Clinical Biochemistry of Domestic Animals (Sixth Edition) - UMK ...

II. Iron Distribution
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plasma of humans ( Beutler, 2006b ). Dog and horse ferritin
H and L subunit cDNA clones have been prepared and
sequenced from various tissues. The amino acid residues
involved in the ferroxidase center and in iron nucleation
were conserved in H and L subunits of canine and equine
ferritins, respectively ( Orino et al ., 2005 ).
Although iron is generally stored in cells as ferritin, iron
overloaded cells (macrophages in normal animals) contain
another storage form of iron called hemosiderin, which
represents partially degraded ferritin that forms after uptake
by lysosomes ( Anderson and Frazer, 2005 ; Ponka et al .,
1998 ). Surprisingly little is known about how ferritin releases
its iron in cells. It is possible that intracellular iron release
requires catabolism in lysosomes ( Ponka et al ., 1998 ).
Free cytoplasmic ferritin molecules are water soluble
and visible by electron microscopy, but they are not visible
by light microscopy unless present in large aggregates.
Hemosiderin is insoluble and visible by light microscopy.
Hemosiderin appears gray to black in aspirate smears stained
with routine bloodstains, golden-brown in H & E -stained
tissue sections, and blue when stained with Prussian blue
stain. There is a fairly good correlation between Prussian
blue staining of tissues and the measurement of storage
iron chemically, but staining is less sensitive in detecting
storage iron ( Blum and Zuber, 1975 ; Franken et al ., 1981 ).
Storage iron can be present as ferritin that is not identified
by Prussian blue staining ( Blum and Zuber, 1975 ).
Major sites of iron storage include the liver, spleen, and
bone marrow. The amount stored in a given tissue may vary
by species. The liver appears to be the major site of iron storage
in humans, but the spleen is reported to be more important
in horses and cattle ( Blum and Zuber, 1975 ; Franken
et al ., 1981 ; Kolb, 1963 ). The relative contributions of ferritin
and hemosiderin to the storage iron pool vary with species,
organ, and amount of iron stored. In normal adults,
ferritin concentration generally equals or exceeds hemosiderin
concentration in the liver, but hemosiderin concentration
typically exceeds ferritin concentration in the spleen. When
iron stores are high, hemosiderin tends to accumulate more
than ferritin, and when iron stores are low, ferritin tends
to be higher than hemosiderin ( Kolb, 1963 ; Underwood,
1977 ). Total body iron stores vary at birth between species,
with considerable iron stores in humans, dogs, cattle,
goats, horses, and rabbits; intermediate iron stores in cats
and rodents; and minimal iron stores in pigs ( Keen et al .,
1981 ; Kolb, 1963 ; Underwood, 1977 ). Iron stores decrease
in nursing animals when demands for growth exceed iron
absorption from milk ( Blum and Zuber, 1975 ; Harvey et al .,
1987a ; Keen et al ., 1981 ; Kolb, 1963 ).
C. Myoglobin
Myoglobin is an oxygen-binding protein located primarily
in muscles. It contains one heme group per molecule
and has a structure similar to that of hemoglobin monomers.
Myoglobin serves as a local oxygen reservoir that
can temporarily provide oxygen when blood oxygen
delivery is insufficient during periods of intense muscular
activity. Iron within the heme group must be in the Fe 2
state to bind oxygen. If iron is oxidized to the Fe 3 state,
metmyoglobin is formed. The total amount of myoglobin
in an animal depends on body weight, degree of muscle
development, and the myoglobin concentration in muscle,
which varies between muscle types (red muscle is rich
in myoglobin and white muscle is myoglobin poor). The
myoglobin concentration in muscle varies with species.
For example, the myoglobin concentration in muscles of
racehorses is about 6 times that of human muscles ( Kolb,
1963 ). Because much of the iron in muscle is in myoglobin,
the percentage of total body iron present in muscle is
higher in horses than in humans.
D. Tissue Iron
In addition to the need for iron in hemoglobin and myoglobin,
iron is vital in various metabolic processes. Nearly half
of the enzymes in the citric acid cycle either contain iron or
need it as a cofactor. Heme-containing proteins include catalase,
peroxidase, and cytochromes. Cytochromes a, b, and c
are required for oxidative phosphorylation in mitochondria.
Other cytochromes, such as cytochrome P450 located in
the endoplasmic reticulum, function in oxidative degradation
of endogenous compounds and drugs. Nonheme iron,
in Fe-S compounds and metalloflavoproteins, is required for
enzymes including succinate dehydrogenase, cytochrome
c reductase, nicotinamide adenine dinucleotide dehydrogenase,
xanthine oxidase, and aconitase ( Smith, 1997 ).
E. Plasma Iron
Nearly all of the iron present in plasma in healthy animals
is bound to apotransferrin, a 72 to 83 kDa glycoprotein
(containing 2 to 4 sialic acid residues/molecule) to form
transferrin ( Welch, 1990 ). Apotransferrin is a bilobar protein
with two binding sites for Fe 3 that is synthesized by
the liver. It is believed to have evolved from duplication
of a primordial gene coding for a protein with one ironbinding
site. The concomitant binding of a bicarbonate
anion is required for each Fe 3 molecule bound to apotransferrin
( Ponka et al ., 1998 ). Normally 25% to 50% of the
plasma apotransferrin binding sites are saturated with iron.
When one or two Fe 3 ions are bound, the protein is referred
to as monoferric or diferric transferrin, respectively. There
is a random distribution of iron on binding sites ( Fig. 9-3 ),
with apotransferrin predominant at low plasma iron concentrations
and diferric transferrin predominant at high plasma
iron concentrations ( Huebers et al ., 1984 ). Monoferric
transferrin is predominant at 50% saturation of transferrin