Binding Affinity of Inorganic Mercury and Cadmium to Biomimetic Erythrocyte Membranes

Inorganic mercury and cadmium are becoming increasingly prevalent due to industrial activity and have been linked to cardiovascular disease and diabetes. The binding affinity of Hg, Cd, and their mixtures to biomimetic erythrocyte membranes was investigated by isothermal titration calorimetry in physiologically relevant media (100 mM NaCl, pH 7.4, 37 °C). The thermodynamic parameters were not expressed per mole of lipid but as metals binding to liposomes. To our knowledge, this method is novel and provides a more intuitive approach to understand such interactions. The results demonstrated that Hg interacted with membranes in the following order: PC (phosphatidylcholine) > 85:15 PC/PE (phosphatidylethanolamine) > 85:15 PC/PS (phosphatidylserine), with the binding constants ranging from 10 to 233 M–1. In contrast, Cd interacted most readily with negatively charged PC/PS membranes but not with the remaining systems. Metal mixtures bind less to PC/PE membranes than the individual constituents. The large entropy contribution from these interactions suggests possible water release and/or reorganization upon Hg and Cd binding to membranes. ζ-Potential data indicate that the process may be electrostatically driven. It is imperative to consider the chemical speciation of these metals in the presence of chloride to better understand metal–lipid interactions and their impact on biomembranes.