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

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
appear to be a death signal for most causes of hemolytic
anemia ( Boas et al. , 1998 ). Macrophages contain a number
of antibody-independent receptors that can bind to oxidized
lipoproteins, anionic phospholipids, polysaccharides, and
polyribonucleotides ( Sambrano et al. , 1994 ). Evidence has
been presented to indicate that several scavenger receptors
may be involved in the recognition and phagocytosis of oxidatively
damaged RBCs ( Terpstra and van Berkel, 2000 ).
The cross-linking of band 3 molecules by hemichromes
during oxidative denaturation of Hb results in clustering of
band 3 molecules and greatly enhanced binding of autologous
IgG ( Cappellini et al. , 1999 ; Low, 1991 ). Because
macrophages contain Fc receptors on their surfaces, the
damaged RBCs would presumably become bound to the surface
of macrophages once a sufficient number of clusters of
band 3 had formed. Sheep with chronic copper toxicity may
become direct Coombs’ test positive, indicating a possible
in vivo role of antibody-mediated removal of oxidatively
injured RBCs ( Wilhelmsen, 1979 ).
B . RBC Aging and Normal Life Spans
Most RBCs circulate in blood for a finite period (survival
time or life span) that varies from 2 to 5 months in domestic
animals, depending on the species ( Table 7-1 ). Methods
for measurement of RBC life spans and results of life span
determinations from many species have been compiled
( Vacha, 1983 ). RBC life spans are related to body weight
(and consequently metabolic rate) with the smallest animals
(highest metabolic rate) having the shortest RBC life
spans ( Vacha, 1983 ). The RBC life span can be prolonged
by reducing the metabolic rate within an individual animal.
Examples include thyroidectomy in rats, hibernation in
hibernating mammals, and reduced ambient temperatures in
poikilotherms ( Landaw, 1988 ). A variety of changes occur
in RBCs as they get older ( Kosower, 1993 ; Low, 1991 ;
Siems et al. , 2000 ), but the nature of the factor(s) that initiates
age-related changes and the mechanism(s) of removal of
senescent RBCs from the circulation require further clarification.
Exposure of phosphatidylserine on the external surface,
modified external membrane carbohydrate residues (e.g.,
desialation of sialoglycoproteins), or modified membrane
proteins (e.g., partially degraded band 3) are possible signals
for removal ( Boas et al. , 1998 ; Kay, 2005 ; Kosower, 1993 ).
1 . Metabolic Impairment
Because RBCs cannot synthesize new enzymes, one theory
is that one or more critical enzymes, involved in generating
ATP or in the protection against oxidants, may decrease
to a point that metabolic impairment results in irreversible
RBC membrane changes ( Piomelli and Seaman,
1993 ). Although the inability to maintain normal ATP and
NADPH concentrations appears to account for premature
destruction of RBCs in patients with hereditary enzyme
deficiencies of the EMP and PPP, respectively, the theory
that metabolic impairment occurs as a result of age-related
decreases in critical enzymes has been questioned ( Beutler,
1985 ; Zimran et al. , 1990 ). Much of the decline in activities
of enzymes, considered in the past to be correlated
with RBC age, occurs as reticulocytes mature into RBCs.
Even if metabolic impairment is not the primary factor
responsible for RBC senescence, it may render the aged
RBC vulnerable to events in the circulation that require a
burst of metabolic activity ( Piomelli and Seaman, 1993 ).
2 . Cumulative Oxidant Damage
Oxidative injury, especially near the end of their life span,
appears to be responsible for normal RBC aging and removal
( Kay, 2005 ; Kiefer and Snyder, 2000 ; Rettig et al. , 1999 ;
Seppi et al. , 1991 ). Senescent dog RBCs exhibit increased
membrane-bound, denatured globin and only 30% of normal
GSH ( Rettig et al. , 1999 ). Even reported decreases in RBC
enzymes may be the result of oxidative damage ( Stadtman,
1992 ). The presence of HzB in RBCs from splenectomized
humans, horses, and dogs, and in RBCs from nonsplenectomized
cats ( Jain, 1986 ; Low, 1989 ), provides evidence
for ongoing oxidant injury in vivo . The inverse correlation
between RBC life span and metabolic rate discussed previously
may result from differences in endogenous oxidant
generation, but other factors such as differing amounts of
mechanical stress should also be considered as a possible
cause of this relationship ( Landaw, 1988 ).
Oxidative stress promotes eryptosis that leads to externalization
of phosphatidylserine. The damaged RBCs can
be phagocytized by macrophages following binding to
phosphatidylserine receptors such as CD36 ( Kiefer and
Snyder, 2000 ; Terpstra and van Berkel, 2000 ). An increased
exposure of phosphatidylserine on the surface of aged rabbit
RBCs correlates with their removal from the circulation
(Boas et al. , 1998 ).
3 . Senescent Cell Antigen
The appearance of a senescent cell antigen may be an
important signal in the removal of senescent RBCs ( Kay,
2005 ). This senescent cell antigen is derived from the
band 3 anion transporter. The specific alteration required
for band 3 to become antigenic remains to be clarified, but
oxidative mechanisms are probably involved. Low (1991)
has demonstrated copolymerization between hemichromes
and band 3 and has suggested that the senescent antigen
represents clusters of band 3. In contrast, studies by Kay
(2005) suggest that the senescent cell antigen results from
band 3 degradation rather than clustering.
A natural antibody against the senescent cell antigen is
present in human plasma. This antibody binds to senescent
cell antigens on the surface of aged cells and, together with
bound complement, promotes the phagocytosis of aged
RBCs by macrophages that exhibit Fc and C3b surface