Catalytic properties and inhibition of hepatic cholesterol-epoxide hydrolase

Hepatic cholesterol-epoxide hydrolase is a microsomal enzyme which appears to be catalytically distinct from the epoxide hydrolase responsible for the catabolism of a wide variety of aromatic and aliphatic epoxides. The diastereomeric forms of cholesterol epoxide, cholesterol 5 alpha,6 alpha-, and cholesterol 5 beta,6 beta-epoxides are converted to cholestane-3 beta,5 alpha,6 beta-triol with equal facility. Kinetic analysis of cholesterol-epoxide hydrolase demonstrated that both diastereomers bind to a common catalytic site. Apparent Km values of 3.69 and 4.42 microM were derived for cholesterol 5 alpha,6 alpha- and cholesterol 5 beta,6 beta-epoxide, respectively. In addition, enzyme activity with both diastereomers was product-inhibited by cholestanetriol through a competitive mechanism with the apparent Ki for cholestanetriol being 10.8 and 6.8 microM against cholesterol alpha- and beta-epoxides, respectively. This inhibitory effect of cholestanetriol may account for the difference observed in the hydration rates for the cholesterol epoxide isomers when they are incubated together in the presence of liver microsomes. Inhibitors of epoxide hydrolase were studied, and three oxidation products were found to be particularly effective against cholesterol-epoxide hydrolase while producing no significant inhibition of styrene-epoxide hydrolase. These inhibitors were 7-ketocholesterol, 6-ketocholestanol, and 7-ketocholestanol, the latter displaying an apparent Ki lower than the Km for either cholesterol epoxide isomer. None of the xenobiotic epoxide hydrolase inhibitors or activators studied affected cholesterol-epoxide hydrolase activity.