Reactions of oxymyoglobin with NO, NO2, and NO2- under argon and in air.

Abstract

Oxymyoglobin under argon reacts with NO2- and NO2 (N2O4) to produce metmyoglobin in a spectrally clean process with clear isosbestic points. In both cases, the reactant is NO2-. The second-order rate constant for NO2- or N2O4 is the same: d(Mb+)/dt = k(MbO2)(NO2-) where k = 0.21 +/- 0.02 L mol-1 S-1. The reaction of MbO2 with NO under argon is a complex process and entails the generation of Mb+ and OONO- (peroxynitrite) in the first step. The latter (lambda MAX, 302 nm) was poorly resolved from more intense protein absorption in the 300-nm region. However, at pH 9, the change in absorbance corresponded exactly to a quantitative production of the OONO-ion. Hydroxy radicals from it were trapped with ethylene-1, 2-(13) C. The initial step is followed in sequence by the rapid formation of MbNO+. The iron(III)-nitrosyl adduct hydrolyzes slowly to MbII and NO2- (k = 8.0 +/- 0.8 x 10(-5) S-1. MbII then rapidly associates with NO, and MbNO is the final product of this reaction. Oxymyoglobin is inert to NO3-. In contrast to the results under argon, in air the reactions of MbO2 with NO2-, NO, and NO2 (N2O4) all proceed in the same autocatalytic fashion with kAVE (for the autocatalytic rates) approximately equal to 9 +/- 5 L mol-1 s-1. Nitrite is the initial reactant in all cases. Isosbestic points are not observed in the visible spectrum, and additional porphyrin iron-ligated species are intermediates. Based upon work with iron porphyrins.