Metabolism of Iron and Heme
Heme, an iron-porphyrin complex, is the prosthetic
group of many important proteins. The central role of
hemoglobin and myoglobin in oxygen transport and stor-
age was discussed in Chapter 28. Heme proteins or en-
zymes are involved in redox reactions (e.g., cytochromes)
and participate in many oxidation reactions needed for
synthesis of metabolically important compounds as well
as for degradation and detoxification of waste products
and environmental toxins.
Ionic forms of iron (referred to hereafter as iron) also
participate in a variety of enzymatic reactions as nonheme
irons, which are present as iron-sulfur clusters (e.g., mito-
chondrial electron transport). There are also both storage
and transportable forms of iron that are bound to pro-
teins. Under normal physiological conditions only trace
amounts of free iron exist. In the body, if iron exceeds the
sequestration capacity of the iron-binding proteins present
in different physiological compartments, the free iron can
cause tissue damage. Cellular injury is caused by reactive
oxygen species that are produced from H
in a reaction
catalyzed by iron. Thus, iron homeostasis in the body is
in a delicate balance. Either the deficiency or the excess
results in abnormalities and presents as a common cause
of human diseases.
Iron Metabolism
Total-body iron of a 70-kg adult is about 4.2-4.4 g. The
distribution of iron in various body compartments is given
in Table 29-1. The key players of iron metabolism in-
clude iron-responsive elements of appropriate mRNAs,
iron regulatory proteins divalent metal transporter
major histocompatibility complex (MHC) class I-like pro-
tein designated as HFE protein, /3
-microglobulin, trans-
ferrin, transferrin receptor, and ferritin.
Absorption of Iron from the Diet
The dietary requirement for iron depends on the amount
and composition of the food, the amount of iron lost from
the body, and variations in physiological state such as
growth, onset of menses, and pregnancy. The average
North American diet contains about
mg of iron per
1000 calories and supplies about 10-15 mg/d. Of that in-
gested, 8-10% (1-1.5 mg/d) is absorbed. Thus, dietary
factors that affect absorption are more important than the
iron content of the diet and may be more important for
correction of iron deficiency than addition of iron to the
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