Iron uptake and metabolism by hepatocytes.

The hepatocytes form part of the iron storage system of the body. In serving this function they exchange iron bidirectionally with the plasma iron transport protein transferrin (Tf). Iron uptake involves binding of the iron-Tf complex to cell membrane receptors and endocytosis into low-density vesicles, where the iron is released from its carrier protein before the Tf is returned undegraded to the extracellular medium. Two components of the iron uptake process can be distinguished, one saturable at low concentrations of diferric Tf and the other not saturable by increasing the Tf concentration. Both result in net uptake of iron by the cells and both appear to depend on specific binding to the cell membrane and endocytosis. Hepatocytes also obtain some iron from haptoglobin-hemoglobin, heme-hemopexin, and ferritin (Fn), in each case by interaction with membrane receptors and endocytosis. Within the cell iron from all sources enters one or more transit pools, where it is available for exchange with the iron storage protein Fn, and for release from the cell to plasma Tf or to iron chelators administered therapeutically or experimentally. Chelator-mediated iron release occurs to the plasma and/or to the bile, depending on the nature of the chelator and the source of the iron.