CYTOCHROME-P450 CYP27-CATALYZED OXIDATION OF C-27-STEROID INTO C-27-ACID

Rabbit liver cytochrome P450 CYP27 has been previously shown to catalyze the complete conversion of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol into 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid. This study compares some properties of the reactions involved, the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol and the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. The K-m was the same for the two substrates, whereas the V-max was three times higher for 27-hydroxylation than for the oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. Ketoconazole inhibited both reactions, whereas disulfiram did not. Carbon monoxide inhibited the 27-hydroxylation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha-triol but not the further oxidation of 5 beta-cholestane-3 alpha,7 alpha,12 alpha,27-tetrol. There was no difference in sensitivity to varying oxygen concentrations between the two reactions. The present study shows that CYP27 also converts, although less efficiently, 5 beta-cholestane-3 alpha,7 alpha-diol into 3 alpha,7 alpha-dihydroxy-5 beta-cholestanoic acid and cholesterol into 3 beta-hydroxy-5-cholestenoic acid. The rate of oxidation of cholesterol into C-27-acid was very low-less than 1% of that with the other C-27-steroids.