Selenium Flashcards
What is Selenium?
non-metal
exists in several oxidation states including Se2-, Se4+,and Se6+
chemistry similar to sulphur
20 mg in the body
What is the structure of Selenium?
analogues of sulphur-containing amino acids (because of the similarity b/w Se and sulphur)
What are some forms of Selenium?
Se occurs naturally in food in organic forms:
selenomethionine (primarily plant) and selenocysteine (animal)
inorganic forms of Se: selenide (H2Se or HSe-), selenite (H2SeO3), and Selenate (H2SeO4): not found in large quantities naturally in food
What are some other sources of Selenium?
Fortificants to animal feed: sodium selenite or sodium selenate
Supplements for humans: selenomethionine, selenate, or selenite
What are some environmental sources of Selenium?
tremendous variation in soil throughout the world
esp. low conc. in New Zealand, Finland, Denmark, and parts of China and Russia
Se content of plants reflects content in the soil
What are some food sources of Selenium?
meat (esp. fish) best soruces
if fish contains mercury
–>bioavailability may be decreased (formation of unabsorbable mercury-selenium complex)
How is selenium absorbed?
Selenium is absorbed throughout the small intestine:
- Se-amino acids absorption occurs via aa transport systems
- Selenate via active sodium-dependent transport
- Selenite via passive diffusion
Does not appear to be regulated: vitamin C, A, E reduced glutathione ENHANCE absorption
Heavy metals (mercury), phytic acid inhibit absorption via chelation and precipitation
how is selenium transport in the body?
selenoamino acids probably transported across basolateral membrane using amino acid transporters
travel free in portal blood
how is selenium uptake and store?
Liver takes up ~50% of Se on first pass
in liver: Se incorporated into Selenoprotein P which contains most selenium in plasma (50-80%)
Receptors for selenoprotein P are found in the cell membrane of some tissues
Apoprotein E receptor governs selenoprotein P uptake in the brain and testes
–>taken up and stored in high conc. in thyroid glandm kidney, liver, heart, pancreas, muscle
(lungs, brain, bone, RBCs)
How is selenium metabolized?
Selenomethionine: can be stored in aa pool–> used in protein syn. or converted to other forms of Se
Selenocysteine: degraded to free selenide
Free Se converted to selenide: selenate from diet converted to selenide
Free Se converted to selenide: selenate from diet converted to selenite to selenide
Selenide: excreted or converted to selenophosphate
What are some functions of selenium?
- Glutathione peroxidase: antioxidant
- Selenoprotein P: antioxidant
- Selenophosphate synthetase 2: selenoprotein synthesis
- Thioredoxin reductase: cellular redox state maintenance
- Iodothyronine 5’-deiodinases: thyroid hormone synthesis and metabolism
- Methionine sulfoxide reductases: oxidative damage repair
- Other selenoproteins: antioxidant, intracellular protein processing and folding, calcium regulation
