Interaction of Myoglobin and Cytochrome c*

Spectrophotometric studies of various mixtures of reduced and oxidized forms of myoglobin and cytochrome c revealed that oxymyoglobin is capable of reducing ferricytochrome c at a molar ratio of 1. Other combinations, i.e. oxymyoglobin and ferrocytochrome c; metmyoglobin and ferricytochrome c; or metmyoglobin and ferrocytochrome c showed no spectral change during the time course studied. The pH optimum of the reaction was found to be 7.2. The Qlo was about 5 and the activation energy was about 29 kcal per mole. Aging of oxymyoglobin tended to increase the rate of the reaction, but metmyoglobin added to the reaction mixture had no effect on the system. Antimycin A, amytal, and rotenone were without effect on the reaction. The partial carboxymethylation of histidyl residues of myoglobin increased the reaction rate to twice that of the control. The formation of a complex between oxymyoglobin and cytochrome c was not detected by various physical methods. Oxygen had an inhibitory effect on the reaction of the oxymyoglobin-ferricytochrome c system. This suggested that deoxymyoglobin rather than oxymyoglobin is the reacting species and that the rate-limiting step is the dissociation of oxygen from oxymyoglobin. This was confirmed by measurements showing that deoxymyoglobin reacted with ferricytochrome c at rates significantly greater than those of oxymyoglobin-cytochrome c systems. This reaction was shown to proceed in stoichiometric fashion under equilibrium conditions. Carbon monoxide myoglobin was unreactive.