The effect of bile acids on "solubilized" cholesterol 7 alpha-hydroxylase activity in swine

Optimal assay conditions for cholesterol 7 alpha-hydroxylase activity in swine liver microsomes were determined. The enzyme activity is induced three-fold by feeding cholestyramine to the swine. This suggests that cholesterol 7 alpha-hydroxylase is likely to be the rate-limiting enzyme for biosynthesis of bile acids in swine. The effects of various bile acids on cholesterol 7 alpha-hydroxylase in swine microsome and "solubilized" cholesterol 7 alpha-hydroxylase activity have been studied. There is no significant reduction of native microsomal enzyme activity. However, except for chenodeoxycholic acid, most of the bile acids tested exerted significant inhibition on "solubilized" cholesterol 7 alpha-hydroxylase. This finding suggests that bile acids could interact with and regulate the rate-limiting enzyme for bile acid formation in swine.