Regulatory Aspects of Endogenous Glutamate in Brain
Glutamate is a major amino acid in the mammalian brain and together with the other members of the glutamine family, glutamine, GABA and aspartate, it constitutes 70–80% of the free amino acid nitrogen (Timiras et al., 1973). In human cerebral cortex the concentration of glutamate, glutamine and GABA is 10.8 mM, 4.4 mM and 2.1 mM, respectively (Perry et al., 1971). In synaptosomes the dominant amino acid is glutamate, followed by glutamine, aspartate, GABA and taurine (Bradford and Thomas, 1969 and Kontro et al., 1980). Glutamate is taken up by high affinity system in neuronal constituents and astrocytes. There is ample evidence that glutamate is compartmentalized in at least two metabolic pools in brain, a small, probably glial pool, which has a rapid turnover into a large glutamine pool, and alarge, probably neuronal pool. This has formed the basis for the hypothetical glutamine cycle, assuming that glutamate is taken up by glial cells, converted to glutamine by the glutamine synthetase reaction, which is predominantly localized in glial cells, and the newly formed glutamine enters neuronal cells to form glutamate and GABA (Balázs et alt, 1970, Benjamin and Quastel, 1972 and Van den Berg et al., 1975 ) • Additional evidence for neuronal compartmentalization of glutamate has also been produced. Thus synaptosomes contain sufficient glutamate to inhibit phosphate activated glutaminase, but this glutamate appears not to be available to the enzyme, in contrast to exogenous glutamate (Kvamme and Lenda, 1981). Furthermore, Storm Mathisen et al. (1983) have demonstrated using immunohistochemical methods, that glutamate most likely is stored in neuronal vesicles.
KeywordsGlutathione Arginine Glutamine Tate Taurine
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