The Enzyme-Enzyme Complex of Transaminase and Glutamate Dehydrogenase

Abstract

Glutamate dehydrogenase decreases the distribution coefficient of glutamate oxalacetate transaminase in Sephadex G-200. This is consistent with previous results which suggested that a complex is formed between these two enzymes. These gel filtration as well as kinetic experiments suggest that transaminase can react with monomeric but not polymer forms of glutamate dehydrogenase. When the levels of both mitochondrial enzymes are too low to form a complex, there is little TPNH and NH4+ generated by the combined aspartate aminotransferase and glutamate dehydrogenase reactions. This is because oxalacetate is a potent product inhibitor of transaminase, the reaction with glutamate and glutamate dehydrogenase is slow and α-ketoglutarate, oxalacetate, and aspartate all inhibit this latter reactions. When the levels of both enzymes are sufficiently high to form a complex, asparate can be dehydrogenated quite rapidly even in the absence of α-ketoglutarate. Furthermore, the aspartate dehydrogenase reaction catalyzed by the enzyme-enzyme complex is not markedly inhibited by oxalacetate, or activated by α-ketoglutarate, and can take place even in the presence of rather high levels of glutamate. Kinetic and gel filtration experiments suggest that the dissociation constant of the enzyme-enzyme complex is considerably lower than that of these substrates for the free enzymes. These results suggest that an important physiological function of the enzyme-enzyme complex is to catalyze the asparatate dehydrogenase reaction in organs as brain, liver, and kidney, where the mitochondrial levels of these enzymes are sufficiently high to form a complex. An advantage of catalysis by the complex over transamination with aspartate followed by dehydrogenation of glutamate is that the complex is not markedly inhibited by low levels of oxalacetate. When tyrosine is the substrate and the levels of these two enzymes are too low to form a significant amount of complex, the tyrosine aminotransferase and glutamate dehydrogenase reactions are coupled. This can occur with tyrosine but not aspartate because hydroxyphenylpyruvate is not a potent inhibitor. However, when the levels of both enzymes are sufficiently high to form a significant amount of the enzymeenzyme complex, the tyrosine dehydrogenase reaction can be catalyzed by this complex and again α-ketoglutarate has no effect on the reaction. Similar relationships occur when phenylalanine is the substrate. Thus, the enzyme-enzyme complex can facilitate dehydrogenation of these amino acids which do not react with glutamate dehydrogenase.