Table 1 Potential and actual human iron transporters

DMT1

Divalent metal (ion) transporter

It is the major duodenal importer of ferrous iron and other metal ions; it is also responsible for exit of ferrous iron from endosomes during the Tf cycle; this transporter also participates in Tf-independent iron entry into cells (non-Tf-bound iron uptake) and possibly in transcytosis of iron across enterocytes

DCT1

Divalent cation transporter

Nramp2

Natural resistance–associated macrophage protein 2

Slc11a2

Solute carrier 11a2

Fpn

Ferroportin

It is the only known cellular iron exporter in mammals; this distinction is physiologically important even if the ability of DMT1 to support endosomal iron exit makes it also a semantic point

MTP1

Metal transporter protein

Ireg1

Iron-regulated transporter

Slc40

Solute carrier 40

Nramp1

Natural resistance–associated macrophage protein 1

Identified initially by the phenotype of resistance to many microbial infections, this transporter probably participates in the exit of iron and manganese from phagosomes

Tf

Transferrin

It carries ferric iron in the plasma, lymph, and cerebrospinal fluid behaving as an iron transporter in conjunction with TfR1

TfR1

Tf receptor 1

It binds Tf to deliver iron into cells with the transport process due to receptor-mediated endocytosis

TfR2

Tf receptor 2

It may also deliver Tf-bound iron and even iron not bound to Tf to some cells

Ft

Ferritin

It is primarily an iron storage protein, but H ft may also participate in iron transport by binding to TfR1

Zip14

Zrt- and Irt-like

Although it was initially identified as a Zn transporter, Zip 14 is responsible for at least a part of iron not bound to Tf uptake in the liver. More speculatively, Zip8 may also be an iron transporter

Slc39a14

Protein 14

Ngal 24p3 lipocalin(-2) uterocalin

Neutrophil gelatinase-associated lipocalin

This protein binds iron bound to an internal siderophore that is yet to be identified. It and its receptor are involved in iron withholding during infection and early kidney development

None

Ca channels

Some Ca channels may also be routes for iron uptake. In addition, there is evidence that the G185R mutation of DMT1 leads to enhanced behavior as a Ca channel for DMT1

TRPML1

Transient receptor potential mucolipidosis–associated protein

Recently shown to act as a channel for iron exit from endosomes and lysosomes, it is best known as a protein in which mutations can cause mucolipidosis type IV disease

MCOLN1

Mucolipin 1

None

Mobilferrin

It was postulated to be a form of calreticulin but also a major iron transporter, but whether it is or not is not known