Oxidation of Tetrahydrostilbazole by Monoamine Oxidase A Demonstrates the Effect of Alternate Pathways in the Kinetic Mechanism

The steady-state kinetics for the oxidation of 1-methyl-1,2,3,6-tetrahydrostilbazole (MTHS) by purified human liver monoamine oxidase A yielded 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 oxidative pathways available to monoamine oxidase A [Ramsay, R. R. (1991) Biochemistry 30, 4624–4629]. At low substrate concentrations, the slower reoxidation of the free enzyme (second-order rate constant was 4000 M⁻¹ s⁻¹) predominates, but at higher concentrations the faster reoxidation of the reduced enzyme-substrate complex (38 300 M⁻¹ s⁻¹) becomes significant. Computer simulation using this model predicts that similar biphasic curves could be obtained for the oxidation of the neurotoxin, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, but that nonlinearity would be obvious only at concentrations above 200K_m.