Deoxycholate alters the order of acyl chains in freeze-thaw extrusion vesicles of L-alpha-dipalmitoyl phosphatidylcholine: Study of the 1,6-diphenyl-1,3,5-hexatriene steady-state fluorescence anisotropy

The destabilization of phosphatidylcholine bilayer membranes by the bile salt sodium deoxycholate (DOG) was studied from steady-state fluorescence anisotropy measurements. Freeze-thaw extrusion vesicles (FATVETs) of L-alpha-dipalmitoyl phosphatidylcholine (DPPC) composition were prepared by sequential extrusion through polycarbonate membranes and characterized for their overall inner volume, average size and size distribution, and lamellarity. Interactions between acyl chains in the lipid matrix, which reflect in the rotational diffusion motion in the 1,6-diphenyl-1,3,5-hexatriene (DPH) molecules, are perturbed by the presence of bile salt in the medium (even at low concentrations) below and above the main transition phase temperature of pure DPPC bilayers. Its effects on the lipid matrix are clearly reflected in the DPH steady-state fluorescence anisotropy (r(s)) measurements. The resolution of r(s) into its static (r(infinity)) and dynamic component (r(d)) show that DOC affects both the amplitude and the velocity of DPH movements. However, at temperatures below the gel <-> liquid crystal phase transition point, the static component (that reflects chain order) is more markedly affected than is the dynamic component (which reflects bilayer fluidity). Thus, at 31 degrees C, angle theta, a measure of the amplitude of DPH oscillations, rises from 23 degrees to 52 degrees over the DOC concentration range from zero to 2.0 mM (equivalent to an effective molar ratio in the bilayer of R(e)(25 degrees) = 0.12); at 45 degrees C, however, it varies from 62 degrees to 77 degrees. These changes in the bilayer packing status may be responsible for the alteration in the retention ability of liposomal formulations of the cytostatic agent 5-fluorouracil at sub-solubilizing concentrations of deoxycholate (Elorza et al., J. Pharm. Sci., 1996, in press).