STUDIES ON GAMMA-AMINOBUTYRATE AMINOTRANSFERASE (GABA-T) ACTIVITIES IN HUMAN AND RODENT BRAIN HOMOGENATES

Differences in the kinetic properties of brain gamma-aminobutyrate aminotransferase (GABA-transaminase; GABA-T) in different species are described in the present investigation. In both rat and human brain enzymes, the effect of temperature on the activity was studied. The maximal activity, for a 30-min incubation period, was attained at an incubation temperature of 45-degrees-C for rat and 56-degrees-C for human brain tissue. The addition of plasma or plasma proteins was found to induce a two-fold increase of the activity of rat brain GABA-T, whereas a slight inhibitory effect on human brain enzyme and no effect on mouse brain enzyme was observed. The species differences are shown to be the results of differences in the binding of the cofactor pyridoxal phosphate to the apoprotein, which are revealed when the free concentration of pyridoxal phosphate is reduced by binding to serum albumin.