Binding of Magnesium and Chloride Ions to Human Hemoglobin A

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Abstract
Ultrafiltration and direct determination of [Mg2+] by ion exchange were used to study the binding of Mg2+ and Cl- to isoionic human Hb. A value of 58.8 .+-. 1.7 l/mol was determined for the volume of hydrated Hb from ultrafiltration of Hb solutions containing 0.15-0.8 M glucose. In solutions with 5.7 mmol Hb, 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 Hb, respectively. A value of .apprx. 11 l/mol was determined for the association constant of Mg2+ to Hb monomer. Free Mg2+ concentrations were measured in Hb 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. Only small changes of free Mg2+ levels in red cells apparently are caused by physiological changes of p[partial pressure]O2.