Page 295 - Color Atlas of Biochemistry

286 Tissues and organs

Iron metabolism very tightly. Similar iron transport proteins
are found in secretions such as saliva, tears,
and milk; these are known as lactoferrins
A. Distribution of iron
(bottom right). Transferrin and the lactofer-
Iron (Fe) is quantitatively the most important rins maintain the concentration of free iron in
–1
trace element (see p. 362). The human body body fluidsatvaluesbelow 10 –10 mol L .
contains4–5giron,whichisalmostexclusively This low level prevents bacteria that require
presentinprotein-boundform.Approximately free iron as an essential growth factor from
three-quarters of the total amount is found in proliferating in the body. Like LDLs (see
heme proteins (see pp. 106, 192), mainly he- p. 278), transferrin and the lactoferrins are
moglobin and myoglobin. About 1% of the taken up into cells by receptor-mediated
iron is bound in iron–sulfur clusters (see endocytosis.
p. 106), which function as cofactors in the Excess iron is incorporated into ferritin and
respiratory chain, in photosynthesis, and in stored in this form in the liver and other
other redox chains. The remainder consists organs. The ferritin molecule consists of 24
of iron in transport and storage proteins subunits and has the shape of a hollow sphere
(transferrin, ferritin; see B). (bottom left). It takes up Fe 2+ ions, which in
the process are oxidized to Fe 3+ and then
deposited in the interior of the sphere as fer-
B. Iron metabolism
rihydrate. Each ferritin molecule is capable of
Iron can onlybe resorbedbythe bowel in storing several thousand iron ions in this way.
2+
bivalent form (i. e., as Fe ). For this reason, In addition to ferritin, there is another storage
reducing agents in food such as ascorbate form, hemosiderin, the function of which is
(vitamin C; see p. 368) promote iron uptake. not yet clear.
Via transporters on the luminal and basal side
of the enterocytes, Fe 2+ enters the blood,
where it is bound by transferrin. Part of the Further information
iron that is taken up is stored in the bowel in Disturbances of the iron metabolism are fre-
the form of ferritin (see below). Heme groups quent and can lead to severe disease pictures.
can also be resorbed by the small intestine. Iron deficiency is usually due to blood loss,
Most of the resorbed iron serves for the or more rarely to inadequate iron uptake.
formation of red blood cells in the bone mar- During pregnancy, increased demand can
row (erythropoiesis,top). As discussed on also cause iron deficiency states. In severe
p. 192, it is only in the final step of hem bio- cases, reduced hemoglobin synthesis can
synthesis that Fe 2+ is incorporated by ferro- lead to anemia (“iron-deficiency anemia”). In
chelatase into the previously prepared tetra- thesepatients, theerythrocytes are smaller
pyrrol framework. and have less hemoglobin. As their membrane
In the blood, 2.5–3.0 g of hemoglobin iron is also altered, they are prematurely elimi-
circulates as a component of the erythrocytes nated in the spleen.
(top right). Over the course of several months, Disturbances resulting from raised iron
the flexibility of the red blood cells constantly concentrations are less frequent. Known as
declines due to damage to the membrane and hemochromatoses, these conditions can
cytoskeleton. Old erythrocytes of this type are have genetic causes, or may be due to re-
taken up by macrophages in the spleen and peated administration of blood transfusions.
other organs and broken down. The organic As the body has practically no means of ex-
part of the heme is oxidized into bilirubin (see creting iron, more and more stored iron is
p. 194), while the iron returns to the plasma deposited in the organs over time in patients
pool. The quantity of heme iron recycled per with untreated hemochromatosis, ultimately
day is much larger than the amount resorbed leading to severe disturbances of organ func-
by the intestines. tion.
Transferrin,a β-globulin with a mass of
80 kDa, serves to transport iron in the blood.
This monomeric protein consists of two sim-
ilar domains, each of which binds an Fe 2+ ion

Koolman, Color Atlas of Biochemistry, 2nd edition © 2005 Thieme
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