Iron Metabolism

List the two major functions of iron in the body.

1. Stabilize oxyhemoglobin for oxygen transport.

2. Functions in oxidation-reduction enzyme systems such as cytochromes, peroxidase, and catalase.

These enzymes act as electron transfer agents in oxidation-reduction reactions.

Cytochromes

Give the average body iron content of adult men and women.

Men = ~4 g

Women = 2-3 g

What percentage of body iron is found in hemoglobin?

60-70%

What percentage of body iron is in storage form?

20-30%

What are the two storage forms of iron?

Ferritin and hemosiderin

Why is serum ferritin useful in diagnosis of iron deficiency?

Serum ferritin is found in minute quantities, but these quantities are proportional to total body-iron stores, making it useful as an indicator of depleted stores.

When is hemosiderin formed? Where is it predominately found?

Formed when ferritin is broken down in secondary lysosomes. Found predominately in cells of the liver, spleen, and BM.

What percentage of total body iron is found in myoglobin?

4-10%

What protein binds iron for transport in the serum? What is the iron-protein complex called?

1. Apotransferrin

2. Transferrin

How is the amount of iron in the body regulated? Why?

There is no physiological "planned" excretion of iron, so iron balance is regulated by the amount of iron absorbed.

Intestinal mucosal cells absorb more iron than the body will take up. How much dietary iron is actually taken up into the blood stream?

5-10%

Why does a high intestinal pH interfere with iron absorption?

Alkaline pH causes the formation of insoluble compounds from the interaction of bile acids with iron salts.

List 6 factors affecting the absorption of iron.

1. Alkaline intestinal pH

2. Conversion of iron from ferric (Fe3+) to ferrous (Fe2+) form for entry into mucosal cells

3. Ascorbic acid, some sugars, and amino acids

4. Type of iron in diet; heme iron versus non-heme iron

5. State of iron stores

6. Hematopoietic activity

What effect do ascorbic acid (vit. C), sugars, and amino acids have on iron absorption.

Enhance

What oxidation state is heme iron in? What effect does this have on iron absorption?

Ferrous (Fe2+), attached to heme molecule; the entire heme molecule can easily passively diffuse into the intestinal mucosal cell, where the iron is split away

What oxidation state is non-heme iron in? What effect does this have on iron absorption?

Ferric (Fe3+) - it must be reduced before it can be absorbed; it is not as readily absorbed as heme iron.

What process allows iron to be absorbed into intestinal mucosal cells?

Passive diffusion of the heme molecule or free ferrous iron

What happens to the ferrous iron that enters the mucosal cells?

It is immediately oxidized to the ferric state to be bound to apoferritin, forming ferritin.

What are the iron stores present in intestinal mucosal cells referred to as? Why?

Referred to as a labile pool, as the mucosal cells may potentially be sloughed off at any time.

What is the transport form of iron in the blood stream?

Transferrin

How much iron can apotransferrin bind?

Two atoms

What oxidation state must iron be in in order to bind to apotransferrin?

Ferric

What type of protein is transferrin? (Review question - think back to protein lectures)

Beta one globulin

How much iron can apoferritin bind?

4,000 iron atoms

Describe what happens physiologically when iron stores are high to decrease iron absorption.

Production of apotransferrin decreases, so there are fewer carrier molecules to transfer iron from intestinal mucosal cells. The iron will be stored as ferritin within the mucosal cells and lost when the cell is sloughed off.

Describe what happens physiologically when iron stores are depleted to increase iron absorption, and the feedback mechanism involved when iron stores are once again sufficient.

Synthesis of apotransferrin increases, allowing for more iron to be absorbed from the intestinal mucosal cells. As iron stores in the form of ferritin build up in the liver, apotransferrin synthesis decreases again.

List the steps involved including iron oxidation states during absorption of iron into the intestinal mucosal cells until it is put into circulation.

