William Gibb

Steric, chiral and conformational aspects of the 3-hydroxy- and 20-hydroxysteroid dehydrogenase activities of cortisone reductase preparations

Abstract The 3-hydroxysteroid dehydrogenase activity of cortisone reductase (20-dihydrocortisone:NAD+ oxidoreductase, EC 1.1.1.53) was known to be 3α activity in the case of certain 5α-steroids in which 3α was 3R and axial. It is now shown to be 3α activity also in a 5α-steroid in which 3α is 3S and axial. In the 5β series, both 3α, 3R, equatorial activity and 3β, 3S axial activity are now described, though the compounds were very poor substrates. The view is discussed that interactions between the all-trans-5α-steroids and the enzyme distinguish the α side of the steroid from the β side; and that the angled A/Bcis-steroids of the 5β series are either bound somewhat differently, or are possibly bound in more than one way. The 20-hydroxysteroid dehydrogenase activity of cortisone reductase was known to be 20β activity in various cases where 20β was 20R. It is now shown to be 20β activity also in a case where 20β is 20S. In the case of 20 activity, the orientation around C-20 at the moment of hydrogen transfer is evidently fixed relative to both the steroid nucleus and the hydrogen donor or acceptor (presumably enzyme-bound NADH or NAD+) even though rotation about the C-17/C-20 bond is possible.

5α-Dihydrotestosterone sulphate and cortisone reductase

Abstract 5α-Dihydrotestosterone sulphate (3-oxo-5a-androstan-17β-yl sulphate) was shown to be a much worse substrate than 5α-dihydrotestosterone (17β-hydroxy-5α-androstan-3-one) for cortisone reductase (EC 1.1.1.53). This contrasted with a previous finding that pregnenolone (3β-hydroxy-5-pregnen-20-one) and pregnenolone sulphate (20-oxo-5-pregnen-3β-yl sulphate) were both quite good substrates for the same enzyme. The slow enzymic reduction of 5α-dihydrotestosterone sulphate gave 3α-hydroxy-5α-androstan-17β-yl sulphate and a smaller amount of 3β-hydroxy-5α-androstan-17β-yl sulphate. This contrasted with the rapid reduction of 5α-dihydrotestosterone, which gave 3α, 17β-dihydroxy-5α-androstane. 5α-Dihydrotestosterone sulphate inhibited the reduction of 17β-hydroxy-1α-methyl-5α-androstan-3-one,z and it seems likely that one complex active centre is involved in these reactions.

The different effects of an epoxy and a methylene group on enzymic reductions of a vicinal oxo group

Abstract In the case of two 3-hydroxysteroid dehydrogenases, 1 α,2α-epoxy-17β-hydroxy-5α-androstan-3-one was shown to be a substrate, whereas 17β-hydroxy-1 α,2α-methylene-5α-androstan-3-one scarcely served as a substrate and was an inhibitor. This is unlikely to be a simple steric effect, but its electronic basis is not clear.