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

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 solubilization. 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 the size of the entrapped sugars. Cholesterol stabilizes the membrane, whereas, in spite of enhanced membrane order, sphingomyelin 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.

Structural determinants of monohydroxylated bile acids to activate β1 subunit-containing BK channels

“…A preferred form of bile acid insertion into membranes is as small aggregates consisting of two to four molecules of bile acids with their hydrophilic sides facing inwards, bound by hydrogen bonds between the hydroxyl groups (18,42,43). The pattern of bile acid effectiveness on BK channel function (cholate,deoxycholate,LC) argues against the possibility that channel activation results from channel protein interactions with this type of bile acid oligomer, as discussed in detail elsewhere (4).…”

Biomimetic PVPA in vitro model for estimation of the intestinal drug permeability using fasted and fed state simulated intestinal fluids

“…The interaction between sodium taurocholate and unilamellar vesicles has earlier been studied and according to Schubert and co-workers low bile salt concentration appears to bind to the vesicles and interact with several membrane lipids, and particularly with lecithin (Schubert et al, 1986;Schubert and Schmidt, 1988 (Fischer et al, 2012). However, the loss of sodium taurocholate from FaSSIF at the different time intervals exhibited no significant differences compared to FeSSIF which is in accordance with earlier studies suggesting that sodium taurocholate binding to the membrane is hampered as the bile salt concentration increases, probably due to bile salts interacting with each other (Nichols, 1986;Schubert et al, 1986). This is also in agreement with the release of phospholipids; no significant differences in the release of phospholipids from the barriers when comparing the presence of FaSSIF and FeSSIF were observed (Figure 2).…”

“…The bile salt sodium taurocholate, present in both FaSSIF and FeSSIF in different concentrations, is known to interact with the membrane lipids at low concentrations (Jantratid et al, 2008;Schubert et al, 1986). PB pH 6.2, FaB and FeB, which do not contain the bile salt, were therefore included to compare the barrier integrity and further evaluate any possible interaction of FaSSIF and FeSSIF with the PVPAbiomimetic barrier.…”

Membrane/Water Partition Coefficients of Bile Salts Determined Using Laurdan as a Fluorescent Probe

“…By separating free from membrane-associated detergent by ultracentrifugation, equilibrium dialysis, or gel chromatography, one can obtain direct access to the association constants, binding sites, and partition coefficients in intact liposomes and in mixed micelles (18,28,29). Because the data for the membrane-bound BS portion can be acquired directly, we used this method as a reference in this study to evaluate the usefulness of the Laurdan fluorescence method.…”

“…1). The results obtained with Laurdan for Ch, CDC, and GCDC at pH 7.4 were compared with those obtained using a direct method in which membrane-bound BSs were separated from free BSs by ultracentrifugation (18). Furthermore, Laurdan offers the possibility of following the kinetics of BS membrane adsorption and transbilayer movement (19).…”