Reaction of ferrous lactoperoxidase with hydrogen peroxide and dioxygen: an anaerobic stopped-flow study

Lactoperoxidase (LPO) is found in mucosal surfaces and exocrine secretions including milk, tears, and saliva and has physiological significance in antimicrobial defense which involves (pseudo-)halide oxidation. LPO compound III (a ferrous-dioxygen complex) is known to be formed rapidly by an excess of hydrogen peroxide and could participate in the observed catalase-like activity of LPO. The present anaerobic stopped-flow kinetic analysis was performed in order to elucidate the catalytic mechanism of LPO and the kinetics of compound III formation by probing the reactivity of ferrous LPO with hydrogen peroxide and molecular oxygen. It is shown that ferrous LPO heterolytically cleaves hydrogen peroxide forming water and oxyferryl LPO (compound II). The two-electron oxidation reaction follows second-order kinetics with the apparent bimolecular rate constant being (7.2 ± 0.3) × 104 M−1 s−1 at pH 7.0 and 25 °C. The H2O2-mediated conversion of compound II to compound III follows also second-order kinetics (220 M−1 s−1 at pH 7.0 and 25 °C). Alternatively, compound III is also formed by dioxygen binding to ferrous LPO at an apparent bimolecular rate constant of (1.8 ± 0.2) × 105 M−1 s−1. Dioxygen binding is reversible and at pH 7.0 the dissociation constant (KD) of the oxyferrous form is 6 μM. The rate constant of dioxygen dissociation from compound III is higher than conversion of compound III to ferric LPO, which is not affected by the oxygen concentration and follows a biphasic kinetics. A reaction cycle including the redox intermediates compound II, compound III, and ferrous LPO is proposed, which explains the observed (pseudo-)catalase activity of LPO in the absence of one-electron donors. The relevance of these findings in LPO catalysis is discussed.