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(IV)oxo species HbFe(IV)z=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 HbFe(IV)z=O with NO(*), 2.4 x 10(7) M(-1)s(-1) at pH 7.0 and 20 degrees C. The reaction proceeds via the intermediate HbFe(III)ONO, which then dissociates to metHb and nitrite. As these products are not oxidizing and because of its large rate, the reaction of HbFe(IV)z=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 HbFe(IV)z=O with nitrite is significantly lower (7.5 x 10(2) M(-1)s(-1) at pH 7.0 and 20 degrees C), but increases with decreasing pH (1.8 x 10(3) M(-1)s(-1) at pH 6.4 and 20 degrees C). Thus, under acidic conditions as found in ischemic tissues, this reaction may also have a physiological relevance.