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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
ECHA InfoCard 100.000.270
Jmol 3D model Interactive image
KEGG C01420 YesY
PubChem 67678
Molar mass 240.29 g·mol−1
Safety data sheet External MSDS
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Infobox references

Cystine is the oxidized dimer form of the amino acid cysteine and has formula (SCH2CH(NH2)CO2H)2. 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 3-dimensional structure.[1]

Formation and reactions[edit]

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.[2] Human hair and skin contain approximately 10–14% cystine by mass.[3] It was discovered in 1810 by William Hyde Wollaston.


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). Meaning that in reductive conditions (Cytoplasm, Nucleus, etc.) cysteine is favorably found. The disulfide link is readily reduced to give the corresponding thiol cysteine. Typical thiols for this reaction are mercaptoethanol and dithiothreitol:



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.[5] In humans the excretion of high levels of cystine crystals can be indicative of cystinosis, a rare genetic disease.

Biological transport[edit]

Cystine serves as a substrate for the cystine-glutamate antiporter. This transport system, which is highly specific for cystine and glutamate, increases 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, induce release of glutamate into the extracellular space.

Cystine hair nutritional supplements[edit]

Cysteine supplements are sometimes marketed as anti-aging products with claims of improved skin elasticity. Cysteine is more easily absorbed by the body than cystine, so most supplements contain cysteine rather than cystine. N-acetyl-cysteine (NAC) is better absorbed than other cysteine or cystine supplements.

See also[edit]


  2. ^ "cystine". Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007
  3. ^ Gortner, R. A.; W. F. Hoffman, W. F. (1941). "l-Cystine". Org. Synth.  ; Coll. Vol., 1, p. 194 
  4. ^ M.A. Aslaksena; O.H. Romarheima; T. Storebakkena; 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. 
  5. ^ Gahl, William A.; Thoene, Jess G.; Schneider, Jerry A. (2002). "Cystinosis". New England Journal of Medicine. 347 (2): 111–121. doi:10.1056/NEJMra020552. ISSN 0028-4793. PMID 12110740.