Protein-independent lead permeation through myelin lipid liposomes

We have investigated the permeability of protein-free myelin lipid liposomes to inorganic lead by using the fluorescent probes fura-2, oxonol V, pyranine, and carboxyfluorescein. Inorganic lead readily crossed the lipid bilayer, as detected with fura-2, to an extent that depended on the external pH and the total nominal lead concentration in the assay medium. Lead entry generated an internally positive transmembrane potential, which could be detected by oxonol V fluorescence quenching, and dissipated a transmembrane pH gradient by alkalinization of the intravesicular space, as measured with pyranine. These results cannot be explained by lead-mediated nonspecific damage to membrane lipids, based on the following results 1) lead exposure did not increase carboxyfluorescein leakage from liposomes, 2) it did not increase the permeability of the lipid bilayer to glucose or KCl, 3) it did not generate peroxidation products in contact with myelin lipids, and 4) it did not induce chemical hydrolysis or modification of any myelin lipid class. We conclude that the principal molecular mechanism of lead permeation through a pure lipid bilayer is the passive diffusion of Pb(OH)+. We discuss the toxicological relevance of these findings for cells in general and for myelin in particular and suggest that this mechanism might contribute significantly to the total lead entry into the cells.