EXCHANGE TRANSAMINATION AND THE METABOLISM OF GLUTAMATE IN BRAIN

Abstract

Experiments were performed to throw light on why the incorporation of 14C from labeled carbohydrate precursors into glutamate was more marked in brain than in other tissues. Rapid isotope exchange between labeled glutamate and un-labeled a-oxoglutarate was demonstrated in brain and liver mito-chondrial preparations. In the presence but not in the absence of a-oxoglutarate the yield of 14CO2 from [1-14C]glutamate exceeded the net glutamate removal, and the final relative specific activities of the 2 substrates indicated that complete isotopic equilibration had occurred. Also, when in a brain preparation net glutamate removal was inhibited by malonate, isotope exchange between [l-14c]glutamate and a-oxoglutarate and the formation of 14CO2 were unaffected. The time-course of isotope exchange between labeled glutamate and un-labeled a-oxoglutarate was followed in uncoupled brain and liver mito-chondrial fractions, and the rate of exchange calculated by a computer was 3-8 times more rapid than the maximal rate of utilization of the 2 substrates. The physiological situation was imitated by the continuous infusion of small amounts of [alpha]-oxo[1-14C]glutarate into brain homogenate containing added glutamate. The fraction of 14C infused that was retained in the glutamate pool depended on the size of the latter, and the final relative specific activities of the 2 substrates in dicated almost complete isotope exchange. Isotopic equilibration also occurred when a-oxoglutarate was generated from pyruvate through the tricarboxylic acid cycle in a brain mitochondrial preparation containing [l-14C]glutamate. The differences in the incorporation of 14C from labeled 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 labeled [alpha]-oxoglutarate in the 2 tissues. It is concluded that the differences between tissues in the incorporation of glucose C into glutamate reflect features of their metabolism largely unrelated to that of glutamate.