Studies on the beta-oxidation of valproic acid in rat liver mitochondrial preparations.

Incubation of valproic acid (VPA) with freshly isolated rat liver mitochondria led to the identification of the beta-keto acid, 3-oxo-VPA, together with three unsaturated metabolites, viz. delta 2(E)-VPA, delta 3-VPA, and delta 2(E),3'(E)-VPA. The corresponding beta-hydroxy acid, 3-hydroxy-VPA, was not detected as a metabolite of VPA in this in vitro system. Studies with subcellular fractions of rat liver and with inhibitors of fatty acid metabolism demonstrated that these metabolites are products of the beta-oxidation system in mitochondria, and are not formed to any significant extent by the related peroxisomal beta-oxidation complex. When incubations with mitochondria were carried out in a medium enriched in H218O, metabolites were labeled in a manner consistent with the involvement of coenzyme A thioester intermediates. Incubation of the authentic metabolites with mitochondrial preparations showed that delta 2(E)-VPA, delta 3-VPA, and delta 2(E),3'(E)-VPA are interconverted by isomerization and reduction processes, and that all three serve as precursors of 3-oxo-VPA. It is concluded that VPA is a substrate for beta-oxidation in liver mitochondria, and that competition by VPA for enzymes and/or cofactors of the fatty acid beta-oxidation complex may contribute to VPA-mediated inhibition of fatty acid metabolism and associated hepatic steatosis.