Autoxidation of Oxymyoglobins

Abstract

Abstract During the autoxidation of oxymyoglobin to metmyoglobin, there is an evolution of oxygen. The experimental results support the conclusion that 0.25 mole of the oxygen in oxymyoglobin is used for the oxidation and 0.75 mole is released. This finding differs significantly from earlier reports of oxygen consumption during the oxidation. The latter results are shown to be attributable to interference by oxidation products of sodium hydrosulfite, which is used as a reducing agent. In the present system, we remove such products completely by means of an ion exchange column. This paper includes a design for a completely sealed cell for oxygen electrode measurements. In view of the finding of interference by hydrosulfite, we have reexamined the effects of temperature, pH, and oxygen level on rates of oxidation of oxymyoglobin. Absolute rates in systems free of hydrosulfite differ substantially from those previously reported, but the relative effects are the same, i.e. there is a direct dependency on hydrogen ion concentration, particularly over the pH range from 5 to 7, and an indirect dependency on oxygen level. There is a marked effect of temperature on the reaction, with Q10 values approximating 5. Activation energies obtained with myoglobins from different sources and at different pH values range from 24,000 to 27,800 calories. Carboxymethylation of available histidine residues in undenatured sperm whale and yellowfin tuna myoglobins has no effect on the oxidation rate. The presence of diphosphoglycerate increases the reaction rate. Under equivalent conditions, the tuna oxymyoglobin oxidizes much faster than those from sperm whale or bovine muscle.