IRON ABSORPTION AND ITS REGULATION
Notes:
1) Iron in heme is absorbed directly and then the iron is released from the heme inside the cell and is combined with nonheme iron.
2) Nonheme iron bound to components of food must be liberated enzymatically. Also see below for the many factors influencing the bioavailability of iron.
3) Iron is absorbed best in the ferrous Fe2+ form. This is mainly due to higher solubility.
4) The actual mechanism(s) for iron to cross the cell membrane are still under study. some iron seems to be "carried" into the cell by cysteine or histidine. A membrane iron binding protein (MIBP) also seems to be involved. This protein has been partially characterized. It is also possible that transferrin-iron complexes bind to receptors and enter the cell. Some iron may enter by passive diffusion.
5) Once inside, binding to apotransferrin seems to facilitate its entry.
6) Depending on the level of iron stores and blood levels of iron, the iron can be stored inside the epithelial cell or
7) The iron is transported out of the cell into the plasma. Once in the plasma, the iron is oxidized to the ferric form by ceruloplasmin and is then taken up by transferrin.