Iron Storage and Toxicity (Chapter 18)

Study Notes

Iron Storage and Toxicity

Free iron is highly toxic and must be sequestered to prevent its toxic effects.
Storage iron is bound to either ferritin or hemosiderin to prevent its toxicity.

Ferritin

Ferritin is a ubiquitous protein-iron complex found at highest levels in the liver, spleen, bone marrow, and skeletal muscles.
In the liver, most ferritin is stored within parenchymal cells, while in other tissues like the spleen and bone marrow, it is found mainly in macrophages.
Hepatocyte iron is derived from plasma transferrin, whereas storage iron in macrophages is derived from the breakdown of red cells.
Intracellular ferritin is located in the cytosol and lysosomes.

Hemosiderin

Hemosiderin is formed from the partial degradation of ferritin protein shells in lysosomes.
Iron in hemosiderin is chemically reactive and turns blue-black when exposed to potassium ferrocyanide (Prussian blue stain).
With normal iron stores, only trace amounts of hemosiderin are found in the body, mainly in macrophages in the bone marrow, spleen, and liver.
In iron-overloaded cells, most iron is stored in hemosiderin.

Iron Regulation and Balance

Plasma ferritin levels correlate well with body iron stores, with levels below 12 µg/L in iron deficiency and up to 5000 µg/L in iron overload.
The storage iron pool can be readily mobilized if iron requirements increase, such as after blood loss.
Iron is both essential for cellular metabolism and highly toxic in excess, so total body iron stores must be regulated meticulously.
Iron balance is maintained by regulating the absorption of dietary iron in the proximal duodenum.
There is no regulated pathway for iron excretion, which is limited to 1 to 2 mg lost each day through the shedding of mucosal and skin epithelial cells.
As body iron stores increase, absorption falls, and vice versa.

Description

This quiz covers the importance of sequestering storage iron to prevent toxicity, and how ferritin and hemosiderin bind to iron in the storage pool. It also discusses the distribution of ferritin in different tissues.