Interaction between the pyruvate dehydrogenase complex and citrate synthase.

Kinetic studies of the individual reaction of pig heart pyruvate dehydrogenase complex (pyruvate dehydrogenase (pyruvate:lipoamide oxidoreductase (decarboxylating and acceptor-acetylating), EC 1.2.4.1); dihydrolipoamide reductase(NAD+) (NADH:lipoamide oxidoreductase, EC 1.6.4.3); dihydrolipoamide acetyltransferase (acetyl-CoA:dihydrolipoamide S-acetyltransferase, EC 2.3.1.12)), citrate synthase (citrate oxaloacetate-lyase (pro-3S-CH2COO- leads to acetyl-CoA), EC 4.1.3.7) and the pyruvate dehydrogenase complex-citrate synthase coupled system show that the KmCoA value of pyruvate dehydrogenase complex and KmCoASAc value of citrate synthase decrease in the coupled system when compared to those in the individual enzyme reactions. The explanation for this interaction may be an association between the two enzymes. When it was centrifuged with 150 000 x g for 140 min, 30% of the citrate synthase sedimented in the presence of the pyruvate dehydrogenase complex, while no sedimentation was observed in the absence of the pyruvate dehydrogenase complex. Sedimentation of cytoplasmic malate dehydrogenase, phosphotransacetylase, hemoglobin and Blue albumin were negligible under the same condition. In gel chromatography experiments a significant peak of citrate synthase activity co-migrated with the pyruvate dehydrogenase complex peak. This observation also suggests the possible association of two enzymes.