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, Bu^tOOH, 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.