STUDIES OF GLUCONEOGENIC MITOCHONDRIAL ENZYMES .V. EFFECT OF TRANSAMINASES ON REACTIONS CATALYZED BY GLUTAMATE DEHYDROGENASE

Transaminases (the glutamate-oxaloacetate from either bovine liver mitochondria or pig heart and the glutamate-pyruvate from pig heart) are activators of reactions catalyzed by glutamate dehydrogenase. Unlike activation by many ligands as steroids and purine nucleotides, transaminase activates reactions with both monoand dicarboxylic substrates and mainly activates by decreasing the Michaelis constant for ammonium ions. The intact transaminase with pyridoxal phosphate is required for activation. There is much less activation when the concentration of glutamate dehydrogenase is low (reaction with α-ketoglutarate) or in the presence of GTP than when enzyme concentration is high (reaction with pyruvate or oxaloacetate) and in the presence of ADP. Other results are consistent with the concept that glutamate dehydrogenase has transaminase dehydrogenase activity. That is in the presence of TPNH, ammonium chloride and glutamate-oxaloacetate transaminase, glutamate dehydrogenase catalyzes the conversion of the transaminase in the pyridoxal phosphate form to the pyridoxamine phosphate form. Keto-acids can rapidly react with the pyridoxamine form of the transaminase producing the pyridoxal form plus an amino acid. Therefore, in the presence of keto-acids the overall reaction is an amino acid dehydrogenase. This is apparently why transaminases activate reactions catalyzed by glutamate dehydrogenase. The transaminase is not an activator but in the presence of transaminase there are two amino acid dehydrogenase reactions. The transaminase dehydrogenase reaction is significant only in the presence of high concentrations of both enzymes and when α-ketoglutarate and glutamate are either absent or present in low concentrations. It seems that pyridoxal phosphate on the transaminase is bound to the α-ketoglutarate site of glutamate dehydrogenase. Also the transaminase is probably preferentially bound to polymeric forms of glutamate dehydrogenase. Some possible physiological implications of the transaminase dehydrogenase reaction are discussed. This reaction could be of some physiological value when the concentration of amino acids are too low to convert a significant amount of the transaminase into the pyridoxamine form. © 1969.