Metabolism Hector

F. DeL uca,2

of vitamin

D: current

Ph.D.

Since 1965 much new information has become available on the functional metabolism of vitamin D3. Following the initial breakthrough, advances have been so rapid that considerable confusion exists concerning what is known about the function of the various metabolites, their biological activity, and their regulation. This brief review will attempt to summarize the status of our information concerning known pathways and their relevance (Fig. 1). Inasmuch as a great deal remains to be learned concerning the metabolism of vitamin D and the regulation of its active metabolites, indications of the information still lacking will also be given. More complete reviews are available (1 -6), and readers are instructed to use the present report as a means of updating the more comprehensive reviews. Production

of vitamin

D3 in skin

(7)

No new information has become available in this important area in the past decade. It is well known that 7-dehydrocholesterol exists in the epidermis and that ultraviolet light will penetrate to this site. Based on the in vitro conversion of 7-dehydrocholesterol to vitamin D3 in organic solvents by ultraviolet light, it is assumed but not proved, that this occurs in the epidermis. It has been demonstrated that man subjected to large amounts of sunlight will show increased circulating levels of 25-hydroxyvitamin D3 (25-OH-D3); this indicates that vitamin D3 production takes place as a result of skin irradiation (8). However, much remains to be learned concerning this biological process and its possible regulation.

25-OH-D3, primarily because of a reaction supported by NADPH and molecular oxygen. This reaction occurs in the endoplasmic reticulum (14, 15), but its mechanism is unknown. It is not blocked by carbon monoxide, lipid peroxidation inhibitors, cytochrome P-450 inhibitors, and is not induced by phenobarbital (16). There may exist an additional, less specific 25-hydroxylase that hydroxylates dihydrotachysterol and cholesterol as well as vitamin D3 on carbon 25. Additional work is needed to elucidate the nature of the 25hydroxylase and whether there is in fact more than one 25-hydroxylase. Tucker et al. (18) have demonstrated the conversion of vitamin D3 to 25-OH-D3 with homogenates of chick intestine and kidney. Although this work has in part been confirmed (19), hepatectomized animals produce little or no 25-OH-D3 (20, 20a). The small amount of 25-OH-D3 produced in the hepatectomized rat is associated with high circulating levels of vitamin D3. Such circulating levels of vitamin D3 are not seen at physiological doses in intact animals. Since vitamin D3 accumulates in the liver and since there is little production of 25-OH-D3 in the absence of this organ, it is likely that the liver is the major, if not sole, site of vitamin D3-25hydroxylation, at least in mammals. There is some debate over whether the 25-hydroxylase is regulated (18, 21, 22). There is little doubt that the administration of vitamin D3 to vitamin D-deficient animals suppresses the level of the 25-hydroxylase as measured in vitro (22). The degree and length of suppression depend upon the dose of the vitamin given. Although it has been suggested 1

The

25-hydroxylation

of vitamin

D3

of

Following the administration of vitamin D3 either orally or intravenously there is a rapid accumulation of the vitamin in the liver (9-13). There the vitamin is converted to

1258

The American

status1

Journal

of Clinical

Nutrition

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

Supported

Grant 2

man, tural son,

by

the

Harry

Steenbock

Research

Fund

Research Foundation and AM-14881 from the National Institutes of Health. Harry Steenbock Research Professor and ChairDepartment of Biochemistry, College of Agriculand Life Sciences, University of Wisconsin-MadiMadison, Wisconsin 53706.

the

Wisconsin

29:NOVEMBER

Alumni

1976, pp. 1258-1270.

Printed

in U.S.A.

METABOLISM

OF

VITAMIN

D: CURRENT

STATUS

1259

OH

Liver

?

,CH2

H2

HO D3

25,26-(OH)2D3

25-OH-D3

OH

I) PTH

2) Low Pi Kidney

HO’ (24R)-24, 25-(OH)2

D3

H

(24

R)-I,

24,25-

(OH)3 D3

1,25

-(OH)2

D3

FIG. I. Diagrammatic representation of the known pathways of vitamin D metabolism. Starting D-deficient animal, the pathway of metabolism is vitamin D3 to 2S-OH-D3 to I,25-(OH)2D3 l,24,25-(OH)3D3, following induction of the 24-hydroxylase by the presence of l,25-(OH)2D3.

that this type of regulation does not occur in the chicken (18, 21), more recent work has shown that when doses of 20 or more units of vitamin D3 are administered to the chick, there occurs a clear suppression of the hepatic 25-hydroxylation reaction (19). This suppression can be overcome by the administration of large amounts of vitamin D. It is unknown whether this is due to competition between vitamin D3 and the suppressant hepatic level of 25-OH-D3 or whether it is due to the 25-hydroxylation of vitamin D3 brought about by the cholesterol 25-hydroxylase. Additional work is required to clarify the nature and importance of this regulation. When given daily to vitamin D-deficient rats or chicks, 25-OH-D3 is three to five times as active as vitamin D3 (23; J. Omdahl, L. Baxter, and H. F. DeLuca, unpublished results). First reports showed that 25-OH-D3 is 1.4 times as active as vitamin D3 (24). However, those studies utilized the standard

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

with a vitamin and further to

antirachitic rat assay in which a single dose of the vitamin or metabolite is administered to rachitic rats and bone ash accumulation is measured 7 days later (25). Since 25-OH-D3 has a shorter life-time in the body than vitamin D3, the low estimate of biological activity may have been due to more rapid turnover of 25-OH-D3. Renal hydroxylation 1,25-dihydroxyvitamin

of 25-OH-D3 to D3 (1,25-(OI-I )2D3)

At physiological doses 25-OH-D3 does not function directly in any of the known systems responsive to vitamin D3. Large concentrations of the metabolite will stimulate bone resorption in culture (26), will stimulate calcium binding protein production in intestinal cultures (27) and calcium transport in vascularly perfused intestine (28). However, nephrectomized animals, which are unable to metabolize 25-OH-D3 to l,25-(OH)2D3, do

1260

DELUCA

not respond to physiological doses of 25-OH-D3; this demonstrates the essentiality of the kidney (29-3 1 ). 25-OH-D3 is converted to l,25-(OH)2D3 in kidney mitochondria (32-35). This reaction is supported by Krebs cycle substrates and molecular oxygen (32--35). NADPI-I cannot be used because of its failure to penetrate intact mitochondria. When mitochondria are swollen to permit NADPH entry, then this compound can serve as the reductant in the hydroxylation reaction (35). The l-hydroxylase reaction is a mixedfunction oxidase as demonstrated by oxygen 18 experiments (36). Furthermore, inhibition by metyrapone, glutethimide, carbon monoxide, and the reversal of carbon monoxide inhibition by 450 nm light suggests it to be a cytochrome P-450 dependent reaction (35, 37). The existence of cytochrome P-450 in chicken kidney mitochondria was recently demonstrated by spectral means (38). Proof that the 1-hydroxylase is indeed a P-450 dependent reaction was provided by the demonstration that the cytochrome P-450 from rachitic chick renal mitochondria will produce l,25-(OH)2D3 from 25-OH-D3 when combined with beef adrenal ferredoxin, NADPH, and beef adrenal ferredoxin reductase (38). Recently this has been further investigated by the isolation of renal ferredoxin from rachitic chick kidney mitochondna, renal ferredoxin reductase and more highly purified renal cytochrome P-450 (J. Pedersen, J. Ghazarian, and H. F. DeLuca, in preparation). When these compounds are recombined only one product is produced from 25-OH-D3, namely l,25-(OH)2D3, which is obtained in excellent yield. Detailed study of this system has shown that all three components are essential for the 1-hydroxylation of 25-OI-I-D3. This provides unequivocal evidence for the hydroxylation mechanism demonstrated in Figure 2. Additional work is required to isolate the cytochrome P-450 in pure form, to study its binding capacity for 25-OH-D3 and l,25-(OH)2D3, and its interaction with the renal ferredoxin. I ,25-(OH)2D3 is 10 times more active in the rat than vitamin D3 itself (23, 39). In the chick the estimated biological activity is 2 to 4 times that of vitamin D3 (40, 41). This has been estimated by comparing frequent dosages of vitamin D3. Previous estimates of

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

COMPONENTS

OF

25-OH-D3-lct-HYDROXYLASE

OF

CHICK

KIDNEY RENAL

NADPH

FP

Cytochrome

-FERREDOXIN

P5O’

*

lcz,25-(OH)2D3 FIG. 2. Diagrammatic transfer system involved 25-OH-D3.

