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

The reaction of oxymyoglobin with H2O2 has been examined at pH 7.2 and 20(+/-2) degrees over a range of [H2O2] 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 [H2O2]. In addition, the stoichiometric relationship between the reactants varies as [H2O2] 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 H2O2, in competition with the formation of the ferryl derivative. In this way, oxymyoglobin catalytically decomposes excess H2O2. Deoxygenated ferromyoglobin is substantially more reactive with H2O2 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 H2O2 with oxymyoglobin.