophosphocholine (see below) or by preparative TLC on silica gel H using solvent B. The yield of chromatographically pure products is 80%, based on the original amount of plasmalogens present in choline and ethanolamine phospholipids. The plasmalogens are accompanied by alkylacylglycerophospholipids ( - 5 mol %).
I-0-1 '-Alkenylglycerophosphocholines To a solution of 5 g choline glycerophospholipids in 200 ml chloroform, a methanolic solution of NaOH (200 ml of 0.1 N NaOH) is added. After standing for 2 hr at 25 ° the mixture is chilled in an ice bath and neutralized with 200 ml of 0.1 N acetic acid in water. After addition of 200 ml chloroform the phases are separated, and the upper phase is reextracted with 400 ml chloroform-methanol (2 : 1, v/v). The lower phases are combined, washed with 0.2 volumes of methanol-water (1 : 1, v/v), and then evaporated under vacuum. The residue (4.6 g) is purified by MPLC on a 3 x 30 cm column using chloroform-methanol (1 : 1, v/v) as the solvent. With collection of 200-ml fractions, 1 g of pure lysoplasmalogen (63% based on choline plasmalogens) is obtained from fractions 16-30. TLC with solvent B shows a single spot, Rf 0.15. The product, 1-O-l'-alkenylglycerophosphocholine, contains approximately 5% of the 1-O-alkyl analog.
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Chromatographic Separation of 1-O-l'-Alkenylglycerophosphocholines Alkenylglycerophosphocholine obtained from beef heart phospholipids (see above) can be resolved into molecular species by reversed-phase HPLC on Ultrasphere ODS. l° Using isocratic elution with methanol-water-acetonitrile (57 : 23 : 20, by volume) containing 20 mM choline chloride, the main species, 1-O-l'-hexadecenylglycerophosphocholine, elutes at 40 min, using a flow rate of 2 ml/min. Micromole quantities can be separated on a 4.6 mm x 25 cm stainless steel column, with UV absorbance at a wavelength of 203 nm as the method of detection.
1-O-l '-Alkenyl-2-oleoyl-sn-glycero-3-phosphocholine Acylation is carried out by a combination of methods described by Gupta et al. 18and Selinger and Lapidot. 19To a solution of 1-O-l'-alkenylglycerophosphocholine (300 mg, 0.624 mmol) and oleic acid (352 mg, 1.25 mmol) in 8 ml chloroform, dicyclohexylcarbodiimide (579 mg, 2.81 mmol) and dimethylaminopyridine (305 mg, 2.5 mmol) are added. After stirring for 17 hr at 40 ° the mixture is cooled, and 52 ml chloroform plus 30 ml methanol are added. The solution is washed with 18 ml methanol-water (I : 1, v/v) followed by removal of the solvent under vacuum. Crude l'alkenylacylglycerophosphocholine (500 mg) is purified by MPLC on a 2 x 22 cm column filled with silica gel 60. Lipids dissolved in 20 ml chloroform-methanol-25% ammonia (80 : 20 : 0.5, by volume) are applied to the column, and elution is performed with 80 ml of the same solvent mixture, f o l l o w e d by 80 ml chloroform-methanol-25% ammonia (70 : 30 : 0.5, by volume), 150 ml chloroform-methanol-ammonia (60 : 40:0.5, by volume), and 200 ml chloroform-methanol-ammonia (50: 50:0.5, by volume). Fractions of 15 ml are collected with the product (350 mg, 76% yield) eluting in fractions 22-30.
Preparation of l-O-l'-Alkenyl-2-oleoyl-sn-glycero-3phosphoethanolamine by Phospholipase D A convenient source of phospholipase D is white cabbage, from which the enzyme can be prepared by extraction with water. 13All operations are performed at 0o-5 °. Leaves of white cabbage (1.5 kg) are cut in small pieces and homogenized for 5 min in the presence of 250 ml distilled water. The homogenate is filtered by suction, and the turbid filtrate is centrifuged at 25,000 g for 20 min. The pellet is discarded, and the clear supernatant 18 C. M. Gupta, R. Radhakrishnan, and H. G. Khorana, Proc. Natl. Acad. Sci. U.S.A. 74, 4315 (1977). 19 Z. Selinger and Y. Lapidot, J. Lipid Res. 7, 174 (1966).
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is adjusted to a protein concentration of 3 mg/ml, which can directly be used for transphosphatidylation. The enzyme solution can be stored at - 2 5 ° for more than 2 years without loss of phospholipase D activity. Transphosphatidylation can be carried out on any scale between milligrams and hundreds of grams. The following procedure has been used for the preparation of radioactively labeled 1-O- 1'-alkenyl-2-[ 1'-14C]oleoyl-snglycero-3-phosphoethanolamine. 1-O- l'-Alkenyl-2-oleoyl-sn-glycero-3phosphocholine (4.7 mg) is dissolved in I ml diethyl ether. To this solution 3 ml buffer2° containing ethanolamine (20%) and crude phospholipase D (6 mg protein) are added. The mixture is vigorously stirred with a magnetic stirrer for 10 hr at 40°. Diethyl ether is removed from the reaction mixture by evaporation at reduced pressure, and the residue is extracted 3 times with 2-ml portions of chloroform-methanol (2: 1, v/v). The solution is washed twice with 1 ml methanol-water (1 : 1, v/v). The lower phase is separated and evaporated to dryness. The lipid is dissolved in chloroform-methanol (9 : I, v/v), and the product is isolated by preparative TLC on two 10 × 10 cm plates coated with 0.2 mm silica gel H. Plates are developed with solvent D, and the product is visualized by spraying the plates with water. The corresponding band at Rf 0.5 is scraped off, and the product is eluted from the wet silica gel with chloroform-methanol (I : 4, v/v). Unreacted choline plasmalogen migrates to an Rf of 0.25 and can be recovered. The yield of pure l'-alkenylacylglycerophosphoethanolamine is 3 mg (64%), which shows a single spot on TLC using solvents B or D.
1-O-l '-Alkenyl-2-oleoyl-sn-glycero-3-phosphate If the reaction described in the previous section is carried out in the absence of ethanolamine, 1-O-l'-alkenyl-2-acyl-sn-glycero-3-phosphate is formed in quantitative yield. Thus, a solution of 200 mg of l'-alkenylacylglycerophosphocholine dissolved in 200 ml diethyl ether stirred for 10 hr in the presence of 20 ml buffer and 600 mg crude phospholipase D gives 170 mg (97% yield) of the phosphatidic acid analog. This product is chromatographically pure when checked on TLC with solvent system B (Rf 0 . 1 ) .
Preparation of 1-O-Alkyl-sn-glycero-3-phosphocholine by Catalytic Hydrogenation of 1-0-1 '-Alkenyl-sn-glycero-3-phosphocholine Platinum dioxide (30 rag) is added to a solution of alkenylglycerophosphocholine (100 mg) in 25 ml dry tetrahydrofuran in a flask equipped with inlet and outlet tubes and a magnetic stirrer. The flask is flushed with 20 The buffer contains 0.2 M sodium acetate, 0.1 M calcium chloride, 20% ethanolamine, adjusted to pH 5.6 with HCl.
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argon, then filled with hydrogen, and the outlet tube is closed. After 3 hr of stirring under hydrogen (35 psi) the hydrogenation is complete. The platinum catalyst is removed by filtration over a thin layer of Hyflo, 21 and the solution is evaporated to dryness. The product is obtained in pure form with quantitative yield. On TLC it migrates to the same Rf (0.15 in solvent B) as the lysoplasmalogen, but it does not show a color reaction when sprayed with dinitrophenylhydrazine. Individual molecular species of alkylglycerophosphocholine can be separated by reversed-phase HPLC following the protocol described above for the separation of the 1'-alkenylglycerophosphocholine species.
1-O-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (Platelet-Activating Factor) Dry alkylglycerophosphocholine (500 mg, ~-1 retool) is suspended in a mixture of 7 ml benzene and 5 ml acetonitrile, to which 0.4 ml ( - 4 retool) of acetic anhydride is added, and the mixture is warmed until a clear solution is obtained. 22 After cooling to room temperature, 150 mg (1.2 retool) of dimethylaminopyridine is added, and the suspension is kept at room temperature for 4-5 hr. The solvents are removed in a rotary evaporator at 40°, and the yellowish residue is dissolved in a small volume of chloroform and purified on preparative TLC plates coated with 0.5-ram layers of silica gel H, using methanol-water (2 : 1, v/v) as the solvent. The fraction containing the alkylacetylglycerophosphocholine is scraped off, and the product is eluted from the silica gel with methanol. After distilling off the solvent at reduced pressure, 10 ml of acetone is added to the colorless residue. The solution is kept at - 1 0 ° for a few hours with occasional stirring, at which time the colorless precipitate is collected and dried in a vacuum desiccator over calcium chloride. The saturated alkylacetylglycerophosphocholine is obtained in an amount of 0.35 g (64% yield). The alkyl moieties consist of approximately 62% C16:0 and 18% Cl8: 0, with smaller quantities of C14: 0, C15:0, and C17: 0. Platelet-activating factor with a defined alkyl chain can be obtained by an analogous procedure starting from synthetic alkylglycerophosphocholines (see below).
