The interaction of oxymyoglobin with hydrogen peroxide: A kinetic anomaly at large excesses of hydrogen peroxide

The reaction of oxymyoglobin (MbO 2) with H 2O 2 has been examined at pH 7.2 and 20(±2) °C for reactant ratios of [H 2O 2]:[MbO 2] > ∼15:1. Under the conditions of large excesses of H 2O 2, the reaction is characterized by an increase in the rate of loss of MbO 2 as [H 2O 2]is increased, for which a value of k(MbO 2 + H 2O 2) ∼ 3 M −1 s −1 is obtained. This kinetic behavior contrasts the saturation kinetics observed previously at lower values of [H 2O 2]. The change in kinetics at increasing excesses of H 2O 2 is accompanied by a progressive tendency toward the direct formation of ferrimyoglobin at the expense of ferrylmyoglobin formation. A mechanism is proposed in which an initially formed intermediate produces the ferryl derivative in competition with the formation of ferrimyoglobin through the interaction of further H 2O 2. Overall, the H 2O 2 is catalytically decomposed by the MbO 2. This mechanism is integrated with that determined previously at low excesses of H 2O 2 into a complex general scheme that applies over the entire studied range of [H 2O 2]:[MbO 2]. No evidence is obtained for the conversion of ferrylmyoglobin to oxymyoglobin by the large excesses of H 2O 2, regardless of whether the ferryl derivative is the product of the reaction of H 2O 2 with the oxy or ferri derivative of myoglobin.