Dietary iron acquisition
Dietary iron is found in two basic forms :
- haem iron : found in meat and meat products
- non-haem iron : present in cereals, vegetables, pulses, beans, fruits, etc.
The absorption of both haem and non-haem iron takes place almost exclusively in the duodenum and the bioavailability of iron from these sources is influenced by a number of variables: the iron content of foods, the type of iron present, haem or non-haem, and other dietary constituents. Absorption is also regulated in line with metabolic demands that reflect the amount of iron stored in the body, and the requirements for red blood cell production.
In western countries haem is the most bioavailable source of iron. In contrast, the bioavailability of non-haem iron is low and is profoundly influenced by other dietary components that can enhance or inhibit non-haem iron bioavailability. The most potent enhancer is ascorbic acid (vitamin C), which acts by reducing ferric iron to the more soluble and absorbable ferrous form, while phytates are the most potent dietary inhibitors of non-haem iron absorption [P Sharp, SK Srai, 2007].
Mechanisms involved in intestinal iron trasport
Both haem and non-haem iron are taken up in the proximal region of the small intestine, though their transport across the apical membrane of the enterocytes occurs through totally independent pathways (fig.1):
Fig.1 The cellular mechanisms involved in intestinal iron absorption . Dietary non-haem iron (mostly ferric) is reduced by the actions of the ferric reductase Dcytb and reducing agents in the diet to yield Fe2+, which subsequently enters the enterocytes via DMT1 . Haem is absorbed via HCP1 , broken down by haem oxygenase 1 (HO) to liberate Fe2+ (this joins a common pool with iron from the non-haem pathway) and bilirubin (which might be removed from the cell by the efflux proteins FLVCR and ABCG2 ). If body iron stores are high, iron may be diverted into ferritin and lost when the cell is shed at the villus tip. Alternatively, iron passes into the labile iron pool (LIP) and is subsequently processed for efflux via IREG1 (as Fe2+). The exiting iron is re-oxidised to Fe3+ through hephaestin (Hp) to enable loading onto transferrin (Tf) .
The majority of dietary non-haem iron enters the gastrointestinal tract in the ferric form.
Fe3+ is thought to be essentially non-bioavailable and, therefore, it must first be converted to ferrous iron prior to absorption.
There are numerous dietary components capable of reducing Fe3+ to Fe2+, including ascorbic acid and amino acids such as cysteine and histidine.
However, ferric iron reaching the duodenal enterocytes may still be reduced by the cells endogenous reducing activity, the brush-border surface of duodenal enterocytes possess ferric reductase enzymic activity: Dcytb ( for duodenal cytochrome b ), a homologue of cytochrome b561.
Like cytochrome b561, Dcytb is a haemcontaining protein with putative binding sites for ascorbate and semi-dehydroascorbate. The protein is expressed on the brush border membrane of duodenal enterocytes, the major site for the absorption of dietary iron [P Sharp, SK Srai, 2007].
Following reduction either by Dcytb or dietary reducing agents, the resulting Fe2+ becomes a substrate for the divalent metal transporter DMT1 -also known as the divalent cation transporter DCT1 , and natural resistance associated macrophage protein Nramp2 . DMT1 is an electrogenic transporter that transports divalent cations in symport with a single proton. Thus, it requires a pH gradient (at least pH 6 vs. pH 7.4: pH optimum 5.5) for transport and is not functional at neutral pH [R Fuchs et al, 2004].
The relatively low pH of the proximal duodenum together with the acid microclimate present at the brush border membrane stabilises iron in the ferrous form and provides a rich source of protons that are essential for driving iron uptake across the apical membrane of the intestinal epithelium. [P Sharp, SK Srai, 2007].
The mechanisms involved in haem iron absorption are only just beginning to emerge. Haem binds to the duodenal brush border membrane and is absorbed as an intact molecule.