1-O-Alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine Alkylglycerophosphocholine (7.7 rag, 15.3/zmol), arachidonic acid (9.2 rag, 30/xmol), dicyclohexylcarbodiimide (19.2 nag, 73/zmol), and 4-dimethylaminopyridine (9.6 rag, 78 ~mol) are dissolved in 300/xl anhydrous chloroform and kept overnight at 40° under anhydrous conditions in an 21 Reduced platinum catalyst will ignite when drying on filter paper in the presence of oxygen. 22 T. Muramatsu, N. Totani, and H. K. Mangold, Chem. Phys. Lipids 29, 121 (1981).
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atmosphere of argon, z3 Then 1 ml chloroform is added, and the reaction mixture is applied to a small column (a Pasteur pipette plugged with glass wool and filled with 350 mg silica gel to a height of 4 cm). Lipids are eluted from the column with 4 ml chloroform, followed by mixtures of chloroform-methanol in the ratios 9 : 1 (3 ml), 8 : 3 (2 ml), 7 : 3 (1 ml), 6 : 4 (1 ml), and 1 : 1 (10 ml). Fractions of I ml are collected, and the product (11 mg, 91% yield) is recovered from fractions 12-17. It is pure as judged by TLC using solvents B, C, and D. In an analogous manner and with identical yields, the eicosapentaenoyl and docosahexaenoyl derivatives as well as the choline plasmalogen analogs have been prepared.
Chemical Synthesis of Alkyl Ether Substrates The chemical synthesis of 1-O-alkylglycerols has been described in a previous volume of this series, z4 The procedure described below follows essentially the same route, but, owing to some modifications ,25the reaction times are considerably shorter, the reaction conditions are milder, and the workup of products has been simplified. Educts for the synthesis of chiral ether phospholipids are the commercially available 1,2- or 2,3-isopropylidene-sn-glycerols; racemic products are obtained starting from racemic isopropylidene glycerol. Introduction of the triphenylmethyl group in position 3 of alkylglycerols, followed by acylation in the 2-position gives l(3)O-alkyl-2-acyl-3(l)-triphenylmethylglycerols, which on removal of the protecting group are converted to l(3)-O-alkyl'-2-acylglycerols. Removal of the triphenylmethyl group is best achieved with boron trifluoride/methanol at low temperature.Z6 The product is used for the following phosphorylation step without purification in order to avoid acyl migration. Coupling alkylacylglycerophosphate with the appropriate (protected) head group alcohol, using triisopropylbenzenesulfonyl chloride as the condensing agent, 27 gives the final alkylacylglycerophospholipid (after deprotection, if necessary). Radioactively labeled fatty acids can be introduced by deacylation-reacylation of the respective choline or N-protected ethanolamine derivatives. Deacylation-reacylation is also recommended for the introduction of other expensive (e.g., fluorescently labeled) or oxygen23 Owing to the oxygen sensitivity of polyunsaturated fatty acids all operations should be carried out under an atmosphere of inert gas, preferentially argon. An antioxidant such as di-tert-butyl-p-cresole (0.1 mg/ml solvent) can be added to the reaction mixture. 24 G. A. Thompson, Jr., and V. M. Kapoulas, this series, Vol. 14, p. 668. 25 R. Franzmair, personal communication (1989). 26 A. Hermetter and F. Paltauf, Chem. Phys. Lipids 29, 191 (1981). 27 R. Aneja, J. S. Chadha, and A. P. Davies, Tetrahedron Lett. 48, 4183 (1969).
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sensitive polyunsaturated fatty acids in order to reduce losses of expensive material and/or to avoid oxidative damage of the products. Phospholipase D-catalyzed transphosphatidylation of alkylacylglycerophosphocholines gives easy access to other phospholipid classes, and N-methylation of alkylacylglycerophosphoethanolaminesz8 with [3H]- or [14C]methyl iodide allows the synthesis of alkylacylglycerophosphocholines radioactively labeled in the polar head group. Choline glycerophospholipids radioactively labeled in the head group have also been prepared by demethylation to the corresponding N-dimethylethanolamine phospholipid with sodium benzenethiolate, followed by quaternization with [14C]methyl iodide. 29 The "unnatural" sn-1 isomers of alkylacylglycerophospholipids are accessible by treatment of the respective racemic phospholipids with phospholipase A 2,3° which leaves the sn-1 isomers intact. The sn-3 lysophospholipids formed can then be acylated to give sn-3 alkylacylglycerophospholipids.
Alkylmethane Sulfonates To a suspension of 1-O-octadecanol (100 g, 0.37 mol) in 500 ml dichloromethane, 44.9 g (0.444 mol) triethylamine is added at room temperature. 25 Then 50.8 g (0.443 mol) methane sulfochloride in 100 ml dichloromethane is added dropwise over a period of 50 min while cooling with water and stirring. After stirring for an additional 1 hr at room temperature, the dichloromethane is removed by distillation under reduced pressure, and 700 ml ethanol-water (1 : 1, v/v) is added to the residue. The product is precipitated in crystalline form and is collected by filtration after 30 min of stirring at room temperature. If an unsaturated alkylmethane sulfonate is prepared, the product will not crystallize after the addition of ethanol-water. In this case the oily residue is collected by centrifugation, washed with water, and then dried by dissolving it in benzene followed by evaporation of the solvent under reduced pressure. The alkylmethane sulfonates are obtained in 99% yield and can be used for subsequent reactions without further purification.
1-O-Hexadecyl-sn-glycerol The procedure described here is a modification25 of the method described by Baumann and Mangold. 31 The isomeric purity of the commercially available chiral educt for the synthesis of 1-O-hexadecyl-sn-glycerol, 28 K. M. Patel, J. D. Morrisett, and J. T. Sparrow, Lipids 14, 596 (1979). 29 W. Stoffel, this series, Vol. 35, p. 533. 30 F. Paltauf, Chem. Phys. Lipids 17, 148 (1976). 31 W. J. Baumann and H. K. Marigold, J. Org. Chem. 29, 3055 (1964).
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2,3-isopropylidene-sn-glycerol, should be checked by measuring its optical rotation ([oq~] - 14.3°). 3z To a solution of 2,3-isopropylideneglycerol (10 g, 75.7 mmol) in 85 ml dry dimethyl sulfoxide, powdered potassium hydroxide (7.3 g, 130. I mmol) is added. The suspension is stirred for 1 hr at 50° with protection from atmospheric moisture. Then hexadecylmethane sulfonate (15.9 g, 49.6 mmol) is added, and stirring at 50° is continued. After 3 hr the reaction mixture is cooled to room temperature, 170 ml water is added, and the product is extracted with 3 portions (100 ml) of light petroleum. The solution is washed 3 times with water, then dried over anhydrous sodium sulfate, and the solvent is removed under reduced pressure. On TLC the product shows an Rf of 0.8 with light petroleum-diethyl ether (7 : 3, v/v) as the developing solvent. Small quantities of impurities (mainly dihexadecyl ether) do not interfere with the subsequent reaction and need not be removed. The crude 1-O-hexadecyl-2,3-O-isopropylidene glycerol (21.5 g) is hydrolyzed by stirring for 3 hr at room temperature in a solution of 80 ml of 10% aqueous hydrochloric acid in 500 ml methanol. After the addition of 500 ml water the product is extracted with 2 portions (200 ml each) of diethyl ether. The combined ether extract is washed consecutively with 3 portions of water (100 ml each) and dried over anhydrous sodium sulfate. The solvent is evaporated, and the product is isolated after recrystallization from 80 ml light petroleum. The yield of pure product is 15.3 g (80% based on hexadecylmethane sulfonate), mp 65.6 °. It shows a single spot on TLC (Re 0.15) with light petroleum-diethyl ether (3:7, v/v) as the developing solvent.
1-O-Hexadecyi-3-O-triphenylmethyl-sn-glycerol A mixture of 15.3 g 1-O-hexadecyl-sn-glycerol(48.3 mmol) and 27 g triphenylmethyl chloride (97 mmol) in 200 ml anhydrous pyridine is stirred for 24 hr at 50° under anhydrous conditions. 33The solution is poured into ice-cold water (400 ml), and the mixture is extracted with 3 portions (200 ml each) of light petroleum. Undissolved triphenylmethanol is removed by filtration. The filtrate is washed 3 times with water (100 ml) and dried over anhydrous sodium sulfate. The solvent is evaporated, and light petroleum, bp 400-60 ° (150 ml), is added to the residue. On standing overnight at 4° additional triphenylmethanol is precipitated. The solid is filtered off, and the filtrate is evaporated to dryness. TLC analysis (solvent A) of the resulting product (30.6 g) shows that the alkylglycerol has completely 32 G. Hirth, H. Saroka, W. Bannwarth, and R. Barner, Helv. Chim. Acta 66, 1210 (1983). 33 G. K. Chacko and D. J. Hanahan, Biochim. Biophys. Acta 164, 252 (1968).
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reacted. The crude 1-O-hexadecyl-3-O-triphenylmethyl-sn-glycerolis directly used for the next reaction step.
