On the stereospecificity of cholestanetriol 26-monooxygenase.

Both diastereoisomers, 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 26-tetrol (25S isomer) and 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 27-tetrol (25R isomer), were chemically synthesized, and the method to separate these epimers by thin layer chromatography was established. Applying this method, the stereospecificity of cholestanetriol 26-monooxygenase (EC 1.14.13.15) was studied as follows. 5 beta-[3H]Cholestane-3 alpha, 7 alpha, 12 alpha-triol was incubated with the reconstituted system of rat liver mitochondria, which consisted of the partially purified cytochrome P-450, bovine NADPH-adrenodoxin reductase, bovine adrenodoxin, and NADPH. The product was identified as 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha, 27-tetrol (25R isomer). This clearly demonstrates that the partially purified cytochrome P-450 of rat liver mitochondria catalyzes hydroxylation of the 27-methyl group (25-pro-(S) methyl group) of 5 beta-cholestane-3 alpha, 7 alpha, 12 alpha-triol.