STUDIES ON THE REGULATION OF GABA SYNTHESIS: SUBSTRATE-PROMOTED DISSOCIATION OF PYRIDOXAL-5'-PHOSPHATE FROM GAD

The association between glutamate decarboxylase (GAD) and its cofactor, pyridoxal-5′-phos-phate (pyridoxal-P), was studied using 20,0000 supernatant of rat brain. In this preparation GAD required added pyridoxal-P to maintain a linear reaction rate beyond 5 min of incubation. Following exhaustive dialysis the enzyme was more than 83% saturated with cofactor indicating that the cofactor was tightly bound to the enzyme. When incubations were performed in the presence of glutamate and without added pyridoxal-P there was a progressive inactivation of the enzyme which was dependent on the glutamate concentration. This lost activity was almost completely recovered by addition of pyridoxal-P to the dialyzed glutamate-inactivated enzyme. The results suggest that glutamate inactivates GAD by promoting the dissociation of pyridoxal-P from the enzyme thereby producing inactive apoen-zyme which can be reactivated by combining with available pyridoxal-P. This interpretation is supported by the finding that progress curves for the reaction were accurately described over a 30 min incubation period and 10-fold glutamate concentration range by an integrated rate equation which takes the glutamate-promoted dissociation of cofactor into account. The progressive inactivation could not be attributed to denaturation of the enzyme, impurities in the substrate, effects of pH, depletion of substrate, protein concentration, sulfhydryl reagents or product inhibition. The results presented here also show that certain precautions must be adopted to accurately measure GAD activity in the absence of added pyridoxal-P as has been widely done in studies of drug action. Specifically, measurements must be made at short times of incubation and low concentrations of glutamate to minimize the glutamate-promoted inactivation of the enzyme.