The mechanism of indole-3-acetic acid oxidation by horseradish peroxidases.

The oxygen-consuming oxidation of indole-3-acetic acid (IAA) occurred much faster in the presence of horseradish peroxidase C (neutral isoenzyme) than in the presence of horseradish peroxidase A (acidic isoenzyme). An intermediate oxidation product of IAA was found to be a hydroperoxide species that reacted with the ferric enzymes to form Compound I at second order rate constants of 6.8 X 10(3) M-1--S-1 for peroxidase A and 2.0 X 10(6) M-1--S-1 for peroxidase C at pH 4.4 The hydroperoxide concentration reached about one-half of the initial IAA concentration at the end of the oxygen-consuming reaction and then decreased slowly. The main intermediate of the enzyme observed during the oxygen-consuming reaction was Compound II, which oxidized IAA to its free radical at rate constants of 1.5 X 10(3) M-1--S1 for peroxidase A and 1.2 times 10(4) M-1--S-1 for peroxidase C at pH 4.4 The results supported the mechanism that the oxygen consumption occurs mainly through the reaction of oxygen with the IAA free radical formed from the peroxidatic oxidation of IAA. The ferric enzymes were not reduced by IAA under strict anaerobic conditions in the presence of carbon monoxide but were reduced upon addition to a small amount of oxygen or hydrogen peroxide to the systems. The results suggested that the ferric enzyme is reduced by the IAA free radical but not by IAA itself. From a comparison of reactivities of oxyperoxidase and Compound II we concluded that the catalytic cycle of ferrous and oxyperoxidases is not involved in the IAA oxidase reaction.