Binding of sodium ethylene diamine tetraacetate (EDTA) to human hemoglobin A

Effects of anions on the ligand-linked subunit assembly of human hemoglobin: the mutual effects of Cl- and EDTA

We have studied the mutual effects of chloride ion and EDTA on the dimer-tetramer assembly of human deoxyhemoglobin and oxyhemoglobin. It is found that these two anions have similar but interdependent effects. In low C1- (.01 M) increasing concentrations of EDTA are found to decrease both forward and reverse rate constants for deoxyhemoglobin, whereas no effect is observed at 0.1 M C1-. These results suggest that binding of anions at the alpha 1 beta 2 intersubunit contact may stabilize both the dimeric and tetrameric forms of the deoxy molecule, thus inhibiting both the dissociation and reassociation reactions. The overall effects of EDTA and low C1- on the dimer-tetramer equilibrium constants are found to be distinctly different in deoxy and oxyhemoglobins with a major effect on the oxy form. These findings establish validity of the results from previous thermodynamic studies carried out in approximately physiological C1- concentrations along with the small amounts of EDTA which are used to minimize artifacts of oxidation. As observed for deoxyhemoglobin, it is found that in 0.1 M C1- ion there is no further effect of EDTA on the oxyhemoglobin dimer-tetramer equilibrium.

The binding of lactate and chloride ions to human adult hemoglobin

The effects of sodium lactate (Lact) on the oxygen affinity and the Bohr effect of purified human adult hemoglobin solutions have been compared to the effects of sodium chloride (Cl). Changes in the affinity for oxygen have been estimated from the variations of log(O2)50 with pH and at various salt concentration from 0.005 up to 2.0 mol.l-1. (O2)50 was calculated as alpha.P0.5 where alpha is the solubility coefficient of oxygen in the solutions at various salt concentrations. Variations of log(O2)50 with pH at constant salt concentration and variations of log(O2)50 with anion concentration at constant pH have been studied according to the linked-functions theory (Wyman, 1968). Bohr curves and salt binding curves were calculated from standard iterative curve fitting procedures and various parameters relevant to the effects of salts on hemoglobin function were estimated. It is shown that Lact and Cl increase (O2)50 and the alkaline Bohr effect in a comparable way at low salt concentration. At high concentration the effect of Lact predominated over that of Cl. The amount of oxygen linked Lact was larger than that of Cl. Binding constants for both anions to deoxy and oxy Hb were estimated. Lact and Cl have comparable binding constants to deoxy hemoglobin. By contrast Lact binds to oxy hemoglobin to a lesser extent than Cl. This may account for the differences observed in the effects of Lact and Cl on the function of hemoglobin. The reason for the low affinity of oxy hemoglobin for Lact may be related to steric differences between the two anions.