Abstract
(22 R)-Cholest-5-ene-3 β,7 α,22-triol and the isomeric (22 R)-cholest-5-ene-3 β,7 β,2-triol were 7-dehydroxylated by rat liver microsomes, after addition of NAD + to the incubations. The product from both sterols was identified as (22 R)-22-hydroxycholesta-4,6-dien-3-one by gas chromatography-mass spectrometry. The overall conversion of the 7α-compound had an apparent V max of 5 nmol/mg protein per h, about 3-times higher than that of the 7β-isomer. K m was about 0.018 mmol/l in both reactions. NAD + was required for the 7-dehydroxylation to proceed, conceivably by serving as cofactor in formation of the intermediate 7β,2-dihydroxy-4-cholesten-3-one. EDTA had a stimulatory effect upon the product formation. 7α-Dehydroxylation of the bile acid precursor 7α-hydroxy-4-cholesten-3-one has been demonstrated in liver tissue, but a 7β-hydroxy-C 27-steroid dehydroxylating enzyme has previously not been identified. The results are discussed in relation to the marked differences in effect on neoplastic growth by the two isomeric hydroxysterols.
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