COMPETITION BETWEEN LI+ AND MG2+ FOR THE PHOSPHATE GROUPS IN THE HUMAN ERYTHROCYTE-MEMBRANE AND ATP - AN NMR AND FLUORESCENCE STUDY

We investigated the mechanism of competition between Li+ and Mg2+ in Li+-loaded human red blood cells (RBCs) by making Li-7 and P-31 NMR and fluorescence measurements. We used Li-7 NMR relaxation times to probe Li+ binding to the human RBC membrane and ATP; an increase in Mg2+ concentration caused an increase in both Li-7 T1 and T2 values in packed Li+-loaded RBCs, in suspensions of Li+-loaded RBC ghosts, in suspensions of Li+-containing RBC membrane, and in aqueous solutions of ATP, indicating competition between Li+ and Mg2+ for binding sites in the membrane and ATP. We found that increasing concentrations of either Li+ or Mg2+ in the presence of human RBC membrane caused an increase in the P-31 NMR chemical shift anisotropy parameter, which describes the observed axially symmetric powder pattern, indicating metal ion binding to the phosphate groups in the membrane. Competition between Li+ and Mg2+ for phosphate groups in ATP and in the RBC membrane was also observed by both fluorescence measurements and P-31 NMR spectroscopy at low temperature. The ratio of the stoichiometric binding constants of Mg2+ to Li+ to the RBC membrane was approximately 20; the ratio of the conditional binding constants in the presence of a free intracellular ATP concentration of 0.2 mM was approximately 4, indicating that Li+ competes for approximately 20% of the Mg2+-binding sites in the RBC membrane. Our results indicate that, regardless of the spectroscopic method used, Li+ competes with Mg2+ for phosphate groups in both ATP and the RBC membrane; the extent of metal ion competition for the phosphate head groups of the phospholipids in the RBC membrane is enhanced by the presence of ATP. Competition between Li+ and Mg2+ for anionic phospholipids or Mg2+-activated proteins present in cell membranes may constitute the basis of a general molecular mechanism for Li+ action in human tissues.