The extent of transmembrane phospholipid movement in mixed phosphatidylcholine-phosphatidylserine vesicles

A hypothesis of rapid coupled translocation of egg phosphatidylcholine and bovine brain phosphatidylserine across a mixed vesicle membrane was tested. Homogeneous preparations of sonicated phospholipid vesicles containing four different compositions of the above two phospholipids were prepared. Trinitrobenzene-sulfonate derivatization of vesicle-phosphatidylserine was used to determine the outside-inside ratio of this phospholipid. No change in the ratio was observed over a 2-hr period. The transmembrane distribution of phosphatidylcholine was quantitated using 600 MHz 1H NMR; this distribution was found to be identical after a 24-hr incubation at pH 5 or pH 8. Thus, rapid exchange of phosphatidylcholine for phosphatidylserine is not observed under the conditions of these experiments.