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 o xidative 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 per oxidase isolated from transgenic Nicotiana sylvestris have been used i n experiments in vitro designed to determine the mechanism of IAA oxid ation. In particular, the initial reduction of ferric to ferrous enzym e, 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 o xygen that is kinetically competent both at the initiation stage and d uring the catalytic cycle of IAA oxidation. A general scheme describin g the oxidative cycles of both anionic and cationic peroxidases is pro posed that includes native ferric enzyme and compound II as kineticall y competent intermediates. For anionic peroxidases, addition of hydrog en peroxide switches on the oxidative cycle thereby promoting IAA oxid ation. 2-Methyl-IAA is not a substrate of the oxidase reaction, sugges ting a specific interaction between plant peroxidases and IAA.