Reduction of ferryl- and metmyoglobin to ferrous myoglobin by menadione-glutathione conjugate. Spectrophotometric studies under aerobic and anaerobic conditions

Both metmyoglobin (Mb III) and ferrylmyoglobin (Mb IV) are reduced by the menadiol-glutathione conjugate (GS-Q 2−) to oxymyoglobin (Mb IIO 2) or deoxymyoglobin (Mb II), depending whether the assay is carried out under aerobic or anaerobic conditions, respectively. Under aerobic conditions, the reduction of Mb III to Mb IIO 2 by GS-Q 2− is associated with O 2 consumption. The latter process is accounted for by (a) the autoxidation of the conjugate yielding H 2O 2 and (b) the rapid binding of O 2 to Mb II to yield Mb IIO 2. The ratio [O 2] consumed/[Mb IIO 2] formed is approximately 1.5 at the time when Mb IIO 2 formation is maximal (at about 0.8 min). This ratio, higher than the unit, indicates that there is more than one O 2-consuming reaction in this experimental model. The ratio of initial rates of O 2 consumption and Mb IIO 2 formation is close to the unit [(− dO 2/ dt)/(+ dMb IIO 2/ dt) = 1.1]. The formation of H 2O 2 originating during the autoxidation of the GS-Q 2− is substantially lower in the presence of Mb III, probably due to the heterolytic cleavage of the O-O bond of the peroxide by the hemoprotein. Although the latter reaction should yield Mb IV, this species is not observed in the absorption spectrum, probably due to its rapid reduction by GS-Q 2−. Mb IV is reduced to Mb IIO 2 by the GS-Q 2−. Whether this reaction takes place in one-electron transfer steps, that is, the sequence: Mb IV → Mb III → Mb IIO 2 is difficult to evaluate by absorption spectral analysis, due to the rapid rate of the [Mb IV → Mb IIO 2] transition. Under anaerobic conditions, the reduction of either Mb III or Mb IV by GS-Q 2− yields Mb II as a stable molecular product. Anaerobic conditions prevent any further interaction of Mb II with intermediates of O 2 reduction derived from GS-Q 2− autoxidation.