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

III. Developing Erythroid Cells
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coordinate protoporphyrin IX formation with the availability
of iron by increasing the synthesis of eALAS (rate limiting
enzyme in porphyrin synthesis) when the LIP is high and
decreasing eALAS synthesis when the LIP is low.
5 . Siderotic Inclusions in Erythroid Cells
Anucleated RBCs containing siderotic (iron-positive)
inclusions are called siderocytes. Nucleated siderocytes
have been called sideroblasts in human hematology, in
which terminology used for RBC precursors is generally
different from that conventionally used in veterinary hematology
( Bottomley, 2004 ). Siderotic inclusions in erythroid
cells may consist of cytoplasmic ferritin aggregates or ironloaded
mitochondria. Ferritin aggregates can occur normally
in nucleated erythroid cells of humans ( Cartwright
and Deiss, 1975 ), dogs ( Feldman et al. , 1981 ), and pigs
( Deiss et al. , 1966 ), but the presence of iron-loaded mitochondria
is a pathological finding ( Cartwright and Deiss,
1975 ). Electron microscopy is used to definitively identify
the nature of siderotic inclusions ( Fresco, 1981 ; Hammond
et al. , 1969 ); however, the location of iron-positive inclusions
in a ring around the nucleus of a nucleated siderocyte
(termed ringed sideroblast in human hematology)
strongly suggests the presence of iron-loaded mitochondria
( Bottomley, 2004 ). Conditions resulting in the pathological
iron accumulation in mitochondria may induce the synthesis
of a novel mitochondrial ferritin ( Torti and Torti, 2002 ).
Except for iron deficiency, disorders in heme synthesis
have the potential to cause excess iron accumulation in
mitochondria ( Beutler, 1995b ; Fairbanks and Beutler, 1995 ).
Experimental pyridoxine deficiency and experimental chronic
copper deficiency have both resulted in mitochondrial iron
overload in nucleated erythroid cells in bone marrow of
deficient pigs ( Hammond et al. , 1969 ; Lee et al. , 1968a ).
Drugs or chemicals reported to cause siderocytes or nucleated
siderocytes in dogs include chloramphenicol ( Harvey
et al. , 1985 ), lead, hydroxyzine, zinc ( Harvey, 2001 ), and
an oxazolidinone antibiotic ( Lund and Brown, 1997 ).
Siderotic inclusions in erythroid cells have been recognized
in some dogs and cats with myeloproliferative disorders
( Blue et al. , 1988 ; Weiss and Lulich, 1999 ). Acquired
dyserythropoiesis with siderocytes have been reported in
dogs in which specific etiologies could not be determined,
although some of these animals had inflammatory disorders
( Canfield et al. , 1987 ; Weiss, 2005 ). Congenital anemias
with ringed nucleated siderocytes have been reported
in humans ( Bottomley, 2006 ). Persistent siderotic inclusions
have been recognized in microcytic hypochromic erythrocytes
from an English bulldog. Erythrocytes also contained
Heinz bodies and rare hemoglobin crystals ( Harvey et al. ,
2007 ). A congenital defect resulting in mitochondrial iron
overload and secondary oxidant injury was suspected, but
not identified. Refer to Chapter 9 for more information concerning
iron metabolism.
C . Hb Synthesis
Hb is a tetrameric protein consisting of four polypeptide
globin chains each of which contains a heme prosthetic
group within a hydrophobic pocket. The molecule consists
of two identical alpha and two nonalpha chains that are
generally classified as beta chains in adults.
1 . Heme Synthesis and Metabolism
Heme is a planar molecule composed of the tetrapyrrole protoporphyrin
IX, containing a central ferrous molecule. The
initial rate-controlling step in heme synthesis, the eALAS
reaction, occurs within mitochondria (see Chapter 8) .
Glycine and the Krebs cycle intermediate succinyl-CoA
are utilized as substrates, and vitamin B 6 , as pyridoxal
phosphate, is required as a cofactor. The ALA formed is
transported to the cytoplasm where a series of reactions
results in the formation of coproporphyrinogen III, which
must enter the mitochondria, presumably using an ATPbinding
cassette transporter ABCB6 ( Krishnamurthy et al. ,
2006 ) for the final steps in heme synthesis. The final reaction,
catalyzed by heme synthetase, involves the insertion
of ferrous iron into protoporphyrin IX.
Following synthesis, heme must be transferred from
mitochondria to the cytoplasm for combination with globin
chains to complete the synthesis of Hb. The mitochondrial
heme exporter has not been identified at this time. Free
heme is poorly soluble in water and can bind to and damage
cellular components ( Kumar and Bandyopadhyay, 2005 ). It
apparently is bound to cytosolic proteins for transport to
sites of globin chain synthesis ( Kumar and Bandyopadhyay,
2005 ; Taketani et al. , 1998 ). Sometime after its synthesis,
the iron moiety of heme is oxidized (presumably spontaneously)
to the ferric state and is then more specifically called
ferriheme ( Schulman et al. , 1974 ).
Heme affects erythroid cell metabolism in different
ways depending on the stage of maturation. It stimulates
iron uptake and heme synthesis in early erythroid cells, but
it inhibits iron uptake and heme synthesis in reticulocytes
(Abraham, 1991 ; Battistini et al. , 1991 ). A cellular heme
exporter termed feline leukemia virus subgroup C cellular
receptor (FLVCR) is up-regulated on colony-forming unitserythroid
(CFU-E) progenitor cells. It may provide a safety
mechanism to prevent the accumulation of toxic amounts of
cytoplasmic heme before globin synthesis is initiated.
2 . Globin Synthesis
The synthesis of polypeptide globin monomers occurs in
association with ribosomes and polyribosomes in the cytoplasm.
Evidence has been provided indicating that binding
of a partially unstructured apo- β chain to a tightly folded
holo- α chain to form a heme-deficient dimer is the initial
step of Hb assembly. Such binding locks the β chain in a
highly ordered conformation, which allows for an efficient