25-OH-D3 representation of the electron in the I-hydroxylation of

13-fold are based on the response to a single dose of l,25-(OH)2D3 with attendant errors resulting from differences in time course relative to that of vitamin D3. In nephrectomized animals l,25-(OH)2D3 produces increased intestinal calcium absorption (29, 3 1), increased bone calcium mobilization (30), and increased intestinal phosphate transport (42). This suggests that it or a further metabolite must be the metabolically active form of vitamin D. l,25-(OH)2D3 is extremely effective in the mineralization of bone in the chick and the rat. So far there is no evidence to suggest that another metabolite of vitamin D3 functions in the calcification of bone. However, this important issue requires additional, more refined studies. The 24-hydroxylation

of vitamin

D compounds

Another important hydroxylation reaction that occurs in the kidney was discovered as a result of the regulation of the 1-hydroxylase (see also below). Whenever the 1-hydroxylase is suppressed, as in normal animals, there is a stimulation of 24-hydroxylation reaction (I, 2). The 25-OH-D3-24-hydroxylase is also located in the mitochondria, is supported by Krebs cycle substrates, and molecular oxygen (43). However, unlike the 1-hydroxylase, the 25-OH-D3-24-hydroxylase does not appear to be inhibited by carbon monoxide and other cytochrome P-450 inhibitors. It is, however, supported by NADPH in calcium swollen mitochondria; this suggests that it is similar in some respects to the 1-hydroxylase system (43). Little more is known concerning this hydroxylase and much work is needed to

METABOLISM

OF

VITAMIN

decipher the nature of the electron transport chain involved in this reaction. There are two possible stereoisomers of 24,25-dihydroxyvitamin D3 (24,25-(OH )2D3). The S and R configurations on carbon 24 are illustrated in Figure 3. Two groups have synthesized the S and R isomers of this metabolite (44, 45) and of 24-hydroxyvitamin D3 (24-OH-D3) (46), a synthetic analog. Using the 24,25-(OH)2D3 S and R isomers, it has been possible to show by cochromatography on high-pressure liquid columns that the natural product is the R isomer (47). Furthermore, the 24R,24,25-(OH)2D3 has biological activity approaching 25-OH-D3, at least in the rat, whereas the S isomer has only slight activity in intestinal calcium transport and very low activity in the bone calcium mobilization system (47, 48). Its activity is somewhere in the neighborhood of 1/10 that of the R isomer. In the bird, however, the 24R,24,25-(OH)2D3 as well as the 24S,24,25-(OH)2D3 has low biological activity (49). Metabolic studies using radioactive 24,25-(OH)2D3 have revealed that this compound persists for long periods in the vitamin D-deficient rat, whereas it is rapidly metabolized and excreted in the chicken. In the rat nephrectomy prevents 24,25-(OH)2D3 from stimulating intestine and bone (49, 50). The product of 24,25(OH)2D3 metabolism has been isolated in pure form and demonstrated to be 1,24,25trihydroxyvitamin D3 (1 ,24,25-(OH )3D3), which is synthesized in the kidney (51). The l,24,25-(OH)3D3 also has S and R isomers and presumably the correct or natural product is the 24R,l,24,25-

D: CURRENT

(OH)3D3. From preliminary results it appears that the S isomer of 1,24,25-(OH)3D3 has activity equal to that of the R isomer; this may mean that discrimination may involve the 1-hydroxylation site (Y. Tanaka and H. F. DeLuca, unpublished results). In intestine the l,24,25-(OH)3D3 is approximately 60% as active as l,25-(OH)2D3. It appears to be considerably less active than l,25-(OH)2D3 in the mobilization of calcium from bone in the rat. Inasmuch as the 1,24,25-(OH)3D3 isomers are now available by virtue of successful synthesis by Milan Uskokovic at Hoffmann-LaRoche (personal communication) and by Professor N. Ikekawa in Tokyo (52), it seems that this area will be clarified considerably in the next year. A major question remains as to the role of the 24-hydroxylation reaction. Is it a signal for excretion or does it represent a functional pathway? The persistence of 24,25-(OH)2D3 in vitamin D-deficient mammals and the significant biological activity of the 1,24,25-(OH)3D3 in the rat suggests that it may have functional importance. On the other hand, the low biological activity in the bird and its rapid turnover in that species suggests that it may be an excretory product (49). It has been suggested that the chicken discriminates against vitamin D2 and vitamin D4 because of the methyl substitution on carbon 24, which may be interpreted as an analog of the normal excretory 24-hydroxylated vitamin D metabolites. Certainly labeled vitamin D2 and vitamin D4 are rapidly excreted in birds (53, 54).

Regulation In

24

R 24.25.(OH)D3

24S

24,25-(OH)203

FIG. 3. Structures of the 24R and 24S isomers of 24,25-(OH)2D3. Determination by cochromatography of the natural product with synthetic S and R isomers has revealed the R as the configuration in the natural product.

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

1261

STATUS

of the renal

hydroxylases

1971 Boyle et al. (55) first recognized that dietary and serum calcium concentration play an important role in the regulation of 1,25-(OH)2D3 production. In animals given a source of vitamin D they were able to show that low calcium diets, accompanied by low serum calcium, stimulate the production of l,25-(OH)2D3 and its accumulation in tissues and blood. On the other hand, diets high in calcium, leading to increased serum calcium concentrations, shut down the 1-hydroxylase and stimulate the production of 24,25-(OH) 2D3 (56). An examination of this relationship

