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SELENIUM
3 levels of biological activity:
1) Trace concentrations required for normal
growth and development
2) Moderate concentrations can be stored
3) Elevated concentrations result in toxic effects
CHEMICAL PROPERTIES
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Atomic number 34 Chemical symbol: Se
Essential trace element
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4 oxidative states
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Component of glutathione peroxidase
Elemental Se (0)
Selenide(-2)
Selenite (+4)
Selenate (+6)
Inorganic forms: selenate, selenite (found in water)
Organic forms: selenomethionine, selenocysteine(found
in veggies and cereal)
PHYSICAL PROPERTIES
-Found in various forms ranging from a
grey metallic to a reddish glassy
appearance
-Found in various rocks and minerals,
coal, and oil
-Melting point 494 K
(221°C, 430°F)
Boiling point 958 K
(685°C, 1265°F)
- Heavy metal Selenides(-2) and elemental Se =
INSOLUBLE in water
-Inorganic alkali selenites(+4), and
selenates (+6) = soluble in water
greater bioavailability
-selenites less soluble than selenates (both
predominate in water)
HISTORY, USES, and
APPLICATIONS
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1817- Jons Jacob Berzelius- isolated and
identified Se
Marco Polo- recorded first observation of toxicity
“hoof rot”
Medical use: dietary supplement
Applications:
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manufacturing of ceramics, glass, pigments,
semiconductors, and steel
Used in photography and pharmaceutical production
Water:
MODE OF ENTRY
Into the Environment:
-Through surface and subsurface draining
-Wet and dry deposition from the atmosphere
-Selenates and selenites predominate
Sea water :[Se]: low .04-.12µg/l
Ground, surface water: .06-400µg/l
Air:
-Natural source: Volcanic gas
-Combustion of fossil fuels and Coal- attaches to fly ash
-Also incineration of rubber, municipal waste and paper.
Soil:
-weathering and leaching of parent bedrock
-Se in sediment can be cycled into food chain for
decades from the soil
Industrial and agricultural activity has hastened
release of Se from geological stores making it
available to fish and wildlife
MODE OF ENTRY
into the Organism
PLANTS:
-Absorb via high affinity sulfate permeases.
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Do not actively absorb selenites
-Preferentially accumulate selenates (less bound to soil
particles)
ANIMALS:
-In vitro studies show organic selenium
(selenomethionine) more readily accumulated in
organisms vs. selenate and selenite
MODE OF ENTRY
into Organism
-Uptake from Diet or from water
-Accumulated in the food chain
-Water-soluble Se by fish and wildlife through:
-gills
-epidermis
-gut
-Dietary uptake- dominant pathway
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Sandholm et al. (1973)-first to determine Se accumulation from
dietary sources greater than accumulation from the water
Fish: Planktonic and detrital food pathways: 770 and 510-1395X
waterborne exposure route
Se has surfaced as an element of primary concern due to its ability
to bioaccumulate within base of food webs
MOLECULAR MODE OF TOXIC
INTERACTION
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Toxicity occurs from a flaw in protein synthesis.
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Recall that sulfur is a key component of proteins.
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Sulfur disulfide bonds required for proper folding of
protein (tertiary structure)
Disulfide bonds are between strands of amino acids
Structure=Function
Cells do not discriminate well between Se and
Sulfur during protein synthesis
MOLECULAR MODE OF TOXIC
INTERACTION 2
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Se is substituted for sulfur
 formation of triselenium linkage (SE-SESE) or a selenotrisulfide linkage (S-Se-S)
-Prevent the formation of necessary disulfide
bonds
-Results in distorted, dysfunctional enzymes and
proteins
impairs normal cellular biochemistry
TOXICITY
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Teratogenic
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biomarker of Se toxicity in birds and fish at
embryo/larval stage
Acute- fatal: hypertension, respiratory
depression
Chronic: change in hair, nails, garlic odor on
breath (expiration of dimethyl selenide)
Selenite more toxic than selenate (fish
selenomethionine more toxic)
TOXICITY
-[Se] >2-5µg of dissolved Se/L water may cause
bioaccumulation in food chain with potential to
cause adverse effects
-Greatest increase in Se occurs at lower end of the
food chain (between algae and water)
Study on marine fish showed:
.5µg sodium selenite/kg body weight injected: half
lives equaled
 Liver 2.1 days
 Gills 2.1 days
 Skin 5.3 days
Inhalation of selenious acid:
excrete about 50%(mostly in urine) with a half life
of 1.2 days
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Fish: reduced growth or survival 3µg/g
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G.pseudolimnaeus LC50=1180µg/l
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G.lacustris LC50=3054µg/l
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H. azteca LC50=2480µg/l
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D.pulex LC50= 8111-10123µg/l
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D.magna LC50= 570-5300µg/l
ELIMINATION
1) Reduction of selenite by cellular glutathione to
selenide
2) Incorporation of selenide into
selenoproteins(glutathione peroxidase, type I
5’iodothyronine deiodinase) via selenocysteine
3) Methylation of selenide to metabolites
(dimethyl diselenide, dimethyl selenide (excreted into
expired air), trimethylselenonium)
eliminated through urine (mostly)
Belews Lake
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Most extensive and prolonged case of Se
poisoning in fresh water fish
19 out of 20 species of fish were eliminated
(Mosquito fish survived)
Eggs-primary point of impact
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Se consumed in diet of adult fish deposited in eggs
Metabolized by larval fish after hatching
1986 stopped draining into lake, 1996 still saw
developmental abnormalities in young fish
References
Barceloux, Donald G. Selenium. Clinical Toxicology 37(2) (1999):145-172.
Brix, Kevin V., Henderson, Douglas G., Adams, William J., Reash, Robin J.,
Carlton, Richard G., McIntyre, Dennis O. Acute Toxicity of Sodium Selenate
to Two Daphnids and Three Amphipods. 2000:142-149.
Hamilton, Steven J., Review of selenium toxicity in the aquatic food chain.
Science of the Total Environment 326 (2004):1-31.
Hoang, Tham C., Klaine, Stephen J. Characterizing the toxicity of pulsed
selenium exposure to Daphnia magna. Chemosphere 71(2008) 429-439.
Lemly, A. Dennis. Symptoms and implications of selenium toxicity in fish: the
Belews Lake case example. Aquatic Toxicology 57(2002): 39-49.