A COMPARATIVE STUDY OF L-GLUTAMATE DECARBOXYLASE FROM MOUSE BRAIN AND BOVINE HEART WITH PURIFIED PREPARATIONS1

Abstract— L-Glutamate decarboxylase (EC 4.1.1.15) (GAD), the enzyme responsible for the formation of GABA, has been purified to homogeneity from mouse brain (Wu et at., 1973) and antibodies specific for neuronal GAD have been obtained (SAITO et al., 1974a). The present report describes the purification of GAD from bovine heart more than 2000-fold over the homogenate by initial solubilization with Triton X-100. subsequent fractionation with ammonium sulfate, column chromatography on DEAE cellulose, calcium phosphate gel, and DEAE-Sephadex, and gel filtration. At least two forms of GAD have been observed in bovine heart preparations; one of them appears as a high molecular weight form (Peak I, MW 360,000) and the other one as a low molecular weight form (Peak II, MW 105,000). Cysteine sulfinic acid and cysteic acid, both precursors of taurine, had no effect on the purified heart enzyme or on neuronal GAD at 10 mM, suggesting that cysteine sulfinic acid and cysteic acid probably are not substrates for any species of GAD described above. The heart enzyme and neuronal GAD differ in several respects. First, they are different immunochemically as judged by the lack of cross reactivity between the purified heart enzyme and the antibody against purified neuronal GAD. Second, they are different biochemically. 5,5′-Dithiobis[2-nitrobenzoic acid] (DTNB). one of the most potent inhibitors of neuronal GAD [K_i= 1.0 × 10^?8M] inhibits the heart enzyme only to a small extent at 1 mM. On the other hand, pyruvic acid, which inhibits the heart enzyme to an extent of 90% at 10 mM, only inhibits the neuronal enzyme slightly. Third, they are different in their substrate specificity. The neuronal enzyme can catalyze α-decarboxylation of both L-glutamate and L-aspartate while the heart enzyme can use only L-glutamate as substrate. Moreover, an unidentified product probably derived from L-glutamate is obtained in the reaction mixture of the heart enzyme but is not observed with the brain enzyme, suggesting that the heart enzyme may catalyze a reaction converting L-glutamate to products other than GABA. It is therefore concluded that heart GAD and neuronal GAD are two different entities. Work is in progress to determine whether the heart enzyme is related to the glial enzyme. Should the antibody against the heart enzyme cross-react with the glial enzyme, the role of the glial enzyme in GABA function can then be studied by immunochemical and immunocytochemical methods.