The transport of copper

In summary, one concern of copper transport is to identify the serum component(s) that deliver copper to cells. Another, to learn how copper ions penetrate cells and engage enzymes. Ceruloplasmin and albumin have received major attention as transport agents but there could be others. Copper transport operates within a system that is sensitive to copper concentrations and poised to engage copper ions should they build up to toxic levels. Uptake studies with cultured cells suggest the presence of specific membrane channels for passing copper ions and specific binding proteins that work against concentration barriers to deliver the copper internally. Diffusion, aqueous-phase pinocytosis, may also play roles in transporting copper into cells. Most work to date supports the existence of specific transporters in the membrane, yet no such carriers have been isolated or characterized outside the environment of the cell. Model system work has opened up new and intriguing ideas that link membrane potential to transmembrane transport. Studies with lower life forms have confirmed that there may be other copper transport mechanisms functional in cells. Finally, we must recognize that nearly all the copper ions in fluids and cells exists in a protein-bound form and, to a much lesser extent, as complexes with amino acids. The metal must dissociate from its protein environment to be functional. Amino acids may perform this function with albumin-bound copper. Cells, however, appear also to have membrane receptors for binding ceruloplasmin as well as mechanisms for reducing and dissociating the copper atoms bound. In this way cells are able to tap the richest excellular source of copper for metabolic purposes. We have yet to understand the finer details of that mechanism.