Iron Storage and Recycling

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Iron Storage - Iron Vaults Unlocked

Iron is stored intracellularly, preventing toxicity. Key forms ensure availability.

  • Storage Proteins:

    FeatureFerritinHemosiderin
    CompositionApoferritin shell + $Fe^{3+}$ core (micelles)Aggregates of denatured ferritin, iron, lipids
    SolubilitySolubleInsoluble
    Iron AvailabilityReadily availableSlowly available
    AbundancePrimary store; reflects body ironIncreases in iron overload
    VisualizationElectron microscopyLight microscopy (Perls' Prussian blue stain)
  • Major Storage Sites:

    • Liver (hepatocytes, Kupffer cells)
    • Bone marrow (macrophages)
    • Spleen (macrophages)

📌 Mnemonic: "Ferritin is Friendly & Fast (soluble, available); Hemosiderin is Hard & Held-up (insoluble, less available)."

Hemosiderin deposits with Perls' Prussian blue stain

⭐ Serum ferritin is an acute phase reactant; its levels can be ↑ in inflammation, independent of iron status. Normal range: Men 20-250 ng/mL, Women 10-120 ng/mL.

Iron Transport & Regulation - Iron Express Lane

  • Key Transporters:
    • Transferrin (Tf): Primary plasma $Fe^{3+}$ carrier; delivers iron to cells via transferrin receptors (TfR1). Normal saturation: 20-50%.
    • Ferroportin (FPN1): Sole iron exporter from cells (enterocytes, macrophages, hepatocytes) to plasma-bound transferrin.
    • Haptoglobin & Hemopexin: Bind free Hb & heme respectively, salvaging iron.
  • Master Regulator: Hepcidin
    • Peptide hormone synthesized mainly in the liver.
    • 📌 Hepcidin 'hides' iron: Binds ferroportin → internalization & degradation → ↓ iron absorption & ↓ release from macrophages.

    ⭐ Hepcidin is the master iron regulatory hormone, primarily synthesized in the liver.

  • Hepcidin Regulation & Action Flowchart:

Systemic and cellular iron metabolism

  • Regulation Summary:
    • Hepcidin ↑ by: Iron overload, inflammation (IL-6).
    • Hepcidin ↓ by: Iron deficiency, hypoxia, increased erythropoiesis.

Iron Recycling - Recycle, Reuse, Re-Iron!

The body efficiently reclaims iron from aged red blood cells (RBCs), meeting most daily needs.

  • Sites & Source:
    • Macrophages (RES): Spleen, liver, bone marrow.
    • Senescent RBCs (~120 days): Primary source.
  • Mechanism:
    • Macrophages phagocytose old RBCs.
    • Heme oxygenase: Heme $\rightarrow$ Fe$^{2+}$ + Biliverdin + CO.
    • Iron (Fe$^{2+}$) exported by ferroportin (FPN1).
    • Hepcidin: Degrades FPN1 $\rightarrow$ ↓ iron release.
    • Transport: Fe$^{3+}$ (oxidized) + transferrin $\rightarrow$ bone marrow (erythropoiesis) or liver (ferritin storage).

Macrophage Iron Metabolism and Recycling

⭐ The majority of daily iron needs (approx. 20-25 mg/day) are met through recycling of iron from senescent red blood cells by macrophages.

Clinical Correlates - Iron Imbalance Issues

Key differences in iron storage and recycling disorders:

FeatureIron Overload (Hereditary Hemochromatosis - HH)Iron Deficiency (Storage/Recycling Defects)
Primary IssueExcessive iron absorption (e.g., HFE mutation)Impaired iron release from macrophages/stores (e.g., ACD, ferroportin disease)
Serum Ferritin↑↑ (often > 1000 ng/mL)↓ (absolute deficiency) or Normal/↑ (functional deficiency, e.g., ACD)
Transferrin Sat.↑↑ (> 45%)
Symptoms📌 "Bronze diabetes", liver damage, cardiomyopathy, arthropathy.Fatigue, pallor. Specific signs (e.g., koilonychia) if severe.

⭐ Mutations in the HFE gene are the most common cause of hereditary hemochromatosis.

High‑Yield Points - ⚡ Biggest Takeaways

  • Ferritin: Primary soluble iron storage protein; serum levels reflect body iron stores.
  • Hemosiderin: Insoluble iron aggregate, seen in iron overload; stains Prussian blue positive.
  • Transferrin: Plasma protein that transports two Fe3+ ions to erythroid precursors.
  • RES Macrophages: Phagocytose old RBCs, releasing iron via heme oxygenase for recycling.
  • Hepcidin: Liver-derived peptide hormone; master iron regulator, blocks ferroportin.
  • Iron Recycling: Efficiently conserves iron, with most daily iron needs met by recycled iron from senescent RBCs.

Practice Questions: Iron Storage and Recycling

Test your understanding with these related questions

All of the following decrease in iron deficiency anemia except:

1 of 5

Flashcards: Iron Storage and Recycling

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Primary _____ is due to mutations in the HFE gene on chromosome 6, leading to increased expression of DMT1 and decreased synthesis of Hepcidin

TAP TO REVEAL ANSWER

Primary _____ is due to mutations in the HFE gene on chromosome 6, leading to increased expression of DMT1 and decreased synthesis of Hepcidin

hemochromatosis

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