Interactions between Glutamate Dehydrogenase, Transaminases, and Keto Acids

Data are presented which are consistent with the concept that transaminases can be bound to glutamate dehydrogenase. In the presence of TPNH and ammonium ions the transaminase in this enzyme-enzyme complex can be converted to the pyridoxamine phosphate form. Keto acids bind to and react with the transaminase in this complex to yield the pyridoxal phosphate form of the transaminase, TPN, and the corresponding amino acid. The specific structure of the keto acid required for binding to the enzyme-enzyme complex is determined by the transaminase, and not glutamate dehydrogenase. For example, oxalacetate is bound better than pyruvate to the complex of glutamate-oxalacetate transaminase and glutamate dehydrogenase while the opposite is the case when glutamate-pyruvate transaminase is part of the complex. The conversion of oxalacetate to aspartate occurs more readily with the complex between glutamate-oxalacetate transaminase and glutamate dehydrogenase than with the latter enzyme alone. With the enzyme-enzyme complex the specific activity with respect to glutamate dehydrogenase is higher (in some cases over 70-fold); the concentration of oxalacetate required to produce maximal activity is more than 1000-fold lower, and the concentration of ammonium ion required to produce maximal activity is more than 10-fold lower. Also, in the presence of the enzyme-enzyme complex, ADP is a significant activator. The activity of the complex can be regulated by several factors. For example, the specific activity and activation by ADP decreases when the concentration of glutamate dehydrogenase exceeds about 0.1 mg per ml. If the concentrations of keto acid and glutamate dehydrogenase are both high, then the reaction is similar to that of the keto acid with glutamate dehydrogenase alone. If the concentrations of the transaminase and glutamate dehydrogenase are both high, the pyridoxamine phosphate form of the transaminase is produced faster than the capacity of the keto acid to react and TPN is produced in excess of the amino acid.