Calcium ion fluorescence detection using liposomes containing Alexa-labeled calmodulin.

This paper describes the preparation and characterization of calcium ion sensitive fluorescent liposomes and their application for the determination of calcium ions in aqueous samples. Calmodulin (CaM), a calcium ion-binding protein labeled with the fluorophore Alexa-488 is embedded in the membrane of unilamellar liposomes. Upon calcium ion binding, calmodulin undergoes a conformational change that exposes its hydrophobic core and affects the fluorescence intensity of the attached fluorophore. Characterization studies of Alexa-CaM-containing liposomes reveal that embedding calmodulin molecules in the bilayer membrane of liposomes extends the lifetime of the calcium ion binding activity of calmodulin by about fourfold compared to the lifetime of its calcium-binding activity in free solution. Moreover, the calcium ion response of Alexa-CaM-containing liposomes is about threefold higher than the calcium ion response of Alexa-CaM in solution. The improvement in the calcium ion detection properties is attributed to the interaction between calmodulin, a membranal protein, and the hydrophobic phospholipids of the liposomes. The analytical properties of the calcium ion sensitive fluorescent liposomes are discussed.