Fig. 2

Enterocyte iron absorption. The critical entry transporter on the apical surface of the enterocyte is DMT1. Given that DMT1 is specific for Fe²⁺ (as opposed to Fe³⁺), Fe³⁺ must be reduced prior to or concomitant with entry. The ferrireductase, duodenal cytochrome B (Dcytb), accomplishes this but does not appear to be an absolute requirement. Subsequently, iron must get to where it exits the cell, a process celled transcytosis (dashed arrow). The exit transporter, ferroportin (Fpn, but also called MTP1, Ireg1 or Slc40), probably acts on Fe²⁺ but reoxidation to Fe³⁺ by hephaestin (Hp) or ceruloplasmin (Cp) as well as binding of the resultant Fe³⁺ to apo-transferrin (Tf) could occur in a relatively concerted fashion. The Tf receptor (TfR1) is probably also a part of the exit process although its role here, the reverse of the Tf cycle (below), is not yet well defined. Also yet to be well defined is the role of DMT1 (here shown with a ?) in possible basolateral uptake of iron noted in basolateral membrane vesicles [24] and recently supported [37]. Núñez et al. also propose that Fpn can allow iron to exit back into the lumen of the GI tract (also shown here with a ?). The figure is modified from Fig. 1 of the earlier review [14] with permission