pH-sensitive liposomes: acid-induced liposome fusion.

Abstract

Sonicated unilamellar liposomes [as all membrane models] containing phosphatidylethanolamine and palmitoylhomocysteine fuse rapidly when the medium pH is lowered from 7 to 5. Liposome fusion was demonstrated by mixing of the liposomal lipids as shown by resonance energy transfer, gel filtration and EM. The pH-sensitive fusion of liposomes was observed only when palmitoylhomocysteine (.gtoreq. 20 mol %) was present in the liposomes. Presence of phosphatidylethanolamine in the liposomes greatly enhanced fusion; presence of phosphatidylcholine inhibited fusion. During fusion of liposomes containing phosphatidylethanolamine and palmitoylhomocysteine (8:2, mol/mol), almost al of the encapsulated calcein was released. Inclusion of cholesterol (40 mol %) in the liposomes substantially decreased leakage without impairing fusion.