OXIDATION OF INDOLEACETIC ACID BY AN APPARENTLY HOMOGENEOUS PEROXIDASE FROM THE FLAVEDO OF WASHINGTON NAVEL ORANGES (CITRUS SINENSIS)

Orange flavedo peroxidase was purified by ammonium sulfate fractionation, gel filtration and ion-exchange chromatography. The anionic fraction, representing 51% of the total peroxidase activity, was an apparently homogeneous isoenzyme with an pI of 4.0. The cationic fraction was composed of two isoenzymes with pI of 10.0 and 11.5.
The anionic peroxidase oxidized indoleacetic acid in the presence of H₂O₂ and 2,4-dichlorophenol. This reaction was inhibited by Mn²⁺ and enzyme activity decreased at increasing pH in the range of 4.0 to 5.5. This isoenzyme also oxidized indoleacetic acid in the reaction mediated by 2,4-dichlorophenol, Mn²⁺ and phosphate. In this case the optimum pH was 4.5 and the reaction was inhibited by H₂O₂.
Study of the characteristics of both reactions indicates that, even though they could share some common steps and have similar end products, they probably differ in their reaction mechanism. The possible physiological significance of the H₂O₂ mediated oxidation of indoleacetic acid is briefly discussed.