Effect of Nucleotides and Other Inhibitors on the Inactivation of Glutamate Decarboxylase

The effects of 17 nucleotides and nucleotide analogs and 11 other compounds on the glutamate‐promoted inactivation of brain glutamate decarboxylase were examined. Among the nucleotides, the major determinant of potency was the polyphosphate chain, Glutamate‐promoted inactivation was strongly enhanced by low concentrations (<100 μM) of adenosine tetraphosphate and all eight nucleoside triphosphates tested. Nucleoside diphosphates enhanced inactivation, but were much less effective than the nucleoside triphosphates; nucleoside monophosphates were not effective. Modification of the polyphosphate chain of the nucleoside triphosphates also affected potency; adenylylimidodiphosphate and α,β‐methylene ATP were about as effective as nucleoside diphosphates, but α,β‐methylene ATP was nearly as effective as ATP. The nucleoside base had only a small effect on potency; purine nucleotides were more potent than pyrimidine nucleotides, and one nucleotide with a tricyclic base, 1, N 6 ‐etheno ATP, was as effective as the purine nucleoside triphosphates. The 2′‐hydroxyl group of ribose was unimportant, since deoxy ATP was as effective as ATP. Three nonnucleotide polyanions were strong promoters of inactivation; inositol hexasulfate and 5‐phosphorylribose 1‐pyrophosphate were at least as effective as ATP; inositol hexaphosphate (phytate) was as effective as the nucleoside diphosphates. These results suggest that a major determinant of potency was a strong negative charge on the molecule. Negative charge was not sufficient, however, since fructose 1,6‐bisphosphate did not promote inactivation. Inactivation by all of these compounds was slow, requiring more than 20 min for full effect. Two competitive inhibitors, chloride and glutarate, acted immediately and also reduced rather than enhanced glutamate‐promoted inactivation.