This word is not acceptable for play in the US & UK dictionaries that are being used in the following games:

• Lexulous
• Wordscraper
• Wordosaur

n. Any of a family of antioxidant proteins that are found in all mammalian cells

"Selenium: A trace mineral that is a cofactor for production of active antioxidant enzymes such as glutathione peroxidases and thioredoxin reductase."