Kinetics of the reactions of nitrogen monoxide and nitrite with ferryl hemoglobin

Hemoglobin released in the circulation from ruptured red blood cells can be oxidized by hydrogen peroxide or peroxynitrite to generate the highly oxidizing iron(image)oxo species HbFeimage=O. Nitrogen monoxide, produced in large amounts by activated inducible nitric oxide synthase, can have indirect cytotoxic effects, mainly through the generation of peroxynitrite from its very fast reaction with superoxide. In the present work we have determined the rate constant for the reaction of HbFeimage=O with NO•, 2.4 × 107 M−1s−1 at pH 7.0 and 20°C. The reaction proceeds via the intermediate HbFeimageONO, which then dissociates to metHb and nitrite. As these products are not oxidizing and because of its large rate, the reaction of HbFeimage=O with NO• may be important to remove the high valent form of hemoglobin, which has been proposed to be at least in part responsible for oxidative lesions. In addition, we have determined that the rate constant for the reaction of HbFeimage=O with nitrite is significantly lower (7.5 × 102 M−1s−1 at pH 7.0 and 20°C), but increases with decreasing pH (1.8 × 103 M−1s−1 at pH 6.4 and 20°C). Thus, under acidic conditions as found in ischemic tissues, this reaction may also have a physiological relevance.