Reactions of oxymyoglobin with NO, NO sub(2), and NO sub(2) super(-) under argon and in air

• Published in: Chemical research in toxicology 1996-12, Vol.9 (8), p.1382-1390
• Main Authors: Wade, R S, Castro, CE
• Format: Article
• Language: English
• ISSN: 0893-228X

Oxymyoglobin under argon reacts with NO sub(2) super(-) and NO sub(2) (N sub(2)O sub(4)) to produce metmyoglobin in a spectrally clean process with clear isosbestic points. In both cases, the reactant is NO sub(2) super(-). The second-order rate constant for NO sub(2) super(-) or N sub(2)O sub(4) is the same: d(Mb super(+))/dt = k(MbO sub(2))(NO sub(2) super(-)) where k = 0.21 plus or minus 0.02 L mol super(-1) s super(-1). The reaction of MbO sub(2) with NO under argon is a complex process and entails the generation of Mb super(+) and OONO super(-) (peroxynitrite) in the first step. The latter ( lambda sub(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 super(-) ion. Hydroxyl radicals from it were trapped with ethylene-1,2- super(13)C. The initial step is followed in sequence by the rapid formation of MbNO super(+). The iron(III)-nitrosyl adduct hydrolyzes slowly to Mb super(II) and NO sub(2) super(-) (k = 8.0 plus or minus 0.8 x 10 super(-5) s super(-1)). Mb super(II) then rapidly associates with NO, and MbNO is the final product of this reaction. Oxymyoglobin is inert to NO sub(3) super(-). In contrast to the results under argon, in air the reactions of MbO sub(2) with NO sub(2) super(-), NO, and NO sub(2) (N sub(2)O sub(4)) all proceed in the same autocatalytic fashion with k sub(ave) (for the autocatalytic rates) approximately equal to 9 plus or minus 5 L mol super(-1) s super(-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, it is proposed that ozone may be an intermediate in the autocatalysis.