Seven peroxidase isozyme fractions were isolated from rust-infected resistant wheat leaves by means of ammonium sulfate fractionation, pH precipitation, ion exchange chromatography and gel filtration. Three isozymes showed a single peroxidative band in the electrophoretic gels. The catalytic activity of the enzymes on non-physiological substrates was comparable to that of commercial horseradish peroxidase. When compared to isozyme 9, isozyme 10 had twice the activity on guaiacol and eugenol but only one-fifth the oxidative activity onp-phenylenediamine ando-dianisidine. The IAA oxidation activity was compared among purified enzyme fractions. Isozyme 10 was the only enzyme which could destroy the auxin without phosphate and manganese cofactors. All the other enzymes, including isozyme 9, showed the activity only when both cofactors were present. The possible involvement of these peroxidases in IAA destruction in the resistant tissue is discussed.
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