A predictive kinetic model for inhibitory effect of nitrite on myeloperoxidase catalytic activity towards oxidation of chloride

Abstract

Myeloperoxidase (MPO) is a neutrophil enzyme that employs hydrogen peroxide (H₂O₂) to catalyze the oxidation of chloride (Cl^–) to hypochlorous acid (HOCl). Accepted mechanism is based on rapid reaction of native MPO with H₂O₂to produce Compound I (MPO-I) which oxidizes Cl^– through a 2e– transition generating MPO and HOCl. MPO-I also reacts with H₂O₂ to generate Compound II (MPO-II) which is inactive in 2e oxidation of Cl^–. Nitrite ( NO₂^-) inhibits the 2e oxidation of Cl^– by reaction with MPO-I through 1e transition generating MPO-II and nitrite radical. H₂O₂ consumption during steady- state catalysis was monitored amperometrically by a carbon fiber based H₂O₂-biosensor at 25^oC. Results demonstrated that in absence of NO₂^- reactions were monophasic and rapid (complete H₂O₂ consumption occurs in < 10 s). As concentration of NO₂^- increases, reactions change to biphasic (rapid step followed by a slow step) and both steps have been inhibited by NO₂^- . A predictive kinetic model describing the inhibittory effect of NO₂^- was developed and applied to experimental results The model is based on the assumption that MPO–I cannot be detected during steady-state catalysis. Calculated rate constants are in agreement with those obtained from pre-steady state kinetic methods.