Pulse Radiolysis Studies of the Reactions of Carbonate Radical Anion with Myoglobin and Hemoglobin

摘要

The reactions of carbonate radical anion [systematic name:trioxidocarbonate(centre dot 1-)] with different forms of myoglobin and hemoglobin were studied by pulse radiolysis in N_2O-saturated 0.25 M sodium bicarbonate solutions at pH 10.0 and room temperature.The reactions of CO_3~(centre dot-) with metMb and metHb involve only amino acid residues of the globin and no oxidation of the iron is observed.The second-order rate constants measured are (4.7+-0.3)x10~7 and (1.9+-0.3)x10~8 M~(-1) s~(-1),for metMb and metHb,respectively.The carbonate radical anion-mediated oxidation of oxyHb proceeds in two steps:First,CO_3~(centre dot -) generates radical(s) in the globin which then,over a longer time scale,oxidize the iron center to finally produce approx40% of metHb.The rate constants obtained for the two steps are (2.1+-0.1)x10~8 and (1.0+-0.2)x10~2 s~(-1),respectively.For the reaction between CO_3~(centre dot-) and oxyMb,at all wavelengths studied we obtained kinetic traces that could be fitted to a single-exponential expression.Two distinct two step mechanisms were proposed to explain the kinetic data.The reaction of CO_3~(centre dot -) with oxyMb proceeds either according to a mechanism identical to that observed for the reaction with oxyHb but with a significantly faster rate of electron transfer from the globin radical(s) to the iron (>6x10~4 s~(-1))or according to a concurring mechanism in which CO_3~(centre dot -) oxidizes directly both approx 50% of the iron center and amino acid residue(s) of the globin.