JouRNAL OF BACTERIOLOGY, Nov. 1975, p. 718-723 Copyright C) 1975 American Society for Microbiology
Vol. 124, No. 2 Printed in U.S.A.
Factors Affecting the Palmitoyl-Coenzyme A Desaturase of Saccharomyces cerevisiae HAROLD P. KLEIN* AND CAROL M. VOLKMANN Planetary Biology Division, Ames Research Center, NASA, Moffett Field, California 94035
Received for publication 19 June 1975
The activity and stability of the palmitoyl-coenzyme A (CoA) desaturase complex of Saccharomyces cerevisiae was influenced by several factors. Cells, grown nonaerobically and then incubated with glucose, either in air or under N2, showed a marked increase in desaturase activity. Cycloheximide, added during such incubations, prevented the increase in activity, suggesting de novo synthesis. The stability of the desaturase from cells grown nonaerobically was affected by subsequent treatment of the cells; enzyme from freshly harvested cells, or from cells that were then shaken under nitrogen, readily lost activity upon washing or during density gradient analysis, whereas aerated cells, in the presence or absence of glucose, yielded stable enzyme preparations. The loss of activity in nonaerobic preparations could be reversed by adding soluble supernatant from these homogenates and could be prevented by growing the cells in the presence of palrritoleic acid and ergosterol, but not with several other lipids tested. Cell particulates have long been implicated strain LK2G12, were grown in standing cultures as as the sites for the oxidative desaturation of described previously (12, 15). After harvesting, cells long-chain fatty acids. A number of investiga- were washed and then suspended in a buffer, pH 7.5, 0.25 M mannitol, 2 mM tris(hydroxytions, using preparations from such diverse or- containing and 2 mM MgCl2. Suspenganisms as fungi (2), insects (25), and higher methyl)aminomethane, sions of cells were disrupted in a Braun homogenizer plants and animals (5, 17, 26), have implicated (18) by the method of Schatz (20), and the resulting the microsomal fraction as the environment crude homogenates were freed of unbroken cells and within which aerobic desaturation to unsatu- large particulates by two 10-min centrifugations at rated fatty acids takes place. It has been demon- 3,300 x g, after which further fractionation was strated that this enzyme system is also particu- carried out to yield mitochondrial, microsomal, and late in the yeast Saccharomyces cerevisiae (3, soluble supernatant fractions as previously de12) and that oxygen is required for desatura- scribed (13). By these procedures, the resultant crude mitochondrial pellets contained virtually all tion. of the succinic dehydrogenase, cytochromes, and cyIn the present study, the desaturation of pal- tochrome oxidase of the initial homogenates, mitoyl-coenzyme A (CoA) was investigated, us- whereas 70% of the original ribonucleic acid was ing a particulate fraction of S. cerevisiae. Large found in the crude microsomal fractions. Under increases in the activity of the palmitoyl-CoA these conditions of centrifugation, the palmitoyldesaturase could be obtained by incubating CoA desaturase distributed into both particle fracthese preparations in the presence of glucose. tions. However, the crude mitochondrial fraction Furthermore, the stability of this enzyme com- routinely contained more than half the total desatuplex was observed to vary considerably depend- rase activity of the homogenates. Therefore, this ing upon the conditions under which it was fraction was the source of enzyme for all the experiments to be described below, unless otherwise speciformed.