1-O-Hexadecyl-2-oleoyl-3-O-triphenyimethyl-sn-glycerol Oleic acid (15.5 g, 55 mmol) and carbonyldiimidazole (9.8 g, 60 mmol) in 100 ml water-free tetrahydrofuran are stirred at room temperature for 1 hr. The freshly prepared solution of oleoylimidazolide is added to l-Ohexadecyl-3-O-triphenylmethyl-sn-glycerol (15.3 g, 27.4 mmol). Tetrahydrofuran is removed by evaporation under reduced pressure, and the residue is dissolved in 100 ml dimethyl sulfoxide. After addition of the catalyst, which is prepared by dissolving metallic sodium (1.35 mg, 58.7 mg atoms) in 100 ml dimethyl sulfoxide, the reaction mixture is kept at room temperature under anhydrous conditions and slightly shaken at intervals of about 10 min. The reaction mixture is then cooled in an ice bath and rapidly neutralized with 160 ml of 0.2 N acetic acid in water. The product is extracted with 3 portions (20 ml) of chloroform-methanol (2 : 1, v/v). The extracts are combined, washed with 0.2 volumes methanol-water (1 : 1) containing 1.5% concentrated ammonia, then with 0.2 volumes methanol-water (1 : 1), and dried over anhydrous sodium sulfate. After evaporation of the solvent under reduced pressure, the crude product (23 g) is purified by MPLC. The sample, dissolved in I00 ml light petroleum (bp 40°-60°), is applied to a silica gel column (3 × 30 cm) 34and eluted with 300 ml light petroleum and 1 liter of light petroleum-diethyl ether (9 : 1, v/v); 100-ml fractions are collected. Pure alkylacyltriphenylmethylglycerol is obtained from fractions 4-7. The yield is 20.5 g (91% based on alkyltriphenylmethylglycerol). The product gives a single spot (Rf 0.6) on TLC developed with light petroleum-diethyl ether (9 : 1, v/v).
1-O-Hexadecyl-2-oleoyl-sn-glycerol 1-O-Hexadecyl-2-oleoyl-3-O-trityl-sn-glycerol (19 g, 23 mmol) is dissolved in 170 ml anhydrous dichloromethane containing 80 ml of a 14% (w/v) solution of boron trifluoride in methanol. 26The mixture is stirred at 0° for 60 rain and then extracted 3 times with ice-cold water. The organic phase is dried over anhydrous sodium sulfate, and the solvent is removed by evaporation under reduced pressure. An oily residue is obtained which is used without further purification for the preparation of the corresponding phosphatidic acid analog. 34 The column is preconditioned with light petroleum saturated with aqueous concentrated ammonia to deactivate the solid phase. On activated silicic acid partial hydrolysis of the triphenylmethoxy group would occur.
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1-O-Alkyl-2-oleoyl-sn-glycero-3-phosphate The following procedure is a modification 3° of the method described by Berecoechea et al. 35 A solution of 11.3 g (20 mmol) alkyloleoylglycerol and anhydrous pyridine (3.2 g, 40 mmol) in 60 ml anhydrous tetrahydrofuran is added dropwise with stirring to freshly distilled phosphorus oxychloride (3.3 g, 22 mmol) dissolved in 20 ml tetrahydrofuran. Stirring is continued for 3 hr at 0 °. Then 100 ml of 10% sodium bicarbonate is added at once, and the mixture is stirred for 15 min at 0°. The solution is then poured on ice water, acidified with HC1, and extracted with diethyl ether. After washing the ether phase with water and drying it over anhydrous sodium sulfate, the solvent is removed under reduced pressure, and 11.5 g of the product is obtained. TLC with solvent B shows the product at Rf 0.1 and slight impurities, at Rf 0.7, corresponding to dipyrophosphatidic acid. 35 The product is used for the subsequent reaction without purification.
1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphocholine 1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphate (1.65 g, 2.5 mmol) and choline toluene sulfonate 36 (1.4 g, 5 mmol) prepared according to Brockerhoff and Ayengar 37 are dried over phosphorus pentoxide under vacuum for 12 hr. The mixture is dissolved in 45 ml anhydrous pyridine, and 2,4,6-triisopropylbenzene sulfonyl chloride (1.82 g, 6 mmol) is added. The solution, protected from atmospheric moisture, is stirred for 1 hr at 70° and then for 4 hr at room temperature. After the addition of 2 ml water, the solvents are removed under vacuum on a rotary evaporator, and pyridine is removed by repeated evaporation in the presence of toluene. The residue is extracted with diethyl ether, and the solutions are decanted from undissolved material (mainly triisopropylbenzenesulfonic acid) and evaporated to dryness. The solid material is dissolved in 50 ml chloroform-methanol (2: 1), and the mixture is successively extracted with 10-ml portions of 3% aqueous Na2CO3, methanol-water (1 : 1), 2% HCI, and finally methanol-water (1 : 1). The organic phase is dried over sodium sulfate and evaporated under reduced pressure. The crude product (2 g) dissolved in 20 ml chloroform is applied to a 2 x 30 cm column for MPLC purification. The following solvents are used for chromatography: 200 ml chloroform, 50 ml each 35 j. Berecoechea, M. Faure, and J. Anatol, Bull. Soc. Chim. Biol. 50, 1561 (1968). 36 To a 40% solution of choline hydroxide in water an equimolar amount of toluene sulfonic acid is added, and the solvents are removed under reduced pressure. The choline toluene sulfonate is recrystallized from acetone. 37 H. Brockerhoff and N. K. N. Ayengar, Lipids 14, 88 (1979).
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of chloroform-methanol 9 : 1, 8 : 2, 7 : 3, and 6 : 4, followed by 500 ml chloroform-methanol 4 : 6. Fractions of 15 ml are collected, and the product (1.3 g, 70% yield) is obtained in fractions 28-42. On TLC with solvents B, C, and D it shows a single spot corresponding to authentic phosphatidylcholine from egg yolk.
1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphoethanolamine A mixture of N-triphenylmethylethanolamine (360 rag, 1.2 mmol; prepared 38 according to Aneja et a1.27), triisopropylbenzenesulfonyl chloride (450 mg, 1.5 mmol), 1-O-hexadecyl-2-oleoyl-sn-glycero-3-phosphate (400 rag, 0.6 mmol) in 12 ml anhydrous pyridine, and 5 ml dry chloroform is stirred at room temperature for 3 hr. Then 2.5 ml of water is added, the mixture is evaporated under vacuum, and the residue is extracted 3 times with diethyl ether. After evaporation of the combined diethyl ether extracts, the crude product is purified by MPLC on a 3 x 35 cm column using the following solvents for elution: 200 ml chloroform, 200 ml chloroform-methanol (95:5), and 400 ml chloroform-methanol (9: 1); 50-ml fractions are collected. The product (450 rag, 80% yield) is obtained from fractions 14 and 15. TLC shows a single spot, Rf 0.6, with chloroform-methanol (9: 1) as the solvent. For removal of the triphenylmethyl protecting group, trifluoroacetic acid (15 ml) is added to an ice-cold solution of hexadecyloleoylglycerophospho(N-triphenylmethyl)ethanolamine (450 mg) in 15 ml dichloromethane under protection from atmospheric moisture. The reaction mixture is left at 0° for 5 min and then poured rapidly into a vigorously stirred solution (50 ml) of 6% ammonia in water. The aqueous phase is separated and extracted twice with chloroform (100 ml). The combined organic phases are washed with water and evaporated to dryness. The crude product is purified by MPLC on a silica gel column using a chloroform-methanol gradient for elution. The product (270 mg, 80%) is eluted at a chloroform-methanol ratio of 7 : 3. TLC with solvent B shows a single spot, Rf 0.6.
Chemical Synthesis of Diether Substrates The occurrence of 1,2-di-O-alkylglycerophosphocholines in nature has been reported, but obviously they constitute only a minute proportion of the choline glycerophospholipids isolated, for example, from bovine 38 N-Triphenylethanolamine is prepared by reacting triphenylmethyl bromide and ethanolamine in chloroform in the presence of triethylamine.
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heart) 9 Because they do not contain an acyl ester linkage, 1,2-dialkylglycerophospholipids cannot be hydrolyzed by phospholipase A or B, by lipases, or by other acyl ester hydrolases. However, they can serve as substrates for phospholipase C or D. 3 In addition, for studies on mechanisms and kinetics of phospholipases they can be used as inert matrices in which the appropriate cleavable diacylglycerophospholipid substrates are embedded at varying concentrations but in a constant total surface presented to the enzyme. Racemic di-O-alkylglycerols with two identical alkyl chains are preferentially synthesized from tetrahydropyranylglycerol.4° The enantiomers containing saturated alkyl chains are accessible via 1- or 3-O-benzylglycerols 41 which are commercially available. Di-O-alkylglycerols with two different alkyl chains (saturated or unsaturated) can be prepared from l(3)alkyl-3(1)-trityl-sn-glycerols42 (see above) by reaction with alkylmethane sulfonates. By the same route optically active dialkylglycerols with identical, or different, saturated or unsaturated alkyl chains can be prepared. The subsequent steps of phosphorylation and attachment of head group alcohols are the same as with alkylacyl substrates (see preceding sections).
