The anomalous kinetics of coupled aspartate aminotransferase and malate dehydrogenase. Evidence for compartmentation of oxaloacetate

Abstract

Cytoplasmic aspartate aminotransferase (EC 2.6.1.1) and malate dehydrogenase (EC 1.1.1.37) were purified from pig heart. Kinetic parameters were determined for the separate reaction catalyzed by each enzyme and used to predict the course of the coupled reaction: .**GRAPHIC**. Although a lag phase should have been easily seen, none was detected. The same coupled reaction was also carried out by using radioactive aspartate in the presence of unlabeled oxaloacetate. The reaction was quenched with HClO4 after 70 ms and the specific radioactivity of the malate produced in this system was found to be essentially the same as that of the original aspartate. Oxaloacetate produced by the aspartate aminotransferase is converted into malate dehydrogenase before it equilibrates with the pool of unlabeled oxaloacetate. The results are consistent with a proposal that the enzymes are associated in a complex. No physical evidence of the existence of a complex was found. An alternative means of compartmentation of the intermediate as an unstable isomer is considered.