IRON

content of iron in the body – male: 4g 70% in hemoglobinfemale: 3,5 g children: 3g

requirement: 1 – 1,5 mg/24 h., intake 10mgIn increased requirement/increased loss: increased absorption to 3 – 3,5 mg/24 h.Tolerated limit of intake: 45 mg/24 h.

Excretion of iron: minimum, daily loss: 0,01% (1-2mg) (by intestine, bleeding, skin. Hair

pregnancy, menstrual bleeding)

Distribution of iron:

Tissue Form % of iron

erytrocytes hemoglobin 70,5

muscle myoglobin 3,2

Stores – liver

spleen

ferritin

hemosiderin

26,0

Blood plasma transferrin 0,1

other cytochromes

catalase

peroxidase

Fe - proteins

0,15

Iron Significance

Essential Element

Key functional component of O2 transportation and storage molecules (Hb, Mb)Many redox enzymes (cytochromes)Production of various metabolic intermediatesHost defence (NADPH oxidase)Iron is toxic to human cells

Iron Balance

Strictly regulated

Total body iron of 3 – 4gMinimal loss 1 – 2mg/dErythropoietic iron requirement only 20mg/d

Important homeostatic mechanisms prevent excessive ironabsorption in duodenum and regulate rate of iron release from RES

Chemical forms of iron:

Ferric (3+) iron: insoluble at physiological pH

Ferrous (2+) iron: dangerous if free, forms free radicals

Since free iron is insoluble or toxic, it must be bound to proteins

Cytochromes of the mitochondrial respiratory chain(100 mg of iron)

Hemoglobin: more than one half of total body iron (2.5 grams)

Myoglobin: about 0.3 grams Fe, muscle oxygen storage protein

Cytochrome P450: most abundant hemeprotein of the liver (about 1 mg)detoxifies foreign compoundsis strongly inducible by drugs (phenobarbital)Its induction forms the basis of drug-drug interactions

Hemeproteins

Ferritin - iron storage protein

Transferrin: iron transport protein

Non - heme iron proteins

Ferritin: iron storage proteinIn men, contains up to 1 gram of iron

Reflects the amount of BODY IRON STORES

Transports iron in the blood

Contains only 2 atoms of iron (Fe3+)

Transferrin is the only source of iron for hemoglobin

Transferrin saturation is clinically useful for iron metabolism studies(iron-saturated Tf / total Tf)

Transferrin

Transferrin saturation:

Normal about 30-50 %,

Transferrin saturation under 15 % = Iron deficiency

Cellular Iron Homeostasis

Concerned with each cells requirements for iron

Systemic Iron Homeostasis

Concerned with the body’s need for iron

Regulation of iron metabolism:

There is no pathway for iron excretion from the bodytherefore

Total body iron level is regulated at the level of iron absorption from the small intestine

Intestinal iron absorption increases with

•Decreased iron stores

•Increased erythropoietic activity

•Anaemia

•Hypoxia

Intestinal iron absorption decreases in inflammation

Excess iron absorption relative to body iron stores – hereditary haemochromatosis

Absorption of iron

Factors affecting absorption:

1. Quality of iron in food: heme iron (meat, liver) – 10-30%

non-heme iron (plant origin) – 1-5%

2. Form of iron: heme Fe2+ Fe3+

3. Other components of food: - inhibitory: salts of Ca2+, phosphate, milk, cheese,

tea (tanin), coffee, oxalates

- stimulatory: ascorbate, citrate, amino acids

enterocyte

Fe2+

Fe2+ Fe3+

ferroxidaseferritin

heme

heme

DMT1

ferroportin

Fe3+ transferrin

ferroxidase

hephaestin – in enterocytes ceruloplasmin – in blood

spleen

heme

erytrocytes

Fe2+feroportin

20mg/d.

1-2mg/d.

cells

TfR1

Fe2+

heme

Fe3+

ferritin

hemosiderin

HCP1

HCP1 – „hem carrier protein“DMT – „divalent metal transporter“

Regulatory places of iron homeostasis

Blood stream

Iron Absorption (into enterocyte)Luminal surface

Dietary free iron (Fe3+) is reduced to Fe2+Occurs at brush border by duodenal ferric reductase (Dcytb)

Transluminal transportDMT1/Nramp2 (divalent metal transporter 1) Dietary haeme iron via transporter and released from heme or absorbed into the circulation.

