Inorganic mercury (Hg2+) transport through lipid bilayer membranes

Summary Diffusion of inorganic mercury (Hg²⁺) through planar lipid bilayer membranes was studied as a function of chloride concentration and pH. Membranes were made from egg lecithin plus cholesterol in tetradecane. Tracer (²⁰³Hg) flux and conductance measurements were used to estimate the permeabilities to ionic and nonionic forms of Hg. At pH 7.0 and [Cl^–] ranging from 10–1000mm, only the dichloride complex of mercury (HgCl₂) crosses the membrane at a significant rate. However, several other Hg complexes (HgOHCl, HgCl₃^– and HgCl₄^2–) contribute to diffusion through the aqueous unstirred layer adjacent to the membrane. The relation between the total mercury flux (J_Hg), Hg concentrations, and permeabilities is: 1/J_Hg=1/P^ul[Hg^t]+1/P^m[HgCl₂], where [Hg^t] is the total concentration of all forms of Hg,P^ul is the unstirred layer permeability, andP^m is the membrane permeability to HgCl₂. By fitting this equation to the data we find thatP^m=1.3×10^–2 cm sec^–1. At Cl^– concentrations ranging from 1–100mm, diffusion of Hg^t through the unstirred layer is rate limiting. At Cl^– concentrations ranging from 500–1000mm, the membrane permeability to HgCl₂ becomes rate limiting because HgCl₂ comprises only about 1% of the total Hg. Under all conditions, chemical reactions among Hg²⁺, Cl^– and/or OH^– near the membrane surface play an important role in the transport process. Other important metals, e.g., Zn²⁺, Cd²⁺, Ag⁺ and CH₃Hg⁺, form neutral chloride complexes under physiological conditions. Thus, it is likely that chloride can facilitate the diffusion of a variety of metals through lipid bilayer and biological membranes.