<P> Cystine is the oxidized dimer form of the amino acid cysteine and has the formula (SCH CH (NH) CO H). It is a white solid that is slightly soluble in water . It serves two biological functions: a site of redox reactions and a mechanical linkage that allows proteins to retain their three - dimensional structure . </P> <P> It is common in many foods such as eggs, meat, dairy products, and whole grains as well as skin, horns and hair . It was not recognized as being derived of proteins until it was isolated from the horn of a cow in 1899 . Human hair and skin contain approximately 10--14% cystine by mass . It was discovered in 1810 by William Hyde Wollaston . </P> <P> It is formed from the oxidation of two cysteine molecules, via the formation of a disulfide bond . In cell biology, cystine (found in proteins) can only exist in non-reductive (oxidative) organelles, such as the secretory pathway (ER, Golgi, lysosomes, vesicles and ECM). Under reductive conditions (in the cytoplasm, nucleus, etc .) cysteine is predominant . The disulfide link is readily reduced to give the corresponding thiol cysteine . Typical thiols for this reaction are mercaptoethanol and dithiothreitol: </P> <Dl> <Dd> (SCH CH (NH) CO H) + 2 RSH → 2 HSCH CH (NH) CO H + RSSR </Dd> </Dl>

Why is the formation of cysteine from cysteine said to be an oxidation reaction
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