STEREOCHEMICAL STUDIES ON THE BETA-OXIDATION OF VALPROIC ACID IN ISOLATED RAT HEPATOCYTES

Stereochemical aspects of the biotransformation of valproic acid (VPA) to four compounds believed to represent products of mitochondrial bet a-oxidation, ViZ. DELTA2(E)-VPA, DELTA3-VPA, 3-hydroxy-VPA, and 3-oxo- VPA, were examined in freshly isolated rat hepatocytes. Following incu bation of the individual enantiomers of [5-C-13]VPA and analysis of pr oducts by GC/MS techniques, it was possible to determine for each meta bolite the relative populations of molecules that had been formed by o xidation on the pro-R vs. the pro-S propyl group of the drug. Metaboli sm was found to exhibit a slight preference (approximately 1.3:1) for attack on the pro-S side-chain for all four compounds, consistent with the hypothesis that this group shares a common metabolic origin. In c ontrast, the hepatotoxic terminal olefin, DELTA4-VPA, was formed with marked enantiotopic differentiation (approximately 3.8:1) favoring the pro-R side-chain. The reason for the surprisingly low stereoselectivi ty displayed by the products of beta-oxidation was investigated with t he aid of [3-H-2]DELTA2(E)-VPA as metabolic substrate. Following incub ation with rat hepatocytes, 35% of the substrate remaining after 2 hr was found to have been isomerized to [3'-H-2]DELTA2(E)-VPA. Because DE LTA2(E)-VPA is known to be formed from VPA-CoA through the action of 2 -methyl-branched-chain acyl-CoA dehydrogenase, it is proposed that the three-carbon side-chains of both parent drug and DELTA2(E)-VPA are in terconverted as a consequence of reversibility in the second half-reac tion of this enzymatic process.