Lecture 16 Iron Metabolism Flashcards
Iron Compartments
68% hemoglobin, 18% storage, 10% myoglobin
Heme iron
- mainly from meat
- absorbed more efficiently
Nonheme iron
- sources are nonmeat such as legumes and leafy vegetables
- accounts for 90% of dietary iron
- only 2-20% absorbed depending on iron status
- absorption enhanced by citrate and acidic foods
- substances that inhibit absorption are phosphates, phytates, oxalates, calcium, foods that form insoluble iron complexes
Ferroportin
- transports iron across enterocyte into blood circulation
Ferritin
storage form of ferric iron
- apoferritin with iron
- can store up to 4500 iron atoms
- primary iron storage, readily available
- found in blood, bone marrow, liver and spleen
- reflective of amount of storage iron in body
Transferrin
- blood iron transporter
- ferric iron bound to transferrin to be transported from duodenum enterocytes to bone marrow and tissues
Hepcidin
- antimicrobial peptide produced in the liver
- negative regulator of intestinal iron
- degrades ferroportin
Transferrin receptors
- present in large numbers of RBC normoblasts and other rapidly dividing cells (placenta, liver)
- glycoproteins located on virtually all cells except mature RBCs
- binds transferrin and takes it in by endocytosis
- also plays role in release of iron from transferrin in cell
- synthesis induced by iron deficiency
- when fully saturated, deposited in liver for storage
Excretion and Regulation of iron
- humans have no effective means to excrete iron and rely on controlling absorption to regulate iron levels
- iron absorbed inversely proportional to iron stores
Iron loss
- mainly by feces
- perspiration, natural skin exfoliation, lactation, menstruation
Iron-responsive protein (IRP) and Iron-responsive elements
- responsible for increasing Hgb production when intracellular iron low
1. IRPs bind IRE to increase RNA product’n -> synth of transferrin receptors and ALA synthase
2. More iron uptake -> greater Hgb production
3. More protoporphyrin IX product’n to accomodate increased iron uptake - steps above -> greater Hgb product’n
Hemosiderin
- storage form of iron
- found primarily in macrophages, formed by partial degradation of ferritin
- when there are hi levels of cellular iron, ferritin processed to form hemosiderin
Ferritin and Hemosiderin are stored:
- mostly in the liver
- bone marrow
- spleen
Serum iron
- measure of ferric iron bound to transferrin (excludes iron bound to Hgb)
- decreased in iron deficiency, inflammatory disorders, acute infection, heart attack
Unsaturated iron binding capacity (UIBC)
iron binding sites not carrying iron
Total iron binding capacity (TIBC)
total available transferrin sites
similar to UIBC, measurement of unbound transferrin
Plasma ferritin
- in equilibrium with stores
- variations in stored ferritin are therefore reflected in plasma
- declines early in iron deficiency anemia
- increases in inflammation regardless of iron stores
Serum transferrin receptors (sTfr) test
- increased numbers in IDA
- decreases with excess iron
Free erythrocyte protoporphyrin test
- excess protoporphyrin due to disorders of heme synthesis
- clinical application for IDA, lead poisoning, sideroblastic anemia
Sideroblasts
nucleated RBC precursor
Siderocytes
non-nucleated RBCs containing iron and would be considered abnormal
- should not be detectable in retics and mature RBCs
Ringed sideroblast
5 or more iron granules at least 30% of circumference of nucleus
