Kinetic and mechanistic studies of the peroxynitrite-mediated oxidation of oxymyoglobin and oxyhemoglobin

Kinetic studies of the peroxynitrite-mediated oxidations of oxymyoglobin (MbFeO(2)) and oxyhemoglobin (HbFeO(2)) showed that the mechanisms of these reactions are more complex than what had previously been reported; both reactions proceed in two steps. For myoglobin, we found that the small amount of deoxymyoglobin (MbFe(II)) which is in equilibrium with MbFeO(2) is first oxidized by peroxynitrous acid to ferryl myoglobin (MbFe(IV)=O). Then, in the second step, MbFe(IV)=O is reduced by peroxynitrous acid to metmyoglobin (metMb). The second-order rate constant values obtained at pH 7.3 and 20 degrees C for the two steps are (5.4 +/- 0.2) x 10(4) and (2.2 +/- 0.1) x 10(4) M-1 s(-1), respectively. Analogous studies with hemoglobin suggest that its reaction with peroxynitrite follows the same mechanism. In this case, the second-order rate constant values measured at pH 7.0 and 20 degrees C for the two steps are (8.8 +/- 0.4) x 10(4) and (9.4 +/- 0.7) x 10(4) M-1 s(-1), respectively. A possible mechanism in the absence as well as in the presence of CO2 and the relevance of these reactions in vivo are discussed.