Reactions of Oxymyoglobin with NO, NO2, and NO2 - under Argon and in Air

• Published in: Chemical research in toxicology 1996-12, Vol.9 (8), p.1382-1390
• Main Authors: Wade, Ruth S, Castro, Charles E
• Format: Article
• Language: English
• Subjects: Air; Animals; Argon; Horses; Kinetics; Myocardium - metabolism; Myoglobin - chemistry; Nitrates - chemistry; Nitrogen Dioxide - chemistry; Nitrous Oxide - chemistry
• ISSN: 0893-228X; 1520-5010
• DOI: 10.1021/tx9600457

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 (λmax, 302 nm) was poorly resolved from more intense protein absorbtion in the 300-nm region. However, at pH 9, the change in absorbance corresponded exactly to a quantitative production of the OONO- ion. Hydroxyl radicals from it were trapped with ethylene-1,2-13C. 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 × 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 k ave (for the autocatalytic rates) ≅ 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 [J. Org. Chem. (1996) 61, 6388−6395 and J. Am. Chem. Soc. (1996) 118, 3894−3895], it is proposed that ozone may be an intermediate in the autocatalysis.