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Role of Phenolic Inhibitors in Peroxidase-mediated Degradation of Indole-3-acetic Acid ^1,2

^a Research Institute, Agriculture Canada, University Sub Post Office, London, Ontario, Canada, N6A 5B7

7-Hydroxy-2,3-dihydrobenzofuran derivatives, metabolites of a carbamate insecticide carbofuran, and five other phenolic inhibitors of indoleacetic acid (IAA) oxidase interfered with IAA-induced spectral change in the Soret band of horseradish peroxidase (HRP). The onset of IAA degradation required transformed HRP intermediates. The inhibitors, when added before IAA, protected HRP from reacting with IAA, thus preventing formation of highly reactive enzyme intermediates, and consequently, IAA degradation. When added after IAA, the inhibitors quickly reversed the IAA-induced spectral change of HRP and inhibited further IAA degradation.

The phenolic inhibitors differed in stability and reactivity. 7-Hydroxy-2,2-dimethyl-2,3-dihydrobenzofuran, 3,7-dihydroxy-2,2-dimethyl-2,3-dihydrobenzofuran, catechol, protocatechuic acid, caffeic acid, ferulic acid, and scopoletin belonged to one group which produced only a temporary inhibition to IAA-induced spectral change of HRP and IAA degradation since the inhibitors were metabolized in the reaction. The length of the lag was dependent on the IAA, inhibitor, and enzyme concentrations. 3-Keto-7-hydroxy-2,2-dimethyl-2,3-dihydrobenzofuran and 3-keto-carbofuran belonged to the other group which produced a persistent inhibition.

Degradation of IAA required both the heme group and apoprotein of HRP. Reconstituted enzyme from bovine hemin and apoprotein or HRP after unfolding by urea or guanidine treatment were inhibited by the inhibitors in a way similar to the native HRP. The inhibition was reversible by higher concentrations of IAA, but the plot of 1/v versus 1/s and 1/v versus i were curvilinear, reflecting the complex nature of a competitive inhibition.

² Contribution No. 653 of the Institute.

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