GABA and Taurine Enzymes in Mammalian Brain

Abstract

γ -Aminobutyric acid (GABA) and taurine are two structurally related amino acids and both have been proposed as inhibitory neurotransmitters or modulators in the mammalian central nervous system. This chapter describes the purification procedures and basic physicochemical and kinetic properties of L-glutamate decarboxylase (GAD), GABA-transaminase (GABA-T), and cysteine sulfinate decarboxylase (CSAD)/ cysteic acid decarboxylase (CAD). The chapter also discusses the species, tissue, and cell specificities of GAD, GABA-T, and CSAD/CAD. GAD and GABA-T were purified from mouse brain by first extracting the enzymes from crude mitochondrial fraction, followed by ammonium sulfate fractionation and a combination of gel filtration, calcium phosphate gel, and DEAE-Sephadex column chromatographies. Eleven steps were employed in the purification of GAD and approximately 1.2% of the total activity was recovered as a purified enzyme preparation representing 940-fold purification over the original homogenate. GABA-T was purified 1200-fold over the brain homogenate with a specific activity of 5.0 units/mg of protein. The molecular weight of GAD and GABA-T from mouse brain was obtained as 85,000 ± 2000 and 109,000, respectively, from sedimentation equilibrium. The molecular weight of the human brain GAD was found to be about 100,000 by Sephadex-gel filtration and about 95,000 by sodium dodecyl sulfate -polyacrylamide gel electrophoresis.