1-O-Hexadecyl-2-O-octadec-9'-enyl-sn-glycerol 1-O-Hexadecyl-3-O-trityl-sn-glycerol (6 g, 10.6 mmol; see above) and powdered potassium hydroxide (6 g) in 100 ml xylene are refluxed for 1 hr in a 500-ml flask fitted with a water-separation head, reflux condenser, dropping funnel, and magnetic stirrer. Then octadecenylmethane sulfonate (4.5 g, 13 mmol; see above) in 50 ml xylene is added dropwise during 15 min, and refluxing is continued for 6 hr. After cooling 100 ml water is added, the xylene phase is removed, and the water phase is extracted twice with 100 ml diethyl ether. The combined organic phases are dried over anhydrous sodium sulfate and evaporated under reduced pressure. The residue (8.6 g) is dissolved in 25 ml light petroleum and purified by MPLC on a 3 × 25 cm column. The following solvents were used for elution: light petroleum (200 ml), light petroleum-diethyl ether, 9 : 1 (100 ml), and light petroleum-diethyl ether, 8 : 2 (300 ml). Fractions (I0 ml) are collected, and the product is recovered from fractions 8-28. The yield of pure product is 8 g (90%). The protecting triphenylmethyl group is removed by treating the product with boron trifluoride-methanol following the procedure described for the alkylacyl analog (see above). Pure 1-O-hexa39 E. L. Pugh, M. Kates, and D. J. Hanahan, J. Lipid Res. 18, 710 (1977). 4o F. Paltauf and F. Spener, Chem. Phys. Lipids 2, 168 (1968). 41 M. Kates, B. Palameta, and L. S. Yengoyan, Biochemistry 4, 1595 (1965). 42 W. J. Baumann and H. K. Mangold, J. Org. Chem. 31, 498 (1966).
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AND PHOSPHATES
decyl-2-octadec-9'-enyl-sn-glycerol (5 g) is obtained in 90% yield after MPLC purification on a 2 x 35 cm column, eluted with a light petroleum-diethyl ether gradient. It shows a single spot, Rf 0.2, on TLC with solvent A.
1-O-Hexadecyl-2-O-octadec-9'-enyl-sn-glycero-3-phosphocholine and -ethanolamine Di-O-alkylglycerophospholipids are prepared from di-O-alkylglycerols by procedures described for the alkylacyl analogs 43 (see above). On TLC using solvents B, C, or D they migrate to the s a m e Rf as the analogous alkylacyl- and diacylglycerophospholipids. Acknowledgments The authors acknowledge the excellent technical assistance of H. Sttitz and thank R. Franzmair for stimulating discussions. The original work described in this chapter has been supported by the Fonds zur F6rderung der wissenschaftlichen Forschung in Osterreich (Project 5746B). 43Because dialkylglycerophospholipids are stable in dilute alkali or acid, workup procedures are more straightforward than with 1'-alkenylacylor alkylacyl analogs. Acidic or basic byproducts can readily be removed by extraction from a chloroform-methanol phase, with dilute sodium hydroxide or hydrochloric acid, respectively.
[13] Inositol P h o s p h o l i p i d s a n d P h o s p h a t e s for I n v e s t i g a t i o n of I n t a c t Cell P h o s p h o l i p a s e C S u b s t r a t e s a n d P r o d u c t s
By
MICHAEL R. HANLEY, DAVID R. POYNER, PHILLIP T. HAWKINS
and
Introduction The receptor-coupled activation of inositol lipid-specific phospholipase C is conventionally analyzed in intact cells, as there has been comparatively limited success in developing cell-free or reconstituted systems. Thus, much of the information on the physiological substrates and resulting enzymatic products has been inferred from the kinetics of changes in the levels of labeled lipids and the production of labeled water-soluble products. The early work on this second messenger pathway used tissue slices or acutely isolated cells, but, where feasible, the recent emphasis has been placed on established cell lines, which have advantages in homoMETHODS IN ENZYMOLOGY, VOL. 197
Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.
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pase which are all present in this protein fraction. The presence of choline is not due to the action of phospholipase D, but rather to further metabolism of phosphocholine and glycerophosphocholine by other lysosomal acid hydrolases. This analysis is confirmed by the fact that the lipid products (Table I) consist only of lysophosphatidylcholine and diglyceride. (Monoglyceride was also produced by the action of a lysosomal lipase.) The absence of phosphatidic acid in the lipid products shown in Table I further emphasizes the absence of phospholipase D in lysosomes. Summary The activity of a phospholipase C or phospholipase D may be assessed by measuring the radioactivity or phosphate released into the aqueous phase of a lipid extract. However, in crude enzyme fractions, this type of analysis may not be possible due to formation of water-soluble metabolites by other enzymatic reactions, as demonstrated here with a crude lysosomal enzyme fraction. In such instances, analysis of both water-soluble and lipid-soluble metabolites, at various times of incubation, may still provide clear identification of phospholipases C or D, even when a variety of lipases and other hydrolases are present.
[12] P r e p a r a t i o n of Alkyl E t h e r a n d Vinyl E t h e r Substrates for Phospholipases
By FRITZ
PALTAUF a n d ALBIN HERMETTER
Introduction Glycerophospholipids containing an alkyl or 1'-alkenyl ether I group in position 1 of sn-glycerol are widely distributed in human and animal cell membranes and in some microorganisms.2 Acyl ester and phospho ester bonds of ionic ether lipids are cleaved by the same phospholipases that hydrolyze the corresponding bonds in diacylglycerophospholipids, although the rates of reaction catalyzed by the same phospholipase may differ considerably depending on the phospholipid subclass (diacyl-, alkyl-
i 1-O-l'-Alkenylacylglycerophospholipids are commonly termed plasmalogens. 2 See respective chapters in "Ether Lipids: Chemistry and Biology" (F. Snyder, ed.). Academic Press, New York and London, 1972.
METHODS IN ENZYMOLOGY, VOL. 197
Copyright © 1991 by Academic Press, Inc. All rights of reproduction in any form reserved.
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acyl-, or l'-alkenylacylglycerophospholipid) serving as the substrate, s Both the l'-alkenyl and alkyl ether bonds resist phospholipases, and specific enzymes exist that are involved in their degradation: Whereas the vinyl ether bond of plasmalogens is rather labile in that it is readily cleaved by acid hydrolysis, the ether bond in alkylglycerolipids is stable under acidic and alkaline conditions. Therefore, synthetic alkyl ether analogs of phospholipids with one or two nonhydrolyzable ether bonds have been used successfully for the assay of phospholipases under conditions where the presence of other acyl ester hydrolases would interfere if 1,2-diacylglycerophospholipids were used as the substrates. 3 For example, 1,2dialkylglycerophospholipids are potential substrates for phospholipases C or D, but cannot be hydrolyzed by phospholipases A and B or by lipases. 1-O-Alkylglycerolipids resist the action of phospholipases Al and of l(3)regiospecific lipases. It is the intent of this chapter to describe procedures for the isolation from natural sources, the semisynthesis, and the total chemical synthesis of selected alkyl ether and 1'-alkenyl ether glycerophospholipids that can serve as substrates for phospholipases A 2, C, or D, or for phosphatidate phosphohydrolase (EC 3.1.3.4, phosphatidate phosphatase). The choice of procedures reflects the experience of the present authors and, because of space limitation, is restricted to the preparation of representative phospholipid classes and species. In addition, variations in standard procedures are reported that facilitate the synthesis of radioactively labeled phospholipids for use in phospholipase assays. These adaptations can also be used for syntheses involving other expensive substrates. Laboratory directions for the synthesis of phospholipids containing ether bonds have been provided previously, 5,6 and the pertinent literature has been reviewed. 7 Semisynthetic Procedures for Preparation of 1-O-Alkylacyl- and 1-O- 1 '-Alkenylacylglycerophospholipids Synthesis starting from natural ether lipid substrates results in products which are heterogeneous with respect to the alkyl or l'-alkenyl chain composition, but it is presently the only practical route for the preparation s F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Mangold and F. Paltauf, eds.), p. 211. Academic Press, New York, 1983. 4 F. Snyder, T.-C. Lee, and R. L. Wykle, in "The Enzymes of Biological Membranes" (A. N. Martonosi, ed.), Vol. 2, p. 1. Plenum, New York, 1985. 5 A. F. Rosenthal, this series, Vol. 35, p. 429. 6 A. Hermetter and F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Marigold and F. Paltauf, eds.), p. 390. Academic Press, New York, 1983. 7 F. Paltauf, in " E t h e r Lipids: Biochemical and Biomedical Aspects" (H. K. Mangold and F. Paltauf, eds.), p. 49. Academic Press, New York, 1983.
