Anions activate the oxidation of indoleacetic Acid by peroxidases from tomato and other sources

Anionic peroxidase from tomato (Lycopersicon esculentum) fruit oxidized indoleacetic acid (IAA) slowly in the presence of Mn²⁺ and dichlorophenol in acetate buffers. The addition of certain anions to the reaction mixture increased the rate of oxidation. Phosphate was one of the effective anions and exerted maximal activation at 0.1 molar. The most effective activator of tomato peroxidase was nitrilotriacetate (NTA) at an optimum concentration of 60 micromolar. Only 0.17 nanomolar peroxidase was needed to oxidize 0.1 micromole IAA/5 minutes in the presence of NTA compared to 650 nanomolar peroxidase for the same rate in the absence of NTA. Other effective anions were oxalate, pyrophosphate, malate, and citrate. Each activator exhibited an optimum concentration and higher concentrations were inhibitory. Anionic peroxidase from horseradish was activated by the same anions. A cationic peroxidase from horseradish and lactoperoxidase oxidized IAA in acetate buffer although anions activated these enzymes severalfold. Microperoxidase and other hematoporphrins also catalyzed IAA oxidation in the presence of anions. It is proposed that IAA oxidation by peroxidase may be important when vacuolar contents mix with peroxidase as during plant injury.