Reaction of Muscimol with 4‐Aminobutyrate Aminotransferase

Abstract

The reaction of muscimol as amino donor substrate for GABA transaminase (GABA-T) was studied using enzyme purified from rabbit brain. Enzyme activity was assayed by measuring the glutamate produced using glutamate dehydrogenase. Kinetic parameters determined at 37.degree. C were for GABA, Km (app) = 1.92 .+-. 0.24 mM, specific activity = 7.33 .+-. 0.27 .mu.mol/min per mg (kcat = 13.7 s-1) and for muscimol, Km (app) = 1.27 .+-. 0.15 mM, specific activity = 0.101 .+-. 0.009 .mu.ol/min per mg (kcat = 0.19 s-1). Addition of muscimol to the enzyme caused the spectral changes associated with conversion of the pyridoxaldimine form to the pyridoxamine form, and the 1st-order rate constant for the reaction showed a dependence on muscimol concentration that followed saturation kinetics, with a K = 1.1 .+-. 0.18 mM and kmax = 0.065 .+-. 0.004 s-1 (19.degree. C). The rate of spectral change observed on addition of muscimol to ornithine transaminase was extremely slow, at least an order of magnitude slower than that seen with GABA-T.