Physical basis for the resolution of intra- and extracellular cesium-133 NMR resonances in cesium(+) loaded human erythrocyte suspensions in the presence and absence of shift reagents

Abstract

For human red blood cells (RBCs) loaded with Cs' and suspended in a shift reagent (SR) free medium, the extracellular 13'Cs+ NMR resonance was shifted upfield from the intracellular resonance. However, in the presence of the SRs Dy(PPP);-, Dy- (TTHA))-, and Tm(DOTP)S- [where Dy3+ and Tm3' denote dysprosium and thulium ions and PPPs-, TTHA6, and DOTPsrepresent the triphosphate, triethylenetetraminehexaacetate, and 1,4,7,10-tetraazacyclododecane-N,N:N""'-tetra~s(methylenephosphonate) ligands, respectively], the extracellular 133C~NrM R resonance was shifted downfield from the intracellular resonance. The magnitudes of the '33Cs+ shifts observed with Tm(DOTP)S- were much larger than those for Dy(TTHA)'- and Dy(PPP)J- at the same concentration. The direction of the IS3Cst shift induced by Dy(PPP)J- was the opposite of that previously reported for 'Lit, 23Nat, and 39K+N MR resonances. The negative sign of the pseudocontact 133Csh ift induced by Dy(PPP)?- is related to the large size of the Cs' cation and its location in the equatorial region formed by the cone around the effective magnetic axis of the triphosphate SR. At physiologically relevant RBC concentrations, 2,3-diphosphoglycerate (DE), of all intracellular phosphates tested, caused the largest '33Cs+ shift. The 13'Cs' resonance in carbonmonoxygenated RBC lysate shifted downfield by approximately 2.0 ppm with increasing hemoglobin concentration, whereas an increase in the diamagnetic susceptibility of the sample induced by hemoglobin is expected to induce an upfield shift of 0.1 ppm. The 13'Cs+ resonance was shifted downfield with increasing concentrations of two unrelated proteins, carbonmonoxyhemoglobin and lysozyme. We conclude that, in the absence of SRs, the physical basis for the resolution of intra- and extracellular NMR resonances in Cs+-loaded human RBC suspensions arises from Cs+ binding to intracellular phosphates, in particular DPG, and from the nonideality of intracellular water induced by hemoglobin.