The events relating to lanthanide ions enhanced permeability of human erythrocyte membrane: binding, conformational change, phase transition, perforation and ion transport

Abstract

The binding and uptake of Gd 3+ ions by human erythrocytes in vitro were studied by determining the Gd contents in membrane and in cytosol by means of particle-induced X-ray emission (PIXE) spectrometry. Results obtained from varied incubation time revealed that the Gd 3+ ions bind to the membrane proteins and lipids at first. Gd 3+ binding to the membrane lipids and proteins lasts 0∼20 and 20∼100 ms respectively, as shown by the stopped-flow studies. Then a fraction of Gd 3+ ions diffuses through the membrane. The kinetics of Gd 3+ binding indicates that the binding to phospholipids is prior to that to the membrane proteins, but a portion of the lipid-bound Gd 3+ redistributed later to the proteins. PIXE studies showed that the entry of Gd 3+ increased the influx of Ca 2+ and Cl −. By monitoring the changes in fluorescence of proteins and that of the Ln 3+, the uptake of La 3+, Eu 3+, Gd 3+ and Tb 3+ was shown to be a process comprising a series of events. Binding to the membrane molecules induces the phase transition of lipid bilayer and conformational changes and aggregation of membrane proteins. Conformational changes of the proteins were characterized by Fourier transform IR spectroscopy (FT-IR) deconvolved spectra, i.e. α-helix content decreases while β-sheet increases. ESR spectra of MSL-labeled proteins reflect the aggregation state related with the conformational change. [ 31P]NMR spectra of membrane lipid bilayer revealed the Ln 3+ ions induced hexagonal (H II) phase formation. Phase transition and aggregation of membrane proteins cause the formation of domain structure and perforation in the membrane. These alterations in membrane structure are responsible for the Ln 3+ enhanced membrane permeability. Thus the previous Ln 3+ binding will facilitate the across-membrane transport of other Ln 3+ ions through the membrane.