Effect of pH on cholesterol retention by multilamellar vesicles

Multilamellar vesicles (MLV) were prepared from 1 mg of dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC) or dipalmitoylphosphatidylethanolamine (DPPE) with equimolar amounts of cholesterol (C) labeled with 4-14C-cholesterol in phosphate buffers from pH 5.8 to 7.8, by agitation with a glass bead. Suspensions of MLV were extracted 3 times with petroleum ether (PE). C remaining in the aqueous phase was determined. Increases in pH caused small, but reproducible, increases in the amount of C retained by DPPE MLV doubled from 220 μg to 450 μg. A plot of C retention over the pH range studied showed a rise beginning at pH 6.2 until a maximum was reached at pH 7.0. Sonicated vesicles, which are smaller than MLV prepared by agitation, retain maximum amounts of C and do not show a similar pH effect. The isoelectric point for both choline and ethanolamine phospholipids has been reported to be pH 6.7. The net charge on phospholipid head groups appears to influence retention of C by MLV made from phospholipids with saturated acyl side chains. The effect of pH on C retention was not observed with MLV prepared from phospholipids with unsaturated side chains