1262

DELUCA

revealed that at normal serum calcium levels the animal produces both l,25-(OH)2D3 and 24,25-(OH)2D3. As the animals are made hypocalcemic, there is preferential stimulation of l,25-(OH)2D3 synthesis. As the animals become hypercalcemic l,25-(OH)2D3 production is suppressed and 24-hydroxylation is stimulated. This has more recently been demonstrated to be due to parathyroid gland intervention (57, 58). It now appears that low serum calcium concentration causes stimulation of parathyroid hormone secretion, which in turn stimulates 1-hydroxylation of 25-OH-D3 and suppression of 24hydroxylation. These results establish l,25-(OH)2D3 as a hormone whose biosynthesis is regulated by the need for calcium through the parathyroid hormone secretion mechanism. A revised scheme of calcium homeostasis is shown in Figure 4. It is important to realize that l,25-(OH)2D3 functtions in the intestine without the help of parathyroid hormone; in fact parathyroid hormone appears to have little or no direct effect on intestinal calcium transport (59). On the other hand, l,25-(OH)2D3 cannot function at physiological doses in the mobilization of calcium from bone unless parathyroid hormone is present (59). It is evident, therefore, that in totally hypoparathyroid patients, l,25-(OH)2D3 can stimulate intestinal calcium absorption, but it cannot stimulate mobilization of calcium from bone. Thus, treatment of hypoparathyroid patients should be most effectively accomplished by the use of dietary calcium plus sufficient levels of l,25-(OH)2D3 or an analog (60). Thyroparathyroidectomized animals, depleted of phosphate by either phosphate deprivation or by glucose loading, will produce I,25-(OH)2D3 (61). Henry et al. (62) have reported that low phosphate diets do not stimulate 1-hydroxylase activity in the chick. However, their diets contained appreciable amounts of phosphate, so that the serum inorganic phosphate of the animals was only partially reduced. Recent experiments in our laboratory have demonstrated that hypophosphatemic chicks have high levels of renal 1-hydroxylase activity, although not as high as in calcium-deprived chicks (62a). In adciition then to regulation by serum calcium and parathyroid hormone, 1-hydroxylation thus

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

in D

FIG. 4. Diagrammatic representation of the calcium homeostatic mechanism involving the vitamin D endocrine system. Please note that the need for calcium stimulates parathyroid hormone secretion which stimulates the production of l,25-(OH)2D3. The l,25-(OH)2D3 functions directly on intestine without further parathyroid hormone participation, whereas the function of l,25-(OH)2D3 in the mobilization of calcium from bone requires the presence of the peptide hormone.

can also be stimulated by phosphate deprivation. These conclusions are strongly supported by the recent report by Hughes et al. (62b) and by Edeistein et al. (62c). It has been suggested (61) but not proved that the mechanism of regulation may involve renal cortical levels of inorganic phosphate and that these levels can be reduced by either dietary deprivation, glucose loading, or parathyroid hormone inhibition of phosphate reabsorption in the renal tubules. It may be, therefore, that the inorganic phosphate level of the cell is the important regulatory determinant. At high cellular phosphate concentrations, biogenesis of 24,25-(OH)2D3 is stimulated, whereas at low inorganic phosphate levels l,25-(OH)2D3 production is favored.

METABOLISM

OF

VITAMIN

The mechanism of renal hydroxylase regulation is as yet unknown. However, the in vivo regulation of these hydroxylases requires many hours which makes it unlikely that regulation involves simple ionic inhibition or activation of existing enzymes. Perhaps cellular ion levels determine biosynthesis or degradation or both the 24-hydroxylase and the 1-hydroxylase systems. Work in progress may help clarify mechanisms of regulation. In vitamin D-deficient animals 25-OH-D3l-hydroxylase is not appreciably altered by changes in dietary calcium or phosphorus levels, while in animals given vitamin D it is sensitive (55). This permissive role of vitamin D can now be attributed specifically to 1,25(OH)2D3 (63). Apparently l,25-(OH)2D3 is required for the appearance of the 24-hydroxylase, the presence of which may be essential for regulation (64). In any case 1,25(OH)2D3 induces some change in the renal cell that in turn permits regulation of the hydroxylases by parathyroid hormone and inorganic phosphate (65). The nature of this change is unknown, however. The kidney is not the only site of 24hydroxylation, inasmuch as nephrectomized animals produce 24,25-(OH)2D3 when large doses of 25-OH-D3 are given (65), but the location of these extrarenal sites is unknown. l,25-(OH)2D3 is a substrate for the 24hydroxylase system (66; Y. Tanaka and H. F. DeLuca, in preparation). Since I ,25-(OH )2D3 induces the 24-hydroxylase (64), the physiological pathway for biosynthesis of 1,24,25(OH)3D3 is by 24-hydroxylation of 1,25(OH)2D3. When the biological activity of I ,24,25-(OH )3D3 is completely understood, the necessity for this reaction may become apparent. However, at this writing the exact role of 24-hydroxylation remains to be established. Metabolism

of 1,25-(OH)2D3

The biological effectiveness of l,25-(OH) 2D3 in nephrectomized rats demonstrated that it is a metabolically active form of the vitamin, but those experiments did not rule out the possibility that the metabolite might be altered further before it functions. Work in two laboratories has shown that at the time the lntestine and bone respond to l,25-(OH) 2D3, no other metabolites can be detected in

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

D: CURRENT

1263

STATUS

the chloroform extract of the tissues (67-69). To eliminate the possibility that the critical label could be lost during conversion, 1,25(OH)2D3 labeled either in the 2 position or in the 26,27 position was used in these studies with identical results (67). However, only about 80% of the total tissue radioactivity could be accounted for as l,25-(OH)2D3. The remainder of the radioactivity was detected in the aqueous phase or bound to the denatured protein. It is, therefore, evident that one cannot yet conclude rigorously that 1,25(OH)2D3 is not further altered before it can function. Other work has shown that l,25-(OH)2D3 disappears rapidly from rats (70). Furthermore, the metabolic effectiveness of l,25-(OH)2D3 5 reduced in the intestine in rats previously given doses of unlabeled l,25-(OH)2D3 (71). All these results suggest rapid metabolism of l,25-(OH)2D3. Experiments with preparations of 25-OH-D3 and l,25-(OH)2D3 labeled on carbons 26 and 27 with carbon 14 (7la) indicate that 25-OH-D3 loses at least one of these two carbons to carbon dioxide (Fig. 5). This loss of carbon dioxide in the expired air is prevented by nephrectomy. With l,25-(OH)2D3 labeled in the same two positions, the loss of carbon dioxide as ‘4CO2 accounts for up to 30% of the administered dose. This loss, not prevented by nephrectomy, illustrates that the substrate for this metabolic reaction is l,25-(OH)2D3. The reaction begins to occur within 4 hr after the administration of l,25-(OH)2D3 and may, therefore, have functional significance, particularly as intestinal calcium transport is not initiated until approximately 3 hr after administration of the compound (67, 72, 73). Metabolism (la-OH-D3)

of la-hydroxyvitamin

D3

Although la-OI-I-D3 is not a metabolite of vitamin D3, it has assumed a position of importance in medicine as an analog of l,25-(OH)2D3. The compound was first prepared (74) as an exercise in the chemical synthesis of l,25-(OH)2D3 (75). Since that time it has been synthesized by several groups and is being tested widely in renal osteodystrophy, hypoparathyroidism, and other bone diseases as a substitute for l,25-(OH)2D3.The

1264

DELUCA

35 30 25

+1,25-

(OH)2

D3NX

24

E

23 22 21 20 9 8

+

I,25-(OH)2D3

Intact

7

6

a.-’ C

15 4

3 I

10

w

9 8 7

6 0 L)

I Intact

5 4

3 2 ./-

1’

04812162024

Time

After

la-OH-D3 is equal to l,25-(OH)2D3 in biological activity in chicks (76), but is only 20 to 50% as active in rats (77). Since it acts almost as rapidly as l,25-(OH)2D3 it has been suggested (78) that la-OH-D3 does not have to be hydroxylated on the 25 position for it to be biologically active. However, with [6-3H 1-1 aOH-D3 (79), it has been demonstrated that the la-OI-I-D3 is converted to la,25-(OH)2D3 before biological activity is initiated in either intestine or bone (79-82). This hydroxylation occurs in the liver (81) and to some extent in the intestine in the case of chicks (81). la-OH-D3 does not bind effectively to what is believed to be the intestinal cytosol receptor (82) for l,25-(OH)2D3, nor does it approach l,25-(OH)2D3’s activity in vitro in the mobilization of bone calcium (83). It, therefore, seems likely that la-OH-D3 is 25-hydroxylated in the liver before it acts. The report of rapid activity of la-OH-D3 and l,25-(OH)2D3 (78) has not been reproduced in any other laboratory and is contrary to results reported for even biologically generated l,25-(OH)2D3 (72, 73).

