Stereospecific dehydrogenation of (25R)- and (25S)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acids by acyl-CoA oxidase in rat liver light mitochondrial fraction.

From a stereochemical point of view, the dehydrogenation mechanism of the biotransformation of 3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholestanoic acid (THCA) into (24E)-3 alpha,7 alpha,12 alpha-trihydroxy-5 beta-cholest-24-enoic acid (delta 24-THCA) has been studied with capillary gas chromatography (GC)/negative ion chemical ionization (NICI)-mass spectrometry. After incubation of (24R,25R)- or (24S,25S)-[24,25-2H2]THCA, synthesized from (24E)-delta 24-THCA by a deuterated diimide reduction, with a rat liver light mitochondrial fraction, 5 beta-cholestanoic acids were extracted and derivatized into a pentafluorobenzyl (PFB) ester-dimethylethylsilyl (DMES) ether. Subsequent resolution into THCA and delta 24-THCA was attained by GC on a cross-linked 5% phenylmethyl silicone fused-silica capillary column monitored with a corresponding characteristic carboxylate anion [M-PFB]- in the NICI mode. The stereospecific elimination of a pro-R hydrogen at C-24 in both (25R)- and (25S)-THCA indicated syn-elimination for the former, whereas anti-elimination for the latter was observed.