Cystine (Cys)

Cystine is the amino acid formed by the oxidation of two cysteine molecules that covalently link via a disulfide bond. This organosulfur compound has the formula (SCH2CH(NH2)CO2H)2. It is a white solid that is slightly soluble in water. Human hair and skin contain approximately 10-14% cystine by mass. Cystine serves as a substrate for the cystine-glutamate antiporter. This transport system, which is highly specific for cystine and glutamate, is used to increase the concentration of cystine inside the cell. In this system, the anionic form of cystine is transported in exchange for glutamate. Cystine is quickly reduced to cysteine. Cysteine prodrugs, e.g. acetylcysteine, increase glutamate release into the extracellular space. Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread.