Prof. of Medical Biochemistry Iron Metabolism Prof. Dr. Maha Sallam Prof. of Medical Biochemistry Extended Modular Program

A well-nourished male has about 3 to 5 grams of iron in his body *A well-nourished male has about 3 to 5 grams of iron in his body. Of this, about 2.5 g are present in hemoglobin.  *Spinach, & other green, leafy vegetables, are rich in iron. 180-g serving of boiled spinach contains 6.43 mg of iron, whereas a 170-g of hamburger contains at most 4.42 mg.  *However, spinach contains iron absorption-inhibiting substances, including high levels of oxalate, which can bind to the iron to form ferrous oxalate and render much of the iron in spinach can not be absorbed by the body.

1.Identify requirements and sources of iron. 2.Understand absorption ,transport, storage and excretion of iron. 3.Understand regulation of iron metabolism.

Iron Metabolism 1- Hemoglobin iron: 75% of the total body iron. Iron is one of the Micro (trace) elements. The total body iron range between 3-5 g and is distributed as : 1- Hemoglobin iron: 75% of the total body iron. 2-Fixed tissue iron: e.g. Myoglobin, cytochrom C (in respiratory chain), catalase and peroxidase enzyme. © 4

3-Labile tissue iron: (changeable) stored iron in the form of ferritin and small amount of hemosidrin. 4- Plasma iron: transport form of iron (trasnferrin). ©

Requirements: The average daily iron requirement is 15-20 mg/day. Only 3-6% of iron intake is absorbed into the blood, which is equal to iron lost by the sloughing of cells (0.6 mg/day). Absorption is increased by increasing demands. To maintain iron homeostasis, the amount of iron absorbed should be equal to iron excreted. Iron homeostasis depends on control of iron absorption from the intestine. Iron losses are generally unregulated. © 6

e.g. pregnancy, lactation, growing children and blood loss.

Iron sources

Heme can be absorbed intact by endocytosis

Normal Iron Absorption & Metabolism Transport form Transferrin of Loss from desqumation of epithelium Hemoproteins Stored form ( Ferritin & hemosiderin ) © 11

Iron Metabolism includes : 1- Absorption (regulatory) 2- Transport 3- Cellular absorption 4- Storage 5- Excretion

1- Absorption of iron In the duodenum

Dietary Iron HCl Vit C transferrin Fe3+ Lost by shedding ferrireductase HCl Vit C transferrin Fe3+ Lost by shedding © 14

Iron Absorption Heme is absorbed by intestinal cells in the duodenum by a heme carrier protein 1 (HCP1). In enterocytes, Fe+2 is released from heme by a heme oxygenase enzyme. Non heme Iron is present in the diet in the Fe3+ state. Cooking of food facilitates breakdown of ligands attached to iron, helping to release it in the gut. It is reduced to Fe2+ by ferrireductase enzyme . The reduction of ferric iron to ferrous iron is facilitated by HCl in the stomach and vitamin C (ascorbic acid) Iron is transported into the enterocyte with a proton via Divalent metal transporter (DMT-1). 6. Ferritin liberates ferric ions into the plasma, where it is carried by transferrin. © 15

Absorbed Fe2+ from heme or non heme sources may have the following fates: Fe2+ iron are oxidized again inside intestinal cells to Fe3+ ions by ferroxidase . Then the ferric ions combine with apoferritin (mucosal cell protein) to form ferritin for temporary storage. Fe2+ is transported out of the enterocyte by the basolateral membrane protein (ferroportin), oxidized by Cu-containing membrane protein (hephaestin), and taken up by the plasma transport protein transferrin (2 Fe3+ / transferrin molecule). Ferroportin protein (the only known exporter of iron from cells to blood) is regulated by the hepatic peptide hepcidin that induces internalization and lysosomal degradation of ferroportin.

Hepcidin It is a hormone released from the liver when iron levels in the body becomes too high. It inhibits further iron passage from the small-intestine epithelial cell into the blood by attaching and inhibiting the ferroportin transporter. A deficiency of hepcidin causes tissue iron overload. Hepcidin has a central role in iron hemostasis (its transcription is suppressed when iron is deficient).

Iron is characterized by : 1-The body has no mechanism of excretion of iron 2- Only minimal amounts of iron are lost by: * bleeding *shedding of cells of intestine *shedding of cells of skin

So Total body Iron is regulated at the level of absorption by enterocytes of the duodenum

4500 Fe3+ ions ferritin Mucosal Block The intestinal content of ferritin is limited so once saturated with iron, the absorption of the iron is inhibited. Mucosal Block © 21

ferritin Fe3+ ions When iron stores (ferritin), in the body are depleted absorption increases.

Factors affecting iron absorption

Vit C (Ascorbic acid) form insoluble complexes with iron Forms a soluble complex Facilitating absorption and helps reduction of Fe3+to Fe2+ *oxalic acid found in spinach *phosphates found primarily in milk, dairy products and egg whites *phytates in beans *tannic acid in tea and coffee form insoluble complexes with iron

Factors affecting iron absorption Requirements of body: An increase rate of erythropoiesis (e.g. after hemorrhage). Vitamin C helps reduction of ferric to ferrous and forming soluble complex. 3. Gastric HCl helps absorption, so absorption decrease in achlorhydria and partial gastrectomy. 4. Some molecules in diet as tannic acid, phytate, phosphate and oxalates forming insoluble complexes with iron. 25

leave at least an hour before and after a meal containing iron. Tea, milk and other dairy products reduce iron absorption, and that is why you should avoid drinking milk and tea with meals. leave at least an hour before and after a meal containing iron.

Because Free iron is toxic so it must be bound to a protein (promote formation of free radicals) so it must be bound to a protein

Iron Metabolism include : 1- Absorption (regulatory) 2- Transport 3- Cellular absorption 4- Storage 5- Excretion

2- Iron Transport Transferrin (Tf): In the plasma, Fe2+ is changed to Fe3+and bound to apotransferrin forming the iron transport protein, transferrin (Tf). Tf is a glycoprotein, synthesized in the liver. Tf transports iron (2 mol of Fe3+ per mol of Tf) in the circulation to sites where iron is required. Tf binds to receptors in cell membrane of erythroblasts, ingested by endocytosis, and iron is delivered to the mitochondria, where heme is synthesized. Transferrin Fe3+ © 29

Total iron binding capacity (TIBC): Represents the capacity of transferrin to bind iron. It measures the free transferrin that is ready to carry Iron. Normally, the protein is only one third(35%) saturated with iron. Low TIBC levels usually indicate high levels of iron in the blood. High TIBC levels typically indicate low levels of iron in the blood.

Iron Metabolism include : 1- Absorption (regulatory) 2- Transport 3- Cellular absorption 4- Storage 5- Excretion

3- Iron absorption by the cells Fe3+ Fe3+ Cell Fe3+ Fe3+

There are receptors (Transferrin receptors ) on the surfaces of many cells for transferrin ( bone marrow cells , placenta). Transferrin binds to these receptors and is internalized by receptor-mediated endocytosis. The acid pH inside the lysosome causes the iron to dissociate from the protein (Apotransferrin) , that is not degraded within the lysosome. Instead, it re-enters the plasma to pick up more iron to cells in need. The translation of the mRNA of transferrin receptors and ferritin is regulated by iron regulatory proteins and iron responsive elements that depend on iron level in the cell.

Iron Metabolism include : 1- Absorption (regulatory) 2- Transport 3- Cellular absorption 4- Storage 5- Excretion

4- Iron Storage 4500 Fe3+ ions ferritin There are 2 intracellular storage forms: 1- Ferritin: is the predominant storage form of iron in cells. It is present in most cells especially in intestinal mucosa , liver, spleen and bone marrow. Apoferritin (the protein part of ferritin ) can take 4000-4500 ferric ions / molecule to form ferritin. Normally, there is a little ferritin in human plasma. However, in patients with excess iron ( e.g hemochromatosis), the amount of ferritin in plasma is markedly elevated. In iron deficency anaemia it is markedly decreased. Ferritin is the primary intracellular iron-storage globular protein. It keeps iron in a soluble and non-toxic form. © 35

4500 Fe3+ ions ferritin 2- Hemosiderin: When iron is in excess, the storage capacity of the apoferritin is exceeded. This leads to iron deposition adjacent to ferritin spheres in the form of brown aggregated deposits, called hemosidrin. 4500 Fe3+ ions ferritin

Iron Metabolism include : 1- Absorption (regulatory) 2- Transport 3- Cellular absorption 4- Storage 5- Excretion

5- iron excretion The high binding capacity of iron to macromolecules, leads to absence of free iron salts. Normally the body guards its own iron content and there is no physiological excretory mechanism So adult male looses about 1-2 mg / day which is replaced by absorption. © 39

Loss of iron occurs only through the normal shedding of tissues: * Epidermis. * Gastrointestinal mucosal cells in stools. * Hair. * Menstrual blood in females.

Iron metabolism (summary)

Trasferrin Iron loss by shedding of cells and bleeding Dietary Iron Ferritin Dietary Iron Fe3+ Fe 3+ Ferritin Fe3+ Fe 3+ Ferritin Fe3+ Fe3+ Feritin Fe3+ Iron loss by shedding of cells and bleeding Trasferrin

Abnormalities of Iron Metabolism Hemochromatosis Iron deficiency anemia

Can occur due to accidental ingestion of iron tablets: Iron overload Can occur due to accidental ingestion of iron tablets: Acute Fe poisoning It is one of the most common causes of poisoning deaths in children under age of 6 years. The lethal dose in children is 200–300 mg/kg body weight and approximately 100 g in adults. High doses of Fe supplements cause gastrointestinal symptoms especially constipation although nausea, vomiting, and diarrhea may occur. Iron overload is treated by an iron chelator.  

Hereditary hemochromatosis Overload can also occur due to genetic defects: Hereditary hemochromatosis Caused by mutations to the high iron gene (HFE). Excess iron is deposited in liver, pancreas, heart and skin; damaging their cells and inducing hyperpigmentation and hyperglycemia (Bronze diabetes). Serum iron and transferrin-saturation are elevated. Treatment is by phlebotomy or use of Fe chelators.

Revision questions The total iron content of the human body is 400–500 mg 1–2 g (C) 2–3 g (D) 3–5 g ©

The iron containing non heme protein is Catalase (B) Cytochrome C (C) Peroxidase (D) Ferritrin ©

Total body Iron is regulated at the level of : A- Absorption B- Transport C- Cellular absorption D Storage E- Excretion ©

Transferrin Ferritin Hemosiderin What are the Non –Heme iron containing proteins ? Transferrin Ferritin Hemosiderin ©

Thank You ©