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Apoferritin – Function, Significance and Clinical Role

Apoferritin is the iron-free protein shell of the storage protein ferritin. It plays a central role in iron metabolism in the human body.

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Things worth knowing about "Apoferritin"

Apoferritin is the iron-free protein shell of the storage protein ferritin. It plays a central role in iron metabolism in the human body.

What is Apoferritin?

Apoferritin is the protein shell of the iron storage protein ferritin, existing in its iron-free form. It is a large, hollow, spherical protein composed of 24 subunits (polypeptide chains) that can store up to 4,500 iron atoms in the form of ferrihydrite (iron(III) oxide-hydroxide phosphate) within its core. Once apoferritin is loaded with iron, the complete molecule is referred to as ferritin.

Biological Function

Apoferritin serves several important roles in the body within the context of iron homeostasis:

Iron storage: Apoferritin captures free iron ions and stores them safely, preventing the toxic accumulation of free iron in tissues.
Iron mobilization: When needed, iron stored within ferritin can be released for use in hemoglobin synthesis and other metabolic processes.
Cellular protection: Free iron can generate reactive oxygen species (ROS) through the Fenton reaction, causing cellular damage. Apoferritin protects cells by sequestering this free iron.

Distribution in the Body

Apoferritin and ferritin are found in virtually all body cells, but are particularly concentrated in:

Liver (main site of hepatic iron storage)
Spleen
Bone marrow
Skeletal muscle

A small amount of ferritin is also present in the blood serum and is used as an important laboratory marker to assess the body's iron stores.

Regulation of Apoferritin Synthesis

The synthesis of apoferritin is regulated at the molecular level by intracellular iron concentrations. When iron levels are low, Iron Regulatory Proteins (IRPs) bind to specific sequences on the apoferritin mRNA called Iron Responsive Elements (IREs), inhibiting translation and reducing apoferritin production. When intracellular iron is abundant, apoferritin synthesis is upregulated to safely store the excess iron.

Clinical Relevance

Since apoferritin forms the structural basis of ferritin, it is relevant to several clinical conditions:

Iron deficiency: When iron stores are depleted, serum ferritin levels fall, reflecting reduced loading of apoferritin. Low ferritin is an early indicator of iron deficiency, often preceding the development of anemia.
Iron overload (Hemochromatosis): In genetic or acquired iron overload disorders, ferritin stores become saturated, potentially leading to organ damage from excess free iron.
Inflammatory marker: Ferritin is also an acute-phase protein. During inflammation, infection, or certain cancers, serum ferritin levels can rise significantly, independent of actual iron stores.
Biomedical research and nanotechnology: The hollow spherical structure of apoferritin is being investigated as a natural nanocontainer for the targeted delivery of drugs and imaging agents.

Apoferritin in Nutrition

In plant-based foods such as legumes and cereals, iron is partially stored in the form of ferritin. This so-called ferritin iron is absorbed in the intestine via a different pathway than free non-heme iron and appears to use a distinct absorption route. The bioavailability of ferritin-bound iron from plant sources is a subject of ongoing research.

References

Theil, E.C. (2011): Ferritin: the protein nanocage and iron biomineral in health and in disease. Inorganic Chemistry, 50(20), 9473-9482. PubMed PMID: 21661724.
World Health Organization (WHO): Iron Deficiency Anaemia: Assessment, Prevention and Control. Geneva, 2001. Available at: https://www.who.int/nutrition/publications/micronutrients/anaemia_iron_deficiency/en/
Muckenthaler, M.U., Rivella, S., Hentze, M.W., Galy, B. (2017): A Red Carpet for Iron Metabolism. Cell, 168(3), 344-361. PubMed PMID: 28129536.

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