The oxidation of myoglobin to metmyoglobin by oxygen. 3. Kinetic studies in the presence of carbon monoxide, and at different hydrogen-ion concentrations with considerations regarding the stability of oxymyoglobin

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Abstract
The oxidation of myoglobin (Mb) to metmyoglobin (MetMb) by O2 is inhibited by CO. The rate of oxidation in O2-CO mixtures at atmospheric pressure, with partial pressure of CO between O and 45 mm, pH 5.7 and 30[degree], is first order with respect to unoxidized Mb. The partial pressure of CO required to halve the uninhibited rate constant was 21 [plus or minus] 2 mm. The partition constant of Mb between CO and O2 enters into the rate equations for the inhibited oxidation. Provided that a value of about 15 is appropriate to the present experimental conditions the results favor a rate equation containing (1 - alpha)2 for the oxidation at high O2 pressures, where (1 - alpha) is the fraction of MbO2. In the pH range 5.35-6.64 at 30[degree], the rate of oxidation increases with hydrogen-ion concentration at both low and high O2 pressures. The effect is small in magnitude, and could be due to the ionization of haem-linked groups on the Mb molecule. A discussion of the available thermochemical data shows that electron transfer between Mb and O2 giving MetMb and O2- is probably a relatively slow reaction since it would be endothermic to about 20 kg calorie/g mol. The electron transfer dissociation of MbO2 into MetMb and O2- would be much less favored, having an endothermicity of about 36 kg calorie/g mol on account of the large exothermic heat of formation of MbO2. The formation of an 02 complex by Mb and hemoglobin in an exothermic reaction is thus in itself a significant factor in protecting the haem from oxidation.

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