Substrate specificity and stabilization by thiamine pyrophosphate of rat liver branched chain α-ketoacid dehydrogenase

Rat liver branched chain α-ketoacid dehydrogenase has been solubilized and used to investigate the substrate specificity, cofactor requirements, and stabilizing properties of thiamine pyrophosphate for this enzyme. Only the branched chain α-ketoacids are oxidatively decarboxylated with apparent K_m values of 30, 32, and 35 μm for α-ketoisovalerate, α-ketoisocaproate and α-keto-β-methylvalerate, respectively. Maximal CO₂ release requires the presence of CoASH, NAD⁺, Mg²⁺ and thiamine pyrophosphate. The ketoacids competitively inhibit one another, activity for all three show identical pH optimum and heat lability which supports the concept of single enzyme complex acting on all three substrates. The activity can be stabilized by thiamine pyrophosphate which provides a rationale for treatment of maple syrup urine disease with pharmacologic doses of thiamine.