OXIDATION OF TETRAHYDROSTILBAZOLE BY MONOAMINE OXIDASE-A DEMONSTRATESTHE EFFECT OF ALTERNATE PATHWAYS IN THE KINETIC MECHANISM

The steady-state kinetics for the oxidation of 1-methyl-1,2,3,6-tetrah ydrostilbazole (MTHS) by purified human liver monoamine oxidase A yiel ded biphasic double-reciprocal plots. Rate constants from stopped-flow studies were determined to show that the apparent stimulation at high substrate concentrations can be explained in terms of the alternate o xidative pathways available to monoamine oxidase A [Ramsay, R. R. (199 1) Biochemistry 30, 4624-4629]. At low substrate concentrations, the s lower reoxidation of the free enzyme (second-order rate constant was 4 000 M-1 s-1) predominates, but at higher concentrations the faster reo xidation of the reduced enzyme-substrate complex (38 300 M-1 s-1) beco mes significant. Computer simulation using this model predicts that si milar biphasic curves could be obtained for the oxidation of the neuro toxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, but that nonlinea rity would be obvious only at concentrations above 200K(m).