Substrate stereospecificity in oxidation of (25S)-3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoyl-CoA by peroxisomal trihydroxy-5 beta-cholestanoyl-CoA oxidase.

Partly purified 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestanoyl-CoA oxidase from rabbit liver peroxisomes was found to convert the 25S- but not the 25R diastereoisomer of 3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholestan-27-oyl-CoA into (24E)-3 alpha, 7 alpha, 12 alpha-trihydroxy-5 beta-cholest-24-en-27-oic acid. In the presence of a peroxisomal THCA-CoA racemase, however, also the 25R isomer was oxidized. Since the mitochondrial steroid-27-hydroxylase, responsible for formation of THCA, is 25R specific a racemase seems to be obligatory for formation of cholic acid by the normal peroxisomal-dependent pathway.