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cystine cys·tine (sĭs'tēn')
A white crystalline amino acid that is found in many proteins, especially keratin, and is the major source of metabolic sulfur.
a crystalline, sulfur-containing amino acid that is formed from two molecules of the amino acid cysteine. Cystine can be converted to cysteine by reduction (in this case, the addition of hydrogen). Discovered in 1810, cystine was not recognized as a component of proteins until 1899, when it was isolated from animal horn. Cystine is particularly abundant in skeletal and connective tissues and in hair, horn, and wool.