2

Much work to 3 years

of 1,25-(OH)2D3

and

has been carried out in the last on the possible mechanism

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

48

Injection

FIG. 5. The exhalation of ‘CO2 by rats given either intravenously at 0 time. Cumulative “CO2 was collected and Nephrectomy prevents “CO2 production from 2S-OH-[26,27-”C]D3

Mechanism of action receptor hypothesis

25-OH-D3NX

32

( Hours)

25-OH-[26,27-’C]D3 measured by liquid but not from

or l,25-(OH)2-[26,27-”C]D3 scintillation counting techniques. I ,2S-(OH )2-[26,27-”CJD3.

whereby l,25-(OH)2D3 stimulates intestinal calcium transport. Work from Haussler’s laboratory has suggested the presence of a 3.0-3.7S receptor protein in chick intestinal cytosol (84-87). This protein and its l,25-(OH)2D3 ligand can then be shown to be transferred to isolated impure chromatin, suggesting a receptor mechanism similar to that for other steroid hormones. The technique has been used as a method of assaying l,25-(OH)2D3 in biological fluids, but its exact role in the intestinal calcium transport mechanism, if any, remains to be determined. The major problem with this hypothesis is that in other species which are responsive to l,25-(OH)2D3, namely the rat, no such 3.0-3.7S protein can be detected and instead the only binder of l,25-(OH)2D3 is a 6S protein which prefers 25-OH-D3 (B. Kream, R. Reynolds, J. Knutson, and H. F. DeLuca, unpublished results; 88). Furthermore, the 6S protein is distributed in large amounts in a wide variety of tissues. This suggests that it is not a receptor protein in the classical sense. Much remains to be learned before it can be concluded that l,25-(OH)2D3 functions by a mechanism generally associated with other steroid hormones. It is important to note that the chromatin association has been shown to be highly specific for l,25-(OH)2D3, with

METABOLISM

OF

VITAMIN

other metabolites of vitamin D3 and analogs being much less favored (87, 89). Of some importance is the fact that actinomycin D does not block the intestinal response of rats to l,25-(OH)2D3 (90). It is, therefore, possible that l,25-(OH)2D3 initiates intestinal calcium transport by a mechanism other than induction of calcium transport proteins. In defense of the calcium transport protein biosynthesis mechanism, however, it should be noted that the calcium binding protein originally discovered by Wasserman and his colleagues has been shown to be assembled de novo in chick tissue and chick preparations (91, 92), although it is not yet clear exactly how 1,25-(OH)2D3 brings about the appearance of this calcium binding protein, nor is it at all clear whether the calcium binding protein even functions in the calcium transport process. It is in this area that a major effort can be expected with the ultimate hope of elucidation of the molecular mechanism of 1,25-(OH)2D3 action in the small intestine. This may be facilitated by the recent in vitro work demonstrating a direct effect of l,25-(OH)2D3 on intestinal cell uptake of calcium (92a). I,25-(OH extracts

)2D3-like

substances

in plant

A series of plants of the Solanum group has been found to contain a substance that causes calcification and death of grazing cattle ,(93). Of particular importance is the plant Solanum malacoxylon, which possesses a material that will stimulate intestinal calcium transport in nephrectomized rats, in chickens fed high strontium diets and in other circumstances previously shown to respond only to I ,25-(OH)2D3 (93 -95). Furthermore, this plant extract stimulates the appearance of calcium binding protein in intestinal organ cultures (95) and binds to the chromatin receptor in a manner similar to 1,25-(OH)2D3 (96). It has, therefore, been concluded that there is in these plants a substance either similar or identical to l,25-(OH)2D3. This interesting idea, however, must await confirmation by chemical isolation and identification of the active principle. There is no question, however, that the plant substance mimics l,25-(OH)2D3 in its action on intes-

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

D: CURRENT

1265

STATUS

tine. It is not yet clear whether it also mimics the action of l,25-(OI-I)2D3 on bone (97). 25,26-Dihydroxyvitamin

D3 (25,26-(

OH

)2D3)

This metabolite of vitamin D3 was isolated and identified from the plasma of pigs given large doses of vitamin D (98). It is also present in the plasma of rats given vitamin D3 in normal amounts (98) which suggests that it is a normal metabolite of vitamin D3. Its site of synthesis has not yet been determined and its importance, if any, is not understood. It has only slight stimulatory action on intestin#{225}l calcium transport. Although it has been chemically synthesized (99), the physiological significance of this compound remains to be determined and at present it must be regarded as a biochemical curiosity. Metabolism

of vitamin

D2

Although vitamin D2 was the first vitamin to be isolated and identified, our understanding of its metabolism has lagged behind that of vitamin D3. It undergoes 25-hydroxylation in the liver much as does vitamin D3 (100). It is converted to 1,25-dihydroxyvitamin D2 (1 ,25-(OH)2D2) or 24,25-dihydroxyvitamin D2 (24,25-(OH)2D2) in the kidney (101). Thus the metabolism of vitamin D2 appears entirely analogous to that of vitamin D3. In the bird, however, vitamin D2 and its metabolites are rapidly metabolized and excreted and have low biological activity. The exact molecular mechanism of this curious discrimination remains unknown at the present time, but has been discussed in connection with the 24hydroxylation system. Excretion

of vitamin

D

Little is known concerning the excretion of vitamin D except that the primary route is through the bile (102, 103). Less than 4% of radioactivity from vitamin D3 appears in the urine (104). The exact nature of the biliary excretion products of vitamin D remains undetermined at the present time. Vitamin D sulfates have been identified, but only in animals given large amounts of vitamin D (105). The route of excretion of the vitamin and its metabolites remains, therefore, to be elucidated.

1266

DELUCA

Summary There has been much progress in our understanding of the metabolism of vitamin D. It is now clear that vitamin D3 can be produced in the skin or ingested in the diet. It accumulates very rapidly in the liver where it undergoes 25-hydroxylation, yielding 25OH-D3, the major circulating metabolite of the vitamin. 25-OH-D3 proceeds to the kidney where it undergoes one of two hydroxylations. If there is a biological need for calcium or for phosphate the kidney is stimulated to convert 25-OH-D3 to the l,25-(OH)2D3, a calcium and phosphate mobilizing hormone. If, however, the animal has sufficient supplies of calcium and phosphate, the 1-bydroxylase is shut down and instead the 25OH-D3 is converted to a 24,25-(OH)2D3. The role of the 24,25-(OH)2D3 remains unknown; it may be an intermediate in the inactivationexcretion mechanism. l,25-(OH)2D3 proceeds to the intestine where it stimulates intestinal calcium transport and intestinal phosphate transport. It also stimulates bone calcium mobilization and probably has other effects yet to be discovered in such tissues as muscle. The 25-OH-D31-hydroxylase, which is located exclusively in renal mitochondria, has been shown to be a three component system involving a flavoprotein, an iron-sulfur protein (renal ferredoxin), and a cytochrome P-450. This system has been successfully solubilized, the components isolated, and reconstituted. The 24-hyd roxylase, however, has not yet been thoroughly studied. l,25-(OH)2D3 is necessary for the appearance of the 24-hydroxylase; parathyroid hormone represses 24-hydroxylation. It is possible that the 24-hydroxylase represents the major regulated enzyme, so that its presence or absence may determine whether l,25-(OH)2D3 is produced. Two metabolic pathways for 1,25-(OH)2D3 are known, conversion by the 24-hydroxylase to l,24,25-(OH)3D3, and conversion of 1,25-(OH)2D3 to an unknown substance. In the latter instance, there occurs loss of a side chain piece, including at least one of the 26 and 27 carbons. Whether l,25-(OH)2D3 must be metabolized further before it carries out all of its functions has yet to be established.

