Binding of magnesium and chloride ions to human hemoglobin A. Mg2+ concentrations in solutions simulating red cell conditions.

Ultrafiltration and direct determination of [Mg2+] by ion exchange were used to study the binding of Mg2+ and Cl- to isoionic human hemoglobin. A value of 58.8 +/- 1.7 l/mol was determined for the volume of hydrated hemoglobin from ultrafiltration of hemoglobin solutions containing 0.15-0.8 M glucose. In solutions with 5.7 mmol hemoglobin, 150 mmol KCl and 0.5-3.5 mmol MgCl2/l total water, 0.6 mol and 2 mol Cl- were bound/mol oxygenated and deoxygenated hemoglobin respectively. A value of about 11 l/mol was determined for the association constant of Mg2+ to hemoglobin monomer. Free Mg2+ concentrations were measured in hemoglobin solutions containing KCl, MgCl2, ATP and D-glycerate-2,3-bisphosphate at concentrations close to those of red cells. The experiments yielded 0.65 mmol/l free Mg2+ after oxygenation and 0.82 mmol/l after deoxygenation. The data indicate that only small changes of free Mg2+ levels in red cells are caused by physiological changes of pO2.