Purification and regulation of L-glutamate decarboxylase

L-Glutamate decarboxylase (GAD), the GABA-synthesizing enzyme, has been purified to homogeneity from mouse brain, bovine brain and catfish brain by a combination of ammonium sulfate fractionation, column chromatographies and preparative gel electrophoresis. The purity of the enzyme preparations was established from the following criteria: First of all, the purified enzyme preparations migrated as a single protein band in both the regular and gradient polyacrylamide gel electrophoresis with the protein band coincident with the enzyme activity; secondly, the purified GAD also appeared to be homogeneous as judged from the sedimentation equilibrium runs in both H₂O and D₂O solutions; thirdly, a single precipitin band was obtained when crude enzyme preparations were tested against the GAD antiserum in both the immunodiffusion and immunoelectrophoresis tests. During the course of purification, we have observed that when GAD was prepared from the hypotonie homogenizing medium, two forms of GAD-low M W (85,000 dallons) and high MW (>200,000) were obtained, while only the low MW form was obtained when GAD was extracted from the crude mitochondrial fraction. Similar results were obtained with other transmitter enzymes, e.g., choline acetyltransferase, cysteic and cysteine-sulfinic acids decarboxylase. The high MW form can be converted to the low MW form by incubation with the crude mitochondrial fraction suggesting that the high MW form may be in the cell body which is transported down to the terminal by axonal transport and converted to the low MW form by factor(s) in the nerve terminal.