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

The primary excretion route of vitamin D3 is via the bile into the feces. Urinary excretion appears small in magnitude and no excretion products have yet been identified positively. Much remains to be learned concerning the metabolism and function of vitamin D and its metabolites. This should, therefore, prove to be a fruitful area of investigation for many years to come, especially since l,25-(OH)2D3, 25-OH-D3, and la-OH-D3 have been shown to be effective in a number of metabolic bone disease states. El References 1. DELUCA, and

H.

F., AND

mechanism

Biochem.

45:

H.

K. SCHNOES.

of action

of vitamin

Metabolism

D. Ann.

Rev.

631, 1976.

2.

DELUCA, (March):l,

H. F. Vitamin D today. Disease-a-Month 1975. 3. DELUCA, H. F. Calcium metabolism. Acta Orthop. Scand. 46: 386, 1975. 4. WASSERMAN, R. H., AND A. N. TAYLOR. Metabolic roles of fat-soluble vitamins D, E, and K. Ann. Rev. Biochem. 41: 179, 1972. 5. KOD1CEK, E. The story of vitamin D, from vitamin to hormone. Lancet 1:325, 1974. 6. NORMAN, A. W., K. SCHAEFER, H-G. GRIGOLEIT, D. VON HERRATH AND E. RITZ. Vitamin D and Problems Related to Uremic Bone Disease. Berlin: Walter de Gruyter, 1975. 7.

H. F., J. W. BLUNT AND H. RIKKERS. Biogenesis. In: The Vitamins, edited by W. H. Sebrell and R. S. Harris. New York: Academic Press, 1971, p. 213. HADDAD, J. G., JR., AND T. J. HAHN. Natural and synthetic sources of circulating 25-hydroxyvitamin D in man. Nature 244: 515, 1973. PONCHON, G., AND H. F. DELUCA. The role of the liver in the metabolism of vitamin D. J. Clin. Invest. 48: 1273, 1969. NORMAN, A. W., AND H. F. DELUCA. The preparation of H3-vitamins D2 and D3 and their localization in the rat. Biochemistry 2: 1160, 1963. NEVILLE, P. F., AND H. F. DELUCA. The synthesis of [1,2-3H1 vitamin D3 and the tissue localization of a 0.25 g (10 IU) dose per rat. Biochemistry 5: 2201, 1966. KODICEK, E. Metabolic studies on vitamin D. In: Ciba Foundation Symposium on Bone Structure and Metabolism, edited by G. W. E. Wolstenholme and C. M. O’Connor. Boston: Little, Brown and

8.

9.

10.

II.

12.

DELUCA,

Co., 13.

1956,

BLUMBERG, SCHONHOLZER.

ergocalciferol rhesusaffen.

p. 161. A.,

H. Das

AEBI,

H.

verteilungsmuster

HURNI

AND von

G. C”beim

(vitamin D2) bei der ratte und Helv. Physiol. Acta 18: 56, 1960. 14. BHATTACHARYYA, M., AND H. F. DELUCA. Subcellular location of rat liver calciferol-25-hydroxylase. Arch. Biochem. Biophys. 160: 58, 1974. 15. HORSTING, M., AND H. F. DELUCA. In vitro

METABOLISM production

of 25-hydroxycholecalciferol. Commun. 36: 251,

Biophys.

16.

Res.

M. The enzymatic

HORSTING, calciferol

to

17.

Wisconsin,

studies

rence chem.

19.

Vitamin and

apparent

Biophys.

BHATTACHARYYA, regulation

thesis

Biochem. 33.

of chole-

H., on

AND the

H. F.

25-hydroxylation

AND

lack

of

regulation.

155: 47, 1973. M. H., AND

ofcalciferol-25-hydroxylase

M.

H.

Chem.

R. HAUS-

tissue Arch.

F. DELUCA. in the

occurBio-

35.

The chick.

Biophys. Res. Commun. 59: 734, 1974. 20. PONCHOS, G., A. L. KENNAN AND H. F. DELUCA. “Activation” of vitamin D by the liver. J. Clin. Invest. 48: 2032, 1969. 20a. OLsoN, E. B., JR., J. C. KNUTSON, M. H. BHATTACHARYYA AND H. F. DELUCA. The effect of hepatectomy on the synthesis of 2S-hydroxyvitamin D3 . J. Clin. Invest. 57: 1213, 1976. 21. NORMAN, A. W. 1,25-Dihydroxyvitamin D3: a kidney-produced steroid hormone essential to calcium homeostasis. Am. J. Med. 57: 21, 1974. 22. BHATTACHARYYA, M. H., AND H. F. DELUCA. The regulation of rat liver calciferol-25-hydroxylase. J. Biol. Chem. 248: 2969, 1973. 23. TANAKA, Y., H. FRANK AND H. F. DELUCA. Biological activity of 1,25-dihydroxyvitamin D3 in the rat. Endocrinology 92:4 17, 1973. 24. BLUNT, J. W., Y. TANAKA AND H. F. DELUCA. The biological activity of 25-hydroxycholecalciferol, a metabolite of vitamin D3. Proc. NatI. Acad. Sci. U.S. 61: 1503, 1968. 25. U. S. PHARMACOPOEIA. 17th Revision, Easton: Mack Publishing Co., 1965, p. 891. 26. TRUMMEL, C. L., L. G. RAISZ, J. W. BLUNT AND H. F. DELUCA. 25-Hydroxycholecalciferol: stimulation of bone resorption in tissue culture. Science 163: 1450, 1969. 27. CORRADINO, R. A. Embryonic chick intestine in organ culture: response to vitamin D3 and its metabolites. Science 179: 402, 1973. 28. OLSON, E. B., AND H. F. DELUCA. 25-Hydroxy-

Biochem.

cholecalciferol: direct effect on calcium transport. Science 165: 405, 1969. 29. BOYLE, I. T., L. MIRAVET, R. W. GRAY, M. F. HOLICK AND H. F. DELUCA. The response of intestinal calcium transport to 25-hydroxy and 1,25-dihydroxy vitamin D in nephrectomized rats. Endocrinology 90: 605, 1972. 30. HOLICK, M. F., M. GARABEDIAN AND H. F. DELUCA. 1,25-Dihydroxycholecalciferol: metabolite of vitamin D3 active on bone in anephric rats. Science 176: 1146, 1972. 31. WONG, R. G., A. W. NORMAN, C. R. REDDY AND J. W. COBURN. Biologic effects of 1,25-dihydroxycholecalciferol (a highly active vitamin D metabolite) in acutely uremic rats. J. Clin. Invest. 51: 1287, 1972. 32. FRASER, D. R., AND E. KODICEK. Unique biosyn-

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

of a biologically

Nature

228:

active

764,

vitamin

D

1970.

GRAY, R. W., J. L. OMDAHL, J. G. GHAZARIAN H . F. DELUCA. 25-Hydroxycholecalciferolsubcellular

Chem.

