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

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Chapter | 22 Trace Minerals
of the animal at the time of incorporation of selenocysteine
into GPx, and that at higher Se concentrations there is an
increase in activity as compared to tissue Se levels, which
can plateau ( Reilly, 2004 ). Few of these criticisms apply to
selenoprotein P, which reflects current history and directly
responds to recent Se intake ( Burk and Hill, 2005 ). Also, it
is important when measuring GPx activity that hydrogen
peroxide be used as the substrate and azide used to inhibit
catalase activity if the purpose of the assay is to determine
Se-dependent GPx activity. For diagnosis of Se toxicosis,
whole blood, hepatic, or renal tissue Se concentrations can
be of value. Urinary excretion of trimethyl Se can also be
of value in severe Se toxicosis (Subcommittee on Mineral
Toxicity in Animals, 1980).
VII . ZINC
A . Zinc Distribution
A large animal can contain as much as 1.4 to 2.3 g (0.024
to 0.035 mmol) of Zn. Next to calcium and magnesium, Zn,
is the most abundant intracellular cation. A large amount
of Zn is presented in bone and muscle ( 70%), but Zn is
not easily mobilizable in response to Zn deficiency ( Keen
et al. , 2003 ; Park et al. , 2004 ; Watson, 1998 ). Thus, a
Zn-deficient diet can significantly reduce certain tissue
pools, such as plasma. In the rat, for example, consumption
of a Zn-deficient diet can result in a 50% reduction
in plasma Zn within 24 h. Eventually, muscle catabolism
can result in a significant release of Zn into the circulation.
Moreover, very high Zn concentrations are found in
integumental tissues (skin, hair, wool, and nails), retina,
and male reproductive organs. Typical plasma or serum
Zn concentrations for most species range between 0.5 and
1.5 microgram/ml (7.6 to 22.9 micromol/liter), with whole
blood concentrations being about 10 times higher ( Keen
et al. , 2003 ; Park et al. , 2004 ; Watson, 1998 ).
B . Functions of Zinc
Zn is essential for the function of more than 200 enzymes.
Zn-containing enzymes are found in all of the major metabolic
pathways involved in carbohydrate, lipid, protein,
and nucleic acid metabolism ( Keen et al. , 2003 ; O’Dell
and Sunde, 1997 ). Zn can function as a structural component
of an enzyme (entasis), as a proton donor at the active
site of an enzyme, and as a bridging atom between the substrate
and the enzyme. Mammalian Zn enzymes include
carboxypeptidases, alkaline phosphatase, alcohol dehydrogenase,
carbonic anhydrase, and superoxide dismutase.
Given the variety of enzymes that contain Zn, a cellular
deficiency of Zn would be expected to have profound
consequences.
In addition to its enzymatic roles, Zn is also thought
to be involved in stabilizing the structures of RNA, DNA,
and ribosomes ( Keen et al. , 2003 ). Zn has been shown to
promote conformational transformations of DNA from the
beta to the Z form. A large family of nuclear binding proteins
has Zn binding domains (Zn-binding fingers). The
structure of each individual finger is highly conserved and
consists of about 30 amino acid residues, constructed as a
ββα fold and held together by the Zn ion. Many transcription
factors, regulatory proteins, and other proteins that
interact with DNA contain Zn fingers (e.g., at the major
groove along the double helix of DNA in which case the
Zn fingers are arranged around the DNA strand). In young
developing animals, even a short-term Zn deficiency can
have a profound effect on transcriptional regulation important
to development.
In addition to its roles in enzymes and nucleic acids,
Zn is also important to the stabilization of biomembranes.
Membrane-bound Zn alters the fluidity and stabilization
of membranes ( O’Dell and Sunde, 1997 ). Zn deficiency
contributes to oxidative stress to membranes, because of
structural strains, altered activities of membrane-bound
enzymes, and changes in membrane receptors. An example
is the increase in the sensitivity of erythrocytes from Zndeficient
animals to osmotic shock that quickly reverses
upon Zn repletion. Membrane lipid damage can also be
an important component in the teratogenic pathology of
Zn deficiency ( Keen et al. , 2003 ). In addition to membrane
proteins, Zn has also been shown to specifically bind to
cytoskeletal proteins, such as tubulin, and cause polymerization.
Such interactions have been linked to abnormal
cell signaling ( Mackenzie et al. , 2002 ).
C . Dietary Zinc
Nutritional Zn deficiency has been well documented in a
number of species including humans, cattle, dogs, and
sheep. Similar to Cu, the uptake of dietary Zn is influenced
by a variety of dietary factors, and conditioned Zn deficiencies
are common. Foods that are high in Zn include shellfish
( 200 microgram/g; 3.0 micromol/g), other seafood
and meat (20 to 50 microgram/g; 0.31 to 0.76 micromol/
gram), and whole grains, legumes, and nuts (20 to 30
microgram/g; 0.31 to 0.46 micromol/g) ( Ammerman et al. ,
1995 ; Spears, 2003 ). Food items considered low in Zn ( 1
microgram/g; 0.015 micromol/g) include dairy products,
fruits, and vegetables. Typical Zn concentrations in pastures
in nonindustrial areas range from 20 to 50 microgram/
g (0.31 to 0.76 micromol/g). Cereal grains of pig and poultry
rations typically contain 20 to 40 microgram/g (0.31
to 0.61 micromol/g). Soybean, peanut, and linseed meals
contain 50 to 70 microgram/g (0.76 to 1.15 micromol/g),
and fish and meal can contain up to 100 microgram/g (1.53
micromol/g) ( Ammerman et al. , 1995 ; Spears, 2003 ).