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of vinyl ether glycerophospholipids (plasmalogens). Total chemical syntheses of plasmalogens have been reported by several groups 7 but to our knowledge have not been adopted by the "users" of this phospholipid subclass. A convenient source from which to isolate choline or ethanolamine plasmalogens are beef heart choline or ethanolamine phospholipids, which contain approximately 50% plasmalogens. The diacyl subclass can be selectively removed either by very mild alkaline hydrolysis8 or by treatment with a regiospecific lipase, such as Rhizopus lipase, 9 which attacks only the acyl ester bond in the sn-1 position of diacylglycerophospholipids and converts them to lysophospholipids. The plasmalogens thus obtained are accompanied by small quantities (-5%) of the alkylacyl subclasses. By mild alkaline hydrolysis of choline glycerophospholipids, the diacyl subclass is completely hydrolyzed, whereas plasmalogens as well as alkylacylglycerophosphocholines are converted to the respective lyso derivatives, which can be separated into molecular species by reversed-phase high-performance liquid chromatography1°(HPLC). The 1'-alkenyl groups in beef heart choline plasmalogen are mainly C16:0(62%), C18:0(11%), and C~8:~ (7%), with the remaining 20% containing C~7:0, C15:0, and C~4:0.11 Acylation of the lysoplasmalogens yields choline plasmalogens with a defined acyl chain composition. Catalytic hydrogenation of the vinyl ether double bond converts plasmalogens or lysoplasmalogens to alkyacylglycerophosphocholines and alkylglycerophosphocholines, respectively. Acylation of the latter, for example, with acetic acid or fatty acid anhydrides, provides a straightforward method for synthesizing platelet-activating factor or alkylacylglycerophosphocholines of defined acyl chain composition. Modification of the head group, for instance, replacement of choline with ethanolamine, is achieved by phospholipase D-catalyzed transphosphatidylation. 12'~3By this procedure primary aliphatic alcohols or polyols with a chain length not exceeding 6 carbon atoms can be introduced. General Procedures and Reagents Anhydrous solvents required for syntheses are prepared by chromatography of commercially available solvents on basic alumina (0.063-0.2 mm, activity I from Merck, Darmstadt, FRG). Medium-pressure liquid 8 0 . Renkonen, Acta. Chem. Scand. 17, 634 (1963). 9 F. Paltauf, Lipids 13, 165 (1978). 10 M. H. Creer and R. W. Gross, J. Chromatogr. 338, 61 (1985). u H. H. O. Schmid and T. Takahashi, Biochim. Biophys. Acta 164, 141 (1968). 12 p. Comfurius and R. F. A. Zwaal, Biochirn. Biophys. Acta 488, 36 (1977). 13 H. Eibl and S. Kovatchev, this series, Vol. 72, p. 632.
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chromatography TM(MPLC) is carded out on columns packed with silica gel 60 (0.04-0.06 mm; Merck). Purity of products is checked by thin-layer chromatography (TLC) on plates precoated with 0.2 mm silica gel 60 using the following developing solvents: solvent A, light petroleum (bp 40°-60°)-diethyl ether-acetic acid (80: 20: 2, by volume); solvent B, chloroform-methanol-25% ammonia (65 : 35 : 5, by volume); solvent C, chloroform-acetone-methanol-acetic acid-water (50 : 20 : 10 : 10 : 5, by volume); solvent D, chloroform-methanol-water (65 : 25 : 4, by volume). Lipids are detected on TLC plates by exposure to iodine vapor, by charring after spraying with 50% H 2 8 0 4 , by spraying with molybdic acid reagent ~5 (for phospholipids), by heating after spraying with ninhydrin (0.2% in methanol; for phospholipids containing amino groups), or by spraying with 2,4-dinitrophenylhydrazine (0.4% in 2 N HC1; for plasmalogens) or with Dragendorff's reagent ~6(for choline-containing phospholipids). Phospholipids are stored either in the dry state at - 2 0 ° or, if they contain polyunsaturated fatty acids, in toluene-ethanol (2 : 3, v/v) under argon at - 20°. The vinyl ether linkage is sensitive to acid-catalyzed hydrolysis; therefore, all operations with plasmalogens should be carried out at neutral or slightly alkaline pH.
Isolation of Phospholipidsfrom Beef Heart Beef hearts obtained immediately after slaughter are freed of pericardial fat, and the tissue is homogenized in the presence of chloroform-methanol (2: 1, v/v; 10 liters solvent/kg tissue). ~7After standing for 10 hr at 4° the mixture is filtered, and the residue is reextracted with chloroform-methanol (2: 1, v/v; 3 liters/kg tissue). The extracts are combined and kept in the cold for 1-2 hr for the separation of water and organic phases. The upper water phase is removed, and the lower phase is washed with 0.2 volumes of water. After evaporation of the organic phase to dryness, the residue (~45 g) is dissolved in a 6-fold (v/w) volume of chloroform. From this solution phospholipids are precipitated by the addition of a 10-fold volume of acetone and incubation for 3 hr at 4 °. The solvent is decanted and the precipitate isolated by centrifugation at 4 °. Then total phospholipids (30 g) are dissolved in 150 ml chloroform, and the resulting solution is filtered through a thin (0.5 cm) layer of silica gel and applied to a column (5.5 × 45 cm) for MPLC separation. The 14 H. Loibner and G. Seidl, Chromatographia 12, 600 (1979). 15 j. C. Dittmer and R. L. Lester, J. LipM Res. 5, 126 (1964). 16 H. Wagner, L. H6rhammer, and P. Wolff, Biochem. Z. 334, 175 (1961). 17 Chloroform and methanol are toxic; therefore, all operations with these solvents must be carried out under a hood or in an efficiently ventilated laboratory.
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phospholipids are eluted in 500-ml fractions using the following solvents: 4 liters chloroform-methanol (9:1, v/v), 4 liters chloroform-methanol (7 : 3, v/v), and 26 liters chloroform-methanol (1 : 1, v/v). Cardiolipin (3.3 g, fractions 7-16), ethanolamine glycerophospholipids (10.7 g, fractions 27-36), and choline glycerophospholipids (7.1 g, fractions 43-66) are obtained. Fractions are analyzed by TLC with solvent D.
Treatment of Choline and Ethanolamine Glycerophospholipids with Lipase from Rhizopus arrhizus Identical procedures 9 can be followed for the treatment of either choline or ethanolamine glycerophospholipids with lipase. The phospholipid (100 mg) is dispersed in 20 ml of buffer [40 mM Tris-HCl, pH 7.6, 0.1 M NaC1, 3 mg/ml sodium deoxycholate, 4.5 mg/ml bovine serum albumin (Calbiochem, San Diego, CA), 5 mM CaC12] by shaking for 10 min, followed by sonication for 20 sec at 00-5 °. Then lipase (1 mg, 600 IU) from Rhizopus arrhizus (Boehringer, Mannheim, FRG) is added, and the mixture is incubated with stirring at 25° for 2 hr. After incubation the lipids are extracted with I00 ml chloroform-methanol (2 : 1, v/v), and the lower organic phase is recovered and evaporated under reduced pressure. Lipids can be isolated by silica gel chromatography on small columns according to the procedure described for the isolation of l'-alkenyloleoylglycerophosphocholine (see below) or by preparative TLC on silica gel H using solvent B. The yield of chromatographically pure products is 80%, based on the original amount of plasmalogens present in choline and ethanolamine phospholipids. The plasmalogens are accompanied by alkylacylglycerophospholipids ( - 5 mol %).
I-0-1 '-Alkenylglycerophosphocholines To a solution of 5 g choline glycerophospholipids in 200 ml chloroform, a methanolic solution of NaOH (200 ml of 0.1 N NaOH) is added. After standing for 2 hr at 25 ° the mixture is chilled in an ice bath and neutralized with 200 ml of 0.1 N acetic acid in water. After addition of 200 ml chloroform the phases are separated, and the upper phase is reextracted with 400 ml chloroform-methanol (2 : 1, v/v). The lower phases are combined, washed with 0.2 volumes of methanol-water (1 : 1, v/v), and then evaporated under vacuum. The residue (4.6 g) is purified by MPLC on a 3 x 30 cm column using chloroform-methanol (1 : 1, v/v) as the solvent. With collection of 200-ml fractions, 1 g of pure lysoplasmalogen (63% based on choline plasmalogens) is obtained from fractions 16-30. TLC with solvent B shows a single spot, Rf 0.15. The product, 1-O-l'-alkenylglycerophosphocholine, contains approximately 5% of the 1-O-alkyl analog.
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Chromatographic Separation of 1-O-l'-Alkenylglycerophosphocholines Alkenylglycerophosphocholine obtained from beef heart phospholipids (see above) can be resolved into molecular species by reversed-phase HPLC on Ultrasphere ODS. l° Using isocratic elution with methanol-water-acetonitrile (57 : 23 : 20, by volume) containing 20 mM choline chloride, the main species, 1-O-l'-hexadecenylglycerophosphocholine, elutes at 40 min, using a flow rate of 2 ml/min. Micromole quantities can be separated on a 4.6 mm x 25 cm stainless steel column, with UV absorbance at a wavelength of 203 nm as the method of detection.
1-O-l '-Alkenyl-2-oleoyl-sn-glycero-3-phosphocholine Acylation is carried out by a combination of methods described by Gupta et al. 18and Selinger and Lapidot. 19To a solution of 1-O-l'-alkenylglycerophosphocholine (300 mg, 0.624 mmol) and oleic acid (352 mg, 1.25 mmol) in 8 ml chloroform, dicyclohexylcarbodiimide (579 mg, 2.81 mmol) and dimethylaminopyridine (305 mg, 2.5 mmol) are added. After stirring for 17 hr at 40 ° the mixture is cooled, and 52 ml chloroform plus 30 ml methanol are added. The solution is washed with 18 ml methanol-water (I : 1, v/v) followed by removal of the solvent under vacuum. Crude l'alkenylacylglycerophosphocholine (500 mg) is purified by MPLC on a 2 x 22 cm column filled with silica gel 60. Lipids dissolved in 20 ml chloroform-methanol-25% ammonia (80 : 20 : 0.5, by volume) are applied to the column, and elution is performed with 80 ml of the same solvent mixture, f o l l o w e d by 80 ml chloroform-methanol-25% ammonia (70 : 30 : 0.5, by volume), 150 ml chloroform-methanol-ammonia (60 : 40:0.5, by volume), and 200 ml chloroform-methanol-ammonia (50: 50:0.5, by volume). Fractions of 15 ml are collected with the product (350 mg, 76% yield) eluting in fractions 22-30.
