THRESHOLD EFFECT FOR THE INHIBITION OF TH E PEROXIDASE-CATALYZED IAA OXIDATION BY CAFFEIC ACID

The aerobic peroxidase-catalyzed oxidation of indole-3-acetic acid was investigated by chemiluminescent method. The threshold effect was fou nd for the inhibition of this reaction by its natural inhibitor - caff eic acid. If the concentration of the inhibitor was less than the thre shold value, we found only temporary decrease of the intensity of chem iluminescence and the rate of reaction. If the inhibitor concentration was more than the threshold value then the chemiluminescence ended an d the reaction stopped. Enzyme addition did not reinitiate the reactio n. Reaction reinitiation took place when the reaction mixture was irra diated by visible light in the presente of a photosensitizer - rose be ngal. The reaction proceeded without light after initiation. To explai n the obtained experimental data we suppose, that, for the given exper imental conditions the oxidation of IAA is an enzymatic, free-radical, chain branched reaction and caffeic acid acts as a trap of free radic als, participating in this chain reaction.