Journal of Lipid Research, Vol 18, 6-13, Copyright © 1977 by Lipid Research, Inc.

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Cholic acid biosynthesis: conversion of 5beta-cholestane- 3alpha,7alpha,12alpha,25-tetrol into 5beta-cholestane-3alpha,7alpha, 12alpha,24beta,25-pentol by human and rat liver microsomes

FW Cheng, S Shefer, B Dayal, GS Tint, T Setoguchi, G Salen and EH Mosbach

This paper describes the conversion of 5beta-cholestane- 3alpha,7alpha,12alpha,25-tetrol into 5beta-cholestane- 3alpha,7alpha,12alpha,24beta,25-pentol by liver microsomes. A sensitive radioactive assay for measuring the formation of 5beta-cholestane- 3alpha,7alpha,12alpha,24beta,25-pentol was developed. Optimal assay conditions for human and rat microsomal systems were established. A higher 24beta-hydroxylation activity was detected in rat than in human liver under the conditions employed. The hydroxylation of 5beta- cholestane-3alpha,7alpha,12alpha,25-tetrol by the rat liver microsomal fraction fortified with NADPH was stimulated about two-fold by administration of phenobarbital. Phenobarbital treatment also stimulated hydroxylations at C-23, C-24alpha, and C-26. Carbon monoxide markedly inhibited all side-chain hydroxylations. In contrast, side- chain hydroxylase activities were not affected in animals deprived of food for 48 hr. These results are consistent with a previously postulated cholic acid biosynthetic pathway involving 5beta-cholestane- 3alpha,7alpha,12alpha,24beta,25-pentol as a key intermediate in man and in the rat.
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