Preparation of l-O-l'-Alkenyl-2-oleoyl-sn-glycero-3phosphoethanolamine by Phospholipase D A convenient source of phospholipase D is white cabbage, from which the enzyme can be prepared by extraction with water. 13All operations are performed at 0o-5 °. Leaves of white cabbage (1.5 kg) are cut in small pieces and homogenized for 5 min in the presence of 250 ml distilled water. The homogenate is filtered by suction, and the turbid filtrate is centrifuged at 25,000 g for 20 min. The pellet is discarded, and the clear supernatant 18 C. M. Gupta, R. Radhakrishnan, and H. G. Khorana, Proc. Natl. Acad. Sci. U.S.A. 74, 4315 (1977). 19 Z. Selinger and Y. Lapidot, J. Lipid Res. 7, 174 (1966).
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is adjusted to a protein concentration of 3 mg/ml, which can directly be used for transphosphatidylation. The enzyme solution can be stored at - 2 5 ° for more than 2 years without loss of phospholipase D activity. Transphosphatidylation can be carried out on any scale between milligrams and hundreds of grams. The following procedure has been used for the preparation of radioactively labeled 1-O- 1'-alkenyl-2-[ 1'-14C]oleoyl-snglycero-3-phosphoethanolamine. 1-O- l'-Alkenyl-2-oleoyl-sn-glycero-3phosphocholine (4.7 mg) is dissolved in I ml diethyl ether. To this solution 3 ml buffer2° containing ethanolamine (20%) and crude phospholipase D (6 mg protein) are added. The mixture is vigorously stirred with a magnetic stirrer for 10 hr at 40°. Diethyl ether is removed from the reaction mixture by evaporation at reduced pressure, and the residue is extracted 3 times with 2-ml portions of chloroform-methanol (2: 1, v/v). The solution is washed twice with 1 ml methanol-water (1 : 1, v/v). The lower phase is separated and evaporated to dryness. The lipid is dissolved in chloroform-methanol (9 : I, v/v), and the product is isolated by preparative TLC on two 10 × 10 cm plates coated with 0.2 mm silica gel H. Plates are developed with solvent D, and the product is visualized by spraying the plates with water. The corresponding band at Rf 0.5 is scraped off, and the product is eluted from the wet silica gel with chloroform-methanol (I : 4, v/v). Unreacted choline plasmalogen migrates to an Rf of 0.25 and can be recovered. The yield of pure l'-alkenylacylglycerophosphoethanolamine is 3 mg (64%), which shows a single spot on TLC using solvents B or D.
1-O-l '-Alkenyl-2-oleoyl-sn-glycero-3-phosphate If the reaction described in the previous section is carried out in the absence of ethanolamine, 1-O-l'-alkenyl-2-acyl-sn-glycero-3-phosphate is formed in quantitative yield. Thus, a solution of 200 mg of l'-alkenylacylglycerophosphocholine dissolved in 200 ml diethyl ether stirred for 10 hr in the presence of 20 ml buffer and 600 mg crude phospholipase D gives 170 mg (97% yield) of the phosphatidic acid analog. This product is chromatographically pure when checked on TLC with solvent system B (Rf 0 . 1 ) .
Preparation of 1-O-Alkyl-sn-glycero-3-phosphocholine by Catalytic Hydrogenation of 1-0-1 '-Alkenyl-sn-glycero-3-phosphocholine Platinum dioxide (30 rag) is added to a solution of alkenylglycerophosphocholine (100 mg) in 25 ml dry tetrahydrofuran in a flask equipped with inlet and outlet tubes and a magnetic stirrer. The flask is flushed with 20 The buffer contains 0.2 M sodium acetate, 0.1 M calcium chloride, 20% ethanolamine, adjusted to pH 5.6 with HCl.
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argon, then filled with hydrogen, and the outlet tube is closed. After 3 hr of stirring under hydrogen (35 psi) the hydrogenation is complete. The platinum catalyst is removed by filtration over a thin layer of Hyflo, 21 and the solution is evaporated to dryness. The product is obtained in pure form with quantitative yield. On TLC it migrates to the same Rf (0.15 in solvent B) as the lysoplasmalogen, but it does not show a color reaction when sprayed with dinitrophenylhydrazine. Individual molecular species of alkylglycerophosphocholine can be separated by reversed-phase HPLC following the protocol described above for the separation of the 1'-alkenylglycerophosphocholine species.
1-O-Alkyl-2-acetyl-sn-glycero-3-phosphocholine (Platelet-Activating Factor) Dry alkylglycerophosphocholine (500 mg, ~-1 retool) is suspended in a mixture of 7 ml benzene and 5 ml acetonitrile, to which 0.4 ml ( - 4 retool) of acetic anhydride is added, and the mixture is warmed until a clear solution is obtained. 22 After cooling to room temperature, 150 mg (1.2 retool) of dimethylaminopyridine is added, and the suspension is kept at room temperature for 4-5 hr. The solvents are removed in a rotary evaporator at 40°, and the yellowish residue is dissolved in a small volume of chloroform and purified on preparative TLC plates coated with 0.5-ram layers of silica gel H, using methanol-water (2 : 1, v/v) as the solvent. The fraction containing the alkylacetylglycerophosphocholine is scraped off, and the product is eluted from the silica gel with methanol. After distilling off the solvent at reduced pressure, 10 ml of acetone is added to the colorless residue. The solution is kept at - 1 0 ° for a few hours with occasional stirring, at which time the colorless precipitate is collected and dried in a vacuum desiccator over calcium chloride. The saturated alkylacetylglycerophosphocholine is obtained in an amount of 0.35 g (64% yield). The alkyl moieties consist of approximately 62% C16:0 and 18% Cl8: 0, with smaller quantities of C14: 0, C15:0, and C17: 0. Platelet-activating factor with a defined alkyl chain can be obtained by an analogous procedure starting from synthetic alkylglycerophosphocholines (see below).
1-O-Alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine Alkylglycerophosphocholine (7.7 rag, 15.3/zmol), arachidonic acid (9.2 rag, 30/xmol), dicyclohexylcarbodiimide (19.2 nag, 73/zmol), and 4-dimethylaminopyridine (9.6 rag, 78 ~mol) are dissolved in 300/xl anhydrous chloroform and kept overnight at 40° under anhydrous conditions in an 21 Reduced platinum catalyst will ignite when drying on filter paper in the presence of oxygen. 22 T. Muramatsu, N. Totani, and H. K. Mangold, Chem. Phys. Lipids 29, 121 (1981).
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atmosphere of argon, z3 Then 1 ml chloroform is added, and the reaction mixture is applied to a small column (a Pasteur pipette plugged with glass wool and filled with 350 mg silica gel to a height of 4 cm). Lipids are eluted from the column with 4 ml chloroform, followed by mixtures of chloroform-methanol in the ratios 9 : 1 (3 ml), 8 : 3 (2 ml), 7 : 3 (1 ml), 6 : 4 (1 ml), and 1 : 1 (10 ml). Fractions of I ml are collected, and the product (11 mg, 91% yield) is recovered from fractions 12-17. It is pure as judged by TLC using solvents B, C, and D. In an analogous manner and with identical yields, the eicosapentaenoyl and docosahexaenoyl derivatives as well as the choline plasmalogen analogs have been prepared.
Chemical Synthesis of Alkyl Ether Substrates The chemical synthesis of 1-O-alkylglycerols has been described in a previous volume of this series, z4 The procedure described below follows essentially the same route, but, owing to some modifications ,25the reaction times are considerably shorter, the reaction conditions are milder, and the workup of products has been simplified. Educts for the synthesis of chiral ether phospholipids are the commercially available 1,2- or 2,3-isopropylidene-sn-glycerols; racemic products are obtained starting from racemic isopropylidene glycerol. Introduction of the triphenylmethyl group in position 3 of alkylglycerols, followed by acylation in the 2-position gives l(3)O-alkyl-2-acyl-3(l)-triphenylmethylglycerols, which on removal of the protecting group are converted to l(3)-O-alkyl'-2-acylglycerols. Removal of the triphenylmethyl group is best achieved with boron trifluoride/methanol at low temperature.Z6 The product is used for the following phosphorylation step without purification in order to avoid acyl migration. Coupling alkylacylglycerophosphate with the appropriate (protected) head group alcohol, using triisopropylbenzenesulfonyl chloride as the condensing agent, 27 gives the final alkylacylglycerophospholipid (after deprotection, if necessary). Radioactively labeled fatty acids can be introduced by deacylation-reacylation of the respective choline or N-protected ethanolamine derivatives. Deacylation-reacylation is also recommended for the introduction of other expensive (e.g., fluorescently labeled) or oxygen23 Owing to the oxygen sensitivity of polyunsaturated fatty acids all operations should be carried out under an atmosphere of inert gas, preferentially argon. An antioxidant such as di-tert-butyl-p-cresole (0.1 mg/ml solvent) can be added to the reaction mixture. 24 G. A. Thompson, Jr., and V. M. Kapoulas, this series, Vol. 14, p. 668. 25 R. Franzmair, personal communication (1989). 26 A. Hermetter and F. Paltauf, Chem. Phys. Lipids 29, 191 (1981). 27 R. Aneja, J. S. Chadha, and A. P. Davies, Tetrahedron Lett. 48, 4183 (1969).
