Non-stoichiometric (excessive) consumption of hydrogen peroxide (H202), which was observed in various lactoperoxidase-catalysed reactions, was tested in the lactoperoxidase/H,O,/iodide system. In preliminary experiments the suitability of the system was tested with special regard to the triiodide (I;) absorption and the I 2 / I i 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 H,S04 medium. The presence of various anions, e.g. OH-and C1-, led to a measurable decrease in I; and I, equilibrium concentrations.These ions are able to displace competitively I-in forming association products with I ' and I,. When I + and I, were generated enzymatically by lactoperoxidase and hydrogen peroxide, additional interactions with H202 were observed. Depending on the enzyme and iodide concentrations, variable amounts of H,O, disappeared nonproductively. Due to its ambivalent redox reactivity, part of the H20, is not reduced to H 2 0 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 I ,/I; generation or (and) the iodination reaction.
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