Immunological and Biochemical Studies on Isozymes of Malate Dehydrogenase and Citrate Synthetase in Castor Bean Glyoxysomes

Rabbit gamma-globulin obtained after the injection of solubilized proteins of castor bean (Ricinus communis L var. Hale) glyoxysomes contains antibodies against some of the glyoxysomal enzymes. The gamma-globulin was shown to inhibit by 50% and 80%, respectively, the in vitro activities of the castor bean glyoxysomal citrate synthetase and malate dehydrogenase. The conditions required for the inactivation are described.The glyoxysomal and mitochondrial citrate synthetase from castor bean endosperm show no significant difference in the above immunological test, in their apparent Michaelis constant values for acetyl coenzyme A and oxaloacetate, or in their sensitivity toward ATP inhibition. In the immunological test, glyoxysomal malate dehydrogenase from castor bean endosperm appeared to be related more closely to the leaf peroxisomal malate dehydrogenase from spinach (Spinacia oleracea L.) and to the glyoxysomal malate dehydrogenase from a variety of other fatty seedlings than to the castor bean mitochondrial or soluble malate dehydrogenase. Unlike the mitochondrial or soluble malate dehydrogenase, the castor bean glyoxysomal malate dehydrogenase is easily inactivated by gentle heat treatment, as was reported previously by other workers for the leaf peroxisomal malate dehydrogenase. Thus, the malate dehydrogenases from various plant microbodies share some common characteristics which distinguish them from other isozymes of malate dehydrogenase.