EUROPIUM CHELATE-LOADED LIPOSOMES - A TOOL FOR THE STUDY OF BINDING AND INTEGRITY OF LIPOSOMES

Using the biotin-streptavidin interaction as a model, we investigated the suitability of lanthanide chelates as encapsulated liposomal label s in liposome-based binding assays. Large unilamellar phospholipid:cho lesterol liposomes containing europium-DTPA chelate and biotinylated p hosphatidylethanolamine were prepared by detergent dialysis. The resul ting Eu-liposomes (empty set 120 nm) bound specifically to streptavidi n in microtiter wells as measured by time-resolved fluorometric assay (TRF). The intensity of fluorescence released from the bound liposomes was dependent on the concentration of biotin in the liposome membrane , the concentration of europium entrapped in the liposomes, the incuba tion time and the amount of liposomes used in the assay. The sensitivi ty of the TRF assay allowed the detection of binding of attomole quant ities of liposomes. The streptavidin-immobilised liposomes subjected t o porcine pancreatic phospholipase A(2) (EC 3.1.1.4) and detergents di splayed a dose-dependent release of the encapsulated europium. Lanthan ide-chelate-liposomes should prove useful for studies addressing bindi ng and stability of liposomes.