fied. Density gradients. Density gradient analyses of MATERIALS AND METHODS the crude enzyme preparations were performed by Materials. Palmitoyl-[1-'4C]CoA (specific activity layering a 1-ml suspension on a linear sucrose gra= 45.5 mCi/mmol) and choline-[methyl-'4C]chloride dient (1.1270 to 1.2575 g/ml density) with a 3-ml (specific activity = 4.6 mCi/mmol) were purchased cushion of 60% sucrose (density = 1.2865 g/ml). The from New England Nuclear Corp. Nicotinamide gradients were centrifuged for 16 h at 56,000 x g and adenine dinucleotide phosphate, reduced form, er- then collected in nine aliquots of 3 ml each with an gosterol, and tris(hydroxymethyl)aminomethane ISCO model D gradient fractionator. On gradients of were products of Sigma Chemical Co. Unlabeled this type, a small amount of material sediments as a fatty acids were purchased from Applied Science pellet, and occasionally this material contained Laboratories. some desaturase activity. Enzyme preparation. Cells of S. cerevisiae, Enzyme assay. The complete system for the as71E
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PALMITOYL-COA DESATURASE OF YEAST
of the palmitoyl-CoA desaturase contained 6 to 10 nmol of palmitoyl-[1-_4C]CoA, 0.5 mM nicotinamide adenine dinucleotide phosphate, reduced form, 100 mM phosphate buffer, pH 7.0, and sufficient protein to yield a linear rate of desaturation, in a total volume of 1 ml. The assay mixture was equilibrated in air at 30 C for 3 min, and the reaction was started by the addition of the enzyme. At the end of 4 min of incubation, the reaction was stopped by the addition of 0.5 ml of saturated KOH. The samples were then saponified, and the fatty acids were extracted and methylated as reported previously (13). After methylation, palmitic and palmitoleic acids were separated by gas chromatography (11), and the radioactivity associated with each peak was counted. Enzymatic activity is expressed as nanomoles of palmitoleic acid produced and was obtained as follows: say
(cpm in palmitoleic acid/cpm in palmitic + palmitoleic acids) x the initial nanomoles of palmi-
toyl-CoA used, where cpm represents counts per minute. Other determinations. Protein was determined as described by Lowry et al. (16) and ribonucleic acid by the orcinol (1) methods. The vegetative "petite" of the parent strain LK2G12, produced by ethidium bromide treatment, was devoid of measurable mitochondrial deoxyribonucleic acid. We thank D. Cryer of the Albert Einstein College of Medicine for supplying us with this organism.
by glucose. However, the addition of cycloheximide completely abolished the increases in enzyme activity, suggesting that de novo synthesis of the desaturase complex is induced by aeration with glucose. Similar levels of stimulation by glucose were obtained whether yeast suspensions were incubated under N2 or in air, the increase in desaturase
activity being at
constant rate up to
about 2 h in both cases. Furthermore, vegetative petite mutants of our strain of yeast, devoid of any measurable respiratory activity, were also found to increase their desaturase content after incubation with glucose. Interestingly, no increase was observed under these conditions in the activity of the fatty acid synthetase complex, according to Kraeger and Klein (unpublished data), or of acetyl-CoA
synthetase (15). Stability of the fatty acid desaturase
complex. To determine whether the desaturase complex produced in the presence of added glucose had different properties, on a density gradient, than those of the original particle preparations, aliquots of material from glucose-treated cells were subjected to equilibrium centrifugation on sucrose gradients. The most striking finding in these studies was not related to glucose at all, but rather to aeration (Table 2). Only those preparations aerated with glucose retained full activity on these gradients. Suspensions that had been shaken with glucose under nitrogen yielded very poor recoveries, as did preparations from nonaerobic cells. That aeration was important in stabilizing the enzyme activity could be shown by simply aerating nonaerobic cells in buffer without glucose, after which the resulting homogenates retained full activity on these gradients. When individual fractions from gradients of zyme
RESULTS Effect of glucose on desaturase activity. The addition of glucose to aerated suspensions of nonaerobic cells caused greater increases in desaturase activity than aeration in phosphate buffer alone. After 2.5-h aeration in the presence of glucose, the cells contained three to four times as much enzyme activity as in the absence of glucose (Table 1). or Luchloramphenicol, at concentrations that affect mitochondrial nonaerated preparations were appropriately refunction (9, 15), had no effect on this stimulation combined, substantial restoration of initial acn-
TABLE 1. Stimulation of desaturase activity by glucose: effect of chloramphenicol and cycloheximidiea % Glucose added 0
Activity per mg of protein
None (control homogenate) L-Chloramphenicol (1,000 ug/ml) Cycloheximide (6.4 ,ug/ml) a
4.0 3.4 2.1
per mg of in protein activity
7.9 8.3 1.3
98 144 0
Activity per mg of
10.6 10.0 1.3
per mg of
165 194 0
12.3 12.0 1.3
208 253 0
Cells were grown nonaerobically, harvested, resuspended with varying amounts of glucose in 0.1 M
phosphate buffer, pH 7.0, and aerated for 2.5 h with other additions as shown. For this experiment, the unfractionated cell-free homogenates were used. The initial specific desaturase activity of the nonaerated cells, before aeration, was 1.3 nmol.
