Page 503 - Veterinary Toxicology, Basic and Clinical Principles, 3rd Edition

470 SECTION | V Metals and Micronutrients

VetBooks.ir He named this element after “Selene,” the green moon nonindicator plants, is in the form of selenomethionine, but
selenocysteine and a variety of other seleno-amino acid
goddess.
Many areas within the northern Great Plains of the
derivatives can also be found (Peterson and Butler, 1962;
United States, such as the Dakotas, eastern Wyoming, Olson et al., 1970; Whanger, 2002). In contrast, the major-
eastern Montana, eastern Colorado, western Nebraska, ity of selenium in indicator plants, such as Astragalus,isa
and western Kansas, have high soil selenium content water soluble Se-methyl-selenocysteine (Shrift, 1973), but
(4 5 ppm selenium or more), resulting in high plant can also have selenocystathionine (Lewis, 1976). Garlic
uptake and subsequent Se toxicosis in herbivores was found to contain significant selenomethionine as well
(Rosenfeld and Beath, 1964). High soil selenium also as glutamyl-Se-methyl-selenocysteine and possibly gamma-
occurs in alkaline soils of some localities in Algeria, glutamyl-selenomethionine (Kotrebai et al., 1999). The
Argentina, Australia, Bulgaria, Canada, China, Columbia, nonprotein associated selenium compounds may be a pro-
Ireland, Israel, Mexico, Morocco, New Zealand, South tective mechanism of the plants to prevent excessive
Africa, the former Soviet Union, Spain, and Venezuela replacement of methionine or other sulfur-containing amino
(NRC, 1983). However, total soil selenium is not the best acids with seleno-amino acids in plant proteins resulting in
indicator of potential selenium poisonings, as Hawaii and loss of disulfide bonds, misfolding and altered protein prop-
Puerto Rico have areas of high soil selenium that is not erties (Peterson and Butler, 1962). Some microbial popula-
available to the plants due to the acidic soil types, which tions, as well as plants, can reduce selenium to volatile
result in lowered water solubility and bioavailable sele- chemical forms (Shrift, 1973).
nium for plant uptake (Lakin, 1961).
Inorganic forms of selenium are the primary form in
soil. Only the water soluble forms are readily available for PHARMACOKINETICS/TOXICOKINETICS
plant uptake, with the greatest absorption being in the form Absorption
of selenate via the sulfate transporter. Elemental selenium
and precipitated metal-selenides are not bioavailable for The majority of ingested selenium compounds are
plant uptake. Some “indicator plants” or “obligatory sele- absorbed from the duodenum, with lesser amounts in the
nium accumulator plants” can accumulate several thousand jejunum and ileum (Wright and Bell, 1966; Whanger
ppm selenium and are often found in selenium-rich areas, et al., 1976). Little to no absorption reportedly occurs
since they require high selenium for growth (Rosenfeld and from the stomach and rumen. However, one report sug-
Beath, 1964). These plants include genera such as gests that minimal absorption of selenomethionine occurs
Astragalus (milk vetch), Xylorhiza, Machaeranthera through the rumen wall and into the blood (Hidiroglou
(woody aster), Haplopappus (golden weed)—formerly and Jenkins, 1973).
known as Oonopsis,and Stanleya (prince’s plume). The chemical form of selenium greatly impacts the
Selenium content as high as 14,990 ppm have been overall absorption. Selenite absorption is via passive dif-
reported for a sample of Astragalus racemosus (Beath, fusion through the brush-border membranes (Vendeland
1937). Although these indicator plants have poor palatabil- et al., 1992, 1994). In contrast, selenate has little affinity
ity, during times of limited forage, they are eaten. for the brush-border membranes. Selenate is absorbed via
Secondary or facultative accumulating plants can survive a sodium cotransport system that is also utilized by sulfate
with high selenium content, but do not require it for (Wolffram et al., 1988). Selenium in the form of seleno-
growth. These plants are often more palatable than the amino acids, selenomethionine and selenocysteine, are
indicator plants and include Aster, Atriplex (salt bush), absorbed through active amino acid transport mechanisms
Castilleja (paintbrush), Gutierrezia (snakeweed), Grindelia and are more bioavailable than selenite or selenate
(gumweeds), Sideranthus (ironweed), Eurotia (winter fat), (McConnell and Cho, 1967; Ammerman and Miller,
Mentzelia, Machaeranthera,and Gyria sp. aswellassome 1974; Vendeland et al., 1994). The selenium status did
crop plants such as western wheat grass, barley, wheat, not affect overall absorption, indicating that absorption
alfalfa, onions, and Swiss chard (Beath et al., 1935; was not under homeostatic regulation.
Williams and Byers, 1936). It should be noted that studies In monogastrics, the relative selenium absorption is
on ingestion of high selenium forages has found that as greater than in ruminants, ranging from 45% to 95%
concentrations increase there appears to be an aversion to (Thomson and Stewart, 1974; Furchner et al., 1975; Bopp
consumption in cattle and sheep, but that initial consump- et al., 1982). And organic forms of selenium are better
tion may not be affected (Pfister et al., 2014). absorbed (Robinson et al., 1978). In ruminants, the rela-
Most of the selenium in nonindicator plants and other tive absorption ranges from 29% to 50% (Wright and
biological matrices is in an organic form, but small Bell, 1966; Suttle and Jones, 1989). The decreased
amounts of inorganic selenate and selenite can also be pres- absorption in ruminants is due to microbial reduction of
ent. The vast majority of plant selenium, especially in selenium forms in the rumen to selenides and elemental