A NA-23 MULTIPLE-QUANTUM-FILTERED NMR-STUDY OF THE EFFECT OF THE CYTOSKELETON CONFORMATION ON THE ANISOTROPIC MOTION OF SODIUM-IONS IN RED-BLOOD-CELLS

Recently, it has been shown that Na-23 double-quantum-filtered NMR spe ctroscopy can be used to detect anisotropic motion of bound sodium ion s in biological systems. The technique is based on the formation of th e second-rank tensor when the quadrupolar interaction is not averaged to zero, Using this method, anisotropic motion of bound sodium in huma n and dog red blood cells was detected, and the effect was shown to de pend on the integrity of the membrane cytoskeleton. In the present stu dy, multiple-quantum-filtered techniques were applied in combination w ith a quadrupolar echo to measure the transverse-relaxation times, T-2 f and T-2s. Line fitting was performed to obtain the values of the res idual quadrupolar interaction, which was measured for sodium in a vari ety of mammalian erythrocytes of different size, shape, rheological pr operties, and sodium concentrations. Human unsealed white ghosts were used to study sodium bound at the anisotropic sites on the inner side of the RBC membrane. Modulations of the conformation of the cytoskelet on by the variation of either the ionic strength or pH of the suspendi ng medium caused drastic changes in both the residual quadrupolar inte raction and T-2f due to changes in the fraction of bound sodium ions a s well as changes in the structure of the binding sites, By combining the two spectroscopic parameters, structural change can be followed. T he changes in the structure of the sodium anisotropic binding sites de duced by this method were found to correlate with known conformational changes of the membrane cytoskeleton. Variations of the medium pH aff ected both the fraction of bound sodium ions and the structure of the anisotropic binding sites. Sodium and potassium were shown to bind to the anisotropic binding sites with the same affinity. (C) 1996 Academi c Press, Inc.