INTERACTION BETWEEN CITRATE SYNTHASE AND MALATE-DEHYDROGENASE - SUBSTRATE CHANNELING OF OXALOACETATE

The interactions between pig heart citrate synthase and mitochondrial malate dehydrogenase or cytosolic malate dehydrogenase were studied us ing the frontal analysis method of gel filtration and by precipitation in polyethylene glycol. This method showed that an interaction betwee n citrate synthase and mitochondrial malate dehydrogenase occurred but no interaction between citrate synthase and cytosolic malate dehydrog enase. Channeling of oxaloacetate in the malate dehydrogenase and citr ate synthase-coupled systems was tested using polyethylene glycol prec ipitates of citrate synthase and mitochondrial malate dehydrogenase, a nd citrate synthase and cytosolic malate dehydrogenase. The effectiven ess of large amounts of aspartate aminotransferase and oxaloacetate de carboxylase, as competing enzymes for the intermediate oxaloacetate, w as examined. Aspartate aminotransferase and oxaloacetate decarboxylase were less effective competitors for oxaloacetate when precipitated ci trate synthase and mitochondrial malate dehydrogenase in polyethylene glycol was used at low ionic strength compared with free enzymes in th e absence of polyethylene glycol or with a co-precipitate of citrate s ynthase and cytosolic malate dehydrogenase. Substrate channeling of ox aloacetate with citrate synthase-mitochondrial malate dehydrogenase pr ecipitate was inefficient at high ionic strength. These effects could be explained through electrostatic interactions of mitochondrial but n ot cytosolic malate dehydrogenase with citrate synthase.