The autoxidation of horse myoglobin was studied in the presence or absence of catalase (EC 1.11.1.6) and/or superoxide dismutase (EC 1.15.1.1) at various pH values (6.6-7.8). Changes in the percentages of oxymyoglobin and metmyoglobin during the reaction were analysed by means of isoelectric focusing on Ampholine gel plates. Oxymyoglobin was decreased in a first-order manner, with an accompanying increase in metmyoglobin, under the various conditions studied. The observed reaction rate constants obtained under these conditions were pH-dependent; however, they were also greatly affected by the presence of the enzymes. The pH-dependence of the overall reaction was explained by the acid-base three-state model of myoglobin proposed by Shikama & Sugawara [(1978) Eur. J. Biochem. 91, 407-413]. The reaction process of myoglobin autoxidation was explained by the model suggested by Winterbourn, McGrath & Carrell [(1976) Biochem. J. 155, 493-502], indicating that superoxide anion and hydrogen peroxide are involved in the reaction mechanism.
