Iron is vital for all living organisms. However, excess iron is hazardous because it produces free radical formation. Therefore, iron absorption is carefully regulated to maintain an equilibrium between absorption and body loss of iron. In countries where heme is a significant part of the diet, most body iron is derived from dietary heme iron because heme binds few of the luminal intestinal iron chelators that inhibit absorption of non-heme iron. Uptake of luminal heme into enterocytes occurs as a metalloporphyrin. Intra-cellularly, iron is released from heme by heme oxygenase so that iron leaves the entero-cyte to enter the plasma as non-heme iron. Ferric iron is absorbed via a 3 integrin and mobilferrin (IMP) pathway that is not shared with other nutritional metals. Ferrous iron uptake is facilitated by DMT-1 (Nramp-2, DCT-1) in a pathway shared with manganese. Other proteins were recently described which are believed to play a role in iron absorption. SFT (Stimulator of Iron Transport) is postulated to facilitate both ferric and ferrous iron uptake, and Hephaestin is thought to be important in transfer of iron from entero-cytes into the plasma. The iron concentration within enterocytes reflects the total body iron and either upregulates or satiates iron-binding sites on regulatory proteins. Entero-cytes of hemochromatotics are iron-depleted similarly to the absorptive cells of iron-deficient subjects. Iron depletion, hemolysis, and hypoxia each can stimulate iron absorption. In non-intestinal cells most iron uptake occurs via either the classical clathrin-coated pathway utilizing transferrin receptors or the poorly defined transferrin receptor independent pathway. Non-intestinal cells possess the IMP and DMT-1 pathways though their role in the absence of iron overload is unclear. This suggests that these pathways have intracellular functions in addition to facilitating iron uptake. Am. J. Hematol. 64:287-298, 2000.

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DMT1 has four names, transports as many as eight metals, may have four or more isoforms and carries out its transport for multiple purposes. This review is a start at sorting out these multiplicities. A G185R mutation results in diminished gastrointestinal iron uptake and decreased endosomal iron exit in microcytic mice and Belgrade rats. Comparison of mutant to normal rodents is one analytical tool. Ectopic expression is another. Antibodies that distinguish the isoforms are also useful. Two mRNA isoforms differ in the 3' UTR: +IRE DMT1 has an IRE (Iron Responsive Element) but -IRE DMT1 lacks this feature. The +/-IRE proteins differ in the distal 18 or 25 amino acid residues after shared identity for the proximal 543 residues. A major function is serving as the apical iron transporter in the lumen of the gut. The +IRE isoform appears to have that role. Another role is endosomal exit of iron. Some evidence indicts the -IRE isoform for this function. In our ectopic expression assay for metal uptake, four metals--Fe2+, Mn2+, Ni2+ and Co2+--respond to the normal DMT1 cDNA but not the G185R mutant. Two metals did not--Cd2+ and Zn2+--and two--Cu2+ and Pb2+--remain to be tested. In competition experiments in the same assay, Cd2+, Cu2+ and Pb2+ inhibit Mn2+ uptake but Zn2+ did not. In rodent mutants, Fe and Mn appear more dependent on DMT1 than Cu and Zn. Experiments based on ectopic expression, specific antibodies that inhibit metal uptake and labeling data indicate that Fe3+ uptake depends on a different pathway in multiple cells. Two isoforms localize differently in a number of cell types. Unexpectedly, the -IRE isoform is in the nuclei of cells with neuronal properties. While the function of -IRE DMT1 in the nucleus is speculative, one may safely infer that this localization identifies new role(s) for this multifunctional transporter. Management of toxic challenges is another function related to metal homeostasis. Airways represent a gateway tissue for metal entry. Preliminary evidence using specific PCR primers and antibodies specific to the two isoforms indicates that -IRE mRNA and protein increase in response to exposure to metal in lungs and in a cell culture model; the +IRE form is unresponsive. Thus the -IRE form could be part of a detoxification system in which +IRE DMT1 does not participate. How does iron status affect other metals' toxicity? In the case of Mn, iron deficiency may enhance cellular responses.

