Iron

Micronutrients :(intake does not exceed 100 mg daily)

Daily intake Body stores

• Zinc 10 mg 2200 mg• Copper 2.5 mg 70 mg• Iron 1-2 mg 4000 mg• Manganese 10 mg• Molybdenum 10 mg• Cobalt 1.5 mg• Chromium 1.5 mg

Body contains:

4 grams of iron (men) 3 grams (women)

2.5 grams of total body iron exist as haemoglobin

Only 1-2 mg of iron is taken up daily from the diet(which contains 10-20mg iron)

Iron metabolism in the body is a closed system

little intake and little loss

Only 1 mg of iron is lost daily from the body(about 0.025% of total body iron)

nonspecific pathways(sloughing of dead cells, iron excretion in bile)

In women, additional 30 mg of iron is lost monthlyby menstruation

(about 1% of total body iron)

Body iron stores are thus greater in men than in women

The basic rule about body iron regulation:

There is no special pathway for iron excretion

The amount of total body iron is determined only at the level of iron uptake from the duodenum

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

Two types of iron-containing proteins:

1) Haemoproteins

2) Non-haem iron proteins

Haemoproteins: contain iron in the form of haem

Haem: iron inserted in a tetrapyrrole ring

Porphyrins:

• They are intensely red• Under ultraviolet light, they display very strong red

fluorescence• Accumulation of porphyrins is harmful, and results in

rare inherited diseases called porphyrias

• Porphyrin plus iron gives Haem• Heme is an exceptional porphyrin compound:

HAEM IS NOT FLUORESCENT

Porphyrin Haem

Iron in Haemoproteins• Cytochromes of the mitochondrial respiratory chain

(100 mg of iron)

• Haemoglobin: 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 haemoprotein of the liver (about 1 mg)

detoxifies foreign compounds

Non - heme iron proteins

• Ferritin - iron storage protein

• Transferrin: iron transport protein

Ferritin: iron storage protein. In men, contains up to 1 gram of iron

450 kDa protein consisting of 24 subunits Inside the ferritin shell, iron ions form crystallites together with phosphate and hydroxide

ions. The resulting particle is similar to the mineral ferrihydrite. Each ferritin complex can store about 4500 iron (Fe3+) ions.

Reflects the amount of BODY IRON STORES

men: 20-275 μg/litrewomen: 5-200 μg/litre

15 μg/litre and less: insufficient iron stores

• Transports iron in the blood

• Contains only 2 atoms of iron

• 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

Ribonucleotide reductase a protein which is necessary for DNA synthesis:

One more iron-containing protein:

Regulation of iron metabolism:

There is no pathway for iron excretion from the body

therefore

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

Disorders of iron metabolism• 1) Increased absorption of iron from the gut:

HAEMOCHROMATOSIS

• 2) Decreased amount of iron in the body:

IRON DEFICIENCY ANAEMIA

• 3) Inflammation-induced change of iron distributrion:

ANAEMIA OF CHRONIC DISEASE

 Primary Haemochromatosis

Excessive absorption of iron from the gut:

 Iron accumulates in the liver, heart and pancreas,

excess iron damages these organs by free radical production

Transferrin saturation increases, serum ferritin increases

Therapy:Phlebotomy (removal of 0.5 l of blood): a decrease of iron in

the circulation leads to iron mobilisation from stores

• Transfusion dependent anemias, for example • thalassemia major

• leukaemia

• Therapy: iron chelators

Secondary Haemochromatosis

Lack of iron in the body: Iron deficiency (anaemia)

(most common anaemia)

Hypochromic microcytic erythrocytes

Serum ferritin decreases (iron stores are depleted)

transferrin saturation decreases (15 % or less)

 

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

men have higher iron stores than women.

Menstruation, pregnancy and birth deplete iron stores,

Iron deficiency is more common in women than in men

Most common cause of iron-deficiency anemia in women:simply lack of iron in the diet.

Inflammation-induced changes of iron distribution:

Anemia of chronic disease

Mild anemia combined with increased iron stores

mild anemia + increased ferritin

Regulation of iron metabolism

Transferrin uptake

Transferrin receptor

Transferrin

Transferrin receptor

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

Transferrin receptor expression is regulated

posttranscriptionally

at the level of transferrin receptor mRNA stability:

Lack of iron stabilises mRNA for transferrin receptor

Regulation of gene expression:

• Transcriptional: • Increasing the amount of mRNA

• Posttranscriptional:• Regulation of mRNA stability (transferrin receptor)

• Regulation of mRNA translation (ferritin)

Recent (2001) look at iron metabolism:

Iron metabolism is regulated mainly at the level of

IRON EXPORT FROM THE CELL

Iron is transported from the cell by

FERROPORTIN

(a recently discovered iron export protein)

Which cells must be able to export iron?

• Macrophages: • they must recycle about 30 mg daily from old erythrocytes

• Enterocytes (endothelial cells in small intestine):• daily uptake and export of about 1 mg of iron from the diet

• Hepatocytes: • Able to mobilise stored iron from ferritin if needed

Hepcidin: Hepatic bactericidal protein

Hepcidin has antibacterial properties

Discovery of HEPCIDIN (2000)

Hepcidin: "iron regulatory hormone"

Hepcidin is produced in the liver, is transported in the blood stream, and

BLOCKS IRON EXPORT FROM THE CELL

Control of Iron Export from Cells:

Hepcidin blocks iron export from:

MACROPHAGES

ENTEROCYTES IN THE SMALL INTESTINE

Pathophysiology of hereditary hemochromatosis

All hereditary hemochromatosis subtypes display

decreased hepcidin levels

Decreased hepcidin allows more iron to be exported from the enterocytes into blood

Juvenile Haemochromatosis (2004):

Extremely severe form of hemochromatosis

caused by mutation of the hepcidin gene

Regulation of hepcidin expression:

• Iron overload increases hepcidin expression

• Iron deficiency decreases hepcidin expression

• Increased erythropoiesis decreases hepcidin expression

• (Vokurka M et al, 2006: Hepcidin mRNA levels in mouse liver respond

to inhibition of erythropoiesis)

Pathophysiology of x-linked sideroblastic anemia:

• A mutation of porphyrin biosynthesis enzyme causes ineffective erythropoiesis

• Ineffective erythropoiesis decreases hepcidin • Lack of hepcidin leads to increased iron absorption• Iron overload damages pancreas and myocardium

• Patients are treated by repeated phlebotomies

Hepcidin is an acute phase protein

(a protein synthesised in the liver, whose synthesis is increased during inflammation)

2001-2002:

Hepcidin expression dramatically increases during inflammation

Hepcidin demonstrates the strong connection between

iron metabolism and defence against pathogens

Bacteria need iron for their ribonucleotide reductase(DNA synthesis)

Host needs iron for his antibacterial enzymes(Nitric oxide synthase and others)

Bacteria and host compete for free iron

Pathophysiology of anemia of chronic disease

1) Inflammation increases hepcidin synthesis

2) Hepcidin decreases iron export from macrophages

3) Iron is locked up inside the macrophages

4) Iron is locked up in enterocytes, and does not enter the body

Pathophysiology of both hemochromatosis and

anemia of chronic disease can be easily explained

by the action of hepcidin.

Hepcidin summary:

Hepcidin is released from the liver according to body iron status: • iron overload increases hepcidin, • iron deficiency decreases hepcidin expression.

Hepcidin blocks iron export from macrophages and enterocytes.

Inflammation increases hepcidin production.