Studies on the 12alpha and 26-hydroxylation of bile alcohols by rabbit liver microsomes

12Alpha--Hydroxylation of two C27-steroids by rabbit liver microsomes was studied. Optimal assay conditions were determined with 7alpha-hydroxy-4-cholesten-3-one and 5beta-cholestand-3alpha, 7alpha-diol as substrates. The rate of 12alpha-hydroxylation of 7alpha-hydroxy-4-cholesten-3-one was found to be greater than that of 5beta-cholestane-3alpha, 7alpha-diol by ca. 60%. Microsomal 26-hydroxylation of 5beta-cholestane-3alpha7alpha-diol was also measured, and the ratio of 26-hydroxylation to 12alpha-hydroxylation of 5beta-cholestane-3alpha, 7alpha-diol was found to be ca. 0.4. Rabbit liver 12-alphahydroxylase was more active than that of three other species (man, rat, monkey), explaining in part the predominance of cholic acid in rabbit bile.