Plant Pyruvate Dehydrogenase Complex Purification, Characterization and Regulation by Metabolites and Phosphorylation

The pyruvate dehydrogenase complex was purified from mitochondria of cauliflower, Brassica oleracea var. botrytis floral buds to a specific activity of 5.4 mumol of NADH/min per mg of protein. The pyruvate dehydrogenase complex required CoASH, NAD+, thiamine pyrophosphate and Mg2+ for the oxidative decarboxylation of pyruvate. The kinetic analysis of the complex gave a series of parallel lines for all substrates. Product interaction patterns showed that NADH is competitive with NAD+; acetyl-CoA is competitive with CoASH; and NADH and acetyl-CoA uncompetitive with pyruvate. These kinetic patterns suggest a multisite ping-pong mechanism as described by Cleveland ((1973) J. Biol. Chem 248, 8353). The noncompetitive inhibition of NADH versus CoASH, and acetyl-CoASH versus NAD are not predicted by this mechanism. Regulation of the complex was more sensitive to the NADH/NAD+ ratio than acetyl-CoA/CoASH ratio. Hydroxypyruvate and glyoxylate inhibited the complex noncompetitively versus pyruvate. The pyruvate dehydrogenase complex was inactivated and phosphorylated by ATP. The ATP dependent inactivation is believed to be enzyme catalyzed by a pyruvate dehydrogenase complex kinase. However, no evidence was found for a plant pyruvate dehydrogenase complex phosphatase. The results suggest that the cauliflower pyruvate dehydrogenase complex is regulated by a phosphorylation-dephosphorylation mechanism.