Selenium

Selenium (chemical symbol Se) is one of the trace elements. It has a vital role in the metabolism of the thyroid hormones and for the functioning of several important enzymes.

Chemistry

Basic chemistry

Selenium has the atomic number  with34 with an atomic weight of 78.96 g/mol. It is a non-metal. It exists and has similar properties to sulfur which lies above it in multiple oxidation states, but physiologicallygroup VI of the trivalentperiodic table. (Cr³⁺) state is the most important. In human toxicology hexavalent chromium (Cr⁶⁺) is key.

Radiochemistry

In nature, selenium exists as three stable isotopes: ⁵²Cr, ⁵³Cr and ⁵⁴Cr. ⁵²Cr is the commonest on Earth, accounting for 83.8% of the total. In total an additional 19 unstable isotopes are known, ranging from the stablest, ⁵⁰Cr with a half life of 1.8 × 10¹⁷ years, and ⁵¹Cr with a half-life of 27.7 days, and the rest with half lives all shorter than a day. Several decaying in seconds only.pending

Nutrition

Brazil nuts, seeds, green vegetables and certain mushrooms contain significant amounts of selenium in areas in which selenium is present in the soil. Fish and meat may also be good sources if the animals dwell in selenium-rich environments. The USA sets the maximal daily selenium allowance as 400 μg per day in adults, in order to avoid chronic toxicity (see below).

Biological function

 forSelenium is essential for the synthesis of the amino acid, selenocysteine, a rare amino acid in which the normal sulfur of cysteine, has been substituted by selenium. This is needed for the production of twenty-five different selenoproteins (proteins containing selenium) coded for by the human genome².

Selenoproteins include ²:

• D₁, D₂ and D₃, are deiodinases required for the metabolism of the thyroid hormones
• glutathione peroxidases (GPX): for cellular antioxidant defence
• thioredoxin reductases

Toxicity

Selenosis, the condition caused by chronic selenium excess, results in deleterious effects on the skin, GI tract, and nervous system ².

Radiological importance

• selenium-75 (⁷⁵Se) is used as a radiotracer for adrenal, parathyroid and parathyroid scintigraphy
• the use of gadolinium contrast agents can result in spuriously raised selenium levels by interfering with the spectrometric assays commonly used ³

History and etymology

In 17

Chromium

• Selenium was discovered by Jöns Jakob Berzelius, a Swedish chemist, in 1817 ⁴.
• Selenium originates from the Ancient Greek word, χρώμα (chromaσελήνη (selene), which means colour.moon (named because it is was discovered at the same time as tellurium, which means 'earth'!)

Related pathology

• selenium deficiency
  • Keshan disease
  • chromium toxicityKashin-Beck disease (KBD)
• selenium toxicity (selenosis)