The frequent clinical association of plumbism and increased alcohol intake has suggested that ethanol may augment lead absorption and toxicity. This investigation was undertaken to determine the effects of acute and chronic ethanol administration on lead absorption and excretion.Materials and methods. Male albino rats of a pathogen-free Wistar strain weighing 200 to 250 g at the time of absorption measurements or intravenous lead injection were used in all experiments. The principles of laboratory animal care as promulgated by the National Research Council were observed. All animals were housed in polypropylene cages containing absorbent bedding in a room provided with automatically controlled temperature and lighting. The rats were given a standard pelleted laboratory chow (Wayne Lab-Blox, Allied Mills, Inc.) fed ad libitum. Demineralized deionized water was supplied to all animals except in some experiments in which 10% ethanol (v/v) was substituted for water.Lead absorption studies were performed by measurement of total body radioactivity in a small animal whole-body liquid scintillation detector (Packard-ARMAC). The radioisotopes utilized were obtained from New England Nuclear as 203Pb acetate (sp act 10-50 mCi/mg of lead) or 210Pb nitrate (sp act 10 mCi/mg of lead). Because of its halflife of 22 years, 'loPb was selected for use in excretion studies only; 203Pb, having a halflife of 2.2 days, was used for all other experiments. All measurements of radioactivity were corrected for radiodecay by comparison to an appropriate standard after subtraction of background radioactivity. Lead absorption experiments were performed in rats fasted overnight from food but not fluids. Under ip pentobarbital anesthesia (4 mg/ 100 g), the urethra was tied with a silk suture to prevent urinary loss of absorbed lead. A laparotomy was performed, the small intestine was iso-lated proximally and distally with umbilical tape, and the bile duct was ligated with silk suture. One milliliter of radiolabeled leadcontaining test solution was injected into the isolated intestinal segment. Injections were accomplished by entering the gut lumen proximal to the proximal ligature with a 21gauge hypodermic needle, passing it intraluminally through the ligature loop, tightening the ligature, and then injecting the test dose into the isolated segment with subsequent withdrawal of the needle and tying of the ligature. In one experiment, animals were administered test doses through an oroesophageal tube following laparotomy. The abdomen was then closed with stainless-steel clips and the tats were placed in 1-quart vented cardboard ice cream containers. Total body radioactivity was measured in a wholebody detector and compared to a 250-ml water-filled plastic bottle containing a test dose equal to that injected into the animals. Four hours after administration of the test dose, each animal was killed by cervical dislocation. Isolated intestinal segments were excised from the carcass and whole-body radioactivity was again quantified and compared to ...

Separate pathways for transport of nontransferrin ferric and ferrous iron into tissue cultured cells were demonstrated. Neither the ferric nor ferrous pathway was shared with either zinc or copper. Manganese shared the ferrous pathway but had no effect on cellular uptake of ferric iron. We postulate that ferric iron was transported into cells via beta(3)-integrin and mobilferrin (IMP), whereas ferrous iron uptake was facilitated by divalent metal transporter-1 (DMT-1; Nramp-2). These conclusions were documented by competitive inhibition studies, utilization of a beta(3)-integrin antibody that blocked uptake of ferric but not ferrous iron, development of an anti-DMT-1 antibody that blocked ferrous iron and manganese uptake but not ferric iron, transfection of DMT-1 DNA into tissue culture cells that showed enhanced uptake of ferrous iron and manganese but neither ferric iron nor zinc, hepatic metal concentrations in mk mice showing decreased iron and manganese but not zinc or copper, and data showing that the addition of reducing agents to tissue culture media altered iron binding to proteins of the IMP and DMT-1 pathways. Although these experiments show ferric and ferrous iron can enter cells via different pathways, they do not indicate which pathway is dominant in humans.