title: Rapid inactivation of brain glutamate decarboxylase by aspartate.
abstract: In the absence of its cofactor, pyridoxal 5'-phosphate (pyridoxal-P), glutamate decarboxylase is rapidly inactivated by aspartate. Inactivation is a first-order process and the apparent rate constant is a simple saturation function of the concentration of aspartate. For the beta-form of the enzyme, the concentration of aspartate giving the half-maximal rate of inactivation is 6.1 +/- 1.3 mM and the maximal apparent rate constant is 1.02 +/- 0.09 min-1, which corresponds to a half-time of inactivation of 41 s. The rate of inactivation by aspartate is about 25 times faster than inactivation by glutamate or gamma-aminobutyric acid (GABA). Inactivation is accompanied by a rapid conversion of holoenzyme to apoenzyme and is opposed by pyridoxal-P, suggesting that inactivation results from an alternative transamination of aspartate catalyzed by the enzyme, as previously observed with glutamate and GABA. Consistent with this mechanism pyridoxamine 5'-phosphate, an expected transamination product, was formed when the enzyme was incubated with aspartate and pyridoxal-P. The rate of transamination relative to the rate of decarboxylation was much greater for aspartate than for glutamate. Apoenzyme formed by transamination of aspartate was reactivated with pyridoxal-P. In view of the high rate of inactivation, aspartate may affect the level of apoenzyme in brain.
mesh_terms: D000818:Animals; D001051:Apoenzymes / Q000378:metabolism; D001224:Aspartic Acid / Q000494:pharmacology*; D001921:Brain / Q000201:enzymology*; D004793:Enzyme Reactivators; D005968:Glutamate Decarboxylase / Q000037:antagonists & inhibitors*; D005971:Glutamates / Q000494:pharmacology; D018698:Glutamic Acid; D007700:Kinetics; D011732:Pyridoxal Phosphate / Q000494:pharmacology; D011733:Pyridoxamine / Q000031:analogs & derivatives / Q000378:metabolism; D013552:Swine; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology
chemical_list: D001051:Apoenzymes; D004793:Enzyme Reactivators; D005971:Glutamates; D001224:Aspartic Acid; D018698:Glutamic Acid; D005680:gamma-Aminobutyric Acid; D011732:Pyridoxal Phosphate; D011733:Pyridoxamine; D005968:Glutamate Decarboxylase; C010627:pyridoxamine phosphate