THE METABOLISM OF GLUTAMATE IN HOMOGENATES AND SLICES OF BRAIN CORTEX

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Abstract
The fates of [l-C14]- and [5-C14]-glutamate was studied in the presence and absence of pyruvate or glucose in homogenates of rat cerebral cortex and slices of guinea pig cerebral cortex. In homogenates 80-90% of the [l-C14]-glutamate removed was converted into aspartate and C14O2- Endogenous glutamate was also converted into aspartate and this accounted for about 50% of the endogenous O2 uptake. Malonate and parapyruvate (gamma-hydroxy-gamma-methyl-[alpha]-oxoglutarate) almost completely inhibited the formation of aspartate and C14O2 from [l-C14]-glutamate. Addition of fumarate removed the inhibitions of asparate formation and in the presense of malonate, but not of parapyruvate, largely restored C14O2 production. Since in homogenates the oxidation of glutamate depends almost entirely on its initial transamination with oxaloacetate. In homogenates pyruvate inhibited the net removal of [1-C14] glutamate by 60%, aspartate formation by 80% and the production of C14O2 by 36%. The formation of C14O2 from [l-C14]glutamate was, in the presence of pyruvate, more than 50% greater than the net glutamate removal, and the specific activity of the residual glutamate decreased. The observations can be explained by a synthesis of unlabelled [alpha]-oxoglutarate through the tri-carboxylic acid cycle and isotope exchange between this unlabelled [alpha]-oxoglutarate and the labelled glutamate. Addition of glucose to homogenate equally decreased glutamate removal and C14O2 production from [l-C14]glutamate (25%). The formation of C14O2 from [5-C14]glutamate in homogenates was equivalent to about 10% of the glutamate removed. Pyruvate trebled C14O2 production. The specific activity of C-4 of aspartate formed from [5-C14]glutamate was 0-46 (expected 0-50) of that of C-5 of the glutamate. In slices of cerebral cortex about 60% of the glutamate removed was converted into aspartate. The formation of C14O2 from [l-C14] glutamate was equivalent to the glutamate removed and that from [5-c14]glutamate to the difference between the glutamate removed and the aspartate formed. The aspartate formed from [l-C14]glutamate was unlabelled whereas that from [5-C14]glutamate was radioactive. The effects of glucose on glutamate metabolism in cortex slices were similar to those of pyruvate in homogenates. In rat-liver homogenate the glutamate removed can be almost entirely accounted for by the formation of aspartate (69%) and alanine (29%). In contrast with homogenate of cerebral cortex addition of pyruvate stimulated glutamate removal and pyruvate replaced oxaloacetate as the [alpha]-oxo acid initiating glutamate oxidation through transamination. The differences between liver and brain can be explained by differences in alanine-amino-transferase activities. The metabolic behavior of glutamate in the preparations of cerebral cortex can be fully accounted for by the tricarboxylic acid cycle and associated reactions. The factors that are responsible for rapid incorporation of isotope from glucose into glutamate and aspartate in brain, both in vivo and in vitro, are discussed and it is emphasized that the rapid labelling of amino acids may be the result of isotope exchanges that are side reactions of the tricarboxylic acid cycle.