The Extent of Transmembrane Phospholipid Movement in Mixed Phosphatidylcholine–Phosphatidylserine Vesicles

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Abstract
A hypothesis of rapid coupled translocation of egg phosphatidylcholine [PC] and bovine brain phosphatidylserine [PS] across a mixed vesicle membrane was tested. Homogeneous preparations of sonicated phospholipid vesicles containing 4 different compositions of the above 2 phospholipids were prepared. Trinitrobenzene-sulfonate derivatization of vesicle-PS was used to determine the outside-inside ratio of this phospholipid. No change in the ratio was observed over a 2-h period. The transmembrane distribution of PC was quantitated using 600 MHz 1H NMR; this distribution was found to be identical aftrer a 24-h incubation at pH 5 or pH 8. Thus, rapid exchange of PC for PS is not observed under the conditions of these experiments.