Mechanism of reaction of horseradish peroxidase with chlorite and chlorine dioxide

It is demonstrated that horseradish peroxidase (HRP) mixed with chlorite follows the whole peroxidase cycle. Chlorite mediates the two-electron oxidation of ferric HRP to compound I (k1) thereby releasing hypochlorous acid. Furthermore, chlorite acts as one-electron reductant of both compound I (k2) and compound II (k3) forming chlorine dioxide. The strong pH-dependence of all three reactions clearly suggests that chlorous acid is the reactive species. Typical apparent bimolecular rate constants at pH 5.6 are 1.4 × 105 M−1 s−1 (k1), 2.25 × 105 M−1 s−1 (k2), and 2.4 × 104 M−1 s−1 (k3), respectively. Moreover, the reaction products hypochlorous acid and chlorine dioxide, which are known to induce heme bleaching and amino acid modification upon longer incubation times, also mediate the oxidation of ferric HRP to compound I (2.4 × 107 M−1 s−1 and 2.7 × 104 M−1 s−1, respectively, pH 5.6) but do not react with compounds I and II. A reaction scheme is presented and discussed from both a mechanistic and thermodynamic point of view. It helps to explain the origin of contradictory data so far found in the literature on this topic.