Oxidation of oxyhemoglobin by nitrite is characterized by the presence of a lag phase followed by autocatalysis. The stoichiometry of the overall reaction is described by the following equation: 4HbO2 + 4NO-+ 4H+ = 4Hb+ + 4NO-+ 02 + 2H20 (Hb denotes hemoglobin monomer). During the oxidation, we detected a free radical at g = 2.005, which is very similar to the methemoglobin free radical generated by the reaction with hydrogen peroxide. Nitrosylhemoglobin was not detected. The oxidation was delayed by the addition of KCN or catalase, but was not modified by superoxide dismutase in phosphate buffer. In bistris buffer, however, superoxide dimutase markedly prolonged the lag phase. The results suggest that during the oxidation, the methemoglobin peroxide compound is generated and converts nitrite into nitrogen dioxide by its peroxidatic activity. Nitrogen dioxide oxidizes oxyhemoglobin to methemoglobin and nitrite, yielding the autocatalytic phase.
To elucidate the molecular conformation of a ligand bound to hemoglobin, nitric oxide derivatives of human adult hemoglobin and its subunits (a and ß) have been studied by electron paramagnetic resonance spectroscopy.The spectrum of hemoglobin -NO and /3-NO are dissimilar and that of erythrocytes NO and human adult hemoglobin NO are the arithmetic mean of the spectrum for the isolated subunits. At room temperatures, the signal of hemoglobin -NO exhibits axial symmetry, whereas that of hemoglobin /3-NO slightly deviates from axial symmetry. Below -50°, however,
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