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

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Chapter | 7 The Erythrocyte: Physiology, Metabolism, and Biochemical Disorders
and Whanger, 1983 ; Caple et al. , 1978 ; Thompson et al. ,
1976 ). Selenium deficiency can be diagnosed by measuring
decreased RBC GPx activity in some species ( Anderson
et al. , 1978 ; Caple et al. , 1978 ); however, caution is indicated
in using this activity as a direct indicator of selenium
status because polymorphism in GPx activity may be present,
as occurs in Finn sheep ( Sankari and Atroshi, 1983 ).
A wide variety of abnormalities or lesions (most notably in
skeletal and cardiac muscle) have been described in association
with selenium deficiency in animals ( Shamberger,
1986 ), but hemolytic anemia rarely, if ever, occurs in the
absence of external oxidant stress. Heinz body (HzB) hemolytic
anemia has been reported during the summer months
in selenium-deficient cattle grazing on St. Augustine grass
growing on peaty muck soils in South Florida ( Morris
et al. , 1984 ). Although HzB formation was greatly reduced
by selenium supplementation, it is likely that these animals
were also exposed to a seasonal oxidant, possibly incorporated
in the grass.
6 . Thioredoxin, Glutaredoxin, and Peroxyredoxin
Thioredoxin (Trx) and glutaredoxin (Grx) are small proteins
with two closely associated cysteines in their active
sites ( Lillig and Holmgren, 2007 ). They may be present in
either oxidized disulfide forms or reduced dithiol forms.
The reduced forms of these proteins interact with and
reduce intramolecular protein disulfides that form in oxidatively
damaged proteins. Resultant oxidized Trx is reduced
to the dithiol form by NADPH and thioredoxin reductase
(together called the thioredoxin system) ( Cha and Kim,
1995 ; Mendiratta et al. , 1998 ). In contrast, oxidized Grx
is reduced by GSH, and the GSSG formed is reduced by
NADPH and GR (together called the glutaredoxin system)
( Papov et al. , 1994 ). Reduced Grx can also reduce mixed
disulfides that form between glutathione and oxidized protein
SH groups (S-glutathionylation) ( Klatt and Lamas,
2000 ). These various reactions regenerate SH groups, presumably
protecting proteins such as Hb and enzymes with
reactive SH groups from irreversible denaturation. Trx
is also important in the scavenging of free radicals (both
directly and indirectly) and as an electron donor for peroxyredoxins.
Grx is also utilized in the reduction of dehydroascorbate.
Both Trx and Grx have additional functions
in other cell types ( Lillig and Holmgren, 2007 ).
Peroxyredoxins are an emerging family of multifunctional
SH-dependent enzymes that reduce H 2 O 2 and alkyl
hydroperoxides to water and alcohol, respectively. Oxidized
peroxyredoxins are regenerated using the Trx system ( Lee
et al. , 2003 ). Peroxiredoxin 2 (Prx2) is the third most abundant
protein in human RBCs, in which it has been shown to
be an efficient scavenger of H 2 O 2 generated endogenously
at low concentrations ( Low et al. , 2007 ). Knockout mice
lacking Prx2 have severe hemolytic anemia characterized
by an increase in RBC reactive oxygen species, leading to
protein oxidation, HzB formation, and decreased RBC life
span ( Lee et al. , 2003 ).
7 . Catalase
Catalase is a heme-containing homotetrameric enzyme that
also destroys H 2 O 2 by conversion to H 2 O and O 2 ( Kirkman
and Gaetani, 2007 ). Except in dogs, mammalian RBCs
generally have high catalase activities ( Allison et al. , 1957 ;
Nakamura et al. , 1998 ; Paniker and Iyer, 1965 ; Suzuki
et al. , 1984 ). Catalase is linked metabolically to the PPP
because NADPH is tightly bound to catalase in mammals
( Kirkman and Gaetani, 2007 ). The oxidation of bound
NADPH to bound NADP results in the accumulation of
an inactive form of catalase (compound II). This inactive
catalase can be returned to normal function by reduction
of bound NADP with unbound NADPH and (to a lesser
degree) unbound NADH using reductase and transhydrogenase
activities, respectively, inherent in catalase ( Gaetani
et al. , 2005 ). This may explain the finding of normal catalase
activity in RBCs from a horse with extremely low NADPH
concentration secondary to G6PD deficiency ( Stockham
et al. , 1994 ). In addition to its benefit to RBCs, the presence
of catalase in RBCs may help protect somatic cells
exposed to high levels of H 2 O 2 , such as in sites of active
inflammation ( Agar et al. , 1986 ).
The relative importance of catalase versus GPx in the
detoxification of low levels of endogenous H 2 O 2 produced
in vivo has been controversial ( Gaetani et al. , 1994 ;
Nagababu et al. , 2003 ; Scott et al. , 1993 ). Dogs have been
identified that lack catalase within their RBCs but have
catalase in other tissues ( Nakamura et al. , 1999 ). RBCs
from acatalasemic humans, mice, and dogs have not been
reported to exhibit increased oxidative damage in vivo
( Goth, 2001 ; Kirkman and Gaetani, 2007 ). RBCs from mice
with GPx deficiency exhibit nearly normal defenses against
H 2 O 2 in vitro , although some loss of membrane sulfhydryls
occurs in vivo (Ho et al. , 2004 ; Johnson et al. , 2000 ). Based
on information available at this time, it appears that Prx2 is
the most important enzyme in protecting RBCs against the
low levels of H 2 O 2 normally produced in vivo (Lee et al. ,
2003 ). Catalase appears to be important in protecting RBCs
against higher levels of exogenous H 2 O 2 (Johnson et al. ,
2005 ; Stagsted and Young, 2002 ). The primary physiological
role of GPx in RBCs appears to be the detoxification of
organic peroxides ( Johnson et al. , 2000 ).
8 . Vitamin E ( α-Tocopherol)
Vitamin E is lipid soluble and the most important antioxidant
in the cell membrane. It donates reducing equivalents
to lipid peroxyl radicals, converting them to less toxic lipid
hydroperoxides (May, 1998 ; Valko et al. , 2007 ). GSHdependent
phospholipid hydroperoxidase can reduce the
lipid hydroperoxides that form. Oxidized vitamin E can be