why is the formation of cysteine from cysteine said to be an oxidation reaction
Cystine 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: