Sources and intestinal barrier
The body obtains iron through two pathways: externally from food (exogenous pathway) and through the breakdown of old erythrocytes (endogenous reutilization). For dietary iron to be absorbed, an acidic gastric environment and the presence of reducing agents are required. The main one is vitamin C, which converts ferric iron into ferrous (Fe2+). It is in this form that the trace element enters enterocytes (intestinal mucosal cells).
Inside the enterocytes, the "mucosal block" mechanism operates. Its task is to prevent excess iron from entering the blood:
- If the body's demand is low, the protein apoferritin is actively synthesized in the cells. It binds iron ions, turning into ferritin (Fe3+). This iron remains in the mucosa and is eliminated with feces during normal epithelial desquamation.
- If there is a deficiency, apoferritin synthesis is inhibited, and ions pass freely through the enterocyte into the bloodstream.
Transport and cellular receptor uptake
Leaving the intestine through the basement membrane, the Fe2+ ion enters the plasma, where it is oxidized again to Fe3+. This is carried out by the copper-containing enzyme ferroxidase (also known as ceruloplasmin). During the reaction, copper is reduced (Cu2+ → Cu+), and the prepared iron binds to the transport glycoprotein apotransferrin. The transferrin-Fe3+ complex is formed, which is distributed throughout the body.
The delivery of iron to target tissues occurs via specific transferrin receptors:
- The complex binds to the receptor on the cell surface.
- A vesicle (endosome) is formed via endocytosis.
- Inside the cell, iron is released (again as Fe2+) and used for metabolic needs.
The number of receptors on the membrane depends directly on intracellular reserves. When iron is lacking, transcription of the receptor gene is upregulated, and the cell captures more transferrin from the blood.
Distribution, storage, and excretion
Iron delivered to tissues is expended on the synthesis of hemoproteins (e.g., hemoglobin in the bone marrow or myoglobin in muscles) and iron-sulfur proteins. Excess is stored in the liver, spleen, and bone marrow.
The primary storage protein is ferritin. However, if too much iron enters and the capacity of ferritin is exhausted, it transforms into hemosiderin. This is a poorly soluble complex capable of storing up to 37% of iron in its structure.
Mass losses of the trace element are normally minimal. The body loses it via feces (including bile and desquamated epithelium), sweat, urine, as well as during menstruation, bleeding, pregnancy, and lactation. At the same time, a significant portion of iron is constantly in circulation: macrophages of the reticuloendothelial system engulf destroyed erythrocytes, release iron from hemoglobin, and return it back to the blood.
Valence rule: "Eat trivalent, absorb divalent, carry trivalent, use divalent." Food (Fe3+) → Intestine (Fe2+) → Blood (Fe3+) → Intracellular (Fe2+).
Class and exam questions
Why is copper needed in iron metabolism?
Copper is a cofactor for ferroxidase (ceruloplasmin). Without this enzyme, iron cannot be oxidized to Fe3+ upon entering the blood and cannot bind to the carrier protein transferrin.
What is the difference between ferritin and hemosiderin?
Ferritin is the physiological soluble iron store. Hemosiderin is formed only in the presence of an excess of the trace element when ferritin reserves are overflowing; it is an insoluble aggregate.
How is the entry of iron into the blood regulated?
Through the "mucosal block" mechanism in enterocytes. When there is an excess of iron in the body, apoferritin is synthesized, which binds iron inside intestinal cells, preventing it from entering the bloodstream.
What else to study on this topic
- Mechanism of redox reactions involving ascorbic acid.
- The role of reticuloendothelial macrophages in hemoglobin reutilization.
- Biochemical structure of heme-containing and iron-sulfur proteins.
- Genetic regulation of transferrin receptor synthesis at the transcriptional level.
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