Formation and metabolism in vitro of 5,6-epoxides of cholesterol and beta-sitosterol.

The formation of 5alpha,6alpha- and 5beta,6beta-epoxides of cholesterol and beta-sitosterol in rat liver subcellular fractions has been studied. The results show that the epoxidation seems to occur only in connection with the nonspecific tissue oxidation of the sterols. The beta-epoxides were formed in three- to fourfold excess over the alpha-epoxides. Both cholesterol epoxides were efficiently converted by a microsomal hydrolase into the 3beta,5alpha,6beta-triol. The conversion was less extensive with beta-sitosterol epoxides, especially the beta-epoxide. The possible biological significance in the formation of the sterol epoxides and the triols was evaluated by their ability to inhibit the microsomal cholesterol 7alpha-hydroxylase. Only the cholesterol epoxides and especially the beta-epoxide were active in this respect.