The interaction of oxymyoglobin with hydrogen peroxide: The formation of ferrylmyoglobin at moderate excesses of hydrogen peroxide

The reaction of oxymyoglobin with H 2O 2 has been examined at pH 7.2 and 20(±2) ° over a range of [H 2O 2] up to an initial excess of 25:1. The reaction is characterized by a direct conversion of oxymyoglobin to ferrylmyoglobin without the intermediacy of the ferri derivative. The initial rate of loss of oxymyoglobin is first-order with respect to [oxymyoglobin], and exhibits saturation kinetics with increasing [H 2O 2]. In addition, the stoichiometric relationship between the reactants varies as [H 2O 2] increases. A complex non-Michaelis-Menten mechanism is proposed in which an intermediate, produced upon the initial interaction of the reactants, regenerates oxymyoglobin by reaction with further H 2O 2, in competition with the formation of the ferryl derivative. In this way, oxymyoglobin catalytically decomposes excess H 2O 2. Deoxygenated ferromyoglobin is substantially more reactive with H 2O 2 in producing the transient intermediate than the oxy analog. Some fundamental similarity is noted between the catalytic mechanism and that of catalase activity. From a detailed examination of the probable nature of the intermediate, conventional Fenton reactivity is rejected for the reaction of H 2O 2 with Oxymyoglobin.