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For the determination of the activity of glutamate: cysteine-gamma-ligase one method was optimized and a new micromethod was developed: a) Utilizing [14C] glutamate and cysteine as substrates, the product [14C] gamma-glutamylcysteine was isolated as its cadmium mercaptide and determined by liquid scintillation counting gamma-glutamyl-[14C] aminobutyrate, which was synthesized from glutamate and [14C] alpha-aminobutyrate was separated from remaining radioactive substrate by paper electrophoresis and counted. Both methods were used to determine the activity of the enzyme in human red blood cells. For 20 parallel determinations a standard deviation of +/- 9% and +/- 5% was obtained for method a and b, respectively. The specific activity of the enzyme in erythrocytes of adults was found to be 0.40 +/- 0.05 U/g hemoglobin with method a, and 0.50 +/- 0.05 U/g hemoglobin with method b.

In mice, the influence of valproic acid (di-n-propylacetate, DPA) on central level of gamma-aminobutyric acid (GABA) as well as the activity of the enzymes regulating GABA metabolism, glutamate decarboxylase (GAD) and GABA-alpha-oxoglutarate aminotransferase (GABA-T) were studied. It was shown that the elevation of GABA by DPA was accompanied by an activation of GAD. GABA-T was inhibited only by toxic doses or high concentrations of DPA in vivo and in vitro, and so was GAD. DPA proved able to antagonize the biochemical alterations induced by convulsant doses of isoniazid, i.e., decreases in the level of GABA and the activity of GAD. These results point to a role of the transmitter pool of GABA regulated by GAD in the anticonvulsant effect of DPA.