Structural changes in membranes of large unilamellar vesicles after binding of sodium cholate

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Abstract
The interaction of the bile salt cholate with unilamellar vesicles was studied. At low cholate content, equilibrium binding measurements with egg yolk lecithin membranes suggest that cholate binds to the outer vesicle leaflet. At increasing concentrations, further bile salt binding to the membrane is hampered. Before the onset of membrane solubilization, diphenylhexatriene fluorescence anisotropy decreases to a shallow minimum. It then increases to the initial value in the cholate concentration range of membrane solubization. At still higher cholate concentrations, a drop in fluorescence anisotropy indicates the transformation of mixed disk micelles into spherical micelles. Perturbation of the vesicle membranes at molar ratios of bound cholate/lecithin exceeding 0.15 leads to a transient release of oligosaccharides from intravesicular space. The cholate concentrations required to induce the release depend on these of the entrapped sugars. Cholesterol stabilizes the membrane, whereas, in spite of enhanced membrane order, sphingomycelin destabilizes the membrane against cholate. Freeze-fracture electron microscopy and phosphorus-31 nuclear magnetic resonance (31P NMR) also reflect a change in membrane structure at maximal cholate binding to the vesicles. In 31P NMR spectra, superimposed on the anisotropic line typically found in phospholipid bilayers, an isotropic peak was found. This signal is most probably due to the formation of smaller vesicles after addition of cholate. The results were discussed with respect to bile salt/membrane interactions in the liver cell. It is concluded that vesicular bile salt transport in the cytoplasm is unlikely and that cholate binding is restricted to the outer leaflet of the canalicular part of the plasma membrane.