On the mechanism of transfer of heavy metals across cell membranes.

In rat jejunal mucosa cells, Cd bound to the outside of the brush border can be distinguished from internalized Cd by its reaction with extracellular chelators; internalized Cd is only released after destruction of the cell. Similar results have been obtained with rabbit erythrocytes and resealed ghost cells. A temperature-dependent process carries Cd from the EDTA-sensitive into the EDTA-insensitive compartment. In enteric cells, only 1/3 of the external binding sites permit subsequent internalization. EDTA does not remove Hg from cell ligands, but internalized Hg can be distinguished from externally bound Hg with the help of dimercaptopropanol; internalization of Hg bound to active membrane sites must be relatively rapid. Internalization of Cd and Hg by ghost cells and by isolated membrane vesicles, as described elsewhere, suggests that the process is independent of pinocytosis. The results therefore support a previously suggested model describing the movements of non-essential and poisonous metals across cell membranes in terms of a non-specific binding step, followed by what may represent the activity of mobile carriers.