Lipid domains in membranes. Evidence derived from structural perturbations induced by free fatty acids and lifetime heterogeneity analysis.

The interaction of free fatty acids with cell membranes and lipid bilayers was studied by monitoring the emission polarization changes of the fluorescent probes 1,6-diphenyl-1,3,5-hexatriene (DPH) and 8-anilino-1-naphthalene sulfonate (ANS). It was found that cis unsaturated fatty acids incorporated into plasma membranes reduced DPH and increased ANS polarization, while trans-unsaturated and saturated fatty acids had no effect on DPH, but did increase ANS polarization. Although in fluid lipid vesicles, both types of fatty acids decreased DPH polarization, in mixed phase vesicles (dilauroyl phosphatidylcho1ine:dipalmitoy1 phosphatidylcholine) polarization changes of ANS and DPH were similar to that observed in plasma membranes. Alteration in the dimyristoyl phosphatidylcholine phase transition as a function of fatty acid type was used to study the fluid-solid partition behavior in lipid bilayers. Using solution theory, the shift in the transition midpoint temperature was used to infer that the el-unsaturated fatty acids partition into fluid domains while the tmns-unsaturated and saturated fatty acids preferentially partition into solid-like domains. The fluorescent lifetime of DPH in membranes and vesicles was measured by the phase-modulation technique. The results were analyzed in terms of site heterogeneity and indicate that while DPH decay in single phase vesicles is monoexponential, a considerable degree of heterogeneity is observed in mixed phase vesicles and plasma membranes. In particular, evidence is obtained for a DPH binding site which corresponds to the interface between lipid domains. These results, together with the free fatty acid effects, are interpreted in terms of lipid domains in membranes.