Editing Cystine (section)

==Formation and reactions==
===Structure===
Cystine is the [[disulfide]] derived from the amino acid [[cysteine]]. The conversion can be viewed as an oxidation:
:{{chem2|2 HO2CCH(NH2)CH2SH  +  0.5 O2  ->  (HO2CCH(NH2)CH2S)2  +  H2O}}
Cystine contains a [[disulfide bond]], two amine groups, and two carboxylic acid groups. As for other amino acids, the amine and carboxylic acid groups exist in rapid equilibrium with the ammonium-carboxylate [[tautomer]]. The great majority of the literature concerns the ''l,l-''cystine, derived from ''l''-cysteine. Other isomers include ''d,d''-cystine and the [[meso isomer]] d,l-cystine, neither of which is biologically significant.

=== Occurrence ===
Cystine 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 (anatomy)|horn]] of a [[cow]] in 1899.<ref>[http://www.britannica.com/eb/article-9028437 "cystine"]. ''Encyclopædia Britannica''. 2007. Encyclopædia Britannica Online. 27 July 2007</ref> Human hair and skin contain approximately 10–14% cystine by mass.<ref>{{OrgSynth | last1 = Gortner|first1=R. A.|first2=W. F.|last2=Hoffman | title = l-Cystine | volume= 5 | page= 39| year = 1925| prep = 10.15227/orgsyn.005.0039}}</ref>

=== History ===
 
Cystine was discovered in 1810 by the English chemist [[William Hyde Wollaston]], who called it "cystic oxide".<ref>{{cite journal |last1=Wollaston |first1=William Hyde |title=On cystic oxide, a new species of urinary calculus |journal=Philosophical Transactions of the Royal Society of London |date=1810 |volume=100 |pages=223–230 |url=https://www.biodiversitylibrary.org/item/213300#page/293/mode/1up}} On p. 227, Wollaston named cystine "cystic oxide".</ref><ref name=":0">{{Citation |last=Bradford Vickery |first=Hubert |title=The History of the Discovery of the Amino Acids II. A Review of Amino Acids Described Since 1931 as Components of Native Proteins |date=1972-01-01 |work=Advances in Protein Chemistry |volume=26 |pages=81–171 |editor-last=Anfinsen |editor-first=C. B. |url=https://www.sciencedirect.com/science/article/pii/S0065323308601400 |access-date=2024-05-13 |publisher=Academic Press |doi=10.1016/s0065-3233(08)60140-0 |editor2-last=Edsall |editor2-first=John T. |editor3-last=Richards |editor3-first=Frederic M.}}</ref> In 1833, the Swedish chemist [[Jöns Jacob Berzelius]] named the amino acid "cystine".<ref>{{cite book |last1=Berzelius |first1=J.J. |last2=Esslinger |first2=Me., trans. |title=Traité de Chimie |date=1833 |publisher=Didot Frères |location=Paris, France |volume=7 |page=424 |url=https://books.google.com/books?id=7kkE6Ol3a8kC&pg=PA424 |language=French}} From p. 424: ''"10. ''Cystine.'' Cette substance a été découverte dans les calculs urinaires par Wollaston, […] je me suis donc permis de changer le nom qu'avait proposé cet homme distingué."'' (10. ''Cystine.'' This substance was discovered in urinary calculi by Wollaston, who gave it the name of "cystic oxide" because it dissolves as much in acids as in alkalis, and it resembles, in this respect, some metallic oxides; but, in a way, the reason [that was] alleged to justify it is not valid: I have therefore taken the liberty of changing the name that this distinguished man had proposed.)</ref> The Norwegian chemist [[Moritz Christian Julius Thaulow|Christian J. Thaulow]] determined, in 1838, the [[empirical formula]] of cystine.<ref>{{cite journal |last1=Thaulow |first1=C. J. |title=Sur la composition de la cystine |journal=Journal de Pharmacie |date=1838 |volume=24 |pages=629–632 |url=https://books.google.com/books?id=LMc0AQAAMAAJ&pg=PA629 |trans-title=On the composition of cystine |language=French}}</ref> In 1884, the German chemist [[Eugen Baumann]] found that when cystine was treated with a reducing agent, cystine revealed itself to be a [[Dimer (chemistry)|dimer]] of a [[monomer]] which he named [[cysteine|"cysteïne"]].<ref>{{cite journal |last1=Baumann |first1=E. |title=Ueber Cystin und Cysteïn |journal=Zeitschrift für physiologische Chemie |date=1884 |volume=8 |pages=299–305 |url=https://babel.hathitrust.org/cgi/pt?id=coo.31924078260563&view=1up&seq=309 |trans-title=On cystine and cysteine |language=German}} From pp. 301-302: ''"Die Analyse der Substanz ergibt Werthe, welche den vom Cystin (C<sub>6</sub>H<sub>12</sub>N<sub>2</sub>S<sub>2</sub>O<sub>4</sub>) verlangten sich nähern, […] nenne ich dieses Reduktionsprodukt des Cystins: Cysteïn."'' (Analysis of the substance [cysteine] reveals values which approximate those [that are] required by cystine (C<sub>6</sub>H<sub>12</sub>N<sub>2</sub>S<sub>2</sub>O<sub>4</sub>), however the new base [cysteine] can clearly be recognized as a reduction product of cystine, to which the [empirical] formula C<sub>3</sub>H<sub>7</sub>NSO<sub>2</sub>, [which had] previously [been] ascribed to cystine, is [now] ascribed.  In order to indicate the relationships of this substance to cystine, I name this reduction product of cystine: "cysteïne".) Note:  Baumann's proposed structures for cysteine and cystine (see p.302) are incorrect: for cysteine, he proposed CH<sub>3</sub>CNH<sub>2</sub>(SH)COOH .</ref><ref name=":0" /> In 1899, cystine was first isolated from protein (horn tissue) by the Swedish chemist Karl A. H. Mörner (1855-1917).<ref>{{cite journal |last1=Mörner |first1=K. A. H. |title=Cystin, ein Spaltungsprodukt der Hornsubstanz |journal=Hoppe-Seyler's Zeitschrift für Physiologische Chemie |date=1899 |volume=28 |issue=5–6 |pages=595–615 |doi=10.1515/bchm2.1899.28.5-6.595 |url=https://babel.hathitrust.org/cgi/pt?id=umn.31951002681071f&view=1up&seq=607 |trans-title=Cystine, a cleavage product of horn tissue |language=German}}</ref> The chemical structure of cystine was determined by synthesis in 1903 by the German chemist [[Emil Erlenmeyer]].<ref>{{cite journal |last1=Erlenmeyer |first1=Emil |title=Synthese des Cystins |journal=Berichte der Deutschen Chemischen Gesellschaft |date=1903 |volume=36 |issue=3 |pages=2720–2722 |doi=10.1002/cber.19030360320 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.cl1i2b&view=1up&seq=72 |trans-title=Synthesis of cystine |language=German}}</ref><ref>{{cite journal |last1=Erlenmeyer |first1=E. jun. |last2=Stoop |first2=F. |title=Ueber die Synthese einiger α-Amido-β-hydroxysäuren. 2. Ueber die Synthese der Serins und Cystins |journal=Annalen der Chemie |date=1904 |volume=337 |pages=236–263 |doi=10.1002/jlac.19043370205 |url=https://babel.hathitrust.org/cgi/pt?id=pst.000067448074&view=1up&seq=246 |trans-title=On the synthesis of some α-amido-β-hydroxy acids. 2. On the synthesis of serine and cystine. |language=German}}  Discussion of the synthesis of cystine begins on p. 241.</ref><ref>Erlenmeyer's findings regarding the structure of cystine were confirmed in 1908 by Fischer and Raske. See: {{cite journal |last1=Fischer |first1=Emil |last2=Raske |first2=Karl |title=Verwandlung des ''l''-Serines in aktives natürliches Cystin |journal=Berichte der Deutschen Chemischen Gesellschaft |date=1908 |volume=41 |pages=893–897 |doi=10.1002/cber.190804101169 |url=https://babel.hathitrust.org/cgi/pt?id=hvd.cl1i2u&view=1up&seq=905 |trans-title=Conversion of ''l''-serine into [optically] active natural cystine |language=German}}</ref>

