The metabolism of gamma-aminobutyric acid (GABA) in the lobster nervous system--glutamic decarboxylase

— The glutamic acid decarboxylase has been purified from the lobster central nervous system. Potassium ion (0-075 m) and β-mercaptoethanol (0-025 m) were essential for enzyme activity. Enzyme had about 60 per cent of its optimal activity in the absence of added pyridoxal phosphate. Carbonyl reagents (10−4m-hydroxylamine or amino oxyacetic acid) would abolish this residual activity. The pH optimum of the enzyme was about 8-0. Standard Michaelis-Menten kinetics were applied to the decarboxylation of glutamate and a Km of 0.02 m was calculated. GABA inhibited the reaction (Ki= 1.25 × 10−3m), but the inhibition showed anomalous behaviour when graphed by the method of Lineweaver and Burk (1934). The GABA inhibition resembled competitive inhibition, but curves rather than straight lines intersecting at a common point on the velocity axis were obtained. This effect remains unexplained. Preliminary studies failed to reveal any subunit structure of the enzyme. The sedimentation coefficient (.S20.w) was 6-55 in a sucrose density gradient in an ultracentrifuge. This was unchanged by the addition of any of the agents that influence enzyme activity. The subcellular localization of the decarboxylase was explored in crude homogenates of lobster central nervous system prepared in various ways. The major proportion (about 90 per cent) of the enzyme activity was in the soluble fraction.‘Particulate’enzyme could be prepared, but gentle suspension of this material in buffer liberated most of the activity. A contaminant in the radioactive substrates led to the production of radioactive GABA without the simultaneous evolution of CO2. In this case, GABA production required active enzyme but was not an exclusive property of the glutamic decarboxylase activity.