(1) An amino acid containing sulfur that is found in most proteins; oxidizes on exposure to air to form cystine
- A sulphur-containing nonessential amino acid C3H7NO2S found in most animal proteins; it readily oxidizes to cystine.
When used as the name of a substance, this noun is uncountable, but when used as the name of individual molecules, it is countable.
- E910 when used as a food additive
- E920 when used as an improving agent
- Cys when viewed as a monomer in proteins