The iron (III) storage form must be reduced to iron (II) in order to cross the plasma membrane.
In the blood, iron (II) is reoxidized to iron (III) by ferroxidase II.
Iron (III) is carried by the serum protein, transferrin.
Transferrin contains two sites that bind iron (III) tightly.
Normally
- about 1/9 of the transferrin molecules have iron bound at both sites
- about 4/9 of them have iron bound at one site, and
- about 4/9 have no iron bound.
This means that transferrin is normally only about 1/3 saturated with iron (the summary of saturation in the list above indicates that about six out of every 18 sites are occupied), and there is a substantial unsaturated plasma iron binding capacity. An unexpected influx of iron can be handled easily.
The iron binding capacity of serum is of clinical interest. It is accounted for almost entirely by transferrin.
- Serum iron is the concentration of iron present. Normally it is about 100 micrograms of iron per 100 milliliters of blood.
- Total iron binding capacity (TIBC) is the maximum amount of iron that can be bound. Normally this is about 200 micrograms per 100 milliliters.
- The unsaturated iron binding capacity (UIBC) is the difference between the TIBC and the serum iron. It is normally about 200.
Iron binding capacity is used in the differential diagnosis of certain diseases.
- In conditions associated with increased need for iron (iron deficiency or late pregnancy) TIBC is increased, but saturation is decreased from the normal 33%.
- In hemochromatosis, TIBC is low, but it is saturated.
- Certain other clinical conditions are associated with their own characteristic patterns of TIBC and percent saturation.