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Cystine

Cystine
Cystine-skeletal.png
Cystine-3D-balls.png
Identifiers
56-89-3 YesY
3D model (Jmol) Interactive image
ChEBI CHEBI:35492 YesY
ChEMBL ChEMBL366563 YesY
ChemSpider 575 YesY
ECHA InfoCard 100.000.270
5413
KEGG C01420 YesY
PubChem 67678
UNII 48TCX9A1VT YesY
Properties
C6H12N2O4S2
Molar mass 240.29 g·mol−1
Hazards
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).
N  (what is YesYN ?)
Infobox references

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 3-dimensional structure.

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.

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:

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.

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


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