A.

NORMAN,

location

and

247: 7528, 1972. W., R. J. MIDGETT,

AND

1-

J.

properties.

J. F.

MYRTLE

H. G. N0wICKI. Studies on calciferol metabolism. I. Production of vitamin D metabolite 4B from 25-OH-cholecalciferol by kidney homogenates. Biochem. Biophys. Res. Commun. 42: 1082, 1971. GHAZARIAN, J. G., AND H. F. DELUCA. 25-Hydroxycholecalciferol1-hydroxylase: a specific requirement for NADPH and a hemoprotein component in AND

of

J. Biol.

D3-25-hydroxylase:

by kidney

hydroxylase:

34.

DELUCA.

1267

STATUS

metabolite.

Ph.D. TheDepartment of Bio-

vitamin D3 and dihydrotachysterol3. 248: 2974, 1973. 18. TUCKER, G., III, R. E. GAGNON SLER.

D: CURRENT

Biol.

M.

BHATTACHARYYA, Comparative

VITAMIN

1969.

conversion

25-hydroxycholecalciferol,

sis. University of chemistry, 1970.

OF

36.

chick

kidney

phys.

160:

GHAZARIAN, DELUCA.

37.

38.

mitochondria.

63,

Mechanism

41.

42.

43.

44.

45.

K.

Biochem.

SCHNOES

AND

Bio-

H.

F.

of 25-hydroxycholecalciferol

J. G., C. R.

GHAZARIAN, W.

H.

Mitochondrial

40.

H.

Arch.

la-hydroxylation. Incorporation of oxygen18 into the lcn position of 25-hydroxycholecalciferol. Biochemistry 12: 2555, 1973. HENRY, H. L., AND A. W. NORMAN. Studies on calciferol metabolism. IX. Renal 25-hydroxyvitamin D3- I -hydroxylase. I nvolvement of cytochrome P.450 and other properties. J. Biol. Chem. 249: 7529, 1974. SON,

39.

1974. J. G.,

ORME-JOHNSON

cytochrome

J. C.

JEFCOATE, AND

P450:

KNUT-

F. DELUCA. a component of

chick kidney 25-hydroxycholecalciferoldroxylase. J. Biol. Chem. 249: 3026, OMDAHL, J. L., AND H. F. DELUCA. vitamin D metabolism and function.

H.

I a-hy1974. Regulation Physiol.

of Rev.

53: 327, 1973. MCNUTT, K. W., AND M. R. HAUSSLER. Nutritional effectiveness of I ,25-dihydroxycholecalciferol in preventing rickets in chicks. J. Nutr. 103: 681, 1973. NORMAN, A. W., AND R. G. WONG. Biological activity of the vitamin D metabolite l,25-dihydroxycholecalciferol in chickens and rats. J. Nutr. 102: 1709, 1972. CHEN, T. C., L. CASTILLO, M. KORYCKA-DAHL AND H. F. DELUCA. Role of vitamin D metabolites in phosphate transport of rat intestine. J. Nutr. 104: 1056, 1974. KNUTSON, J. C., AND H. F. DELUCA. 25-Hydroxyvitamin D3 -24-hydroxylase: subcellular location and properties. Biochemistry 13: 1543, 1974. SEKI, M., N. K0IzUMI, M. MORISAKI AND N. IKEKAWA. Synthesis of active forms of vitamin D. VI. Synthesis of (24R)and (24S)-24,25-dihydroxyvitamin D3. Tetrahedron Letters 1:15, 1975. USKOKOVLC,

M., E. BAGGIOLINI, A. MAHGOUB, T. J. J. PARTRIDGE. Synthesis of vitamin D3 metabolites. In: Vitamin D and Problems Related to Uremic Bone Disease, edited by A. W. Norman, K. Schaefer, H. G. Grigoleit, D. V. Herrath and E. Ritz. Berlin: Walter de Gruyter, 1975, p. 279. 46. IKEKAWA, N., N. MORISAKI, N. KOIzUMI, M. SAWAMURA, Y. TANAKA AND H. F. DELUCA. Synthesis and biological activity of 24k’- and 22NARWID

AND

1268

47.

DELUCA hydroxyvitamin mun. 62: 485, TANAKA, Y.,

D3. Biochem. 1975. H F. DELUCA, AND N. K0IzUMI. configuration

MORISAKI

stereochemical group

48.

49.

50.

of

Res.

Com-

MURRAY.

N.

M.

IKEKAWA,

Determination

of

the

of

24-hydroxyl

D3

and

its

bio-

61.

62.

Regulation

SCHNOES,

M. F., A. KLEINER-BOSSALLER, P. M. KASTEN, 1. T. BOYLE

DELUCA.

1,24,25-Trihydroxyvitamin

62a.

D3:

AND

57.

58.

59.

60.

of

R.,

P.

F.

Stimulation

P. J.

AND

in the rat.

R.

D3

Science

by

190: 578,

BAR

S.

AND

ofvitamin D and low-phosphorus diets. 385: 438, 1975.

fed low-calcium Biophys. Acta

of

The role of 1,25-dihydroxyvitamin D3 and parathyroid hormone in the regulation of chick renal 25-hydroxyvitamin D3-24-hydroxylase. Arch. Biochem. Biophys. 171: 521, 1975. 65. GARABEDIAN, M., H. PAVLOVITCH, C. FELL0T AND S. BALSAN. Metabolism of 25-hydroxyvitamin D3 in anephric rats: a new active metabolite. Proc. Nail. Acad. Sci. U.S. 71: 554, 1974. 66. KLEINER-BOSSALLER, A., AND H. F. DELUCA. Formation of 1,24,25-trihydroxyvitamin D3 from 1,25-dihydroxyvitamin D3. Biochim. Biophys. Acta 338: 489, 1974. 67. FROLIK, C. A., AND H. F. DELUCA. 1,25-Dihydroxycholecalciferol: the metabolite of vitamin D responsible for increased intestinal calcium transport. Arch. Biochem. Biophys. 147: 143, 1971. 68. FROLIK, C. A., AND H. F. DELUCA. Metabolism of 1,25-dihydroxycholecalciferol in the rat. J. Clin. Invest. 51: 2900, 1972. 69. TSAL, H. C., R. G. WONG AND A. W. NORMAN. Studies on calciferol metabolism. IV. Subcellular localization of 1,25-dihydroxyvitamin D, in intestinal mucosa and correlation with increased calcium transport. J. Biol. Chem. 247: 5511, 1972. 70. FROLIK, C. A., AND H. F. DELUCA. The stimulation of I ,25-dihydroxycholecalciferol metabolism in

vitamin

DELUCA,

120: 525, 1967. H. F., M. WELLER,

J. W. BLUNT AND P. F. NEVILLE. Synthesis, biological activity, and metabolism of 22,23-3H-vitamin D4. Arch. Biochem. Biophys. 124: 122, 1968. BOYLE, I. T., R. W. GRAY AND H. F. DELUCA. Regulation by calcium of in vivo synthesis of I .25-dihydroxycholecalciferol and 21 ,25-dihydroxycholecalciferol. Proc. NatI. Acad. Sci. U.S. 68: 2131, 1971. BOYLE, I. T., R. W. GRAY, J. L. OMDAHL AND H. F. DELUCA. Calcium control of the in vivo biosynthesis of 1,25-dihydroxyvitamin D3: Nicolaysen’s endogenous factor. In: Endocrinology 1971, edited by S. Taylor. London: Wm. Heinemann Medical Books Ltd., 1972, p. 468. GARABEDIAN, M., M. F. HOLICK, H. F. DELUCA AND I. T. BOYLE. Control of 25-hydroxycholecalciferol metabolism by the parathyroid glands. Proc. NatI. Acad. Sci. U.S. 69: 1673, 1972. FRASER. D. R., AND E. KODICEK. Regulation of 25-hydroxycholecalciferolI -hydroxylase activity in kidney by parathyroid hormone. Nature New Biol. 241: 163, 1973. GARABEDIAN, M., Y. TANAKA, M. F. HOLICK AND H. F. DELLCA. Response of intestinal calcium transport and bone calcium mobilization to 1,25dihydroxyvitam in D3 in thyroparathyroidectomized rats. Endocrinology 94: 1022, 1974. KooH, S. W., D. FRASER, H. F. DELUCA, M. F. HOLICK, R. E. BELSEY, M. B. CLARK AND T. M.