1. Ferrous iron (Fe2+) passively diffuses into intestinal mucosal cell

2. This ferrous iron binds to apoferritin

3. Once bound, the ferrous iron with this complex is oxidized to ferric iron (Fe3+) for storage as ferritin within the mucosal cell

4. To release iron into the blood stream, ferric iron (Fe3+) in ferritin is reduced to release iron from ferritin

5. The ferrous iron (Fe2+) is oxidized to ferric iron (Fe3+) when it binds with apotransferrin to form transferrin for distribution in the body

List the steps involved in cell uptake of iron.

1. Iron-loaded transferrin (Fe2-Tf) binds to Transferrin receptors (TfR) on the cell surface

2. The transferrin-receptor complex is endocytosed by the cell

3. Protons are pumped into the formed endosome, releasing iron from transferrin

4. Iron is transported out of the acidified endosome into the cytoplasm

5. Cytoplasmic iron either enters the mitchondria for heme synthesis or is stored as ferritin

6. Endosome fuses with cell membrane to release apotransferrin back into the blood stream

Why is free iron typically not found in the body?

It is extremely toxic

Why are the TIBC, UIBC, and % saturation useful in indicating iron stores?

The synthesis of apotransferrin is inversely proportional to the intracellular ferritin levels of hepatocytes. Under normal conditions, transferrin is 20-30% saturated with iron.

Give another name for transferrin.

Siderophyllin

What does the serum iron test measure?

The amount of iron bound to transferrin.

What does the total iron binding capacity (TIBC) measure?

The total amount of iron that transferrin can bind.

What does the unsaturated binding capacity (UIBC) measure?

Unbound transferrin.

Is TIBC usually directly measured?

No, it is calculated by adding the UIBC and serum iron.

What does percent saturation indicate? How is it calculated?

The amount of iron present in the serum vs. the amount of transferrin present.

% sat = serum iron/TIBC x 100

An abnormally low result for this test is diagnostic for IDA.

Serum ferritin

What conditions are associated with elevation of serum ferritin levels out of proportion with the level of iron stores?

Chronic inflammation and active liver disease.

Why is serum ferritin useful in the diagnosis of IDA?

Low serum ferritin indicates depletion of iron stores caused by an inadequate amount of iron absorbed for normal body function, causing anemia.

What does the FEP/ZPP measure?

The protoporphyrins that have been forced to bind zinc due to a lack of iron.

What does FEP and ZPP stand for?

Free Erythrocyte Protoporphyrin

Zinc Protoporphyrin

What does an increased level of serum transferrin receptors indicate?

Tissue iron deficiency

When is it recommended to draw samples for an iron panel? Why?

Morning, fasting specimens are preferred, as iron levels exhibit diurnal variation, with levels highest in the morning.

List four causes of iron deficiency.

1. Malabsorption

2. Increased loss, such as through chronic blood loss

3. An increased demand for iron stores such as in IDA

4. Decreased release from iron stores, such as with a reticuloendothelial infection

List four causes for increased serum iron levels.

1. Hemolytic anemia

2. Decreased iron utilization (lead poisoning, other interferences with hemoglobin synthesis)

3. Increased released from stores (liver disease/inflammation)

4. Defective storage

At this grade of iron deficiency, total body iron is decreased, but there is still enough present to support all hemoglobin synthesis.

Grade I Iron Deficiency

At this grade of iron deficiency, iron supply has become insufficient for normal erythropoiesis, resulting in iron-deficient erythropoiesis.

Grade II Iron Deficiency

At this grade of iron deficiency, the iron supply is no longer able to maintain normal hemoglobin levels.

Grade III Iron Deficiency

What item on an iron panel differs between Grade II and Grade III iron deficiency?

RBC morphology is normal in Grade II, but microcytic hypochromic in Grade III.

What lab value is abnormal in Grade I iron deficiency?

Ferritin is decreased

Chronic inflammatory disorders, infections, or neoplastic disorders have this effect, resulting in abnormal iron metabolism.