Iron Absorption (out of enterocyte)

Basolateral absorption via ferroportin or haeme transporter.Hephaestin facilitates enterocyte iron releaseReleased by way of ferroportin into the circulation

FerroportinPresent on the basolateral membrane of enterocytesPresent on macrophages and other RES cellsPresent on hepatocytes

Iron TansportVia transferrin

Iron Storage (Hepatic - major site)Hepatic uptake of transferrin bound Fe viaclassic transferrin receptor TfR1 (and homologous TfR2)

Hepatocytes are storage reservoir for ironTaking up dietary iron from portal bloodReleasing iron into the circulation via ferroportin in times of increased demand

Iron UtilisationErythropoiesis for heme synthhesis / general cellular respirationvia TfRs on erythroid precursors and other cells

Transferrin uptake - transferrin receptor

Transferrin

Transferrin receptor

Cells which need iron express high number of transferrin receptors on their surface

Transferrin receptor expression is regulatedposttranscriptionally

at the level of transferrin receptor mRNA stability

Lack of iron stabilises mRNA for transferrin receptor

• ferritransferrin binds to receptors at the surface of the cells

• internalization of the complex ito endosome

• in endosome iron is released from tranferrin (by help of low pH & reduction Fe3+ → Fe2+) and is relesed into cytosol

• iron is transported to the place of its utilization resp. Is bound to ferritin (Fe2+ → Fe3+ and stored)

• apotransferrin connected with receptor returns back to the membrane

• apotransferrin releases from receptor into blood plasma for binding of next iron ions

The IRE/IRP Regulatory System

Each cell has the capacity to regulate its own utilisation of iron.

This is co-ordinated via Iron regulatory proteins (IRP) and their binding to Iron regulatory elements (IRE) which are presents on nucleic acids.

In this manner proteins involved in iron storage, erythroid haeme synthesis, the TCA cycle, iron export, and iron uptake are coordinately regulate.

IRP act as the cell sensor to iron availability

Cellular Iron Regulation

IRP1IRP2

lower iron level

3'

5'

binds on IRES

IRP1IRP2

3'

5'

stabilisation of mRNA

stop translation

TfR1DMT1

synthesis of new proteinsfor other iron intake

FerritinALA synthaseFeroportin

lower iron level

binds on IRES

IRP1IRP2

higher iron level

3'

5'

stop translation

stabilisation of mRNA

FerritinALA synthaseFeroportin

TfR1DMT1

Hepcidin

Intestinal iron absorption varies inversely with liver hepcidin expression

Hepcidin decrease the functional activity of ferroportin by directly binding to it and causing it to be internalised from the cell surface and deregulated

Decreases basolateral iron transfer and thus dietary iron absorption

Decrease in iron export by hepatocyte and macrophage and a resultant increase in stored iron

Systemic Iron Homeostasis

Hepcidin

25 aa peptide. Identified 2000

Antimicrobial activity. Hepatic bacteriocidal protein

Main iron regulatory hormone

Factors regulating intestinal iron absorption also regulate the expression of hepcidin •Decreased iron stores•Increased erythropoietic activity•Anaemia•Hypoxia

Cellular targets of hepcidin are villous enterocyte, RE macrophage and hepatocyte

Disorders of iron metabolism

1) Increased absorption of iron from the gut:HEMOCHROMATOSIS

2) Decreased amount of iron in the body:IRON DEFICIENCY ANEMIA

3) Inflammation-induced change of iron distributrion:ANEMIA OF CHRONIC DISEASE

Excessive absorption of iron from the gut:

Hemochromatosis

Iron accumulates in the liver, heart and pancreas,excess iron damages these organs by free radical production

Transferrin saturation increases, serum ferritin increases

Lack of iron in the body:

Iron deficiency anemia

(most common anemia)

Hypochromic microcytic erythrocytes

Serum ferritin decreases (iron stores are depleted)transferrin saturation decreases (15 % or less)

Iron deficiency is more common in women than in men

Menstruation, pregnancy and birth deplete iron stores,men have higher iron stores than women

If iron deficiency anemia is seen in a male patient, the patient should always be checked forblood loss from the gastrointestinal tract

Absorption curve of iron

• dif. dg. od iron deficit • determination of basal level of iron in serum• administration of 200mg Fe2+ p.o. (Ferronat),• blood taken after 2, 4 a 6 hours• determination of iron

Ref. Values of iron in blood serum: 10,5 – 28 mol/l (males) 9,0 – 25,6 mol/l (females)

In case of normal absorption – increase of iron at least to 28 mol/l

Practical part: