Cellular iron processing.

Iron is transported in the blood plasma, mainly bound to transferrin, but in abnormal conditions other iron containing compounds may become important. These include ferritin, haemopexin-haem, haptoglobin-haemoglobin and non-specific non-transferrin-bound iron, all of which are taken up from the circulation by the liver. Transferrin-bound iron can be used by all types of cells in amounts that depend on their complement of transferrin receptors. Immature erythroid cells are the most active in this function. Investigations using reticulocytes as an example of erythroid cells have demonstrated the presence of two mechanisms for the uptake of ferrous iron. One, a high affinity process disappears as reticulocytes mature. It probably represents the mechanism by which iron derived from transferrin is transported into the cytosol after receptor-mediated endocytosis of the iron-transferrin complex. The other mechanism has a lower affinity for iron, is retained when reticulocytes mature and is probably associated with Na+ transport across the cell membrane. The physiological characteristics of the two iron transport processes and the evidence for the above conclusions are summarized in the present paper.