Use of the Caco-2 cell model to assess the relative lead-chelating ability of diasterioisomers of 2,3-dimercaptosuccinic acid.

The purpose of this study was to examine the mechanisms of lead (Pb) uptake by human intestinal cells and to compare the intestinal transport and relative lead-chelating ability of two diastereoisomeric forms (i.e., meso and racemic) of 2, 3-dimercaptosuccinic acid (DMSA). The model used was the human adenocarcinoma (Caco-2) cell monolayer. The Caco-2 cells were cultured in flasks for examination of cellular uptake of lead and subsequent chelation of the lead by the DMSA isomers. For assessment of the comparative intestinal transport of the diastereoisomers, the Caco-2 cells were cultured on semipermeable supports. The effects of N-ethylmaleimide and 1,25-dihydroxyvitamin D3 (vitamin D3) on the uptake of lead by the Caco-2 monolayer were examined to determine the contributions of sulfhydryl-binding and calcium-binding protein, respectively, to the lead uptake process. Analysis of lead was performed using both macro- and micro-proton-induced X-ray emission (PIXE), and DMSA was measured spectrophotometrically following derivatization with 5,5'-dithiobis-2-nitrobenzoic acid. Results from micro-PIXE imaging suggest that lead is bound on the surface of the cell, and that sulfhydryl binding may be an important step in the uptake of lead by the Caco-2 cells. Macro-PIXE results indicate that the racemic form of DMSA may be more effective in chelating lead from within the cell. Comparison of the transport of the two DMSA diastereoisomers indicates that the racemic form is transported across the Caco-2 monolayer more readily than the meso form.