Substrate analogues and divalent cations as inhibitors of glutamate decarboxylase from Escherichia coli

To examine the idea that glutamate decarboxylase from E. coli can be a convenient source for the study of the effects of compounds on GABA synthesis in the nervous system, a series of substrate analogues and divalent cations were tested as potential inhibitors of the bacterial enzyme. Those analogues exhibiting inhibitor activity did so in a competitive manner. The most effective inhibitors were 3-mercaptopropionic acid, 4-bromoisophthalic acid and isophthalic acid which exhibited Ki values of 0.13 mM, 0.22 mM and 0.31 mM, respectively. Eight other analogues produced lesser degrees of inhibition. In addition, seven divalent metal cations were tested as inhibitors of the enzyme. However, only Hg2+, Cd2+, Cu2+ and Zn2+ were effective at a concentration of 0.1mM. When these results were compared to the patterns of inhibition of glutamate decarboxylase from mouse brain, certain differences in the manner in which the enzymes responded to the inhibitors, emerged. Consequently, the bacterial decarboxylase may not be a good model for the study of drug action on brain GABA synthesis.