Lithium-7 NMR relaxation study of lithium binding in human erythrocytes

Abstract

We used 7Li NMR spin-lattice (TI) and spin-spin (Tz) relaxation time measurements to investigate the binding of Li+ in human red blood cell (RBC) suspensions. In RBCs containing 1.4 mM Li+, the intracellular 7Li NMR T2 relaxation value (0.30 f 0.03 s) was much smaller than the corresponding TI value (6.0 f 0.1 s), yielding a ratio of TI to T2 of 20. For 1.5 mM LiCl solutions whose viscosities were adjusted to 5 CP with glycerol, the values of the T1/T2 ratios were as follows: 49 for unsealed RBC membrane (2.0 mg of protein/mL); 4.4 for spectrin (1.9 mg/mL); 1.5 for 5.4 mM 2,3-bisphosphoglycerate (BPG); 2.2 for 2.7 mM carbonmonoxyhemoglobin (COHb); 1.6 for 2.0 mM ATP; and 1.2 for a 50/50% (v/v) glycerol-water mixture. Intracellular viscosity and the electric field gradients experienced by Li+ when traversing the spectrin-actin network therefore are not responsible for the large values of the Tl/ T2 ratios observed in Li+-loaded RBCs. We conclude that the RBC membrane is the major Li+ binding site in Li+-loaded RBCs (KI=, 215 f 36 M-l) and that the binding of Li+ to COHb, BPG, spectrin-actin, or ATP is weak. Partially relaxed 7Li NMR spectra of Li+-containing RBC membrane suspensions indicated the presence of two relaxation components, one broad and one narrow. At the same extravesicular Li+ and protein concentrations, the TI values for right-side-out RBC vesicle suspensions were at least 2-fold larger than those for inside-out RBC vesicle suspensions; the inner layer of the RBC membrane, which has a larger percentage of anionic phospholipids than the outer layer, contributes mostly to Li+ binding.