Ascorbic acid as negative effector of the peroxidase‐catalyzed degradation of indole‐3‐acetic acid

Abstract

Ascorbic acid is a strong inhibitor of indole‐3‐acetic oxidation catalyzed by commercial horse‐radish peroxidase. In the presence of excess ascorbic acid, the indole‐acetic acid oxidation catalysis is apparently blocked. The activity of peroxidase for indoleacetic acid at pH 3.7 and 33°C, in the presence of 2,4‐dichlorophenol and MnCl2 as promotors was measured by polarographic technique. The Km was 0.27 mM and the maximum velocity was 1.02 mmol O2 (mg protein)−1 min−1. Dixon plots lead to an apparent Ki of 1.25 (μM for ascorbic acid and the inhibition was apparently competitive. Ascorbic acid, besides appearing to be a strong inhibitor of the IAA oxidase activity of peroxidase, seemed to protect IAA from total degradation. Addition of more than 5 μM ascorbic acid produced both an exponential increase in the lag time before the onset of reaction and, at the end, an oxidation protection of 26 μM IAA when 111 μM IAA was present at the stawrt. The possibility of ascorbic acid‐IAA auxin from endogenous oxidation in plants, is proposed.