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

The effect of a series of ligands on the binding of the lanthanide, eu ropium (Eu), to rabbit intestinal cell membranes was investigated in v itro. When tested as Eu-ligand complexes (ratio of Eu:ligand, 1:2) of intermediate stability (log stability constant, log K-1, for the react ion Eu + L = EuL, of about 7-12) such as Eu-citrate and Eu-nitrilotria cetate (NTA), Eu was available for uptake in a soluble form by intesti nal brush-border membrane vesicles (BBMV) in phosphate- and bicarbonat e-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 m embranes. Generally, there was a clear relationship between logK(1) of the Eu-ligand complex and the binding of Eu to BBMV. This relationshi p identifies ligands that can effectively donate Eu to vesicles under these conditions. BBMV uptake of Eu was due to binding at two sites. B inding to the diethylenetriaminepentaacetate (DTPA)-sensitive site pre dominated 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 physiologic al concentrations of phosphate and bicarbonate in cell culture medium, Eu was precipitated from most complexes (at 1:2 and 1:5 Eu:ligand rat io) except DTPA and albumin. Eu precipitation could be prevented by in creasing the ligand:Eu ratio. When isolated hepatocytes; in cell cultu re medium were incubated with EuC1(3), about 60% of Eu was bound to th e cells; Err-albumin was not bound by hepatocytes.