Factors influencing the activation and inactivation of glutamate decarboxylase

Abstract

The effects of pyridoxal-P, P i, glutamate and ATP were studied using a partially purified preparation from hog brain. Incubation of apoenzyme with pyridoxal-P resulted in a slow reactivation, the rate of which was greatly increased by 1–10 mM P i. In the absence of glutamate, pyridoxal-P is tightly bound to the enzyme. Addition of glutamate promoted the inactivation of holoenzyme suggesting that there is continuous formation of apoenzyme in vivo. Chromatography on Sephadex G-200 indicated that interconversion of apo- and holoenzyme did not involve subunit dissociation. Inhibition by ATP was observed in the absence of pyridoxal-P showing that ATP does not act only by blocking activation of apoenzyme. P i relieved the inhibition by ATP. Since this effect of P i required pyridoxal-P, it is not attributable simply to competition with ATP. The evidence suggests that regulation of glutamate decarboxylase involves a cycle of formation of apoenzyme and its reactivation by pyridoxal-P.