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Not to be confused with cytosine, cysteine, cytisine, or cytidine.
56-89-3 YesY
ChEBI CHEBI:35492 YesY
ChEMBL ChEMBL366563 YesY
ChemSpider 575 YesY
Jmol-3D images Image
KEGG C01420 YesY
PubChem 67678
Molar mass 240.29 g·mol−1
Safety data sheet External MSDS
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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Infobox references

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. It was discovered in 1810 by William Hyde Wollaston but was not recognized as being derived of proteins until it was isolated from the horn of a cow in 1899.[1]

Properties and nutritional aspects[edit]

Disulfide bonding, along with hydrogen bonding and hydrophobic interactions is partially responsible for the formation of the gluten matrix in bread.[2]

The disulfide link is readily reduced to give the corresponding thiol cysteine. Typical thiols for this reaction are mercaptoethanol and dithiothreitol:


Because of the facility of the thiol-disulfide exchange, the nutritional benefits and sources of cystine are identical to those for the more-common cysteine. Disulfide bonds cleave more rapidly at higher temperatures.[3]

Cystine-based disorders[edit]

The presence of cystine in urine is often indicative of amino acid reabsorption defects. Cystinuria has been reported to occur in dogs.[4] In humans the excretion of high levels of cystine crystals can be indicative of cystinosis, a rare genetic disease.

Cystine formation reaction[edit]

The cystine formation reaction starting from cysteine is the following one:

formation reaction of cystine starting from cysteine

Cystine is formed from the oxidation of two cysteine molecules forming a covalent disulfide bond between the two cysteine R groups, releasing 2 protons and 2 electrons.[5] 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). Meaning that in reductive conditions (Cytoplasm, Nucleus, etc.) cysteine is favorably found.[6] This reductive nature of the cytosol is in part caused by high Glutathione levels.[7]

Biological transport[edit]

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.[citation needed] Cysteine prodrugs, e.g. acetylcysteine, increase glutamate release into the extracellular space.

Cystine hair straightening treatment[edit]

In recent years,[when?] some hair product manufacturers[who?] have moved away from the traditional re-bonding and keratin hair straightening treatment to using cystine as the main ingredient in the formula. Cystine hair straightening treatments are deemed safer than keratin treatments because they do not contain formaldehyde. Cystine is the most representative amino acid present inside the human hair structure, and cystine hair treatments aim to break, reorganize and form new disulphide links. A disulphide link is very strong and links the cystine amino acids together; their alteration will result in change of permanent hair structure, thus making the hair straight.

See also[edit]


  1. ^ "cystine". Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007
  2. ^ Gortner, R. A.; W. F. Hoffman, W. F. (1941). "l-Cystine". Org. Synth. ; Coll. Vol. 1, p. 194 
  3. ^ M.A. Aslaksena, O.H. Romarheima, T. Storebakkena and A. Skrede (28 June 2006). "Evaluation of content and digestibility of disulfide bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources". Animal Feed Science and Technology 128 (3–4): 320–330. doi:10.1016/j.anifeedsci.2005.11.008. 
  4. ^ Gahl WA, Thoene JG, Schneider JA. "Cystinosis" New England Journal of Medicine 2002, vol. 347. pp. 111-121.
  5. ^ Department of Pharmaceutical Sciences, University of Kentucky. Journal of Pharmaceutical Science. February 2005. “Kinetics and mechanism of the reaction of cysteine and hydrogen peroxide in aqueous solution.”
  6. ^ On the cysteine and cystine content of proteins
  7. ^ On the cysteine and cystine content of proteins