| Abstract
| - The reactions of carbonate radical anion [systematic name: trioxidocarbonate(•1−)] with different forms ofmyoglobin and hemoglobin were studied by pulse radiolysis in N2O-saturated 0.25 M sodium bicarbonatesolutions at pH 10.0 and room temperature. The reactions of CO3•- with metMb and metHb involve onlyamino acid residues of the globin and no oxidation of the iron is observed. The second-order rate constantsmeasured are (4.7 ± 0.3) × 107 and (1.9 ± 0.3) × 108 M-1 s-1, for metMb and metHb, respectively. Thecarbonate radical anion-mediated oxidation of oxyHb proceeds in two steps: First, CO3•- generatesradical(s) in the globin which then, over a longer time scale, oxidize the iron center to finally produce ∼40%of metHb. The rate constants obtained for the two steps are (2.1 ± 0.1) × 108 and (1.0 ± 0.2) × 102 s-1,respectively. For the reaction between CO3•- and oxyMb, at all wavelengths studied we obtained kinetictraces that could be fitted to a single-exponential expression. Two distinct two step mechanisms were proposedto explain the kinetic data. The reaction of CO3•- with oxyMb proceeds either according to a mechanismidentical to that observed for the reaction with oxyHb but with a significantly faster rate of electron transferfrom the globin radical(s) to the iron (>6 × 104 s-1) or according to a concurring mechanism in which CO3•-oxidizes directly both ∼50% of the iron center and amino acid residue(s) of the globin.
|
