7a-Hydroxylation of cholestanol bv rat liver microsomes

In a study of the mechanism whereby 5a-bile acids are formed from cholestanol, the 7a-hydroxylation of cholestanol was investigated in rat liver preparations in vitro. I t was found that in the presence of NADPH and oxygen, rat liver microsomes catalyzed the 70-hydroxylation of cholestanol to the same extent as that of cholesterol. The rate of the hydroxylation was enhanced by prior treatment of the experimental rats with cholestyramine (a bile acid sequestrant) or by establishment of bile fistulas-Le., by partial or complete removal of bile acids from the enterohepatic circulation. The 7-hydroxylation reaction was further stimulated by pretreatment of the animals with phenobarbital, a drug known to produce increased biosynthesis of hepatic endoplasmic membranes. The 7a-hydroxylase was inhibited by the reaction product, by sterols with 7-keto or 7P-hydroxyl groups, and also by monoand dihydroxy bile acids of the 5P-series, although cholic acid or taurocholate produced no inhibition unless added in high concentrations. The results of these studies are in accord with the concept that the presence of a A5-double bond is not required for the enzymatic formation of the 7a-hydroxy derivative. The rate of this hydroxylation reaction in vitro appears to depend on the concentration of bile salts in the enterohepatic circulation of the experimental animals from whom the microsomes were obtained.