Aerobic oxidation of indole-3-acetic acid catalysed by anionic and cationic peanut peroxidase

The catalytic properties of anionic and cationic peanut peroxidases with regards to the oxidation of indole-3-acetic acid (IAA) by molecular oxygen at low pH have been studied. Transient kinetic studies demonstrate that only cationic peroxidases (peanut and horseradish) but not anionic peroxidases (such as anionic tobacco and anionic peanut peroxidases) form a stable compound III in the course of IAA oxidation. The failure to observe inhibition in the presence of superoxide dismutase is consistent with the formation of compound III from a ternary complex comprising ferric enzyme, IAA and dioxygen at the initiation step. Product analysis by HPLC showed an enhanced rate of IAA oxidation in the presence of superoxide dismutase. Go-addition of superoxide dismutase and catalase demonstrates that this stimulation is not due to the formation of hydrogen peroxide. The correlation between initial rates of IAA degradation and product accumulation indicates that skatole hydroperoxide is a primary reaction product and indole-3-methanol is the product of its subsequent enzymatic reduction. The relative catalytic activities for IAA oxidation by tobacco:horseradish isoenzyme c:anionic peanut:cationic peanut peroxidase are 28:20:2:1. (C) 1999 Elsevier Science Ltd. All rights reserved.