Exchange Transamination and the Metabolism of Glutamate in Brain.

1. Experiments were performed to throw light on why the incorporation of (14)C from labelled carbohydrate precursors into glutamate has been found to be more marked in brain than in other tissues. 2. Rapid isotope exchange between labelled glutamate and unlabelled alpha-oxoglutarate was demonstrated in brain and liver mitochondrial preparations. In the presence but not in the absence of alpha-oxoglutarate the yield of (14)CO(2) from [1-(14)C]glutamate exceeded the net glutamate removal, and the final relative specific activities of the two substrates indicated that complete isotopic equilibration had occurred. Also, when in a brain preparation net glutamate removal was inhibited by malonate, isotope exchange between [1-(14)C]glutamate and alpha-oxoglutarate and the formation of (14)CO(2) were unaffected. 3. The time-course of isotope exchange between labelled glutamate and unlabelled alpha-oxoglutarate was followed in uncoupled brain and liver mitochondrial fractions, and the rate of exchange calculated by a computer was found to be 3-8 times more rapid than the maximal rate of utilization of the two substrates. 4. The physiological situation was imitated by the continuous infusion of small amounts of alpha-oxo[1-(14)C]glutarate into brain homogenate containing added glutamate. The fraction of (14)C infused that was retained in the glutamate pool depended on the size of the latter, and the final relative specific activities of the two substrates indicated almost complete isotope exchange. Isotopic equilibration also occurred when alpha-oxoglutarate was generated from pyruvate through the tricarboxylic acid cycle in a brain mitochondrial preparation containing [1-(14)C]glutamate. 5. The differences in the incorporation of (14)C from labelled glucose into the glutamate of brain and liver are discussed in terms of the rates of isotope exchange, the glutamate pool sizes and the rates of formation of labelled alpha-oxoglutarate in the two tissues. It is concluded that the differences between tissues in the incorporation of glucose carbon into glutamate reflect features of their metabolism largely unrelated to that of glutamate.