Iron

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Functions

The three major functions of Iron are:

  1. Oxygen transport (uses one of its 6 coordinate bonds)
  2. Electron transport in the cytochromes of the respiratory chain of mitochondria (uses its ability to change valence)
  3. As an essential cofactor in ribonucleotide reductase (a regulator of DNA Synthesis; it uses its ability to change valence).

Iron Diagnostics

Serum Iron

  • Measures iron concentration in serum
  • Measures only iron bound to transferrin
  • The two iron-binding sites are normally < 50% saturated

Total Iron Binding Capacity (TIBC)

  • potentially bound iron for a person's circulatory system
  • assumes that transferrin is completely saturated by iron

Transferrin saturation

Serum Ferritin

  • Detects ferritin that leaks into the circulation
  • Amount of serum ferretin is proportional to the amount of ferretin in the body
  • Therefore, it is an effective measure of iron stores

Hemosiderin

  • Most concentrated form of storage iron
  • Allows for a visual estimation of iron in a tissue sample as a result of the Prussian Blue stain

Absorption

Various factors increase and decrease absorption of iron:

Increase absorption

  • Increased acidity (lower pH)
  • Chelating agents (Organic acids, amino acids)
  • iron deficiency

Decrease absorption

  • Phosphate
  • Phytates
    • binds iron and renders it insoluble
    • very common in vegetables
  • Oxalates
  • Dietary fiber
  • Small bowel diseases
  • The type of iron consumed also affects absorption - from meat >> from plants >> elemental iron

Metabolism

  • Not easily absorbed in the duodenum (most likely as protection)
  • heme Fe and non-heme Fe are absorbed via different mechanisms
  • most dietary Fe is non-heme and inefficiently absorbed
  • Absorption depends on:
    • Chemical form - heme Fe vs. inorganic Fe
    • Binding components - Parts of the diet that prevent iron release or solubility
    • Stomach function - important since acid solubilizes iron
    • Absorptive capacity of the upper intestine - iron is absorbed in the duodenum

Homeostasis

A model of iron homeostasis mechanisms.
  • Body has 4g of iron content
  • Body Fe stores are ~1g
  • Mechanism for homeostasis is not completely understood
  • Basically, 1-2mg are necessarily lost each day, but these are replaced by 1-2mg that are absorbed in their stead.
  • If the body detects low iron, it tries to compensate (through an unknown mechanism) by absorbing more iron in the intestines.
  • There is no excretion mechanism for iron

Deficiency

  • Iron deficiency is common because:
    • Many forms of cereal and vegetable iron are unavailable to large segments of the world population
    • Meat consumption is costly and not wide
    • Many segments of the world population are subjected to parasites that cause intestinal bleeding
    • Some foods bind iron and make it unavailable to the body (e.g., phytate)

Overload

  • Excessive amounts of iron in the body are toxic and can result in damage to the liver, heart and pancreas
  • Usually the result of specific disease processes
  • Free Iron creates lots of free-radicals and steals oxygen

Hemochromatosis

Secondary iron overload

  • Occurs in diseases such as thalassemia' and sideroblastic anemia
  • Iron accumulation as a secondary consequence of heme or globin missynthesis
  • Mainly a side effect of therapeutic blood transfusion

Hemosiderosis

  • Term used to describe stainable Fe deposits in tissues
  • Have to be transfused for treatment

Resources