BINDING OF LANTHANIDES TO CELL-MEMBRANES IN THE PRESENCE OF LIGANDS

The effect of a series of ligands on the binding of the lanthanide, europium (Eu), to rabbit intestinal cell membranes was investigated in vitro. When tested as Eu-ligand complexes (ratio of Eu:ligand, 1:2) of intermediate stability (log stability constant, log K-1, for the reaction Eu + L = EuL, of about 7-12) such as Eu-citrate and Eu-nitrilotriacetate (NTA), Eu was available for uptake in a soluble form by intestinal brush-border membrane vesicles (BBMV) in phosphate- and bicarbonate-free solutions at pH 7.2. Ligands with lower log K, did not maintain Eu in solution whilst those of higher affinity did not donate it to membranes. Generally, there was a clear relationship between logK(1) of the Eu-ligand complex and the binding of Eu to BBMV. This relationship identifies ligands that can effectively donate Eu to vesicles under these conditions. BBMV uptake of Eu was due to binding at two sites. Binding to the diethylenetriaminepentaacetate (DTPA)-sensitive site predominated at 20 degrees C and uptake by the DTPA-insensitive site was enhanced at 37 degrees C. Only trace amounts of the bound Eu appeared to be internalized within the vesicles. In the presence of physiological concentrations of phosphate and bicarbonate in cell culture medium, Eu was precipitated from most complexes (at 1:2 and 1:5 Eu:ligand ratio) except DTPA and albumin. Eu precipitation could be prevented by increasing the ligand:Eu ratio. When isolated hepatocytes; in cell culture medium were incubated with EuC1(3), about 60% of Eu was bound to the cells; Err-albumin was not bound by hepatocytes.