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

268
Chapter | 9 Iron Metabolism and Its Disorders
mice have iron deficiency anemia, but iron accumulates in
the liver, presumably because of the ability of hepatocytes to
take up NTBI ( Verga Falzacappa and Muckenthaler, 2005 ).
Ceruloplasmin is a glycoprotein with a molecular
weight of 100 to 155 kDa depending on species. It migrates
in the α 2 -region in humans but in the α 1 -region in horses
on protein electrophoresis ( Okumura et al ., 1991 ). During
biosynthesis, six atoms of copper are incorporated into
ceruloplasmin late in the secretory pathway ( Hellman and
Gitlin, 2002 ). It contains most of the copper in the circulation
in all species studied, except for the dog in which
ceruloplasmin accounts for only about 40% of the plasma
copper content ( Montaser et al ., 1992 ). It plays no role in
copper transport or delivery to tissues ( Hellman and Gitlin,
2002 ). Ceruloplasmin has ferroxidase enzyme activity that
facilitates the oxidation of Fe 2 to Fe 3 , a process involved
in iron mobilization from liver and other tissue, but not
from enterocytes ( Osaki et al ., 1971 ; Wessling-Resnick,
2006 ). It also appears to function as an antioxidant in
plasma. Ceruloplasmin is a mild to moderate acute phase
protein that increases in concentration in association with
inflammation ( Ceron et al ., 2005 ; Smith and Cipriano,
1987 ). Its synthesis may also be enhanced by iron deficiency,
estrogen, and progesterone. In contrast to iron,
hepatocytes can excrete copper in the bile by a process that
is dependent on the intracellular concentration of copper
( Hellman and Gitlin, 2002 ).
Haptoglobin is a glycoprotein of approximately 80 kDa
molecular weight that contains approximately 20% carbohydrate.
As an acute phase protein, the liver secretes increased
amounts of haptoglobin into the circulation in response to
inflammation (see Chapter 5) . Plasma haptoglobin concentration
may also increase following glucocorticoid administration
in dogs and cattle ( Harvey and West, 1987 ; Higuchi
et al ., 1994 ; Yoshino et al ., 1993 ). Haptoglobin exists in
dimer and polymer forms and is a major component of the
α 2 -protein band identified by electrophoresis in most species.
Lysis of erythrocytes in the circulation (intravascular hemolysis)
releases free hemoglobin into plasma, and hemoglobin
tetramers spontaneously dissociate into α-β dimers that
are bound by haptoglobin. Each haptoglobin monomer can
irreversibly bind a hemoglobin α-β dimer, preventing some
hemoglobin loss (and therefore iron loss) in the urine following
intravascular hemolysis. Macrophages remove hemoglobin-haptoglobin
complexes from plasma, degrade the
protein complex, and subsequently recycle the released iron.
Haptoglobin also assists in the protection against bacterial
infections by binding to free hemoglobin in infected tissues,
limiting iron availability for bacterial growth. In addition,
haptoglobin functions as an antioxidant because free hemoglobin
promotes oxidative injury, which is inhibited by
binding to haptoglobin ( Melamed-Frank et al ., 2001 ).
Hepatocytes synthesize hemopexin, a 60-kDa acute
phase plasma protein with a remarkably high binding affinity
for heme. Free heme may be released in toxic amounts
when intravascular hemolysis, rhabdomyolysis, or internal
hemorrhage occur. Heme binds to cell membranes
and other lipophilic structures, such as low-density lipoproteins,
and promotes oxidative injury and inflammatory
reactions. Binding of heme to hemopexin dampens these
toxic effects. Like macrophages, hepatocytes can endocytose
heme-hemopexin complexes following binding
to CD91 on their surfaces. Once inside the cell, the complexes
are transported to lysosomes for degradation and
release of iron, and receptors are recycled to the cell surface
( Hvidberg et al ., 2005 ).
VII. TESTS FOR EVALUATING IRON
METABOLISM
A. Hematology
When the hemoglobin concentration reaches a certain level
in developing erythroid cells, it appears to signal the cessation
of cell division. Abnormalities in heme or globin synthesis
result in deficient hemoglobin synthesis and a delay
in the signal for cell division to cease. When this happens,
one or more extra cell divisions occur during erythroid cell
development, resulting in the formation of microcytic erythrocytes
( Stohlman et al ., 1963 ). A deficiency in hemoglobin
synthesis can also result in the formation of hypochromic
erythrocytes with decreased hemoglobin concentration.
Iron is essential for heme synthesis; consequently, iron deficiency
results in deficient heme synthesis and the formation
of microcytic, hypochromic erythrocytes. In addition
to true iron deficiency, disorders that result in functional
iron deficiency or defective iron incorporation into heme
in mitochondria may result in the formation of microcytic,
and possibly hypochromic, erythrocytes. These disorders
include the anemia of inflammatory disease, copper deficiency,
myelodysplastic disorders, drug or chemical toxicities,
and possibly portosystemic shunts ( Harvey, 2000 ).
The mean cell volume (MCV) represents the average
volume of a single erythrocyte in femtoliters (fl 10 15 l).
The MCV is determined most accurately with appropriately
calibrated electronic cell counters that determine the size
of individual cells and compute the MCV. The MCV varies
greatly depending on species and, in some cases, depending
on a breed within a species. Some dogs from Japanese
breeds (Akita and Shiba) normally have MCV values below
the reference intervals established for other breeds of dogs
(Gookin et al ., 1998 ), but these dogs are not anemic. The
MCV is a fairly insensitive indicator of the formation of
microcytes because a relatively high percentage of microcytes
must generally be present in blood for the MCV to
decrease below the reference interval.
The mean cell hemoglobin concentration (MCHC)
represents the average hemoglobin concentration
within erythrocytes. It is calculated by dividing the