KLEIN AND VOLKMAN
TABLE 2. Palmitoyl-CoA desaturase activity on sucrose density gradientsa Initial Fraction
1 2 3 4 5 6 7 8 9
After 2.5 h in air
Activ- Pro- Activity tein ity (nmol) (mg) (nmol) (60.0) (11.2) (178.0)
After 2.5 h
Pro- Activ- Protein ity tein (mg) (nmol) (mg) (10.2) (237) (15.2)
0.8 1.2 1.0 1.4 1.4 2.3 2.3 1.7 1.3 3.7
4.2 0.5 0.8 0.8 0.7 0.9 1.0 0.8 0.8 0.8
3.7 4.8 12.9 20.3 23.1 35.4 49.6 5.2 3.5 1.9
2.2 0.8 1.1 1.0 1.2 1.4 1.7 0.6 0.3 0.1
3.8 6.5 8.2 9.2 4.8 5.2 4.3 1.8 1.4 4.2
3.5 1.0 1.4 1.5 1.2 1.7 1.8 1.0 0.8 0.5
a Cells were grown nonaerobically and an aliquot was used to provide material for the control (initial) gradient. The remaining cells were resuspended in 0.1 M phosphate buffer, pH 7.0, containing 5% glucose, stirred in air or under nitrogen, and then further processed. b Total units of activity or milligrams of protein applied to gradient.
tivity resulted. In the experiment summarized in Table 3, fractions obtained from a gradient of this type were recombined in all possible combinations of two fractions. Shown here are the data for combinations involving the top (fraction 1), middle (fraction 5), and bottom (fraction 9) regions of the gradient. It is evident that those combinations containing material from the top and the bottom regions of the gradient were considerably more active than could be accounted for by the simple summation of the two activities. These observations suggest the possibility that some easily dissociable soluble factor, necessary for the activity of the enzyme complex obtained from nonaerated cells, had been removed from that complex during density gradient analysis. This supposition is supported by other experiments in which the crude particulate desaturase complex from nonaerated cells lost activity when simply washed in buffer a number of times. By suspending the crude particulate desaturase preparations, centrifuging, and decanting the suspending fluid three times, the resulting particles showed losses of activity of up to 70%. Virtually no activity was found in the washes under these conditions. However, full activity could be restored by recombining such washed particles with the buffer wash water. The factor(s) re-
leased by washing can also be replaced by the soluble supernatant obtained from nonaerobic cells. This stimulation appears to be specific, since supernatants obtained from aerated cells, or from aerobically grown cells, do not restore the activity of the washed nonaerobic preparations. It should be emphasized that no loss of desaturase activity occurred on density gradients, or upon washing, using particles obtained from aerobically grown cells or from nonaerobic cells that had been aerated. Variation in cellular lipids and stability of the fatty acid desaturase complex. Because oxygen is known to be required in the formation of unsaturated fatty acids (3, 11) and sterols (14) in this strain of yeast, and because these lipids are important components of cellular membranes, it seemed plausible that the basis for the apparent instability of the fatty acid desaturase of nonaerobic cells might be the result of reduced levels of unsaturated fatty acids or sterols, or both, in such cells. In a series of experiments, cells were grown nonaerobically in the presence of added unsaturated fatty acids or ergosterol, or both. After harvesting the cultures, the resulting crude particulate desaturase complex from each of these was subjected to sucrose density gradient fractionation to determine whether the incorporation of any of these lipids conferred stability on the desaturase system. Cells grown without added lipids yielded preparations that lost 70 to 80% of their initial activity on these gradients (Table 4). Palmitoleic or oleic acids were readily incorporated into the yeast cells, resulting in TABLE 3. Stimulation of desaturase activity by recombinations of fractions from density gradient Activity after recombinationa Gradient fraction
2 3 4 5 6 7 8 9
Initial activity (nmol)
1.35 1.27 1.07 1.47 1.68 2.52 2.38 2.70 2.40
0.40 0.85 0.32 0.28 0.63 2.33 1.08 2.29
0.28 0.49 0.56 0.34
(0.64) 0.67 0.26 0.18
Plus fraction 9 (nmol)
2.29 2.26 2.05 1.08 0.18 0.25 0.48 0.62
a Data in these columns indicate increased desaturase activity over that expected by simple summation of the activities of the recombined individual fractions. The figure in parentheses indicates decreased desaturase activity.
