Reduction of metmyoglobin derivatives by dithionite ion

The rate constants for reduction by dithionite of a number of metmyoglobin species Mb+X(X=H2O, imidazole, OH-, F-, N3-, CNO-, SCN-, HCO2-, NO2-, and CN-) were measured at 25 degrees by stopped flow spectrophotometry. The dependence of the rate was [S2O42-]1/2, and the SO2- radical was considered to be the active reductant. Except for X=imidazole and CN-, reduction occurred through dissociation of Mb+X. Values for the dissociative rate constant obtained from dithionite reduction were in good agreement with those obtained directly. Reduction of the dissociated fragment (assumed Mb+H2O) by SO2- is 3+/-1 X 10(6) M-1S-1 at pH 8.2 for all Mb+X species examined. Reduction of Mb+ imidazole and Mb+CN- occurs directly with SO2-, and Mb0CN- (Mb0, deoxymyoglobin) is characterized as an intermediate in reduction of the latter.