Binding of cadmium (Cd2+) to E-CAD1, a calcium-binding polypeptide analog of E-cadherin

Recent studies have shown that Cd²⁺ can damage the Ca²⁺-dependent junctions between renal epithelial cells in culture, and preliminary evidence suggests that this effect may involve the interaction of Cd²⁺ with E-cadherin, a Ca²⁺-dependent cell adhesion molecule that is localized at the adhering junctions of epithelial cells. To determine whether or not Cd²⁺ might bind directly to the E-cadherin molecule, we studied the binding of Cd²⁺ to E-CAD1, a recombinant, 145-residue polypeptide that corresponds to one of the extracellular Ca²⁺-binding regions of mouse E-cadherin. By using an equilibrium microdialysis technique, we were able to show that Cd²⁺ could, in fact, bind to E-CAD1. The binding was saturable, with a maximum of one Cd²⁺ binding site per E-CAD1 molecule. The apparent dissociation constant (K_D) for the binding was about 20 μM, a concentration similar to that which has been shown to disrupt the junctions between epithelial cells. Other results showed that the binding of Cd²⁺ was greatly reduced when excess Ca²⁺ was included in the dialysis solution. These results suggest that Cd²⁺ can interact with the Ca²⁺ binding regions on the E-CAD1 molecule, and they provide additional support for the hypothesis that E-cadherin might be a molecular target for Cd²⁺ toxicity.