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143
sensitive polyunsaturated fatty acids in order to reduce losses of expensive material and/or to avoid oxidative damage of the products. Phospholipase D-catalyzed transphosphatidylation of alkylacylglycerophosphocholines gives easy access to other phospholipid classes, and N-methylation of alkylacylglycerophosphoethanolaminesz8 with [3H]- or [14C]methyl iodide allows the synthesis of alkylacylglycerophosphocholines radioactively labeled in the polar head group. Choline glycerophospholipids radioactively labeled in the head group have also been prepared by demethylation to the corresponding N-dimethylethanolamine phospholipid with sodium benzenethiolate, followed by quaternization with [14C]methyl iodide. 29 The "unnatural" sn-1 isomers of alkylacylglycerophospholipids are accessible by treatment of the respective racemic phospholipids with phospholipase A 2,3° which leaves the sn-1 isomers intact. The sn-3 lysophospholipids formed can then be acylated to give sn-3 alkylacylglycerophospholipids.
Alkylmethane Sulfonates To a suspension of 1-O-octadecanol (100 g, 0.37 mol) in 500 ml dichloromethane, 44.9 g (0.444 mol) triethylamine is added at room temperature. 25 Then 50.8 g (0.443 mol) methane sulfochloride in 100 ml dichloromethane is added dropwise over a period of 50 min while cooling with water and stirring. After stirring for an additional 1 hr at room temperature, the dichloromethane is removed by distillation under reduced pressure, and 700 ml ethanol-water (1 : 1, v/v) is added to the residue. The product is precipitated in crystalline form and is collected by filtration after 30 min of stirring at room temperature. If an unsaturated alkylmethane sulfonate is prepared, the product will not crystallize after the addition of ethanol-water. In this case the oily residue is collected by centrifugation, washed with water, and then dried by dissolving it in benzene followed by evaporation of the solvent under reduced pressure. The alkylmethane sulfonates are obtained in 99% yield and can be used for subsequent reactions without further purification.
1-O-Hexadecyl-sn-glycerol The procedure described here is a modification25 of the method described by Baumann and Mangold. 31 The isomeric purity of the commercially available chiral educt for the synthesis of 1-O-hexadecyl-sn-glycerol, 28 K. M. Patel, J. D. Morrisett, and J. T. Sparrow, Lipids 14, 596 (1979). 29 W. Stoffel, this series, Vol. 35, p. 533. 30 F. Paltauf, Chem. Phys. Lipids 17, 148 (1976). 31 W. J. Baumann and H. K. Marigold, J. Org. Chem. 29, 3055 (1964).
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2,3-isopropylidene-sn-glycerol, should be checked by measuring its optical rotation ([oq~] - 14.3°). 3z To a solution of 2,3-isopropylideneglycerol (10 g, 75.7 mmol) in 85 ml dry dimethyl sulfoxide, powdered potassium hydroxide (7.3 g, 130. I mmol) is added. The suspension is stirred for 1 hr at 50° with protection from atmospheric moisture. Then hexadecylmethane sulfonate (15.9 g, 49.6 mmol) is added, and stirring at 50° is continued. After 3 hr the reaction mixture is cooled to room temperature, 170 ml water is added, and the product is extracted with 3 portions (100 ml) of light petroleum. The solution is washed 3 times with water, then dried over anhydrous sodium sulfate, and the solvent is removed under reduced pressure. On TLC the product shows an Rf of 0.8 with light petroleum-diethyl ether (7 : 3, v/v) as the developing solvent. Small quantities of impurities (mainly dihexadecyl ether) do not interfere with the subsequent reaction and need not be removed. The crude 1-O-hexadecyl-2,3-O-isopropylidene glycerol (21.5 g) is hydrolyzed by stirring for 3 hr at room temperature in a solution of 80 ml of 10% aqueous hydrochloric acid in 500 ml methanol. After the addition of 500 ml water the product is extracted with 2 portions (200 ml each) of diethyl ether. The combined ether extract is washed consecutively with 3 portions of water (100 ml each) and dried over anhydrous sodium sulfate. The solvent is evaporated, and the product is isolated after recrystallization from 80 ml light petroleum. The yield of pure product is 15.3 g (80% based on hexadecylmethane sulfonate), mp 65.6 °. It shows a single spot on TLC (Re 0.15) with light petroleum-diethyl ether (3:7, v/v) as the developing solvent.
1-O-Hexadecyi-3-O-triphenylmethyl-sn-glycerol A mixture of 15.3 g 1-O-hexadecyl-sn-glycerol(48.3 mmol) and 27 g triphenylmethyl chloride (97 mmol) in 200 ml anhydrous pyridine is stirred for 24 hr at 50° under anhydrous conditions. 33The solution is poured into ice-cold water (400 ml), and the mixture is extracted with 3 portions (200 ml each) of light petroleum. Undissolved triphenylmethanol is removed by filtration. The filtrate is washed 3 times with water (100 ml) and dried over anhydrous sodium sulfate. The solvent is evaporated, and light petroleum, bp 400-60 ° (150 ml), is added to the residue. On standing overnight at 4° additional triphenylmethanol is precipitated. The solid is filtered off, and the filtrate is evaporated to dryness. TLC analysis (solvent A) of the resulting product (30.6 g) shows that the alkylglycerol has completely 32 G. Hirth, H. Saroka, W. Bannwarth, and R. Barner, Helv. Chim. Acta 66, 1210 (1983). 33 G. K. Chacko and D. J. Hanahan, Biochim. Biophys. Acta 164, 252 (1968).
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reacted. The crude 1-O-hexadecyl-3-O-triphenylmethyl-sn-glycerolis directly used for the next reaction step.
1-O-Hexadecyl-2-oleoyl-3-O-triphenyimethyl-sn-glycerol Oleic acid (15.5 g, 55 mmol) and carbonyldiimidazole (9.8 g, 60 mmol) in 100 ml water-free tetrahydrofuran are stirred at room temperature for 1 hr. The freshly prepared solution of oleoylimidazolide is added to l-Ohexadecyl-3-O-triphenylmethyl-sn-glycerol (15.3 g, 27.4 mmol). Tetrahydrofuran is removed by evaporation under reduced pressure, and the residue is dissolved in 100 ml dimethyl sulfoxide. After addition of the catalyst, which is prepared by dissolving metallic sodium (1.35 mg, 58.7 mg atoms) in 100 ml dimethyl sulfoxide, the reaction mixture is kept at room temperature under anhydrous conditions and slightly shaken at intervals of about 10 min. The reaction mixture is then cooled in an ice bath and rapidly neutralized with 160 ml of 0.2 N acetic acid in water. The product is extracted with 3 portions (20 ml) of chloroform-methanol (2 : 1, v/v). The extracts are combined, washed with 0.2 volumes methanol-water (1 : 1) containing 1.5% concentrated ammonia, then with 0.2 volumes methanol-water (1 : 1), and dried over anhydrous sodium sulfate. After evaporation of the solvent under reduced pressure, the crude product (23 g) is purified by MPLC. The sample, dissolved in I00 ml light petroleum (bp 40°-60°), is applied to a silica gel column (3 × 30 cm) 34and eluted with 300 ml light petroleum and 1 liter of light petroleum-diethyl ether (9 : 1, v/v); 100-ml fractions are collected. Pure alkylacyltriphenylmethylglycerol is obtained from fractions 4-7. The yield is 20.5 g (91% based on alkyltriphenylmethylglycerol). The product gives a single spot (Rf 0.6) on TLC developed with light petroleum-diethyl ether (9 : 1, v/v).
1-O-Hexadecyl-2-oleoyl-sn-glycerol 1-O-Hexadecyl-2-oleoyl-3-O-trityl-sn-glycerol (19 g, 23 mmol) is dissolved in 170 ml anhydrous dichloromethane containing 80 ml of a 14% (w/v) solution of boron trifluoride in methanol. 26The mixture is stirred at 0° for 60 rain and then extracted 3 times with ice-cold water. The organic phase is dried over anhydrous sodium sulfate, and the solvent is removed by evaporation under reduced pressure. An oily residue is obtained which is used without further purification for the preparation of the corresponding phosphatidic acid analog. 34 The column is preconditioned with light petroleum saturated with aqueous concentrated ammonia to deactivate the solid phase. On activated silicic acid partial hydrolysis of the triphenylmethoxy group would occur.
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1-O-Alkyl-2-oleoyl-sn-glycero-3-phosphate The following procedure is a modification 3° of the method described by Berecoechea et al. 35 A solution of 11.3 g (20 mmol) alkyloleoylglycerol and anhydrous pyridine (3.2 g, 40 mmol) in 60 ml anhydrous tetrahydrofuran is added dropwise with stirring to freshly distilled phosphorus oxychloride (3.3 g, 22 mmol) dissolved in 20 ml tetrahydrofuran. Stirring is continued for 3 hr at 0 °. Then 100 ml of 10% sodium bicarbonate is added at once, and the mixture is stirred for 15 min at 0°. The solution is then poured on ice water, acidified with HC1, and extracted with diethyl ether. After washing the ether phase with water and drying it over anhydrous sodium sulfate, the solvent is removed under reduced pressure, and 11.5 g of the product is obtained. TLC with solvent B shows the product at Rf 0.1 and slight impurities, at Rf 0.7, corresponding to dipyrophosphatidic acid. 35 The product is used for the subsequent reaction without purification.
