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 β‐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 γ‐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