Accelerating Effect of Umbelliferone on Peroxidase-Catalyzed Oxidation of Indole-3-acetic Acid at Neutral pH

Abstract

The acceleration by the phenol umbelliferone (7-hydroxycoumarin) of the horseradish peroxidase (HRP) catalyzed oxidation of indole-3-acetic acid (IAA) was studied at pH 7.4 using spectral and kinetic approaches. For the system 0.1 mM IAA/1 µM HRP/variable umbelliferone concentration, an increase in rate by a factor of 8 was reached in the presence of 1 µM umbelliferone; further increase of the umbelliferone concentration had no further effect. The rate constants for the peroxidase compounds I and II (HRP-I and HRP-II) reductions by umbelliferone in the absence of IAA were measured in the transient state as functions of the umbelliferone concentration. The plot of the pseudo-first-order rate constant k obs vs [umbelliferone] for HRP-I reduction was curved upward. This result implies that umbelliferone catalyzes its own oxidation by HRP-I. The bimolecular rate constant of the reduction of 1 µM HRP-I by 1 µM umbelliferone was estimated to be 1.1 × 10 5 M -1 s -1 . The plot of k obs vs [umbelliferone] was linear for HRP-II reduction, yielding a bimolecular rate constant of 1.7 × 10 5 M -1 s -1 . The influence of umbelliferone on the rates of HRP-I and HRP-II reduction by IAA was also studied. It was found that 1 µM umbelliferone accelerates the reduction of 1 µM HRP-I by a factor of 10 but did not influence the reduction of HRP-II by IAA. The influence of umbelliferone on the HRP-I reduction by umbelliferone and/or IAA suggests that HRP-I can bind umbelliferone at a site different from the active site, where it provides a beneficial conformational change. However, the binding of umbelliferone to native HRP was not observed. A detailed mechanism for the HRP-catalyzed oxidation of IAA, both in the absence and in the presence of umbelliferone, is presented. There are three umbelliferoneinduced accelerating effects: (i) the reduction of the rate-limiting HRP species, HRP-II, by umbelliferone, which increases the rate of enzyme turnover and hence the rate of IAA oxidation, (ii) the nonenzymatic oxidation of IAA by free radicals of umbelliferone formed in the HRP-catalyzed oxidation of umbelliferone, and (iii) umbelliferone-induced acceleration of the HRP-I reduction by IAA. The magnitudes of these three effects are similar.

Distribution of superoxide and hydrogen peroxide in Arabidopsis root and their influence on root development: possible interaction with peroxidases

“…Arabidopsis seeds were sown on media containing various substances known to react with ROS and thereby modify their content in plants. Esculetin and umbelliferone were also tested because of the faculty of coumarins to modulate peroxidase activity (Sirois & Miller, 1972; Krylov & Dunford, 1996). After 7 d of culture on vertical plates, the effect of the treatments was assessed on root morphology and peroxidase activity.…”

Characterizing Sorption and Permeation Properties of Membrane Filters Used for Aquatic Integrative Passive Samplers

“…All experiments described in the following were conducted with water purified with a Millipore 27 is partially (33%) anionic at pH 7.5 and is classified into neutral compounds here. conducted.…”

A kinetic and mechanistic study on the oxidation of indole‐3‐acetic acid by peroxodisulfate

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