L8: Iron metabolism and microcytic anaemias Flashcards
What are microcytic anaemias?
Erythrocytes are smaller
Reduced rate of haemoglobin synthesis
Cells paler (hypochromic)
What causes microcytic anaemias?
Reduced haem synthesis
-Anaemia of chronic disease –> Hepcidin result in functional iron deficiency (plenty of it but cant be used)
-Iron deficiency–> required for haem synthesis
-Lead poisoning–> acquired defect
-Sideroblastic anaemia–> inherited defect in haem synthesis
Reduced globin chain synthesis
- Thalassaemia –> α and β
–> α –> deletion or loss of one or more of α globin genes
–> β –> mutation in β globin genes leading to reduction or absence of the β globin
Mnemonic –> TAILS
What is iron? Whats its function?
Element
Essential in all living cells
Free iron–> potentially toxic to cells
Complex regulatory system–> safe utilisation, absorption and transport
Required for:
- O2 carriers –> haemoglobin in red cells
–> myoglobin in myocytes
-Cofactor in many enzymes–> cytochromes (OP), Krebs cycle enzymes, cytochrome P450 enzymes (detoxification), catalase
Body has no mechanism for excreting iron
What is the difference between ferrous and ferric?
Ferrous Fe2+ –> reduced form–> absorbed from diet in this form
Ferric Fe3+ –> oxidised form
How is ferric reduced to ferrous and vice versa?
Ferric (Fe3+) + e- –> Ferrous (Fe2+) reduction low pH (acid)
Ferrous (Fe2+) –> Ferric (Fe3+) + e- oxidisation high pH (alkaline)
What is the difference between haem and non-haem iron?
Haem–> associate with globin–> haemoglobin
Come from animals–> liver, kidney, steak, beef burgers etc
Easily/readily absorbed
Non-haem –> Ferrous or ferric form–> fortified cereals, raisins, beans, figs, barely, oats, rice and potatoes
Converted to ferrous for absorption
How much iron is needed in the diet and where is it absorbed?
10-15 mg/day
Absorbed in the duodenum and upper jejunum
How is iron absorbed into the bloodstream?
- Chyme enter duodenum/ upper jejunum
- a) Haem–> readily absorbed by the enterocytes
- inside Fe2+ released by haem oxygenase
b) Non Haem–> Fe3+ –> Fe2+ via reductase enzymes in brush border
–> Requires Vit C as electron donor
–> Fe2+ –> enters enterocyte through DMT1 (divalent metal transporter 1)- H+ ion out - Fe2+ stored- storage protein ferritin in Fe3+ form
OR - Enters bloodstream through ferroportin
- Transported- Fe3+ form- converted by Hephaestin
- Fe3+ binds to transferrin—> transported around the blood
What inhibits the role of ferroportin?
Hepcidin
Peptide hormone
Produced by liver
Bind to ferroportin–> degradation
What factors affect the absorption of non-haem iron from food?
Negative influences–> Tannis (in tea), phytates (pulses), fibre and antacids (gavison)
- -> bind to non-heam iron in the intestine reduce absorption
- -> need acidic environment to convert Fe3+–> Fe2+
Positive influences–> Vit C and citrate–> prevent formation of insoluble iron compounds
VitC require for conversion of Fe3+–> Fe2+
What is the difference between functional and stored iron?
Functional iron–> available
- Heamoglobin (2000mg)
- Myoglobin (300mg)
- Enzymes- cytochromes (50mg)
- Transported iron (transferrin) (3mg)
Stored iron (1000mg)
How can iron be stored?
Ferritin–> soluble form
- -> stored in enterocyte
- -> globular protein complex with hollow core–> pores allow iron to enter and be released
Haemosiderin –> insoluble form
- -> Aggregates of clumped ferritin particles, denatured protein and lipid
- -> Accumulates in macrophages, particular in liver and spleen
How is iron taken up into cells?
1- Fe3+ bound trasferrin binds transferrin receptor and enters the cytosol receptor-mediated endocytosis
2- Fe3+ within the endosome released by acidic microenvironment and reduced to Fe2+
3- The Fe2+ transported to the cystol via DMT1
4- Once in cystol Fe2+ can be:
- stored in ferritin
-exported by ferroportin (FPN1) or
-taken up by mitochondria - cytochrome enzymes
What is meant by iron recycling?
Small intake in diet Most (>80%) - recycled from damaged or senescent RBC Phagocytosis by macrophages Splenic macrophages and kupffer cells - Liver Catabolise haem released from RBC AA reused and iron: - exported to blood - transferrin - stored - Ferritin - macrophages
How is iron absorption regulated?
Determined by: dietary factors, iron stores, erythropoiesis
Sensed by enterocytes
Controlled mechanisms
- Regulation of transporter- ferroportin
- Regulation of receptors- Transferrin receptor and HFE protein (homeostatic iron regulator)
- Hepcidin and cytokines
- Crosstalk between the epithelial cells and other cells like macrophages
How does hepcidin regulate iron absorption?
