Abstract

Many biological systems including the oxidative catabolic pathway for branched-chain amino acids (BCAAs) are affected in vivo by valproate therapy. In this study, we investigated the potential effect of valproic acid (VPA) and some of its metabolites on the metabolism of BCAAs. In vitro studies were performed using isovaleryl-CoA dehydrogenase (IVD), isobutyryl-CoA dehydrogenase (IBD), and short branched-chain acyl-CoA dehydrogenase (SBCAD), enzymes involved in the degradation pathway of leucine, valine, and isoleucine. The enzymatic activities of the three purified human enzymes were measured using optimized high-performance liquid chromatography procedures, and the respective kinetic parameters were determined in the absence and presence of VPA and the corresponding CoA and dephosphoCoA conjugates. Valproyl-CoA and valproyl-dephosphoCoA inhibited IVD activity significantly by a purely competitive mechanism with K_i values of 74 ± 4 and 170 ± 12 μM, respectively. IBD activity was not affected by any of the tested VPA esters. However, valproyl-CoA did inhibit SBCAD activity by a purely competitive mechanism with a K_i of 249 ± 29 μM. In addition, valproyl-dephosphoCoA inhibited SBCAD activity via a distinct mechanism (K_i = 511 ± 96 μM) that appeared to be of the mixed type. Furthermore, we show that both SBCAD and IVD are active, using valproyl-CoA as a substrate. The catalytic efficiency of SBCAD turned out to be much higher than that of IVD, demonstrating that SBCAD is the most probable candidate for the first dehydrogenation step of VPA β-oxidation. Our data explain some of the effects of valproate on the branched-chain amino acid metabolism and shed new light on the biotransformation pathway of valproate.