INTERACTION OF FREE FATTY-ACIDS WITH PHOSPHOLIPID-BILAYERS

The partition of free fatty acids (FFA) to egg-phosphatidylcholine (eg g-PC) and egg-phosphatidylethanolamine (egg-PE) vesicles was studied. Upon the addition of FFA to the suspension of vesicles, the pH of the aqueous phase changed depending on the length and saturation of the FF A hydrocarbon chain, as well as on the vesicle composition. The medium pH decreased faster if FFA was added to egg-PE as compared to egg-PC vesicles. The fluorescent free fatty acid indicator (ADIFAB) was used to measure the amount of FFA remaining in the aqueous phase. Most of t he FFA added to the suspension of egg-PE vesicles remained in the aque ous phase, whereas in the presence of egg-PC vesicles the FFA partitio ned preferentially into the lipid phase. The amount of FFA incorporate d into the lipid bilayers was estimated by measuring the changes of pH at the lipid bilayer surface, using fluorescein-PE. At high surface c oncentrations of FFA, decreasing pH at the bilayer surface caused the protonation of FFA, and raised the pK of FFA at the bilayer surface fr om 5 to about 7. The partition of FFA in egg-PE vesicles was an order of magnitude lower than that in egg-PC vesicles. The incorporation amo unt was determined more by the molecular packing than by the nature of lipid headgroups, because steroylcaprioyl-PE, which preferred the bil ayer structure, behaved more like egg-PC than egg-PE. Understanding FF A partition characteristics would help to interpret the hydrolysis mea surements of phospholipids, and to explain many biological activities of FFA.