Threshold Effect during Inhibition by Caffeic Acid of Peroxidase-catalysed Oxidation of Indol Yl-3-acetic Acid*

Chemiluminescence has been used 10 study aerobic peroxidase-catalysed oxidation of indolyI-3-acetic acid (IAA). A threshold effect was found on inhibition of this reaction by its natural inhibitor caffeic acid. At an inhibitor concentration below threshold, only a temporary fall in the intensity of chemiluminescence and the reaction rale was established; at an above-threshold concentration, chemiluminescence ceased, the reaction slopped and was not renewed throughout the observation period. It is shown that the reaction is not fe-initiated by the addition of the enzyme; fe-initiation of the arrested reaction was observed on exposure of the reaction mixture to visible light in the presence of a photosensitizer, Bengal rose; after initiation the reaction proceeded without light. It is assumed thatin the experimental conditions employed enzymatic oxidation of 1M is a branched chain fre~-radica] reaction and caffeic acid acts as a trap of the radicals involved in this reaction.