PALMITOYL-COA DESATURASE OF YEAST
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TABLE 4. Effect of lipid supplerentation on the stability on density gradients of the palmitoyl-CoA desaturase of nonaerobic cellsa Lipid composition of crude desaturase preparations
acid (wt % of total
None (6) None, with aeration (4) Oleic acid Palmitoleic acid Oleic plus palmitoleic acids Ergosterol Oleic acid plus ergosterol Tween 80 plus ergosterol Palmitoleic acid plus cholesterol Palmitoleic acid plus ergosterol (3)
Oleic acid (Wt % Of total fatty
22 ± 2.6 39.3 t 3.5 10.0 41.0 26
14.8 ± 2.9 21.7 t 0.2 40.0 11.0 22
11.0 20.0 46.0 45.0 + 2
39.0 55.0 10.0 9.0 t 1.5
% Recovery of
trl(ggdstrs dmg/wterol (mwt dsaurs (gg
dry wt) 1.7
0 0 0 0 0 0 0 2.3 0
2.9 2.8 3.3 1.6
83.0 ± 9.1 31.0 41.0 40.0 41.0 34.0 40.0 37.0 73.0 ± 6.3
a For these experiments, cells were grown in 2-liter standing cultures, with or without supplements as shown; oleic and palmitoleic acids (in absolute ethyl alcohol) were added to yield a final concentration of 30 mg/liter; ergosterol (in absolute ethyl alcohol) was added to yield a final concentration of 110 mg/liter; Tween 80 was added to give 1.5 mg/ml final concentration. Figures in parentheses refer to replicate experiments; for these the standard deviation of the results are given. Palmitoleic and oleic acid contents were estimated from gas chromatograms; ergosterol content was determined by the method of Shaw and Jefferies (21). Cholesterol was determined by subtracting ergosterol from the total sterol content.
