GABA‐TRANSAMINASES OF HUMAN BRAIN AND PERIPHERAL TISSUES—KINETIC AND MOLECULAR PROPERTIES

— Kinetic experiments with 4‐aminobutyrate‐2‐ketoglutarate transaminase (GABA‐T), partially purified from human brain tissue, supported a Bi Bi Ping‐Pong type of enzyme mechanism in which the enzyme oscillates between forms bound to pyridoxal phosphate and pyridoxamine phosphate. Extrapolated Km values were 0.31 mm for γ‐aminobutyrate, 0.16 mm for α‐ketoglutarate, and 3.8 μm for pyridoxal phosphate. Very similar kinetic parameters were observed with rat brain enzyme. Apparent molecular weight of human GABA‐T by gel filtration was 70,000 ± 3000. Electrofucusing experiments indicated a single ionic form with isoelectric pH = 5.7. Enzyme activity was inhibited by Tris, halides, cadmium and cupric ions, and known GABA‐T inhibitors. GABA‐transaminating enzymes isolated from human kidney and liver were found to be similar to the brain enzyme with respect to substrate affinities, cofactor requirements, isoelectric pH values, molecular weights, and response to inhibitors.