Cadmium Uptake by Caco-2 Cells: Effects of Cd Complexation by Chloride, Glutathione, and Phytochelatins

Short-term cadmium uptake by the highly differentiated TC7 clone of enterocytic-like Caco-2 cells was studied as a function of Cd speciation. For low metal concentrations and with a constant free [Cd 2+ ] = 43 nM, initial uptake rates of 109 Cd increased linearly as a function of increasing concentration of chlorocomplexes (Σ[ 109 CdCl 2−n n ]) over the range from 0 to 250 nM. When normalized as a function of the metal concentration, the absorption rate for the chlorocomplexes was less than that estimated for uptake of the free Cd 2+ cation. Metal absorption decreased upon organic ligand addition in the exposure media, but much less than predicted from the assumption that only inorganic metal species would be transported. Under exposure conditions where the concentration of each of the inorganic species was kept constant, 109 Cd uptake increased with increasing concentrations of cadmium glutathione ( 109 Cd–GSH) or phytochelatin ( 109 Cd–hmPC 3 ) complexes. A specific system of very high affinity but low capacity has been characterized for 109 Cd–GSH transport, whereas accumulation data increased linearly with 109 Cd–hmPC 3 up to 6 μM. Comparison among uptake data for 0.3 μM inorganic 109 Cd, 109 Cd—GSH, or 109 Cd–hmPC 3 yields the following accumulation ratios: Cd–GSH/Cd inorg = 0.2; Cd–hmPC 3 /Cd inorg = 0.5. These results clearly show that Cd 2+ is not the exclusive metal species participating in Cd absorption, though, for comparable Cd concentrations, its contribution to transport would be more important than that of other species. Cadmium bound to thiol-containing peptides may be absorbed via transport systems that differ from those involved in absorption of the inorganic metal species.