Inorganic mercury (Hg2+) transport through lipid bilayer membranes

SummaryDiffusion 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 HgCl42−) contribute to diffusion through the aqueous unstirred layer adjacent to the membrane. The relation between the total mercury flux (JHg), Hg concentrations, and permeabilities is: 1/JHg=1/Pul[Hgt]+1/Pm[HgCl2], where [Hgt] is the total concentration of all forms of Hg,Pul is the unstirred layer permeability, andPm is the membrane permeability to HgCl2. By fitting this equation to the data we find thatPm=1.3×10−2 cm sec−1. At Cl− concentrations ranging from 1–100mm, diffusion of Hgt 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.