Hg- and Cd-induced modulation of lipid packing and monolayer fluidity in biomimetic erythrocyte model systems.

The public health consequences that are associated with the low level exposure of various human populations to Cd(2+) and Hg(2+) are incompletely understood. In order to assess if interactions between these inorganic pollutants and erythrocyte biomembranes may contribute to their chronic toxicity, we have used a Langmuir trough to probe the effect of HgCl2 and CdCl2 on the packing and elasticity properties of biomimetic lipid monolayers using different lipid mixtures. These lipid films were deposited at room temperature on a biologically relevant subphase (1mM phosphate, 100mM NaCl at pH 7.4) in the absence and presence of 100μM HgCl2, CdCl2 and 1:1 mixtures thereof. The interactions of heavy metals with the lipids were monitored as changes in the surface pressure (π)-area (A) isotherms. In addition, metal induced changes to the elastic properties of the model systems were analyzed by area and compressibility data of phosphatidylcholine (PC) systems containing 0, 15, 30, 45 and 100% phosphatidylethanolamine (PE) and phosphatidylserine (PS). These mixtures revealed changes in lateral lipid packing as indicated by area expansion as well as enhanced film rigidity. The results demonstrate that both heavy metals affected the various lipid matrices, but metal mixtures showed the strongest impact. Based on these data, the adverse interaction of Hg(2+) and Cd(2+) with lipid bilayer membranes is identified as a feasible mechanism by which these toxic metals exert toxicity in mammalian cells. Interestingly, these metal interactions were found to depend on the lipid composition.