Iron in intestinal mucosal cells or stored in the liver (see
below) may be transferred into the blood for transport to other tissues.
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.
- 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.
There are three components to the iron binding capacity of serum.
- 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 300 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
- In conditions associated with increased need for iron (iron deficiency or
late pregnancy) TIBC is increased, but saturation is decreased from the normal
- 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.
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Last modified 7/22/97