Properties of liver microsomal cholesterol 5,6-oxide hydrolase

Cholestane 3 beta,5 alpha, 6 beta-triol has been identified as the exclusive product formed on hydration of cholesterol 5,6 alpha- and 5,6 beta-oxide catalyzed by cholesterol oxide hydrolase in liver microsomes obtained from five mammalian species. Highest activities were present in microsomes from rats and humans. Both acid- and base-catalyzed hydrolysis of the two epoxides also produce this product, presumably due to preference for pseudo-axial opening of the oxirane ring to form product with a trans-AB ring junction. Although the beta-oxide is more reactive than the alpha-oxide upon acid-catalyzed hydration, the alpha-oxide is a 4.5-fold better substrate than the beta-oxide as indicated by values of Vmax/Km. The kinetic parameters Vmax and Km for the reaction catalyzed by rat liver microsomes are 1.68 +/- 0.15 X 10(-7) M min-1 and 10.6 +/- 1.5 microM for the alpha-oxide and 1.32 +/- 0.11 X 10(-7) M min-1 and 37.2 +/- 5.5 microM for the beta-oxide at 0.35 mg protein/ml, pH 7.4, 6.35% (v/v) CH3CN, and 37 degrees C. Several imino compounds are competitive inhibitors for the enzyme from rat liver. The most effective of these is 5,6 alpha-iminocholestanol (Ki = 0.085 microM) which was known to be a good inhibitor from previous studies. Inhibition by aziridines is consistent with the participation of acid catalysis in the mechanism of action of the enzyme. Cholesterol oxide hydrolase is a distinct enzyme from oxidosqualene cyclase as well as microsomal epoxide hydrolase (EC 3.3.2.3) and the recently reported mouse hepatic microsomal epoxide hydrolase that catalyzes the hydration of trans-stilbene oxide.