Polyethylene glycol-induced heteroassociation of malate dehydrogenase and citrate synthase

Studies by dynamic and total intensity light scattering, ultracentrifugation, electron microscopy, and chemical crosslinking on solutions of the pig heart mitochondrial enzymes, malate dehydrogenase and citrate synthase (separately and together) demonstrate that polyethylene glycol induces very large homoassociations of each enzyme, and still larger heteroenzyme complexes between these two enzymes in the solution phase. Specificity of this heteroassociation is indicated by the facts that heteroassociations with bovine serum albumin were not observed for either the mitochondrial dehydrogenase or the synthase or between cytosolic malate dehydrogenase and citrate synthase. The weight fraction of the enzymes in the mitochondrial dehydrogenase-synthase associated particles in the solution phase was less than 0.03% with the dilute conditions used in the dynamic light scattering measurements. Neither palmitoyl-CoA nor other solution conditions tested significantly increased this weight fraction of associated enzymes in the solution phase. Because of the extremely low solubility of the associated species, however, the majority of the enzymes can be precipitated as the heteroenzyme complex. This precipitation is a classical first-order transition in spite of the large particle sizes and broad size distribution. Ionic effects on the solubility of the heteroenzyme complex appear to be of general electrostatic nature. Polyethylene glycol was found to be more potent in precipitating this complex than dextrans, polyvinylpyrrolidones, ficoll, and beta-lactoglobulin.