Pseudo‐catalytic degradation of hydrogen peroxide in the lactoperoxidase/H2O2/iodide system

Abstract

Non-stoichiometric (excessive) consumption of hydrogen peroxide (H₂O₂), which was observed in various lactoperoxidase-catalysed reactions, was tested in the lactoperoxidase/H₂O₂/iodide system. In preliminary experiments the suitability of the system was tested with special regard to the triiodide (I⁻₃) absorption and the I₂/I⁻₃ equilibrium. Triiodide equilibrium concentrations evaluated theoretically and experimentally were compared after adding a known amount of iodine (I₂) to solutions containing variable I⁻ concentrations. A close fit of the two methods was only obtained if experiments were carried out in pure aqueous or 0.001 M H₂SO₄ medium. The presence of various anions, e.g. OH⁻ and Cl⁻, led to a measurable decrease in I⁻₃ and I₂ equilibrium concentrations. These ions are able to displace competitively I⁻ in forming association products with I⁺ and I₂. When I⁺ and I₂ were generated enzymatically by lactoperoxidase and hydrogen peroxide, additional interactions with H₂O₂ were observed. Depending on the enzyme and iodide concentrations, variable amounts of H₂O₂ disappeared non-productively. Due to its ambivalent redox reactivity, part of the H₂O₂ is not reduced to H₂O in the enzyme-catalysed generation of iodine, but undergoes oxidation to O₂ by an oxidized iodine compound. This suggests a pseudocatalatic side reaction which can competitively interfere with the I₂ I⁻₃ generation or (and) the iodination reaction.