Negative regulator
Synthesis increased in iron overload
Decreased by high erythropoietic activity
Induces the internalisation and degradation of ferroportin
What is meant by anaemia of chronic disease?
Anaemia caused by inflammation
Inflammatory condition–> cytokine release (IL-6)
Main effect–> increased hepcidin production–> inhibition of ferroportin–> decreased iron released from reticuloendothelial cells and reduced absorption from diet–> plasma iron reduced–> inhibition of erythropoiesis in bone marrow–> anaemia (less RBC)
Minor pathway–> Inhibition of erythropoietin production and inhibition of erythropoeisis–anaemia
What type of deficiency is anaemia of chronic disease?
Functional deficiency
enough iron in body but cannot utilise it
How much iron is lost from the body each day and how? Is iron excretion regulated?
1-2 mg/day - desquamation of epithelia - menstural bleeding - sweat - pregnancy--> 3.5mg/day No--> no mechanisms to control excretion
What is significant about iron deficiency?
Most common nutritional disorder
1/3rd population anaemic- 1/2 iron deficient
Sign not a diagnosis
What are the causes of iron deficiency?
Insufficient iron in diet Malabsorption Bleeding Increased requirement (pregnancy) Anaemia of chronic disease
Why is the iron requirement for females 19-50 greater than males of similar age?
Females of this age lose lots of blood during menstruation each month
What groups are at risk?
Children
Infants
Women of child bearing age
Geriatric age group (elderly)
What are the signs and symptoms of anaemia?
- Tiredness, pallor, reduced exercise tolerance, cardiac (angina, palpitations, development of HF), increased resp rate, headache, dizziness, light-headedness
- Pica (unusual cravings)–> non-nutritive substances
- Cold hands and feet
- Epithelial changes (angular cheilitis, glossy tongue with atrophy of linguinal papillae), Koilonychia (spoon nails)
What are the features on a blood film?
Low mean corpuscular volume (MCV)
Low mean corpuscular haemoglobin concentration (MCHC)
Elevated platelet count (>45,000/microlitres)
Normal or elevated WBC count
Low serum ferritin, serum iron and %transferrin saturation, raised total iron binding capacity
Low reticulocyte haemoglobin content (CHr)
What would a blood smear look like in iron deficiency?
RBC are microcytic and hypochromic in chronic cases
Anisopoikilocytosis: change size and shape
Sometimes pencil and target cells
How do you test for iron deficiency?
Plasma ferritin commonly used marker of total iron status (predominantly a cystolic protein- small amounts secreted into blood)
–> reduced= iron deficiency
Normal or increased does NOT exclude iron deficiency (increase in cancer, infection, inflammation, liver disease, alcoholism)
CHr (reticulocyte haemoglobin content)–> test for functional iron deficiency–> remains low during inflammatory response
Low for thalassaemia pt too
How is iron deficiency treated?
Dietary advice
Oral iron supplements (potential GI side effects)
Intramuscular iron injections
Intravenous iron
Blood transfusion (only used for severe anaemia with imminent cardiac compromise)
What should be the response to treatment?
Improvement in symptoms
20g/L rise in Hb in 3 weeks
Why is iron excess dangerous?
Exceed binding capacity of transferrin
Excess iron deposited in organs–> haemosiderin (insoluble- denatures proteins/lipids)
Iron promoted free radical formation and organ damage
What is the fenton reaction? Why is it damaging?
Creation of free radicals from iron Fe2+ + H2O2--> Fe3+ + OH• + OH- Fe3+ + H2O2--> Fe2+ + OOH• + H+ Hydroxyl and hydroperoxyl radicals cause damage: - Lipid peroxidation - Damage to proteins - Damage to DNA
What is Transfusion Associated Haemosiderosis?
Repeated transfusion–> gradual accumulation of iron
400ml blood= 200mg iron
Problem with transfusion dependent anaemias such as thalassaemias and sickle cell anaemia
Iron chelating agents such as desferrioxamine can delay but not stop effects of overload
What are the effects of transfusion associated haemosiderosis with iron overload?
Liver cirrhosis Diabetes mellitus Hypogonadism Cardiomyopathy Arthropathy Slate grey colour of skin
What is Hereditary Haemochromatosis?
Autosomal recessive disease–> mutation in HFE gene (Chr 6)
HFE protein–> Interacts with transferrin receptor –> reduce affinity for iron bound transferrin
HFE also has increases hepcidin production
Mutated HFE cannot bind to transferrin–> negative influence on uptake is lost
AND –> underproduciton of hepcidin–> increased iron uptake
Iron accumulates in end organs causing damage
Treated with venesection –> removal of blood (1-2pints)
What are the features of hereditary haemochromatosis?
Liver cirrhosis Diabetes mellitus Hypogonadism Cardiomyopathy Arthropathy Increased skin pigmentation
