Cystine

Cystine is the oxidized dimer form of the amino acid cysteine and has the 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 three-dimensional structure.[1]

Cystine
Cystine-from-xtal-2D-skeletal.png
Cystine-from-xtal-Mercury-3D-balls-thin.png
Identifiers
CAS Number
  • 56-89-3 check
3D model (JSmol)
  • Interactive image
ChEBI
  • CHEBI:35492 check
ChEMBL
  • ChEMBL366563 check
ChemSpider
  • 575 check
ECHA InfoCard100.000.270 Edit this at Wikidata
IUPHAR/BPS
  • 5413
KEGG
  • C01420 check
PubChem CID
  • 67678
UNII
  • 48TCX9A1VT check
CompTox Dashboard (EPA)
  • DTXSID2046418 Edit this at Wikidata
InChI
  • InChI=1S/C6H12N2O4S2/c7-3(5(9)10)1-13-14-2-4(8)6(11)12/h3-4H,1-2,7-8H2,(H,9,10)(H,11,12) check
    Key: LEVWYRKDKASIDU-UHFFFAOYSA-N check
  • InChI=1/C6H12N2O4S2/c7-3(5(9)10)1-13-14-2-4(8)6(11)12/h3-4H,1-2,7-8H2,(H,9,10)(H,11,12)
    Key: LEVWYRKDKASIDU-UHFFFAOYAA
SMILES
  • C(C(C(=O)O)N)SSCC(C(=O)O)N
Properties
Chemical formula
C6H12N2O4S2
Molar mass240.29 g·mol−1
Hazards
Safety data sheet (SDS)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

Formation and reactionsEdit

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.

RedoxEdit

It is formed from the oxidation of two cysteine molecules, which results in the formation of a disulfide bond. In cell biology, cystine residues (found in proteins) only exist in non-reductive (oxidative) organelles, such as the secretory pathway (ER, Golgi, lysosomes, and vesicles) and extracellular spaces (e.g., 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:

(SCH2CH(NH2)CO2H)2 + 2 RSH → 2 HSCH2CH(NH2)CO2H + RSSR

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.[4]

Cystine-based disordersEdit

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 transportEdit

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 supplementsEdit

Cysteine supplements are sometimes marketed as anti-aging products with claims of improved skin elasticity.[citation needed] 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. 


This article uses material from the Wikipedia article
 Metasyntactic variable, which is released under the 
Creative Commons
Attribution-ShareAlike 3.0 Unported License.