Reduction of metmyoglobin derivatives by dithionite ion.

E Olivas,D J De Waal,R G Wilkins

doi:10.1016/s0021-9258(17)40229-8

Abstract

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.

Highlights

The rate constants for reduction by dithionite of a number of metmyoglobin species Mb+X (X = H,O, imidazole, OH, F, NT, CNO, SCN, HC02, NOy, and CN-) were measured at 25” by stopped flow spectrophotometry
The spectra of the products showed that the met species Mb+, Mb+OH, Mb+X, and Mb+imid were completely reduced to Mb0 by dithionite ion (Mb+CN, Mb+imid, and Mb+NO, in two stages)
For Mb+imid and Mb+CN- (Fig. 1) and Mb+SCN, Mb+, and Mb+OH, a linear dependence of the pseudo-first order rate constant on [S,0,‘-]1p2 was observed over the whole range of dithionite concentrations with a zero or near-zero intercept

Summary

The abbreviations used are

MbO, deoxymyoglobin; Mb+, metmyoglobin, Mb+OH- and Mb+X-, ionized and anionic adducts of metmyoglobins; Mb+imid, imidazole complex of Mb+. Mb+X- and Mb+X with distinctive spectral characteristics (1, 11). The reduction of these species to Mb” by dithionite can be monitored. The results can be compared with dithionite reduction of another heme-containing protein, namely, ferricytochrome c (6), which participates in electron transfer reactions but does not bind oxygen

AND METHODS

RESULTS

DISCUSSION