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

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Chapter | 9 Iron Metabolism and Its Disorders
investigators using these methods reported ZnPP as free protoporphyrin
( Labbe et al ., 1999 ). Fortunately, ZnPP can now
be measured without extraction in washed erythrocytes using
a hematofluorometer. This dedicated instrument measures the
ratio of ZnPP fluorescence to heme (hemoglobin) absorption
(Labbe et al ., 1999 ). In addition, a new fluorescent method
for the direct and simultaneous measurement of ZnPP, free
protoporphyrin IX, and fluorescent heme degradation product
has been described using human and mouse hemolysates
( Chen and Hirsch, 2006 ).
Increased ZnPP concentrations in circulating erythrocytes
indicates that Fe 2 availability was insufficient in
the mitochondria of their precursors (nucleated erythroid
cells and reticulocytes) and limited heme synthesis in these
precursor cells. Because high ZnPP concentration is only
present in erythrocytes formed during periods of limited
iron availability and erythrocytes have long life spans in
the circulation, conditions of limited iron availability need
to persist for weeks before whole blood erythrocyte ZnPP
concentrations are clearly above reference intervals ( Martin
et al ., 2004 ). Erythrocyte ZnPP concentration is high in
true iron deficiency and in inflammatory disorders in
which iron delivery to erythroid cells is limited ( Feldman
et al ., 1981a ; Labbe et al ., 1999 ; Weeks et al ., 1990 ).
ZnPP concentration is also increased in association with
lead toxicity ( Hawke et al ., 1992 ; Kowalczyk et al ., 1981 ;
Martin et al ., 2004 ). Lead inhibits the ferrochelatase enzyme
to some degree. This inhibition should result in increased free
protoporphyrin, rather than the increase in ZnPP that predominates
in lead toxicity in humans ( Lamola and Yamane, 1974 )
and the 2:1 increase in ZnPP versus free protoporphyrin that
occurs in cattle with lead toxicity ( George and Duncan, 1981 ).
Consequently, it appears that lead impairs iron utilization in
an additional way ( Labbe et al ., 1999 ). Erythrocyte ZnPP
may also increase when iron is being delivered to developing
erythroid cells in the marrow at a rate insufficient to meet the
demands of conditions with accelerated erythropoiesis.
G. Tissue Nonheme Iron
Iron stores (ferritin and hemosiderin) can be determined
directly by measuring nonheme iron concentrations in various
organs. Although it may be desirable to measure total body
nonheme iron stores, this is impossible in clinical patients
and impractical in most research animals. Consequently,
nonheme iron concentration is generally only determined
in the liver and spleen because these organs contain large
quantities of stored iron and are easily biopsied.
Total tissue iron includes heme-containing proteins,
including hemoglobin, myoglobin, and certain enzymes, in
addition to iron stored as ferritin and hemosiderin. Heme
resists acid hydrolysis; consequently, nonheme iron can be
separated from heme iron by extraction in acid and determined
colorimetrically or coulometrically ( Smith, 1997 ).
Nonheme iron stores are decreased in iron deficiency and
increased in iron overload disorders (see Section VII.D).
Nonheme stores are also increased in animals with hemolytic
anemia and in animals with anemia resulting from
decreased erythrocyte production. These conditions are
not necessarily associated with an increase in total body
iron, but rather with a shift of heme iron normally present
in hemoglobin in circulating erythrocytes to nonheme iron
stored as ferritin and hemosiderin within macrophages.
H. Ferrokinetics
Ferrokinetics refers to measurements that are made following
the intravenous injection of transferrin labeled with
radioactive iron or the absorption of radioactive iron from
the diet. Ferrokinetics studies of intravenously administered
radioactive iron provide information concerning plasma iron
turnover and iron incorporation in hemoglobin in circulating
erythrocytes. Radioactive iron is cleared from plasma
with half-time of about 60 to 90min ( Nathanson et al ., 1985 ;
Smith, 1997 ), and from 60% to 95% of the iron present in
plasma is transported to the bone marrow for incorporation
into hemoglobin in developing erythroid cells in animals
(Fillet et al ., 1974 ; Gillis and Mitchell, 1974 ; Kaneko, 1964 ;
Kaneko and Mattheeuws, 1966 ; Smith, 1997 ). Plasma iron
transfer rates, plasma iron turnover rates, and marrow transit
times may also be calculated ( Smith, 1997 ). Ferrokinetic
studies of experimental iron deficiency in dogs revealed that
iron absorption was increased, plasma iron clearance was
shortened, and iron retention in the body was increased compared
to normal dogs ( Nathanson et al ., 1985 ). Ferrokinetic
studies provide useful pathophysiological information, but
they are not practical for use in diagnostic veterinary medicine.
The reader is referred to the fifth edition of this text for
more detailed ferrokinetic information ( Smith, 1997 ).
VIII. DISORDERS OF IRON METABOLISM
A. Iron Deficiency
Iron deficiency may be classified in three stages: storage iron
deficiency, iron-deficient erythropoiesis, and iron deficiency
anemia ( Table 9-2 ). Iron deficiency in domestic animals is
generally not recognized until microcytic anemia is present.
Iron deficiency results from insufficient iron absorption in
the intestine (rare except in nursing animals) or from hemorrhage
and associated iron loss from the body.
1 . Clinical Signs
Clinical signs associated with iron deficiency anemia
include pale mucous membranes, lethargy, weakness, and
weight loss or retarded growth. These signs result not only
from decreased hemoglobin synthesis, but from deficiencies