The effect of sub-solubilizing concentrations of sodium deoxycholate on the order of acyl chains in dipalmitoylphosphatidylcholine (DPPC). Study of the fluorescence anistropy of 1,6-diphenyl-1,3,5-hexatriene (DPH)

Abstract

The destabilization of phosphatidylcholine bilayer membranes by the bile salt sodium deoxycholate was studied from steady-state fluorescence anisotropy measurements. FATVET liposomes of 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 rotational diffusion motion in the DPH molecules, are perturbed by the presence of the 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 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 _∞) and dynamic component (r _d) shows 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°C, angle ϑ, a measure of the amplitude of DPH oscillations, rises from 23° to 52° over the DOC concentration range from zero to 2.0 mM (equivalent to a effective molar ratio in bilayer of R ^25°₃ = 0.12); at 45°C, however, it varies from 62° to 77°. These changes in the bilayer packing status my 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. (submitted)]