The Deoxygenation Kinetics of Hemoglobin Partially Saturated with Carbon Monoxide

The deoxygenation rate of hemoglobin solutions partially saturated with carbon monoxide, in the presence and absence of 2,3-diphosphoglycerate (2,3-DPG) at 23° and pH 7.0 and 8.0 have been measured. When the rate is calculated over the initial 30% of the reaction, solutions containing 98% HbO2 show an increase in the rate as hemoglobin deoxygenates both at neutral and alkaline pH. Partial saturation with CO severely compromises this variation at both pH values. Indeed, for a solution containing about 15% HbO2, no variation in the rate over the initial 30% of the reaction or thereafter, is observed. These results for the initial portion of the reaction are in agreement with the earlier studies of Gibson and Roughton ((1955) Proc. Roy. Soc. London B Biol. Sci. 143, 310). The values of the deoxygenation rate constant calculated between 85 and 40% saturation (k in s-1) at pH 8.0, in the absence of 2,3-DPG, are 19.5 ± 1.0 s-1 for the 98% HbO2 condition and 14.2 ± 0.7 s-1 for the 85% HbCO-15% HbO2 case. 2,3-DPG has no significant effect on k at pH 8.0. At pH 7.0, with no 2,3-DPG, the value of k for the 98% condition is 36.0 ± 1.8 s-1, and for the 85% HbCO-15% HbO2 case, it is 17.9 ± 0.9 s-1. In the presence of a 10:1 molar ratio of 2,3-DPG to hemoglobin tetramer, the effect of partial saturation with CO is even more pronounced as evidenced by a value of 52.4 ± 2.0 s-1 for 98% HbO2 and 18.0 ± 0.8 s-1 for 85% HbCO-15% HbO2. Thus solutions containing Hb4O2(CO)3 as a major initial species show no effect of 2,3-DPG at pH 7.0. The effect of this organic phosphate on k becomes significant as the concentration of the species Hb4O4(CO)2 increases. These results support the view that there is no effect of 2,3-DPG on the release of the first oxygen from the fully liganded conformation. They also suggest that 2,3-DPG exerts its influence on the rate of removal of the second and subsequent oxygen molecules. This kinetic result is consistent with the findings of MacQuarrie and Gibson ((1971) J. Biol. Chem. 246, 5832), that a 2,3-DPG analogue dissociates from its combination with hemoglobin after the binding of the third heme ligand. Since 2,3-DPG binding is thought to be dependent on the quaternary structure of hemoglobin, these results taken together would indicate that the transition from the oxy to the deoxy quaternary structure, at pH 7.0, occurs very early in the deoxygenation process, probably with the loss of the first heme ligand from fully liganded hemoglobin.