Interaction of cadmium (Cd(2+)) with a 13-residue polypeptide analog of a putative calcium-binding motif of E-cadherin.

Previous studies from our laboratory have shown that Cd(2+) can selectively damage the tight junctions between epithelial cells in culture. Recently, we have obtained evidence suggesting that this effect may involve the interaction of Cd(2+) with E-cadherin, a Ca(2+)-dependent cell adhesion molecule that is localized at the adhering junctions of epithelial cells. To begin to determine whether or not Cd(2+) might interact directly with the E-cadherin molecule, we studied the binding of Cd(2+) to peptide B, a synthetic, 13-residue polypeptide that corresponds to one of the extracellular Ca(2+) binding regions of mouse E-cadherin (also known as uvomorulin). The binding of Cd(2+) to peptide B was evaluated by using an equilibrium microdialysis technique and the radioactive isotope (109)Cd(2+). The effects of the binding on the conformation of the peptide were evaluated by circular dichroism (CD) spectroscopy. The results showed that Cd(2+) bound to peptide B, with a maximum of one Cd(2+) binding site per molecule and an apparent dissociation constant (K(d)) of 640 microM. The binding of Cd(2+) was reduced in the presence of excess Ca(2+), an effect that was overcome by raising the concentration of Cd(2+). Both Cd(2+) and Ca(2+) caused a shift in the CD spectrum of the peptide. However, the shift produced by Cd(2+) was about 3 times the magnitude of that produced by Ca(2+). These results indicate that Cd(2+) can interact with the Ca(2+) binding site on the peptide B molecule and distort the secondary structure of the peptide. These findings are consistent with the hypothesis that E-cadherin may be a direct molecular target for Cd(2+) toxicity.