Comparison of 12alpha-hydroxylation of oxygenated 5alpha-cholestanes and allochenodeoxycholate with rat liver microsomes.

[5alpha,6alpha-3H2]-5alpha-Cholestane-3alpha,7alpha-diol (A), (25 R)-3alpha,7alpha-dihydroxyl-[5alpha,6alpha-3H2]-5alpha--cholestan-26-oic acid (B),(25R)-[5alpha,6alpha-3H2]stane-3alpha,7alpha-26-triol (C), and [3beta-3H]allochenodeoxycholic acid (D) were prepared, characterized, and studied with a rat liver microsomal preparation fortified with 1 mM NADPH. The 12alpha-hydroxylated product formed from each of these substrates was identified by isotopic dilution; the relative reactivity of the four substrates was (A) 100; (B) 87; (C) 135; and (D) 40. The microsomal system showed a requirement for added NADPH and other properties similar to those shown for 12alpha-hydroxylation of 7alpha-hydroxycholest-4-en-3-one, which is ultimately converted to cholic acid. Since the coplanar 5alpha-cholestane-3alpha,7alpha-diol is virtually superimposable upon 7alpha-hydroxycholest-4-en-3-one, and the enzymic requirements are comparable, it is suggested that a single enzyme system may be responsible for 12alpha-hydroxylation of these substrates.