Glutamate, decarboxylase transaminase

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

Mechanisms of action of pyridoxal phosphate (a) in glutamate-oxaloacetate transaminase, and (b) in aspartate /3-decarboxylase. 

Figure 34-32 Metabolism of y-hydroxybutyrate and its precursors. /, alcohol dehydrogenase 2, aldehyde dehydrogenase 3, lactonase 4, GHB dehydrogenase J, SSA reductase 6, GABA transaminase 7, glutamate decarboxylase 8, SSA dehydrogenase.

Lin C-T, Li J-Z, Wu J-Y (1983) Immunocytochemical localization of L-glutamate decarboxylase, gamma-aminobutyric acid transaminase, cysteine sulfinic acid decarboxylase, aspartate aminotransferase and somatostatin in rat retina. Brain Res 270 273-283. 

Fonnum, F. (1968) The distribution of glutamate decarboxylase and aspartate transaminase in subcellular fractions of rat and guinea-pig brain. Biochem. J., 106,401-412. 

Subsequent to this work, apoaspartate transaminase was used to assay the stereospecificity of a variety of other transaminases, all of which were shown to involve protonation/deprotonation at the C-4 Si face of the cofactor. These enzymes included pyridoxamine-pyruvate transaminase (EC 2.6.1.30) (26) and a-dialkylamino acid transaminase (27). L-Glutamate decarboxylase (EC 4.1.1.15) catalyzes an abortive transamination reaction when oc-methylglutamate is used as substrate, and this too was shown to occur with protonation at the Si face of C-4 in the intermediate 4d (28) as was the abnormal transamination of D-alanine by serine hydroxymethyltransferase (29). 

Tryptophan 331 is converted to tryptamine 332 by both aromatic L-amino acid decarboxylase (EC 4.1.1.28) and tyrosine decarboxylase (EC 4.1.1.25), and in both instances (334, 335) it was shown, either by use of the pro-R specific monoamine oxidase (335) or by degradation of the labeled tryptamines to glycine and use of the pro-S specific D-amino acid oxidase and pro-R specific glutamate pyruvate transaminase (334), that decarboxylation involved retention of configuration. Hydroxylation that leads to sporidesmin 333 has been shown to involve specific loss of the 3-pro-R hydrogen, and so again hydroxylation involves retention of configuration (102). 

Wong E., Schousboe A., Saito K, Wu J.-Y., and Roberts E (1974) Glutamate decarboxylase and GABA-transaminase from six mouse strains. Brain Res 68, 133-139... 

Collins G G S (1972) GABA-2-oxoglutarate transaminase, glutamate decarboxylase, and the half-life of GABA m different areas of rat brain. Biochem Pharmacol 21, 2849-2853. 

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