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

Y.,

M. BAYLINK.

la,25-dihydroxyvitamin

phosphate

S., A. HARELL, A. The functional metabolism

EDELSTEIN, HURWITZ.

by phos1976.

BRUMBAUGH,

WERGEDAL

of serum

and

in chicks Biochim.

1974.

DELUCA.

D3-la-hydroxylase

J. E.

Regulation

calcium 1975.

H. F.

AND

63.

of

Biophys.

56.

M.

HUGHES,

7584,

J. Biol. Chem. 251: 3158,

depletion.

HAUSSLER,

a metabo-

and

metabolites

D3-1-hydroxylase

249:

Chem.

A., of 25-hydroxyvitamin

62b.

K. H. F.

D. VIII. Synthesis of [24Rjand [24S1-l a,24,25-trihydroxyvitamin D3. Chem. Pharm. Bull. 23: 695, 1975. 53. IMRIE, M. H., P. F. NEVILLE, A. W. SNELLGROVE AND H. F. DELUCA. Metabolism of vitamin D2 and vitamin D3 in the rachitic chick. Arch. Biochem.

55.

Biol.

L.

BAXTER,

phate

H.

with

vitamin D3 effective on intestine. J. Biol. 248: 6691, 1973. IKEKAWA, N., M. MORISAKI, N. K0IzUMI, Y. KATO AND T. TAKESHITA. Synthesis of active forms

lite

54.

hypoparathyroidism

of 25-hydroxyvitamin

J.

in vivo.

TANAKA,

AND

H. F.

24,25-dihydroxyvitamin

Chem.

52.

of

vitamin D: evidence for impaired conversion of 25-hydroxyvitamin D to la,25-dihydroxyvitamin D. New EngI. J. Med. 293: 840, 1975. TANAKA, Y., AND H. F. DELUCA. The control of 25-hydroxyvitamin D metabolism by inorganic phosphorus. Arch. Biochem. Biophys. 154: 566, 1973. HENRY, H. L., R. J. MIDGETT AND A. W. NORMAN.

62c.

1973.

HOLICK,

Treatment

pseudohypoparathyroidism

logical importance. Arch. Biochem. Biophys. 170: 620, 1975. TANAKA, Y., H. FRANK, H. F. DELUCA, N. KolZUMI AND N. IKEKAWA. Importance ofthe stereochemical position of the 24-hydroxyl to biological activity of 24-hydroxyvitamin D3. Biochemistry 14:3293, 1975. HOLICK, M. F., L. A. BAXTER, P. K. SCHRAUFROGEL, T. E. TAVELA AND H. F. DELUCA. Metabolism and biological activity of 24,25-dihydroxyvitamin D3 in the chick. J. Biol. Chem. 251: 397, 1976. BOYLE, I. T., J. L. OMDAHL, R. W. GRAY AND H. F. DELUCA. The biological activity and metabolism of 24,25-dihydroxyvitamin D3. J. Biol. Chem. 248: 4174,

SI.

24,25-dihydroxyvitamin

Biophys.

DELUCA.

D,

Stimulation

production

D3. Science 183: I 198, Y., R. S. LORENC AND H. F.

dihydroxyvitamin

64.

TANAKA,

vitamin

D-deficient

rats

by

by

of

1,25-

1974. DELUCA.

1,25-dihydroxycholecal-

J. Clin. Invest. 52: 543, 1973. TANAKA, Y., H. FRANK AND H. F. DELUCA. ciferol.

71.

of

1,25-dihydroxycholecalciferol

bone

maintenance

of serum

calcium

of

concentra-

J. Nutr. 102: 1569, 1972. D., R. KUMAR, M. F. HOLICK AND H. F. DELUCA. Side chain metabolism of 25hydroxy-[26,27-”Cjvitamin D3 and 1,25-dihydroxy-[26,27-”C]vitamin D3 in vivo. Science 193: 493, 1976. tion

71a.

and

Role

in calcification

in the

HARNDEN,

rat.

METABOLISM 72.

73.

74.

J., F.

OMDAHL,

H.

AND

76.

Y.

SUDA,

Biological

D3. Science E. J., M. F.

180:

la-hydroxycholecalciferol,

the

hormonal

Sci.

USA

HOLICK,

form

70:2248,

M.

analog

D3. Proc.

Nat.

78.

AND

F., P.

BACHER.

79.

80.

E. P., M. M.

TOFFOLON,

H.

K.

Demonstration

GALLAGII mm

D3

[6-3H]vitamin

M.

PECHET

of the

F., T.

SCHNOES. ER. and

E. H.

Synthesis its

89.

90.

HOLICK.

T. activity

AND

rapid

DELUCA

of

K.

ISSEL-

the

92.

to rat.

93.

la,25-dihydroxy-

95.

1020,

81.

82.

83.

84.

85.

86.

ence of hydroxyl substituents in the A ring. Endocrinology, 97: 1552, 1976. BRUMBAUGH, P. F., AND M. R. HAUssLER. la,25Dihydroxyvitamin D3 receptor: competitive binding of vitamin D analogs. Life Sci. 13: 1737, 1973. BRUMBAUGH, P. F., AND M. R. HAUSsLER. la,25Dihydroxycholecalciferol receptors in intestine. I. Association of la,25-dihydroxycholecalciferol with intestinal mucosa chromatin. J. Biol. Chem. 249: 1251, 1974. BRUMBAUGH, P. F., AND M. R. HAUSSLER. ta,2S Dihydroxycholecalciferol receptors in intestine. II.

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

to

R.

The

U.S.

Sci.

R.,

J. W.

induction

of

J. S., D. E. M.

EMTAGE,

of

1,25-dihydroxy-

calcium 68: 1286, HAMILTON

calcium

binding

by vitamin 1970. LAWSON

transport. 1971. ANI) D. V.

AND

protein

D3.

Biochim.

E.

KODICEK.

calin

cell

calcium Science

M.

J. Biol.

hormone

Vitamin D-induced synthesis of mRNA for cium-binding protein. Nature 246: 100, 1973. 92a. FREUND, T., AND F. BRONNER. Stimulation vitro by 1,25-dihydroxy-vitamin D3 ofintestinal

94.

HOLICK,

B.

Acad.

MACGREGOR,

action

intestinal

biosynthesis in intestine Biophys. Acta 222: 482,

of pure

AND

of

on

Natl.

COHN.

la-hydroxy-[6-3H]-vita-

metabolism in

S. A.

TAVELA,

F.

