CONTROL OF GLUTAMATE OXIDATION IN BRAIN AND LIVER MITOCHONDRIAL SYSTEMS

Abstract

Glutamate oxidation in brain and liver mitochondrial systems proceeds mainly through transamination with oxaloacetate followed by oxidation of the [alpha]-oxoglutarate formed. Both in the presence and absence of dinitrophenol in liver mitochondria this pathway accounted for almost 80% of the uptake of glutamate. In brain preparations the transamination pathway accounted for about 90% of the glutamate uptake. The oxidation of [1-C14]- and [5-C14]- glutamate in brain preparations is compatible with utilization through the tricarboxylic acid cycle, either after the formation of [alpha]-oxoglutarate or after decarboxylation to form gamma-aminobutyrate. There is no indication of gamma-decarboxylation of glutamate. The high respiratory control ratio obtained with glutamate as substrate in brain mitochondrial preparations is due to the low respiration rate in the absence of ADP this results from the low rate of formation of oxaloacetate under these conditions. When oxaloacetate is made available by the addition of malate or of NAD+, the respiration rate is increased to the level obtained with other substrates. When the transamination pathway of glutamate oxidation was blocked with malonate, the uptake of glutamate was inhibited in the presence of ADP or ADP plus dinitrophenol by about 70 and 80% respectively in brain mitochondrial systems, whereas the inhibition was only about 50% in dinitrophenol-stimulated liver preparations. In unstimulated liver mitochondria in the presence of malonate there was a sixfold increase in the oxidation of glutamate by the glutamate-dehydrogenase pathway. Thus the operating activity of glutamate dehydrogenase is much less than the "free" (non-latent) activity. The following explanation is put forward for the control of glutamate metabolism in liver and brain mitochondrial preparations. The oxidation of glutamate by either pathway yields [alpha]-oxoglutarate, which is further metabolized. Since aspartate aminotransferase is present in great excess compared with the respiration rate, the oxaloacetate formed is continuously removed by the transamination reaction. Thus [alpha]-oxoglutarate is formed independently of glutamate dehydrogenation, and the question is how the dehydrogenation of glutamate is influenced by the continuous formation of [alpha]-oxoglutarate. The results indicate that a competition takes place between the [alpha]-oxoglutaratedehydrogenase complex and glutamate dehydrogenase, probably for NAD+, resulting in preferential oxidation of [alpha]-oxoglutarate.