Formation of bile acids in man: conversion of cholesterol into 5β-cholestane-3α,7α,12α-triol in liver homogenates*

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Abstract
The mechanisms of the conversion of cholesterol into bile acids in man were studied by examining the metabolism of cholesterol-1,2-3H, cholest-5-ene-3β,7α-diol-7β-3H, tritiumlabeled 7α-hydroxycholest-4-en-3-one, 7α,12α-dihydroxycholest-4-en-3-one, and cholest-5-ene-3β,7α,12α-triol in fractions of liver homogenates. The 20,000 g supernatant fluid catalyzed the conversion of cholesterol into cholest-5-ene-3β,7α-diol, 7α-hydroxycholest-4-en-3-one, 7α-12α-dihydroxycholest-4-en-3-one, and 5β-cholestane-3α,7α,12α-triol. In the presence of microsomal fraction fortified with NAD+, cholest-5-ene-3β,7α-diol was converted into 7α-hydroxycholest-4-en-3-one, and when this fraction was fortified with NADPH small amounts of cholest-5-ene-3β-7α,12α-triol were formed. 7α-Hydroxycholest-4-en-3-one was metabolized into 7α-12α-dihydroxycholest-4-en-3-one in the presence of microsomal fraction fortified with NADPH and into 5β-cholestane-3α,7α-diol in the presence of 100,000 g supernatant fluid. Cholest-5-ene-3β,7α,12α-triol was converted into 7α,12α-dihydroxycholest-4-en-3-one in the presence of microsomal fraction fortified with NAD+. The 100,000 g supernatant fluid catalyzed the conversion of 7α,12α-dihydroxycholest-4-en-3-one into 5β-cholestane-3α,7α,12α-triol. The sequence of reactions in the conversion of cholesterol into 5β-cholestane-3α,7α-diol and 5β-cholestane-3α,7α,12α-triol, the subcellular localization of the enzymes, and the cofactor requirements were found to be the same as those described for rat liver.