Tobacco anionic peroxidase overexpressed in transgenic plants: Aerobic oxidation of indole-3-acetic acid

We have investigated the catalytic properties of the tobacco anionic peroxidase with regards to the oxidation of indole-3-acetic acid (IAA). As judged by oxygen uptake, the homogeneous enzyme was capable of oxidizing IAA in the absence of additional cofactors such as manganese ion, hydrogen peroxide and phenols. Phenolic substrates such as caffeic acid, chlorogenic acids, 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulphonic acid) diammonium salt or phenol inhibited the oxidation of IAA, The spectral changes in the course of IAA oxidation allow us to conclude that the oxidation cycle is functioning separately from peroxidation. The addition of trace amounts of hydrogen peroxide to the reaction eliminates a distinctive lag phase in the consumption of oxygen. This activation by hydrogen peroxide is connected with the formation of compound II, which appears to be a key intermediate in the oxidation process. However, there is still no direct evidence regarding the mechanism of initiation of IAA oxidation, though the appearance of the ferrous form has been clearly some indirect evidence of IAA hydroperoxide formation in the course of aerobic oxidation of IAA by the tobacco enzyme based on the slight inhibition effect of catalase. The results obtained have been rationalized in the reaction scheme, which proposes the existence of an enzyme-substrate complex and is in agreement with the previous data on IAA oxidation catalysed by horseradish and turnip peroxidases.