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

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 K_m was 0.27 m M and the maximum velocity was 1.02 mmol O₂ (mg protein)^?1 min^?1. Dixon plots lead to an apparent K_i of 1.25 (μ M 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 μ M 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 μ M IAA when 111 μ M IAA was present at the stawrt. The possibility of ascorbic acid-IAA auxin from endogenous oxidation in plants, is proposed.