Formation of bile acids in man: conversion of cholesterol into 5-beta-cholestane-3-alpha, 7-alpha, 12-alpha-triol in liver homogenates.

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