DETAILED MODEL OF THE PEROXIDASE-CATALYZED OXIDATION OF INDOLE-3-ACETIC-ACID AT NEUTRAL PH

A mechanistic model of peroxidase-catalyzed oxidation of indole-3-acet ic acid (IAA) at neutral pH has been developed, characterized, and com pared with experiments. The model is based on experimental facts showi ng that IAA is oxidized in the presence of HRP by two pathways: (i) th e standard peroxidase cycle, which is accompanied by (ii) a nonenzymat ic free radical chain reaction, The peroxidase cycle normally requires the addition of a hydroperoxide, whereas IAA oxidation does not. Ther efore, the model includes the enzymatic peroxidase cycle which is init iated by organic hydroperoxide (ROOH) derived from autoxidation of IAA . It also includes a nonenzymatic free radical chain which utilizes ox ygen, oxidizes IAA, and recycles ROOH required for the enzymatic cycle . Available experimental values of rate constants were used, Unavailab le rate constants were initially estimated analytically using the stea dy state assumption and then optimized by computer simulation. There i s a unique set of rate constants which satisfies the model, The averag e deviation of simulated kinetic traces from experimental ones was les s than 5%. Critical values of the rate constants were determined; for values smaller than the critical values IAA oxidation stops. In the pr e-steady state the concentration of ROOH rises exponentially, Thus, a decrease of the initial concentration of ROOH by 3 orders of magnitude gives rise to only a 1 min delay of reaction. For all intermediates e xcept ROOH the steady state was reached in 1 min; for ROOH, 10 min aft er reaction initiation. More than half of the oxidation of IAA occurs by the chain reaction, a nonenzymatic pathway. The model satisfactoril y describes all available experimental kinetic data and predicts some previously unobserved behavior which should stimulate further experime nts.