Reciprocal inhibition of Cd and Pb sulfocomplexes for uptake in Caco‐2 cells

Cadmium‐lead interactions for uptake were studied in the TC7 clone of human enterocytic‐like Caco‐2 cells as a function of inorganic metal speciation. We have previously shown that Cd uptake in these cells involves both the free cation Cd 2+ and chlorocomplex (CdCl   n 2− n ) species. Here we show 1.9 times higher uptake levels for 109 CdCl   n 2− ncompared to 210 PbCl   n 2− n . Reciprocal inhibitions of chlorocomplexes were observed with a much higher inhibitory effect of Cd compared to Pb. Replacing Cl − by NO   3 −increased both the level of aquo ion 109 Cd 2+ and 109 Cd accumulation. In contrast, higher levels of 210 Pb 2+ did not favor 210 Pb uptake. For both metals, higher uptake data were recorded in the presence of SO 4 2− , leading to sulfocomplex formation, compared with Cl − . Reciprocal inhibitions were minimal at high‐cation levels but were significant and comparable in the presence of sulfo‐complexes. We conclude that, in addition to Cd 2+ (but not Pb 2+ ), sulfocomplexes of both metals would preferentially be taken up compared to chlorocomplexes. NRAMP2 is not involved in Pb 2+ uptake, and the NRAMP2‐mediated Cd 2+ uptake is insensitive to Pb. Uptake of Pb chlorocomplexes could involve specific mechanisms but of very low affinity, whereas uptake of Pb sulfocomplexes occurs with high affinity. © 2005 Wiley Periodicals, Inc. J Biochem Mol Toxicol 19:256–265, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/jbt.20085