Effects of various substituted hydrazones and hydrazines of pyridoxal-5'-phosphate on brain glutamate decarboxylase

Abstract

In addition to pyridoxal-5'-phosphate hydrazone (I), the following substituted hydrazones of pyridoxal-5'-phosphate were prepared: l-glutamyl-γ-hydrazone (II), isonicotinylhydrazone (III) and thiosemicarbazone (IV). The pyridoxal-5'-phosphate oxime- O-acetic acid was also prepared. The substituted hydrazines corresponding to I–IV and the reduced oxime- O-acetic acid were prepared by reduction of the hydra-zones with sodium borohydride. All the hydrazones and the oxime were tested against glutamate decarboxylase (GAD) from mouse brain. The hydrazones were found to have coenzymatic activity corresponding to 62–87 per cent of the activity measured for pyridoxal phosphate under identical assay conditions. The coenzymatic activity of the four derivatives was in the following order: III > II > I > IV. The oxime- O-acetic acid not only was devoid of coenzymatic activity, but it actually inhibited GAD activity to the extent of 50 per cent. The substituted hydrazines tested under identical conditions had no coenzymatic or inhibitory effect. The reduced oxime- O-acetic acid also inhibited GAD activity. Injected i.p. I and II caused fatal convulsions and produced a notable decrease in brain GAD if assayed without pyridoxal phosphate. III had a slight inhibitory effect on GAD and no convulsant action. With added pyridoxal 5'-phosphate, the activity was about the same as that recorded for control animals. The thiosemicarbazone. the oxime, and all the substituted hydrazines had little or no effect on GAD activity under these conditions and they did not produce convulsions. I, II and III inhibited brain pyridoxal kinase to some extent. Injected intraperitoneally, only I and II had a significant (30–36 per cent) inhibitory effect on brain pyridoxal kinase. Of the substituted hydrazines, only the one corresponding to II had a slight (9 per cent) inhibitory effect. The results are discussed in relation to the mechanism of activation of GAD in vitro by the hydrazones, and attempts are made to correlate the convulsant action of I and II with their effect in vivo upon GAD and pyridoxal kinase.