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.
Highlights
EDTA (4 mM) blocks the oxidation of iodide to I; Chloroperoxidase was a generous gift from Prof
The assay system contained in a final volume of 3 ml the following reagents: 50 mM sodium acetate buffer, pH 5.25, 1.7 mM KI,a suitable amount of the enzyme to get linear assay, and 0.27 mM H202,which was added last to start thereaction
Evidence is presented to show that this bleaching of 1; is due to enzymatic conversion of I5 to iodide in presence of EDTAand H20zand this involves pseudocatalatic degradation of H202to 02
Summary
EDTA blocked the triiodide formation catalyzed by chloroperoxidase, horseradish peroxidase, and lactoperoxidase. This led us to investigate more critically the effect of EDTA on this reaction using horseradish peroxidase as the model enzyme. We are presenting evidence indicating that EDTA interfered with the enzymatic formation of 13, and led to the formation of iodide catalyzed by the same enzyme. Similar phenomena were formation of iodide from triiodide only in the presence of observed when chloroperoxidase or lactoperoxidase was EDTA and H202.This reaction is associated with concomitant used instead of horseradish peroxidase.
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