Cholesterol alpha- and beta-epoxides as obligatory intermediates in the hepatic microsomal metabolism of cholesterol to cholestanetriol.

A high performance liquid chromatographic method for the good separation and direct determination of cholesterol alpha-epoxide (5,6 alpha-epoxy-5 alpha-cholestan-3 beta-ol) and beta-epoxide (5,6 beta-epoxy-5 beta-cholestan-3 beta-ol) was introduced to the study of microsomal lipid peroxidation-mediated oxygenation of the cholesterol double bond. In the presence of NADPH, FeSO4, and ADP, bovine liver microsomes converted [4-14C] cholesterol to the alpha-epoxide, beta-epoxide, and cholestanetriol (5 alpha-cholestane-3 beta,5,6 beta-triol) in the ratio 1.0:4.3:0.7. Obligatory intermidiacy of both cholesterol alpha- and beta-epoxides and essential role of microsomal cholesterol epoxide hydratease in the conversion of cholesterol to cholestanetriol were established by using the isotope trapping method as well as the cholesterol epoxide hydratase inhibitor, 5,6 alpha-imino-5 alpha-cholestan-3 beta-ol. Hepatic microsomal P-450 played no appreciable role in the epoxidation of cholesterol. Microsomal cholesterol epoxide hydratase was with no doubt found to differ in nature from microsomal xenobiotic epoxide hydratase. Microsomal hydrolysis of styrene oxide and safrole oxide (0.1 mM each) was almost completely inhibited by 3,3,3-trichloro-1-propene oxide (1 mM) but not by 5,6 alpha-imino-5 alpha-cholestan-3 beta-ol (1 mM). However, microsomal hydrolysis of both cholesterol alpha- and beta-epoxides was remarkably accelerated by 3,3,3-trichloro-1-propene oxide and inhibited by 5,6 alpha-imino-5 alpha-cholestan-3 beta-ol.