|Jmol-3D images||Image 1|
|Molar mass||240.30 g mol−1|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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.
Properties and nutritional aspects
- (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.
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.
Cystine formation reaction
The cystine formation reaction starting from cysteine is the following one:
The reaction starts with two cysteine molecules. When the reaction has finished, it produces a cystine molecule and also 2 protons (H+) and 2 electrons (e-), due to the disulfide bond that is formed between the two sulfurs of the two cysteines. 
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. Cysteine prodrugs, e.g. acetylcysteine, increase glutamate release into the extracellular space.
- "cystine." Encyclopædia Britannica. 2007. Encyclopædia Britannica Online. 27 July 2007 www.britannica.com/eb/article-9028437/cystine
- Gortner, R. A.; W. F. Hoffman, W. F. (1941), l-Cystine, Org. Synth.; Coll. Vol. 1: 194
- 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.
- Gahl WA, Thoene JG, Schneider JA. "Cystinosis" New England Journal of Medicine 2002, vol. 347. pp. 111-121.
- 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.”