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RECENT ADVANCES IN OUR UNDERSTANDING OF THE METABOLISM OF VITAMIN D AND ITS REGULATION
Author(s) -
DeLUCA HECTOR F.
Publication year - 1976
Publication title -
clinical endocrinology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.055
H-Index - 147
eISSN - 1365-2265
pISSN - 0300-0664
DOI - 10.1111/j.1365-2265.1976.tb03819.x
Subject(s) - biological activity , vitamin d and neurology , hydroxylation , metabolism , chemistry , vitamin , biochemistry , medicine , stereochemistry , endocrinology , biology , enzyme , in vitro
SUMMARY Potent new methods of separation of vitamin D metabolites and analogues have been developed using high pressure liquid chromatography and silica gel columns. Utility of this new method in the further study of vitamin D metabolism has been demonstrated. With this technique it has been possible to demonstrate that the natural product, 24,25‐dihydroxyvitamin D 3 (24,25‐(OH) 2 D 3 ), has its hydroxyl in the R configuration. With 3 H‐1α‐hydroxyvitamin D 3 (1α‐OH‐D 3 ), which has been recently synthesized in this laboratory, it has been possible to show that it is rapidly converted to 1,25‐dihydroxyvitamin D 3 (1,25‐(OH) 2 D 3 ) predominantly by liver and that this conversion occurs in ample time to account for its biological activity. Chick intestine possesses some 25‐hydroxylating ability whereas rat intestine does not. It seems likely that 1α‐OH‐D 3 is biologically active almost entirely by virtue of its conversion to 1,25‐(OH) 2 D 3 . The 24R,24,25‐(OH) 2 D 3 is almost as biologically active as 25‐hydroxyvitamin D 3 (25‐OH‐D 3 ) in the rat, but has low activity in the chick. The S isomer is essentially without activity in either species except for intestinal calcium transport in the rat. Radioactive 24,25‐(OH) 2 D 3 has been used to demonstrate that in the chick it is rapidly metabolized and excreted suggesting that the 24‐hydroxylation in the chick is an important biological inactivation mechanism. In the rat it is possible that the 24‐hydroxylation has functional significance. Finally, the 1α‐hydroxylase of 25‐OH‐D 3 in chick kidney has been successfully solubilized and the cytochrome P‐450 as well as the renal ferredoxin has been isolated and recombined with beef adrenal flavoprotein to produce a reconstructed 25‐OH‐D 3 ‐1α‐hydroxylase system with very high specificity and high enzymatic activity. The results show conclusively that the 1‐hydroxylase system requires NADPH to reduce a flavoprotein which in turn reduces a renal ferredoxin which in turn reduces the cytochrome P‐450. The cytochrome P‐450 carries out the hydroxylation using molecular oxygen and 25‐OH‐D 3 .