Mechanism of ionophoric transport of indium-111 cations through a lipid bilayer membrane

The use of mobile ionophores to facilitate the transport of 111In through a lipid bilayer membrane has broad applications in liposome technology and cell labeling. However, the mechanism of such ionophore-mediated transport of 111In through a lipid bilayer membrane is not completely clear. The present report describes the correlations of the behaviors of ionophoric loading of 111In into liposomes with the lipophilicity and the indium-binding affinity of three ionophores, namely, 8-hydroxyquinoline, acetylacetone, and tropolone. Our results suggest that the mechanism of the ionophoric transport of 111In through a lipid bilayer membrane involves the rapid exchange of 111In cations among the ionophores in both the aqueous solution and the lipid bilayer. Furthermore, the effectiveness of an ionophore in facilitating the transport of 111In from the external aqueous compartment to the entrapped nitrilotriacetic acid depends not only on the lipophilicity of the [111In]ionophore complex, but also on the lipophilicity of the free ionophore itself and the competition of 111In between nitrilotriacetic acid inside the inner aqueous compartment of the liposome and the ionophore imbedded in the lipid bilayer membrane of the liposome