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This paper describes the conversion of 5β-cholestane-3α,7α,12α,25-tetrol into 5β-cholestane-3α,7α,12α,24β,25-pentol by liver microsomes. A sensitive radioactive assay for measuring the formation of 5β-cholestane-3α,7α,12α,24β,25-pentol was developed. Optimal assay conditions for human and rat microsomal systems were established. A higher 24β-hydroxylation activity was detected in rat than in human liver under the conditions employed. The hydroxylation of 5β-cholestane-3α,7α,12α,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-24α, 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 5β-cholestane-3α,7α,12α,24β,25-pentol as a key intermediate in man and in the rat. |
