Peroxidase activities of ferrihaems: kinetics of the oxidation of iodide by peroxidatically active compounds formed by reaction of deuteroferrihaem with hydrogen peroxide, t-butyl hydroperoxide, and peroxybenzoic acids

Abstract

Deuteroferrihaem peroxide compounds (dpc) have been formed by reaction of deuteroferrihaem with H2O2, ButOOH, and 10 peroxybenzoic acids, and the kinetics of oxidation of iodide by these species have been studied by stopped-flow spectrophotometry at 25 °C and I= 0.1 mol dm–3. Under conditions where the reaction is first order with respect to both [dpc] and [I–], the second-order rate constant k=(1.6 ± 0.3)× 102 dm3 mol–1 s–1 for all the hydroperoxides used at pH > 7.75. This result supports the concept that all the deuteroferrihaem peroxide compounds are the same oxidized form of deuterohaem, independent of the nature of the oxidant progenitor. The rate constant k is independent of pH in the range 7.75–10.0 but shows about a three-fold increase as the pH decreases in the range 7.75–6.5. In contrast, the rate constants for the oxidation of iodide by Compounds I and II of the ferrihaem hydroperoxidase enzyme, Horse Radish Peroxidase (E.C. 1.11.1.7), are directly proportional to [H+] over a wide range. Deuteroferrihaem peroxide compound is a much more effective oxidant (towards I–) than Compound II over the whole pH range studied and has equal reactivity to Compound I at pH 8.5.