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 enteroc ytic-like Caco-2 cells was studied as a function of Cd speciation. For low metal concentrations and with a constant free [Cd2+] = 43 nM, initial uptak e rates of Cd-109 increased linearly as a function of increasing concentrat ion of chlorocomplexes (Sigma[(CdCln2-n)-Cd-109]) over the range from 0 to 250 nM. When normalized as a function of the metal concentration, the absor ption rate for the chlorocomplexes was less than that estimated for uptake of the free Cd2+ cation, Metal absorption decreased upon organic ligand add ition in the exposure media, but much less than predicted from the assumpti on that only inorganic metal species would be transported. Under exposure c onditions where the concentration of each of the inorganic species was kept constant, Cd-109 uptake increased with increasing concentrations of cadmiu m glutathione (Cd-109-GSH) or phytochelatin (Cd-109-hmPC(3)) complexes. A s pecific system of very high affinity but low capacity has been characterize d for Cd-109-GSH transport, whereas accumulation data increased linearly wi th Cd-109-hmPC(3) up to 6 muM Comparison among uptake data for 0.3 muM inor ganic Cd-109, Cd-109-GSH, or Cd-109-hmPC(3) yields the following accumulati on ratios: Cd-GSH/Cd-inorg = 0.2; Cd-hmPC(3)/Cd-inorg = 0.5. These results clearly show that Cd2+ is not the exclusive metal species participating in Cd absorption, though, for comparable Cd concentrations, its contribution t o transport would be more important than that of other species. Cadmium bou nd to thiol-containing peptides may be absorbed via transport systems that differ from those involved in absorption of the inorganic metal species. (C ) 2001 Academic Press.