They block the mobilization of iron stores.

Give the expected iron panel results for a patient with chronic disease.

1. Serum Iron

2. Ferritin

3. Sideoblast Count

4. ZPP or FEP

5. % sat.

6. RBC morphology

7. TIBC

1. SI - Decreased (can't be taken from storage)

2. Ferritin - Increased (overloaded storage areas leak into blood)

3. Sid. Ct. - Decreased

4. ZPP/FEP - Increased

5. % sat - Decreased to normal

6. RBC - normal

7. Decreased (too much iron stored - avoiding absorption)

In this hereditary iron metabolism disorder, the passage of iron through intestinal cells and into the blood stream is relatively unhindered compared to normal, increasing iron absorption.

Hereditary Hemochromatosis

This disorder results in the following lab results:

1. Serum Iron - increased

2. Ferritin - increased

3. FEP/ZPP - normal

4. % sat. - increased

5. RBC morph - normal

6. TIBC - normal to decreased

Hemochromatosis - there is an overall increase in iron, which causes the serum iron, ferritin, and % saturation levels to increase.

A hallmark symptom of hemochromatosis.

Bronzed skin

List the classic triad of symptoms for hereditary hemochromatosis.

1. Bronzing of skin

2. Liver cirrhosis

3. Diabetes

List two causes of non-hereditary hemochromatosis.

1. Abnormally increased intestinal absorption

2. Iron accumulation from multiple transfusions (Thalassemia major and myelodysplastic treatment)

Give the pathophysiology of pernicious anemia and why it affects iron levels.

Lack of intrinsic factor leads to reduced or non existent absorption of B12, affecting DNA synthesis, leading to a decrease in the total number of cells. Because there is decreased erythropoiesis, iron has nowhere to go.

Pernicious anemia, hemolytic anemia, and Thalassemia all share this iron profile.

1. Serum iron - increased

2. Ferritin - increased

3. FEP/ZPP - normal

4. % sat. - increased

5. TIBC - decreased

6. RBC Morph. - abnormal

Stores are built up as there is a normal amount of body iron, but it's not being utilized, thus increasing ferritin and leading to less transferrin is being made (low TIBC); however, what transferrin is present is binding iron, leading to an increased % sat.

Why does thalassemia lead to an abnormal iron profile?

Ineffective hemoglobin synthesis leads to normal amounts of iron present in the body, but the iron is not being utilized.

List the steps of the serum iron test.

1. Disruption of iron-protein complex with strong acid (HCl, H2SO4, TCA)

2. Reaction of iron with chromogen, usually ferrozine, in presence of reducing agent

Why must a reducing agent be used when measuring serum iron?

The chomagen only reacts with with ferrous iron (Fe2+).

List the steps for measuring TIBC.

1. Excess iron added to the serum (typically ferric ammonium sulfate)

2. pH adjusted to 8.0 to stabilize iron-transferrin complex

4. Excess, unbound iron removed with magnesium carbonate

5. Iron measured in the same procedure as serum iron

List the steps for measuring the UIBC.

1. A known excess quantity of ferric iron is added

2. pH adjusted to 8.0

3. Free iron reduced from ferric state to ferrous state. Bound iron is not affected and remains in ferric state

4. Ferrous iron is reacted with the chromagen

5. Added iron - measured iron = UIBC

What metal can interfere with the iron assay? How is this corrected?

Copper - corrected with the addition of thiourea to prevent formation of ferrozine-copper complexes.

Will slight hemolysis, lipemia, or icterus affect the iron assays?

No.

Give the normal values for an iron panel:

1. Serum iron

2. TIBC

3. % sat.

4. Transferrin

5. Ferritin - men and women

1. 65-185 µg/dL

2. 300-500 µg/dL

3. 20-30%

4. 144-511 mg/dL

5. Men: 10-273 ng/mL, Women: 5-99 ng/mL