A number of candidate haem binding proteins have been identified in the intestinal epithelial cells including the haem carrier protein HCP1 that acts as a haem import protein, its high duodenal expression suggests that it may be the protein involved in haem uptake from the diet. However, the precise role of HCP1 in iron metabolism remains to be fully elucidated.
Following absorption, haem is detectable in membrane-bound vesicles within the cytoplasm. Within these vesicles, it is thought that the iron contained with the protoporphyrin ring is excised by the action of haem oxygenase 1 yielding ferrous iron which enters a common intracellular pool along with the iron absorbed via the non-haem transport pathways.
Digestion appears to be complete within the enterocytes, actually there are other 2 haem binding proteins on the basolateral membrane: the ATP-binding cassette protein ABCG2 , the feline leukaemia virus C receptor protein FLVCR and one possibility is that the efflux proteins ABCG2 and FLVCR, also expressed in the duodenum, may act to remove bilirubin formed as a by-product of haem degradation from the enterocytes. [P Sharp, SK Srai, 2007]
Ferritin is the major iron storage protein.
The ferritin iron uptake mechanism is yet to be determined.
One possibility is that ferritin is broken down by protease activity in the upper gastrointestinal tract and the released iron is absorbed via the Dcytb/DMT1 route.
However, studies have shown that ferritin is largely resistant to high temperature, low pH and protein denaturing agents. Therefore, it is possible that ferritin may be absorbed intact and broken down intracellularly (in the lysosomes) to liberate its iron load.
While the presence of ferritin receptors has been postulated on liver and placental plasma
membranes, none has yet been identified in intestinal tissue. [P Sharp, SK Srai, 2007]
Lactoferrin is the human milk protein , an iron-binding protein capable of binding two ferric ions. Specific receptors for lactoferrin have been identified on the brush border surface of foetal enterocytes, these receptors mediate the uptake of lactoferrin-bound iron in intestinal epithelial cell cultures. The lactoferrin receptor may be the principal iron transport pathway in early life. [P Sharp, SK Srai, 2007]
- Intracellular storage - Ferritin
Intracellular iron is either stored in ferritin , the 24-subunit protein that is capable of binding up to 4,600 Fe(III) ions.
If the body stores are replete and there is no increased erythropoietic drive, a significant amount of newly absorbed iron will be stored in the enterocytes as ferritin.
Iron stored in ferritin will be returned to the lumen when mature enterocytes are sloughed off, because duodenal enterocytes turnover very rapidly (their lifespan is approximately 3-4 d). [P Sharp, SK Srai, 2007; BA Syed et al 2006]
Iron liberated from heme or imported into the enterocyte by DMT1 then enters the hypothesized intracellular or ‘labile’ iron pool . The molecular character of this pool in enterocytes remains unknown, but it could consist of low molecular weight chelates or chaperone proteins that bind and transport iron. Iron is delivered to the basolateral membrane, although the proteins, cellular compartments or mechanisms that convey the iron remain unknown.
Intracellular iron passes out of the enterocyte through the basolateral membrane via a transmembrane iron exporter.
Efflux of iron across the basolateral surface of enterocytes is achieved through the co-ordinated action of a transport protein ferroportin , also known as IREG1 and MTP1, and a multicopper ferrioxidase: hephaestin, localized to the basolateral membrane:
The human Ireg1 gene has a predicted open reading frame of 562 amino acids and sequence analysis has revealed the presence of an iron responsive element (IRE) in the 5' translated region (UTR), indicating that expression of this protein is regulated by intracellular iron levels. Both iron regulatory proteins 1 and 2 (IRP1 and IRP2) have been shown to bind to this IRE.
Transferrin contains 2 specific high affinity Fe(III) binding sites. When not bound to iron, it is known as apotransferrin.
The released iron from the enterocyte basolateral membrane is sequestered by transferrin Tf and transported to sites of utilization and storage. Epatocytes are the major storage site of iron. (Transferrin cycle)
