Covalent attachment of horseradish peroxidase to the outer surface of liposomes

We describe a method by which horseradish peroxidase may be attached covalently to the surface of liposomes under conditions which permit minimal non-covalent association of the enzyme with the lipids. The coupling method adopted does not allow the formation of homopolymers of liposomes or peroxidase. For phosphatidylethanolamine/phosphatidylcholine and stearylamine/phosphatidylcholine vesicles, minimal disruption of vesicular structure is observed, whilst for phosphatidylserine vesicles, the lipid-protein complex appears to form structures much smaller than 25 nm in diameter. Stearylamine/phosphatidylcholine vesicles have been shown to retain entrapped inulin, and activity measurements for the peroxidase suggest that it is located exclusively on the external surface of the liposome membrane. Peroxidase can be localized histochemically which has permitted the morphological study of the coated liposomes and their interactions with cells.