Mechanism of indole-3-acetic acid oxidation by plant peroxidases: Anaerobic stopped-flow spectrophotometric studies on horseradish and tobacco peroxidases

Indole-3-acetic acid (IAA) is a powerful plant growth regulator. The oxidative decarboxylation of IAA by plant peroxidases is thought to be a major degradation reaction involved in controlling the in vivo level of IAA. Horseradish peroxidase isoenzyme C and an anionic tobacco peroxidase isolated from transgenic Nicotiana sylvestris have been used in experiments in vitro designed to determine the mechanism of IAA oxidation. In particular, the initial reduction of ferric to ferrous enzyme, a key step in previously proposed mechanisms, has been investigated by rapid-scan stopped-flow spectrophotometry under strictly anaerobic conditions and at defined oxygen concentrations. The data provide the first evidence for a ternary complex comprising peroxidase, IAA and oxygen that is kinetically competent both at the initiation stage and during the catalytic cycle of IAA oxidation. A general scheme describing the oxidative cycles of both anionic and cationic peroxidases is proposed that includes native ferric enzyme and compound II as kinetically competent intermediates. For anionic peroxidases, addition of hydrogen peroxide switches on the oxidative cycle thereby promoting IAA oxidation. 2-Methyl-IAA is not a substrate of the oxidase reaction, suggesting a specific interaction between plant peroxidases and IAA.