Hg2+ and Cd2+ interact differently with biomimetic erythrocyte membranes

In order to characterize the potentially deleterious effects of toxic Hg²⁺ and Cd²⁺ 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 Hg²⁺ and Cd²⁺ was found to be a sensitive tool to probe these interactions at metal concentrations ≤1 μM. We have systematically developed a metal binding affinity assay to screen for the interactions of Hg²⁺ or Cd²⁺ 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 Hg²⁺, which preferentially bound to the negatively charged phosphatidylserine compared to the zwitterionic components, Cd²⁺ bound stronger to the two zwitterionic lipids. Thus, the observed distinct differences in the binding affinity of Hg²⁺ and Cd²⁺ 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.