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 (HbO(2)) 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 contai ning two rate constants - k(f), for fast autoxidation of the alpha-cha in, and k(s), for slow autoxidation of the beta-chain, respectively. W hen tetrameric HbO(2) was dissociated into alpha beta-dimers by diluti on, the value of k(f) increased markedly to an extent comparable with the autoxidation rate of horse heart oxymyoglobin (MbO(2)). The rate c onstant k(s), on the other hand, was found to remain at an almost cons tant 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 autoxidatio n of HbO(2) was monophasic. and no enhancement in the rate was observe d by diluting hemoglobin solutions. Taking into consideration the effe cts of 2,3-diphosphoglyceric acid and chloride anion on the autoxidati on rate of HbO(2), we have characterized the differential susceptibili ty of the alpha- and beta-chains to the autoxidation reaction in aqueo us solution.