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

Abstract

This paper describes the conversion of 5.beta.-cholestane-3.alpha.,7.alpha.,12.alpha.,25-tetrol into 5.beta.-cholestane-3.alpha.,7.alpha.,12.alpha.,24.beta.,25-pentol by liver microsomes. A sensitive radioactive assay for measuring the formation of 5.beta.-cholestane-3.alpha.,7.alpha.,12.alpha.,24.beta.,25-pentol was developed. Optimal assay conditions for human and rat microsomal systems were established. A higher 24.beta.-hydroxylation activity was detected in rat than in human liver under the conditions employed. The hydroxylation of 5.beta.-cholestane-3.alpha.,7.alpha.,12.alpha.,25-tetrol by the rat liver microsomal fraction fortified with NADPH was stimulated about 2-fold by administration of phenobarbital. Phenobarbital treatment also stimulated hydroxylations at C-23, C-24.alpha. and C-26. CO markedly inhibited all side-chain hydroxylations. Side-chain hydroxylase activities were not affected in animals deprived of food for 48 h. These results are consistent with a previously postulated cholic acid biosynthetic pathway involving 5.beta.-cholestane-3.alpha.,7.alpha.,12.alpha.,24.beta.,25-pentol as a key intermediate in humans and rats.