7Li NMR relaxation study of Li+ binding in human erythrocytes.

We used 7Li NMR spin-lattice (T1) and spin-spin (T2) 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 +/- 0.03 s) was much smaller than the corresponding T1 value (6.0 +/- 0.1 s), yielding a ratio of T1 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 T1/T2 ratios observed in Li(+)-loaded RBCs. We conclude that the RBC membrane is the major Li+ binding site in Li(+)-loaded RBCs (Kb = 215 +/- 36 M-1) 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.(ABSTRACT TRUNCATED AT 250 WORDS)