Figure 3 | Cellular iron transport.   There are four cell types that have special functions in iron handling. a | Duodenal enterocytes absorb iron from the diet. Non-haem iron is reduced by a ferric reductase in the brush border and is transported into the cell through the transmembrane iron transporter DMT1 (for divalent metal transporter 1). Some iron is stored within the cell in ferritin; the remainder must pass through the basolateral membrane to reach the plasma. An iron exporter, ferroportin1, probably carries out basolateral iron transfer in cooperation with hephaestin, a possible ferroxidase. Hephaestin is homologous to the plasma multicopper oxidase ceruloplasmin (CP), and might have a function analogous to that of CP in iron export from other cells. Absorbed iron is loaded onto apotransferrin (APO-TF) to give holotransferrrin (HOLO-TF) through a mechanism that is not yet understood. b | Erythroid precursors take up iron through the transferrin cycle, as described in Fig. 2. Erythroid cells probably have no iron-export mechanism; essentially all iron in these cells is incorporated into haemoglobin. c | Hepatocytes take up iron through at least two distinct pathways. They have a functional transferrin cycle and a transport system to take up non-transferrin-bound iron. The molecules important for non-transferrin-bound iron transport have not yet been identified. Hepatocytes store iron in ferritin. When iron is needed elsewhere in the body, they can release it to transferrin. The mechanism of hepatocyte export is not known, but it may involve ferroportin1. CP seems to aid in iron export from hepatocytes, but its precise function has not yet been defined. d | Reticuloendothelial macrophages carry out iron recycling. They ingest senescent red blood cells (RBC) and lyse them in a phagolysosomal compartment. Haemoglobin is degraded and iron is liberated from haem. The enzyme haem oxygenase may participate in this process. Iron is then exported through the cell. The mechanism of macrophage iron export is not known, but may again involve ferroportin1 and CP, similar to iron export from hepatocytes. (TFR, transferrin receptor.)

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