shifts in the ratio of saturated to unsaturated fatty acids of the order of that found in aerated cells, but these added fatty acids only slightly increased the stability of this enzyme complex. Nor did the incorporation of ergosterol result in substantial increases in stability. On the other hand, when both palmitoleic acid and ergosterol were assimilated into the cellular lipids, the desaturase exhibited the properties of aerated preparations on the gradients. Of interest is the fact that combinations of oleic acid plus ergosterol, or palmitoleic acid plus cholesterol, were ineffective in this regard despite the ready incorporation of these lipids into the particulate fraction containing the desaturase. DISCUSSION Palmitoyl-CoA desaturase activity is strongly increased when cells of S. cerevisiae are incubated with glucose even under conditions that severely restrict desaturase activity, i.e., under anaerobic conditions. This increase is probably the result of de novo synthesis of additional enzyme protein, since this effect is completely abolished by cycloheximide. In this regard, these results are analogous to those of Sato and his collaborators, who have reported similar effects of glucose with rat liver preparations (R. Sato, N. Oshino, T. Shimakata, K. Mihara, and Y. Imai, 9th Int. Congr. Biochem. Abstr., p. 322). In yeast, the induction of desaturase by glucose seems to be relatively specific,
since two other enzymes involved in lipid synthesis, fatty acid synthetase and acetyl-CoA synthetase, are inert to this effect. In the case of rat liver microsomes, Shimakata et al. (22) have shown that the synthesis of a specific protein component, CSF protein, of the stearylCoA desaturase complex is induced when starved animals are fed a high carbohydrate diet. During this study, it became clear that the palmitoyl-CoA desaturase complex formed under anaerobic conditions exhibits an inherent instability compared with enzyme produced when the cells are exposed to air. From the experiments described above, it appears that some soluble component(s), essential for activity, is removed from the particulate complex by washing or by the procedures used in density gradient analysis. Soluble supernatant from nonaerated cells is most active in reconstituting such depleted particulate preparations, although washes and low-density material from density gradient preparations of nonaerated cells are also active. At present, there is essentially no information on the nature of this soluble component. Superficially, the results resemble those of Scallen et al. (19), who observed that washing rat liver microsomes three times virtually eliminated the ability of these particles to convert squalene to sterols, and that the addition of protein from the soluble supernatant restored this activity. On the basis of their
KLEIN AND VOLKMAN
results, these authors postulated the existence of carrier proteins, whose role is to keep highly insoluble lipids in solution. Another, more reasonable, possibility is that the inherent instability of the palmitoyl-CoA desaturase of nonaerobic cell preparations represents a structural lability of the desaturase complex itself. It should be noted that several authors working with animal desaturase systems have been able to fractionate this complex by chemical and physical methods. Thus, Holloway (7) found that treatment of hen liver microsomes with deoxycholate and N-ethylmaleimide solubilized a nicotinamide adenine dinucleotide phosphate, reduced form, cytochrome b5 reductase from these particles. Similarly, Shimakata et al. (22) were able to separate and remove this reductase from rat liver microsomes by using detergents. In both cases, the particles resulting from the respective treatments were inactive in the absence of the reductase. By analogy with these findings, it may well turn out that the nonaerobic desaturase complex is more readily separable into its component parts than the animal systems, and that the solubilizable component of all these systems is a reduced pyridine nucleotide-cytochrome reductase. A number of laboratories have reported that modifications of the lipid content of yeast membranes, by introducing specific lipids during growth, result in changes in the physical or catalytic properties of a number of membranebound enzymes (4, 10). Alteration of membrane lipids has also been reported to result in changes in several membrane-dependent activities (6, 24). In the studies reported here, alteration of the cellular lipid composition resulted in significant changes in the properties of the palmitoyl-CoA desaturase activity of yeast. Cells grown in the absence of air, and hence restricted in producing unsaturated fatty acids and ergosterol, yielded desaturase from which some component(s) could be readily solubilized. The reason for the apparently specific requirement for palmitoleic acid and ergosterol to stabilize this complex is obscure. It may be that these lipids, when incorporated into the membranes that bind the desaturase, are more effective than other lipids in maintaining optimal binding of the complex. In the absence of these lipids, partial dissociation of the desaturase complex could occur upon washing or gradient analysis, releasing one or more soluble elements. Another possibility is that the desaturase complex itself contains lipid material. This has been reported, for example, in the case of the stearoyl-CoA desaturase studied by Hollo-
way and Wakil (8), although no role has been ascribed to these lipids. More recently, Strittmatter et al. (23) have purified and described the properties of the rat liver microsomal stearyl-CoA desaturase. In addition to cytochrome b5 reductase, cytochrome b5, and the desaturase, they confirmed a lipid requirement for optimum desaturation. It is possible that the yeast system may also contain lipid material that is an integral part of the desaturase complex. If this were so, one function of the lipids might be to keep the complex together, perhaps by interacting appropriately with the hydrophobic proteins in the complex. Purification of the yeast system will be necessary to obtain a clear resolution of the function of these
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