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In the presence of iodide (I−, 10 mM) and hydrogen peroxide in a large excess (H2O2, 0.1–10 mM) catalytic amounts of lactoperoxidase (2 nM) are very rapidly irreversibly inactivated without forming compound III (cpd III). In contrast, in the absence of I− cpd III is formed and inactivation proceeds very slowly. Increasing the enzyme concentration up to the micromolar range significantly accelerates the rate of inactivation. The present data reveal that irreversible inactivation of the enzyme involves cleavage of the prosthetic group and liberation of heme iron. The rate of enzyme destruction is well correlated with the production of molecular oxygen (O2), which originates from the oxidation of excess H2O2. Since H2O2 and O2per se do not affect the heme moiety of the peroxidase, we suggest that the damaging species may be a primary intermediate of the H2O2 oxidation, such as oxygen in its excited singlet state (1ΔgO2), superoxide radicals (O−2), or consequently formed hydroxyl radicals (OH⋅).

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