Horseradish peroxidase-catalyzed conversion of iodine to iodide in presence of EDTA and H₂O₂

Abstract

EDTA (4 mM) blocks the oxidation of iodide to I-3 (increase of extinction at 353 nm) by H₂O₂ catalyzed by horseradish peroxidase, which is reversed by the addition of an equimolar concentration of Zn₂±. Addition of suboptimal concentration of EDTA (2 mM) not only decreases the rate of forward reaction of I-3 formation but also causes loss of extinction of the same when I-3 is generated. The loss of extinction of I-3 is proportional to the enzyme concentration and is blocked by azide, the inhibitor of the peroxidase. EDTA also causes bleaching of nonenzymatically formed I-3 (from iodide and H₂O₂) only in the presence of horseradish peroxidase, and the effect is reversed by the equimolar concentration of Zn₂±. Both the bleaching of I-3 by EDTA and reversal of EDTA effect by Zn₂± are sensitive to azide. The decrease of extinction of I-3 (formed by dissolving iodine in KI solution) is dependent on EDTA, H₂O₂, and horseradish peroxidase. Molecular iodine is also bleached but at a slower rate than I-3. Evidence is presented to show that this bleaching of I-3 is due to enzymatic conversion of I-3 to iodide in presence of EDTA and H₂O₂ and this involves pseudocatalatic degradation of H₂O₂ to O₂.