Biphasic nature in the autoxidation reaction of human oxyhemoglobin

In comparison with myoglobin molecule as a reference, we have studied the autoxidation rate of human oxyhemoglobin (HbO2) as a function of its concentration in 0.1 M buffer at 35 degrees C and in the presence of 1 mM EDTA. At pH 6.5, HbA showed a biphasic autoxidation reaction that can be described completely by a first-order rate equation containing two rate constants-kf, for fast autoxidation of the alpha-chain, and ks, for slow autoxidation of the beta-chain, respectively. When tetrameric HbO2 was dissociated into alpha beta-dimers by dilution, the value of kf increased markedly to an extent comparable with the autoxidation rate of horse heart oxymyoglobin (MbO2). The rate constant Ks, on the other hand, was found to remain at an almost constant value over the whole concentration range from 1.0 x 10(-3) M to 3.2 x 10(-6) M in heme. At pH 8.5 and pH 10.0, however, the autoxidation of HbO2 was monophasic, and no enhancement in the rate was observed by diluting hemoglobin solutions. Taking into consideration the effects of 2,3-diphosphoglyceric acid and chloride anion on the autoxidation rate of HbO2, we have characterized the differential susceptibility of the alpha- and beta-chains to the autoxidation reaction in aqueous solution.