Nuclear Magnetic Resonance and Oxygen Affinity Study of Cesium Binding in Human Erythrocytes

Abstract

We investigated the interaction of the cesium ion (Cs+) with the anionic intracellular components of human red blood cells (RBCs); the components studied included 2,3-bisphosphoglycerate (BPG), ADP, ATP, inorganic phosphate (Pi), carbonmonoxy hemoglobin (COHb), and RBC membranes. We used spin–lattice (T1) and spin–spin (T2) 133Cs NMR relaxation measurements to probe Cs+ binding, and we found that Cs+ bound more strongly to binding sites in BPG and in RBC membranes than in any other intracellular component in RBCs at physiologic concentrations. By using James–Noggle plots, we obtained Cs+ binding constants per binding site in BPG (66 ± 8 M−1), ADP (19 ± 1 M−1), ATP (25 ± 3 M−1), and RBC membranes (55 ± 2 M−1) from the observed T1 values. We also studied the effect of Cs+ on the oxygen (O2) affinity of purified Hb and of Hb in intact RBCs in the absence and in the presence of BPG. In the absence of BPG, the O2 affinity of Hb decreased upon addition of Cs+. However, in the presence of BPG, the O2 affinity of Hb increased upon addition of Cs+. The O2 affinity of Cs+-loaded human RBCs was larger than that of Cs+-free cells at the same BPG level. 31P NMR studies on the pH dependence of the interaction between BPG and Hb indicated that the presence of Cs+ resulted in a smaller fraction of BPG available to bind to the cleft of deoxyHb. Our NMR and O2 affinity data indicate that a strong binding site for Cs+ in human RBCs is BPG. A partial mechanism for Cs+ toxicity might arise from competition between Cs+ and deoxyHb for BPG, thereby increasing oxygenation of Hb in RBCs, and thus decreasing the ability of RBCs to give up oxygen in tissues. The presence of Cs+ at 12.5 mM in intact human RBCs containing BPG at normal concentrations did not, however, alter significantly the O2 affinity of Hb, thus ruling out the possibility of Cs+–BPG interactions accounting for Cs+ toxicity in this cell type.