Oxysterol 7α-hydroxylase activity by cholesterol 7α-hydroxylase (CYP7a)

Abstract

A 7α-hydroxylation is necessary for conversion of both cholesterol and 27-hydroxycholesterol into bile acids. According to current theories, cholesterol 7α-hydroxylase (CYP7A) is responsible for the former and oxysterol 7α-hydroxylase (CYP7B) for the latter reaction. CYP7A is believed to have a very high substrate specificity whereas CYP7B is active toward oxysterols, dehydroepiandrosterone, and pregnenolone. In the present study, 7α-hydroxylation of various oxysterols in liver and kidney was investigated. Surprisingly, human cholesterol 7α-hydroxylase, CYP7A, expressed as a recombinant in Escherichia coli and COS cells, was active toward 20(S)hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol. This enzyme has previously been thought to be specific for cholesterol and cholestanol. A partially purified and reconstituted cholesterol 7α-hydroxylase enzyme fraction from pig liver showed 7α-hydroxylase activity toward the same oxysterols as metabolized by expressed recombinant human and rat CYP7A. The 7α-hydroxylase activity toward 20(S)-hydroxycholesterol, 25-hydroxycholesterol, and 27-hydroxycholesterol in rat liver was significantly increased by treatment with cholestyramine, an inducer of CYP7A. From the present results it may be concluded that CYP7A is able to function as an oxysterol 7α-hydroxylase, in addition to the previously known human oxysterol 7α-hydroxylase, CYP7B. These findings may have implications for oxysterol-mediated regulation of gene expression and for pathways of bile acid biosynthesis. A possible use of 20(S)-hydroxycholesterol as a marker substrate for CYP7A is proposed.