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

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Chapter | 22 Trace Minerals
VI . SELENIUM
A . Dietary Selenium
A large animal (50 to 100 kg) contains 10 to 20 mg (126 to
253 micromol) of Se. Se is found throughout the body with
highest concentrations normally in the kidney and liver (0.5
to 1.5 and 0.2 to 0.8 micrograms/g [0.0063 to 0.019 and
0.0025 to 0.010 micromol/g], respectively). Skeletal muscle
has a mean Se concentration of about 0.2 microgram/g
(0.0025 micromol/g), and muscle contains about 50% of
the total body pool. Blood Se concentrations are highly
responsive to diet, with values in humans ranging from
0.02 to 7.0 microgram/ml (0.25 to 0.88 micromol/liter) in
low Se and high Se areas, respectively ( Ammerman et al. ,
1995 ; Finley, 2006 ; Hostetler and Kincaid, 2004 ; O’Dell
and Sunde, 1997 ; Spears, 2002).
Plant foods are the major dietary sources of Se in most
countries throughout the world. The amount of Se in soil,
which varies by region, can determine the amount of Se in
the food chain, wherein Se is found as selenomethionine
and selenocysteine. Se is one of the few mineral elements
in which the soil concentration can influence the relative
amounts found in food. Because Se bioavailability varies
markedly with the form of Se ingested and other competing
factors, it has been difficult to define what constitutes either
deficient or toxic amounts. Various dietary forms of Se are
given in Figure 22-6 . Each can accumulate to some degree
in tissue proteins. Accordingly, whole body and tissue
concentrations of Se tend to correlate with environmental
exposure ( Ammerman et al. , 1995 ; Finley, 2006 ; Gunther
et al. , 2002; Hostetler et al. , 2003; O’Dell and Sunde, 1997 ;
Spears, 2002). Suggested Se intakes for a number of species
are given in Table 22-2 . Foods that contain Se include nuts
(0.5 to 10.0μ g/g), fish, poultry and beef (0.5 to 0.8 μg/g) ,
grains (0.2 to 0.4μ g/g can vary with high Se soils), whole
eggs (0.1 to 0.3μg/g), and cheese (0.1 to 0.2μg/g). Se-accumulating
plants can have concentrations that exceed 5mg/g
(63.3 micromol/g), whereas pastures and forages in areas
without Se deficiency syndromes can be as low as 0.1 μg/g
(0.0013 micromol/g). In grazing animals, deficiency signs
occur when feed concentrations are below 0.05 μg/g (0.0063
micromol/g), and adverse effects can occur when dietary
levels exceed 3μ g/g (0.038 micromol/g) ( Ammerman et al. ,
1995 ; Finley, 2006 ; Gunther et al. , 2002; Hostetler and
Kincaid, 2004; O’Dell and Sunde, 1997 ; Spears, 2002) .
B . Selenium Functions
Perspectives regarding the nutritional importance of Se
have changed markedly. In the 1930s, Se was identified as
the toxic agent causing so-called alkali disease in animals
( O’Dell and Sunde, 1997 ). In the 1940s and early 1950s,
research was conducted to identify the specific selenocompounds
causing toxicity. Throughout the 1960s, concerns
FIGURE 22-6 Common forms of dietary selenium.
regarding Se focused on its putative procarcinogenic potential.
However, following the demonstration that Se was an
essential nutrient for laboratory animals, the scope of work
quickly shifted to identifying deficiency syndromes and
signs. Se deficiency was soon identified as a cause of white
muscle disease. However, it was not until 1979 that the
U.S. Food and Drug Administration published regulations
that legalized Se supplementation of diets for dairy cattle
and eventually humans and other animals (Subcommittee
on Mineral Toxicity in Animals, 1980).
1 . Glutathione Peroxidase
The best-defined function of Se is as a component of glutathione
peroxidase (GPx). GPx catalyzes the reduction of
hydrogen and organic peroxides (ROOH) to their respective
alcohols and water ( Herbette et al. , 2007 ). It is now
recognized that there are two different GPx activities in tissues,
one that is Se dependent and a second that is not. The
non-Se-dependent GPx enzymes are referred to as GSH S-
transferases, and their activities can increase under conditions
of severe Se deficiency.
Regarding the Se-containing GPx, there are several
isozymes encoded by different genes that vary in cellular
location and substrate specificity. GPx1 is the most abundant
and is found in the cytoplasm. Although H 2 O 2 is the
preferred substrate (2GSH H 2 O 2 → GS SG 2H 2 O,
where GSH represents reduced monomeric glutathione,
and GS – SG represents glutathione disulfide), fatty acid