Hg2+ and Cd2+ interact differently with biomimetic erythrocyte membranes

In order to characterize the potentially deleterious effects of toxic Hg(2+) and Cd(2+) on lipid membranes, we have studied their binding to liposomes whose composition mimicked erythrocyte membranes. Fluorescence spectroscopy utilizing the concentration dependent quenching of Phen Green SK by Hg(2+) and Cd(2+) was found to be a sensitive tool to probe these interactions at metal concentrations < or =1 microM. We have systematically developed a metal binding affinity assay to screen for the interactions of Hg(2+) or Cd(2+) with certain lipid classes. A biomimetic liposome system was developed that contained four major lipid classes of erythrocyte membranes (zwitterionic lipids: phosphatidylcholine and phosphatidylethanolamine; negatively charged: phosphatidylserine and neutral: cholesterol). In contrast to Hg(2+), which preferentially bound to the negatively charged phosphatidylserine compared to the zwitterionic components, Cd(2+) bound stronger to the two zwitterionic lipids. Thus, the observed distinct differences in the binding affinity of Hg(2+) and Cd(2+) for certain lipid classes together with their known effects on membrane properties represent an important first step toward a better understanding the role of these interactions in the chronic toxicity of these metals.