Cysteine When this compound is found as a single molecule it is called cysteine but when two cysteine molecules are joined together, it is called cystine.
As cystine, it plays a special role in the structure of some proteins such as insulin and immunoglobulins (or antibodies). Generally the proteins so composed are found outside the cell. Cystine is also an important antioxidant within the body's protective cell defences. As a therapeutic agent it is used as an agent to help bring up phlegm in such conditions as cystic fibrosis, chronic bronchitis and bronchiecstasis.
As cysteine this amino acid can perform important functions such as slowing or preventing the action of some enzymes. It is important in the detoxification of some chemicals and like its fellows it is also involved in glucose manufacture.