Correct Answer: D. Divalent metal Transporter 1
Divalent Metal Transporter 1 (DMT1) is the primary iron transporter at the apical membrane of enterocytes, responsible for Fe²⁺ uptake from the intestinal lumen. DMT1 is a proton-coupled transporter that requires an acidic pH gradient (maintained by gastric acid and duodenal secretions) to actively transport ferrous iron across the apical brush border. This transporter is upregulated in iron deficiency states—a critical adaptation in populations with high prevalence of iron-deficiency anemia (IDA), such as in India where dietary iron bioavailability is limited due to high phytate and polyphenol content in vegetarian diets. DMT1 also transports other divalent cations (Zn²⁺, Mn²⁺, Cu²⁺, Pb²⁺), explaining its broad substrate specificity. Once inside the enterocyte, iron is either stored as ferritin or exported via ferroportin at the basolateral membrane. The regulation of DMT1 expression by hepcidin (through ferroportin degradation) indirectly controls iron absorption. Understanding DMT1 is essential for explaining iron supplementation efficacy and why ferrous salts (Fe²⁺) are preferred over ferric (Fe³⁺) forms in Indian clinical practice.
Why the other options are wrong
A. Divalent Metal Transporter 2 — DMT2 is a vesicular transporter involved in intracellular iron trafficking and endosomal recycling, NOT apical membrane uptake. It plays a minor role in intestinal iron absorption and is not the primary transporter for Fe²⁺ entry into enterocytes. This is a distractor that confuses intracellular iron handling with intestinal absorption. B. Ferroportin — Ferroportin is the iron exporter at the basolateral membrane of enterocytes, responsible for transferring iron OUT of the cell into the bloodstream (bound to transferrin). It is the exit transporter, not the entry transporter. NBE pairs this with the absorption question to test understanding of directional iron flux. C. Hephaestin — Hephaestin is a ferroxidase enzyme that oxidizes Fe²⁺ to Fe³⁺ at the basolateral membrane, facilitating iron loading onto transferrin. It is a functional partner of ferroportin, not an iron transporter itself. Students confuse it with transporters because it is essential for iron export, but it catalyzes oxidation, not transport.
High-Yield Facts
- DMT1 is the apical membrane transporter for Fe²⁺ uptake; requires acidic pH and proton gradient for function.
- Ferroportin is the basolateral exporter; hepcidin-regulated iron exit from enterocytes into blood.
- Ferrous (Fe²⁺) salts are better absorbed than ferric (Fe³⁺) because DMT1 specifically recognizes Fe²⁺; ferric requires reduction by duodenal cytochrome b.
- DMT1 upregulation in iron deficiency increases intestinal absorption; downregulation in iron overload reduces absorption (hepcidin-mediated).
- DMT1 also transports Zn²⁺, Mn²⁺, Cu²⁺, Pb²⁺—explaining divalent metal toxicity and competitive absorption inhibition.
Mnemonics
Iron Entry & Exit DMT1 IN (apical, Fe²⁺ entry) → Ferroportin OUT (basolateral, Fe²⁺ exit). Hephaestin oxidizes Fe²⁺ to Fe³⁺ for transferrin binding during exit. DMT1 = Divalent Metal Transporter 1 (Entry) Remember: 1 = apical membrane (first contact with lumen). DMT1 is the first transporter iron meets; ferroportin is the last before blood.
NBE Trap
NBE pairs ferroportin (the correct exit transporter) with the absorption question to trap students who confuse directional iron flux. Students who memorize "ferroportin = iron transport" without understanding basolateral location will select option B.
Clinical Pearl
In Indian patients with IDA (prevalence ~50% in women), ferrous sulfate supplementation works because it provides Fe²⁺ that DMT1 can directly transport. Ferric salts require reduction by duodenal cytochrome b first, making them less bioavailable—hence ferrous is the DOC in Indian guidelines (ICMR, IAP). Hepcidin elevation in chronic inflammation (common in India due to infections, malnutrition) suppresses ferroportin and indirectly reduces DMT1 activity, explaining anemia of chronic disease.
_Reference: Guyton & Hall Textbook of Medical Physiology, Ch. 65 (Iron Metabolism); Harrison's Principles of Internal Medicine, Ch. 405 (Iron Deficiency and Other Hypoproliferative Anemias)_