Oxidation—reduction potentials of high molecular weight heme proteins

Abstract

Previous studies on the oxidation by potassium ferricyanide of the high molecular weight heme proteins of Annelids, erythrocruorins and chlorocruorins, brought out several differences with respect to tetramerit hemoglobins. The nature of the oxidation product depends on the pH of the medium; thus a high molecular weight derivative with the spectral properties of aquo-met-heme proteins can be obtained only at pH values near neutrality. At acid and alkaline pH values a low molecular weight hemichrome is formed. Moreover, complete oxidation is achieved with a stoichiometric amount of ferricyanide only in the case of the deoxygenated derivative; it requires a large excess of oxidant in the case of the oxygenated derivative. In accordance with this observation, the met-form reverts partially into the oxygenated form upon removal of ferroand ferricyanide by dialysis in the presence of oxygen [ 1,2]. The latter data pointed to a higher value of the oxidation-reduction potential in erythroand chlorocruorins with respect to tetrameric hemoglobins. This communication presents results of anaerobic spectrophotometric oxidation-reduction titrations of erythrocruorins performed in the presence of the ferro-ferricyanide system. In all the proteins studied the redox potentials are around 0.22-0.23 V at pH 7 .O and 2O”C, a value which is significantly higher than that of human hemoglobin under similar experimental conditions [3].