1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphocholine 1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphate (1.65 g, 2.5 mmol) and choline toluene sulfonate 36 (1.4 g, 5 mmol) prepared according to Brockerhoff and Ayengar 37 are dried over phosphorus pentoxide under vacuum for 12 hr. The mixture is dissolved in 45 ml anhydrous pyridine, and 2,4,6-triisopropylbenzene sulfonyl chloride (1.82 g, 6 mmol) is added. The solution, protected from atmospheric moisture, is stirred for 1 hr at 70° and then for 4 hr at room temperature. After the addition of 2 ml water, the solvents are removed under vacuum on a rotary evaporator, and pyridine is removed by repeated evaporation in the presence of toluene. The residue is extracted with diethyl ether, and the solutions are decanted from undissolved material (mainly triisopropylbenzenesulfonic acid) and evaporated to dryness. The solid material is dissolved in 50 ml chloroform-methanol (2: 1), and the mixture is successively extracted with 10-ml portions of 3% aqueous Na2CO3, methanol-water (1 : 1), 2% HCI, and finally methanol-water (1 : 1). The organic phase is dried over sodium sulfate and evaporated under reduced pressure. The crude product (2 g) dissolved in 20 ml chloroform is applied to a 2 x 30 cm column for MPLC purification. The following solvents are used for chromatography: 200 ml chloroform, 50 ml each 35 j. Berecoechea, M. Faure, and J. Anatol, Bull. Soc. Chim. Biol. 50, 1561 (1968). 36 To a 40% solution of choline hydroxide in water an equimolar amount of toluene sulfonic acid is added, and the solvents are removed under reduced pressure. The choline toluene sulfonate is recrystallized from acetone. 37 H. Brockerhoff and N. K. N. Ayengar, Lipids 14, 88 (1979).
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of chloroform-methanol 9 : 1, 8 : 2, 7 : 3, and 6 : 4, followed by 500 ml chloroform-methanol 4 : 6. Fractions of 15 ml are collected, and the product (1.3 g, 70% yield) is obtained in fractions 28-42. On TLC with solvents B, C, and D it shows a single spot corresponding to authentic phosphatidylcholine from egg yolk.
1-O-Hexadecyl-2-oleoyl-sn-glycero-3-phosphoethanolamine A mixture of N-triphenylmethylethanolamine (360 rag, 1.2 mmol; prepared 38 according to Aneja et a1.27), triisopropylbenzenesulfonyl chloride (450 mg, 1.5 mmol), 1-O-hexadecyl-2-oleoyl-sn-glycero-3-phosphate (400 rag, 0.6 mmol) in 12 ml anhydrous pyridine, and 5 ml dry chloroform is stirred at room temperature for 3 hr. Then 2.5 ml of water is added, the mixture is evaporated under vacuum, and the residue is extracted 3 times with diethyl ether. After evaporation of the combined diethyl ether extracts, the crude product is purified by MPLC on a 3 x 35 cm column using the following solvents for elution: 200 ml chloroform, 200 ml chloroform-methanol (95:5), and 400 ml chloroform-methanol (9: 1); 50-ml fractions are collected. The product (450 rag, 80% yield) is obtained from fractions 14 and 15. TLC shows a single spot, Rf 0.6, with chloroform-methanol (9: 1) as the solvent. For removal of the triphenylmethyl protecting group, trifluoroacetic acid (15 ml) is added to an ice-cold solution of hexadecyloleoylglycerophospho(N-triphenylmethyl)ethanolamine (450 mg) in 15 ml dichloromethane under protection from atmospheric moisture. The reaction mixture is left at 0° for 5 min and then poured rapidly into a vigorously stirred solution (50 ml) of 6% ammonia in water. The aqueous phase is separated and extracted twice with chloroform (100 ml). The combined organic phases are washed with water and evaporated to dryness. The crude product is purified by MPLC on a silica gel column using a chloroform-methanol gradient for elution. The product (270 mg, 80%) is eluted at a chloroform-methanol ratio of 7 : 3. TLC with solvent B shows a single spot, Rf 0.6.
Chemical Synthesis of Diether Substrates The occurrence of 1,2-di-O-alkylglycerophosphocholines in nature has been reported, but obviously they constitute only a minute proportion of the choline glycerophospholipids isolated, for example, from bovine 38 N-Triphenylethanolamine is prepared by reacting triphenylmethyl bromide and ethanolamine in chloroform in the presence of triethylamine.
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heart) 9 Because they do not contain an acyl ester linkage, 1,2-dialkylglycerophospholipids cannot be hydrolyzed by phospholipase A or B, by lipases, or by other acyl ester hydrolases. However, they can serve as substrates for phospholipase C or D. 3 In addition, for studies on mechanisms and kinetics of phospholipases they can be used as inert matrices in which the appropriate cleavable diacylglycerophospholipid substrates are embedded at varying concentrations but in a constant total surface presented to the enzyme. Racemic di-O-alkylglycerols with two identical alkyl chains are preferentially synthesized from tetrahydropyranylglycerol.4° The enantiomers containing saturated alkyl chains are accessible via 1- or 3-O-benzylglycerols 41 which are commercially available. Di-O-alkylglycerols with two different alkyl chains (saturated or unsaturated) can be prepared from l(3)alkyl-3(1)-trityl-sn-glycerols42 (see above) by reaction with alkylmethane sulfonates. By the same route optically active dialkylglycerols with identical, or different, saturated or unsaturated alkyl chains can be prepared. The subsequent steps of phosphorylation and attachment of head group alcohols are the same as with alkylacyl substrates (see preceding sections).
1-O-Hexadecyl-2-O-octadec-9'-enyl-sn-glycerol 1-O-Hexadecyl-3-O-trityl-sn-glycerol (6 g, 10.6 mmol; see above) and powdered potassium hydroxide (6 g) in 100 ml xylene are refluxed for 1 hr in a 500-ml flask fitted with a water-separation head, reflux condenser, dropping funnel, and magnetic stirrer. Then octadecenylmethane sulfonate (4.5 g, 13 mmol; see above) in 50 ml xylene is added dropwise during 15 min, and refluxing is continued for 6 hr. After cooling 100 ml water is added, the xylene phase is removed, and the water phase is extracted twice with 100 ml diethyl ether. The combined organic phases are dried over anhydrous sodium sulfate and evaporated under reduced pressure. The residue (8.6 g) is dissolved in 25 ml light petroleum and purified by MPLC on a 3 × 25 cm column. The following solvents were used for elution: light petroleum (200 ml), light petroleum-diethyl ether, 9 : 1 (100 ml), and light petroleum-diethyl ether, 8 : 2 (300 ml). Fractions (I0 ml) are collected, and the product is recovered from fractions 8-28. The yield of pure product is 8 g (90%). The protecting triphenylmethyl group is removed by treating the product with boron trifluoride-methanol following the procedure described for the alkylacyl analog (see above). Pure 1-O-hexa39 E. L. Pugh, M. Kates, and D. J. Hanahan, J. Lipid Res. 18, 710 (1977). 4o F. Paltauf and F. Spener, Chem. Phys. Lipids 2, 168 (1968). 41 M. Kates, B. Palameta, and L. S. Yengoyan, Biochemistry 4, 1595 (1965). 42 W. J. Baumann and H. K. Mangold, J. Org. Chem. 31, 498 (1966).
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AND PHOSPHATES
decyl-2-octadec-9'-enyl-sn-glycerol (5 g) is obtained in 90% yield after MPLC purification on a 2 x 35 cm column, eluted with a light petroleum-diethyl ether gradient. It shows a single spot, Rf 0.2, on TLC with solvent A.
1-O-Hexadecyl-2-O-octadec-9'-enyl-sn-glycero-3-phosphocholine and -ethanolamine Di-O-alkylglycerophospholipids are prepared from di-O-alkylglycerols by procedures described for the alkylacyl analogs 43 (see above). On TLC using solvents B, C, or D they migrate to the s a m e Rf as the analogous alkylacyl- and diacylglycerophospholipids. Acknowledgments The authors acknowledge the excellent technical assistance of H. Sttitz and thank R. Franzmair for stimulating discussions. The original work described in this chapter has been supported by the Fonds zur F6rderung der wissenschaftlichen Forschung in Osterreich (Project 5746B). 43Because dialkylglycerophospholipids are stable in dilute alkali or acid, workup procedures are more straightforward than with 1'-alkenylacylor alkylacyl analogs. Acidic or basic byproducts can readily be removed by extraction from a chloroform-methanol phase, with dilute sodium hydroxide or hydrochloric acid, respectively.
[13] Inositol P h o s p h o l i p i d s a n d P h o s p h a t e s for I n v e s t i g a t i o n of I n t a c t Cell P h o s p h o l i p a s e C S u b s t r a t e s a n d P r o d u c t s
By
MICHAEL R. HANLEY, DAVID R. POYNER, PHILLIP T. HAWKINS
and
Introduction The receptor-coupled activation of inositol lipid-specific phospholipase C is conventionally analyzed in intact cells, as there has been comparatively limited success in developing cell-free or reconstituted systems. Thus, much of the information on the physiological substrates and resulting enzymatic products has been inferred from the kinetics of changes in the levels of labeled lipids and the production of labeled water-soluble products. The early work on this second messenger pathway used tissue slices or acutely isolated cells, but, where feasible, the recent emphasis has been placed on established cell lines, which have advantages in homoMETHODS IN ENZYMOLOGY, VOL. 197
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