Two-electron reduction and one-electron oxidation of organic hydroperoxides by human myeloperoxidase

The reaction of native myeloperoxidase (MPO) and its redox intermediate compound I with hydrogen peroxide, ethyl hydroperoxide, peroxyacetic acid, t-butyl hydroperoxide, 3-chloroperoxybenzoic acid and cumene hydroperoxide was studied by multi-mixing stopped-flow techniques. Hydroperoxides are decomposed by MPO by two mechanisms. Firstly, the hydroperoxide undergoes a two-electron reduction to its corresponding alcohol and heme iron is oxidized to compound I. At pH 7 and 15°C, the rate constant of the reaction between 3-chloroperoxybenzoic acid and ferric MPO was similar to that with hydrogen peroxide (1.8×107 M−1 s−1 and 1.4×107 M−1 s−1, respectively). With the exception of t-butyl hydroperoxide, the rates of compound I formation varied between 5.2×105 M−1 s−1 and 2.7×106 M−1 s−1. Secondly, compound I can abstract hydrogen from these peroxides, producing peroxyl radicals and compound II. Compound I reduction is shown to be more than two orders of magnitude slower than compound I formation. Again, with 3-chloroperoxybenzoic acid this reaction is most effective (6.6×104 M−1 s−1 at pH 7 and 15°C). Both reactions are controlled by the same ionizable group (average pKa of about 4.0) which has to be in its conjugated base form for reaction.