CATABOLISM OF INDOLE-3-ACETIC-ACID IN CITRUS LEAVES - IDENTIFICATION AND CHARACTERIZATION OF INDOLE-3-ALDEHYDE OXIDASE

A new enzyme, named indole-3-aldehyde oxidase (IAldO), was identified in citrus (Citrus sinensis L. Osbeck cv. Shamouti) leaves. The enzyme was partially purified by (NH4)2SO4 fractionation, Sephadex G-200 gel filtration and DEAE-cellulose ion exchange chromatography. IAldO catalyzes the oxidation of indole-3-aldehyde (IAld) to indole-3-carboxylic acid (ICA) with the production of H2O2. The enzyme is highly specific for IAld. The apparent K(m) of the enzyme for IAld is 19 muM. The optimum oxidation of IAld occurs at pH 7.5. The molecular mass of the enzyme, as determined by Sepharose-6B gel filtration, is about 200 kDa. Based on inhibitor studies, it is concluded that IAldO is not a flavin-linked oxidase and there is no requirement for free sulfhydryl groups or divalent cations for maximum activity. The enzyme is strongly inhibited by benzaldehyde. Ethylene pretreatment, wounding and aging of leaf tissues did not affect enzyme activity, suggesting that the enzyme is constitutive in citrus tissues.