Study on the Inhibition of Brain Glutamate Decarboxylase by Pyridoxal Phosphate Oxime‐o‐acetic Acid

The kinetics of the inhibition of mouse brain glutamate decarboxylase by pyri‐doxaI‐5′‐phosphate oxime‐O‐acetic acid (PLPOAA) was studied. The inhibition was noncompetitive with regard to glutamic acid; it could be partially reversed by pyridoxal phosphate, but only when the concentration of the latter in the incubation medium was higher than that of pyridoxal‐5′‐phosphate oxime‐O‐acetic acid. The inhibition produced by aminooxyacetic acid, which is remarkably greater than that produced by PLPOAA, was also partially reversed only when an excess of pyridoxal phosphate was added. Both in the presence and in the absence of a saturating concentration of pyridoxal phosphate, the activity of the enzyme was decreased by PLPOAA at a 10−4m concentration to a value of about 50 per cent of the control value obtained without added coenzyme. This activity could not be further reduced even when PLPOAA concentration was increased to 5 × 10−3m. This same minimal activity of glutamate decarboxylase was obtained after dialysis of the enzymic preparation, or after incubation with glutamic acid in the cold followed by filtration through Sephadex G‐25. The addition of pyridoxal phosphate to the dialysed or glutamic acid‐treated enzyme restored the activity to almost the control values. PLPOAA did not affect the activity of glutamate decarboxylase from E. coli or that of DOPA decarboxylase and GABA transaminase from mouse brain. To account for the results obtained it is postulated that brain glutamate decarboxylase has two types of active site, one with firmly bound, non‐dialysable pyridoxal phosphate and the other with loosely bound, dialysable coenzyme; PLPOAA behaves as a weak inhibitor probably because it can combine mainly with the loosely bound coenzyme site, while aminooxyacetic acid is a potent inhibitor probably because it can block both the ‘loosely bound coenzyme’ and the ‘firmly bound coenzyme’ sites.