91.

of

Biochem.

action

Mechanism

cholecalciferol

Chem. 251: 1976. HOLICK, S. A., M. F. HOLICK, T. E. TAVELA, H. K. SCHNOES AND H. F. DELUCA. Metabolism of la-hydroxyvitamin D3 in the chick. J. Biol. Chem. 251: 1025, 1976. ZERWEKH, J. E., P. F. BRUMBAUGH, D. H. HAUSSLER, D. J. CoRK AND M. R. HAUSSLER. Iahydroxyvitamin D3. An analog of vitamin D which apparently acts by metabolism to la,25-dihydroxyvitamin D3. Biochemistry 13: 4097, 1974. STERN, P. H., C. L. TRUMMEL, H. K. SCHNOES AND H. F. DELUCA. Bone resorbing activity of vitamin D mctabolitcs and congcners in viirb: influ-

D3

88.

of

crystalline la-hydroxy vitamin D3 and la,25-dihydroxy vitamin D3 on intestinal calcium uptake. Proc. NatI. Acad. Sci. U.S. 72: 229, 1975. HOLICK, M. F., S. A. HOLICK, T. TAVELA. B. GALLAGHER, H. K. SCHNOES AND H. F. DELUCA. Synthesis of [6-3HJ-ln-hydroxyvitamin D3 and its metabolism in vivo to [3HJ-la,25-dihydroxyvitamin D3. Science 190: 576, 1975. HOLICK,

87.

Acad.

KASTEN-SCHRAUFROGEL,

of the

via a specific cytosol receptor. J. Biol. Chem. 249: 1258, 1974. BRUMBAUGH, P. F., AND M. R. HAUSSLER. Specific binding of la,25-dihydroxycholecalciferol to nuclear components of chick intestine. J. Biol. Chem. 250 1588, 1975. HADDAD, J. G., AND S. J. BIRGE. Widespread, specific binding of 25-hydroxycholecalciferol in rat tissues. J. Biol. Chem. 250: 299, 1975. NORMAN, A. W., R. L. JOHNSON, T. W. OSBORN, D. A. PROCSAL, S. C. CAREY, M. L. HASIM0ND, M. N. MTRA, M. R. PIRIO, A. REGO, R. M. WING AND W. H. OKAMURA. The chemistry and conformational and biological analysis of vitamin D3, its metabolites and analogs. Clin. Endocrinol. 5: 1215, 1976. TANAKA, Y., H. F. DELUCA, J. OMDAHL ANI) M. F.

Proc.

H.

transfer

chromatin

1973.

F. DELUCA. Biological la-hydroxyvitamin D3 in the rat. Arch. Biophys. 166: 63, 1975.

TAVELA

1269

STATUS

Temperature-dependent of

190,

a synthetic ofvitamin

D: CURRENT

TANAKA

activity

1973. SEMMLER, HOLICK, H. K. SCHNOES AND H. F. DELUCA. The synthesis of la,25-dihydroxycholecalciferol-a metabolically active form ofvitamin D3. Tetrahedron Letters40: 4147, 1972. HAUSSLER, M. R., J. E. ZERWEKH, R. H. HESSE, E. RIZZARDO AN[) M. M. PECHET. Biological activity of

77.

T.

HOLICK,

DELUCA.

VITAMIN

I ,25-dihydroxycholecalciferol. Biochemistry 10: 2935, 1971. TANAKA, Y., ANI) H. F. DELUCA. Bone mineral mobilization activity of 1,25-dihydroxycholecalciferol, a metabolite of vitamin D. Arch. Biochem. Biophys. 146: 574, 1971. H0LIcK, M. F., E. J. SEMMLER, H. K. ScuNoEs AND H. F. DELUCA. la-Hydroxy derivative of vitamin D3: a highly potent analog of lcs,25-dihydroxyvitamin

75.

M.

OF

uptake and calcium-binding protein. 190: 1300, 1975. WASSERMAN, R. H. Active vitamin D-like substances in solanum malacoxylon and other calcinogenic plants. Nutr. Rev. 33: 1, 1975. WASSERMAN, R. H. Calcium absorption and calcium-binding protein synthesis: solanum malacoxylon reverses strontium inhibition. Science 183: 1092, 1974. WASSERMAN, R. H., A. BAR, R. A. CORRADINO, A. N. TAYLOR AND M. PETERLIK. Calcium absorption and

calcium

binding

evidence for a factor in solanum lating Hormones, Owen,

96.

97.

98.

synthesis

in the

chick:

J. A. Parsons. Amsterdam: Excerpta 1975, p. 318. WALLING, M. W., D. V. KIMBERG, W. LLOYD, H. WELLS, D. A. PROCSAL AND A. W. NORMAN. Solanum glaucophyllum (malacoxylon): an apparent source of a water-soluble, functional and structural analogue of I-alpha, 25-dihydroxyvitamin D3. In: Vitamin D and Problems Related to Uremic Bone Disease, edited by A. W. Norman, K. Schaefer, H. G. Grigoleit, D. V. Herrath, and E. Ritz. Berlin: Walter de Gruyter, 1975, p. 717. URIBE, A., M. F. HOLICK, N. A. JORGENSEN AND H. F. DELUcA. Action of solanum malacoxylon on calcium metabolism in the rat. Biochem. Biophys. Res. Commun. 58: 257, 1974. SUDA, T., H. F. DELUCA, H. K. SCHNOES, Y. TANAKA AND M. F. HOLICK. 25,26-Dihydroxycholecalciferol, a metabolite of vitamin D3 with intestinal calcium transport activity. Biochemistry 9: 4776, 1970. Medica,

and

protein

I ,25-dihydroxycholecalciferol-like malacoxylon. In: Calcium Reguedited by R. V. Talmage, M.

1270 99.

100.

DELUCA LAM, H-Y., H. K. SCHNOES AND H. F. DELUCA. Synthesis and biological activity of 25,26-dihydroxycholecalciferol. Steroids 25: 247, 1975. JONES,

G., H. K.

SCHNOES

AND

H. F.

DELUCA.

consin,

103. An

in vitro study chick. J. Biol. 101.

102.

of vitamin D2 hydroxylases in the Chem. 251: 24, 1976. JONES, G., H. K. SCHNOES AND H. F. DELUCA. Isolation and identification of 1,25-dihydroxyvitamin D2. Biochemistry 14: 1250, 1975. FROLIK, C. A. Studies on the metabolites of vitamin D present in the bile of chicks and on the metabolism and function of 1,25-dihydroxycholecalciferol. Ph.D. Thesis, University of Wis-

Downloaded from https://academic.oup.com/ajcn/article-abstract/29/11/1258/4649685 by University of Minnesota Law Library user on 10 July 2018

104.

105.

Department of Biochemistry, 1972. P. A., AND E. KODICEK. Investigations on metabolites of vitamin D in rat bile. Separation and partial identification of a major metabolite. Biochem. J. 115: 663, 1969. AvI0LI, L. V., S. W. LEE, J. E. MCDONALD, J. LUND AND H. F. DELUCA. Metabolism of vitamin D3-3H in human subjects: distribution in blood, bile, feces and urine. J. Clin. Invest. 46: 983, 1967. HIGAKI, M., M. TAKAHASHI, T. SUZUKI AND Y. SAHASHI. Metabolic activities of vitamin D in animals. III. Biogenesis of vitamin D sulfate in animal tissues. J. Vitaminol. 11: 261, 1965. BELL,

Metabolism of vitamin D: current status.

Metabolism Hector F. DeL uca,2 of vitamin D: current Ph.D. Since 1965 much new information has become available on the functional metabolism of v...
2MB Sizes 0 Downloads 0 Views