title: The rate of reaction of superoxide radical ion with oxyhaemoglobin and methaemoglobin.
abstract: Superoxide radical ions (O2-) produced by the radiolytic reduction of oxygenated formate solutions and by the xanthine oxidase-catalysed oxidation of xanthine were shown to oxidize the haem groups in oxyhaemoglobin and reduce those in methaemoglobin as in reactions (1) and (2): (see articles) Reaction (1) is suppressed by reaction (8) when [O2-]exceeds 10 muM, but consumes all the O2- generated in oxyhaemoglobin solutions when [oxyhaemoglobin] greater than 160 muM and [O2-]less than 1 nM at pH 7. The yield of reaction (2) is also maximal in methaemoglobin solutions under similar conditions, but less than one haem group is reduced per O2- radical. From studies of (a) the yield of reactions (1) and (2) at variable [haemoglobin] and rates of production of O2-, (b) their suppression by superoxide dismutase, and (c) equilibria observed with mixtures of oxyhaemoglobin and methaemoglobin, it is shown that k1/k2=0.7 +/- 0.2 and k1 = (4 +/- 1) X 10(3) M-1-S-1 At pH7, and k1 and k2 decrease with increasing pH. Concentrations and rate constants are expressed in terms of haem-group concentrations. Concentrations of superoxide dismutase observed in normal erythrocytes are sufficient to suppress reactions (1) and (2), and hence prevent the formation of excessive methaemoglobin.
mesh_terms: D006418:Heme; D006454:Hemoglobins; D007700:Kinetics; D008706:Methemoglobin; D010084:Oxidation-Reduction; D010100:Oxygen; D010108:Oxyhemoglobins; D011830:Radiation Effects; D013482:Superoxide Dismutase; D013481:Superoxides
chemical_list: D006454:Hemoglobins; D010108:Oxyhemoglobins; D013481:Superoxides; D006418:Heme; D008706:Methemoglobin; D013482:Superoxide Dismutase; D010100:Oxygen