Exchange of phosphatidylcholine between small unilamellar liposomes and human plasma high-density lipoprotein involves exclusively the phospholipid in the outer monolayer of the liposomal membrane.

By making use of the capacity of phospholipase A2 to degrade selectively the phospholipid in the outer half of the lipid bilayer of small unilamellar phospholipid/cholesterol vesicles without affecting the retention of a vesicle-encapsulated solute, we demonstrated that the exchange of phosphatidylcholine between such vesicles and human high density lipoprotein involves exclusively the phosphatidylcholine present in the outer monolayer of the vesicle membrane.