Kinetics of calcium-induced mixing of lipids and aqueous contents of large unilamellar phosphatidylserine vesicles |
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Authors: | Leonid Ababei Knut Hildenbrand |
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Institution: | Max-Planck-Institut für Strahlenchemie, D-433 Mülheim/Ruhr F.R.G. |
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Abstract: | Calcium ion-induced fusion events in suspensions of large unilamellar phosphatidylserine (PS) liposomes were monitored by fluorescence methods. Mixing of vesicle contents was studied by measuring the increase in terbium emission intensity due to formation of a complex between Tb3+ ions and dipicolinic acid trapped in the liposomes. Lipid redistribution was determined with the aid of the resonance transfer of excitaton energy using dipalmitoylphosphatidylethanolamine labelled with the donor N-(7-nitro-2,1,3-benzoxadiazol-4-yl) or the acceptor tetramethylrhodamine at the free amino group. The two methods yielded significantly different results. While recombination of contents could not be detected at Ca2+ concentrations below 2.5 mM the threshold concentration for lipid mixing was 1 mM. For saturating Ca2+ concentrations (>5 mM Ca2+) initial rates were higher by almost an order of magnitude for lipid mixing than for recombination of liposome contents. These observations indicate that the observation of rapid lipid mixing phenomena does not allow one to draw conclusions as to the fate of the enclosed volumes. |
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Keywords: | membrane fusion calcium fluorescence DPA dipicolinic acid sodium salt EDTA ethylenediaminetetraacetic acid LUV large unilamellar vesicles PG phosphatidylglycerol PS phosphatidylserine RET resonance energy transfer SUV small unilamellar vesicles Tes To whom correspondence should be addressed |
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