Mechanism of nitrite‐stimulated catalysis by lactoperoxidase

The reactions of lactoperoxidase (LPO) intermediates compound I, compound II and compound III, with nitrite ( ) were investigated. Reduction of compound I by was rapid (k2 = 2.3 × 107 m−1·s−1; pH = 7.2) and compound II was not an intermediate, indicating that radicals are not produced when reacts with compound I. The second‐order rate constant for the reaction of compound II with at pH = 7.2 was 3.5 × 105 m−1·s−1. The reaction of compound III with exhibited saturation behaviour when the observed pseudo first‐order rate constants were plotted against concentrations and could be quantitatively explained by the formation of a 1 : 1 ratio compound III/NO2− complex. The Km of compound III for was 1.7 × 10−4 m and the first‐order decay constant of the compound III/ complex was 12.5 ± 0.6 s−1. The second‐order rate constant for the reaction of the complex with was 3.3 × 103 m−1·s−1. Rate enhancement by does not require as a redox intermediate. accelerates the overall rate of catalysis by reducing compound II to the ferric state. With increasing levels of H2O2, there is an increased tendency for the catalytically dead‐end intermediate compound III to form. Under these conditions, the ‘rescue’ reaction of with compound III to form compound II will maintain the peroxidatic cycle of the enzyme.