The history of cystine and [[cysteine]] is complicated by the dimer-monomer relationship of the two.<ref name=":0" /> The cysteine monomer was proposed as the actual unit by Embden in 1901.

The sulfur within the structure of cysteine and cystine has been subject of historical interest.<ref name=":0" /> In 1902, Osborne partially succeeded in analysing cystine content via lead compounds. An improved colorimetric method was developed in 1922 by Folin and Looney. An iodometric analysis method was developed by Okuda in 1925.

===Redox===
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 ([[endoplasmic reticulum]], [[Golgi apparatus]], [[lysosome]]s, and vesicles) and extracellular spaces (e.g., [[extracellular matrix]]). 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]]:
:(SCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H)<sub>2</sub>  +  2 RSH  →  2 HSCH<sub>2</sub>CH(NH<sub>2</sub>)CO<sub>2</sub>H  +  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.<ref>{{cite journal
|author1=Aslaksena, M.A. |author2=Romarheima, O.H. |author3=Storebakkena, T. |author4=Skrede, A. | title = Evaluation of content and digestibility of disulfide bonds and free thiols in unextruded and extruded diets containing fish meal and soybean protein sources
| journal = Animal Feed Science and Technology
| volume = 128
| issue = 3–4
| pages = 320–330
| date = 28 June 2006
| doi = 10.1016/j.anifeedsci.2005.11.008 }}</ref>

===Cystine-based disorders===
[[File:Urine crystals comparison.png|thumb|Comparison of different types of urinary crystals.]]
The presence of cystine in urine is often indicative of amino acid reabsorption defects. [[Cystinuria]] has been reported to occur in dogs.<ref>{{cite journal|last1=Gahl|first1=William A.|last2=Thoene|first2=Jess G.|last3=Schneider|first3=Jerry A.|title=Cystinosis|journal=New England Journal of Medicine|volume=347|issue=2|year=2002|pages=111–121|doi=10.1056/NEJMra020552|pmid= 12110740}}</ref>
In humans the excretion of high levels of cystine crystals can be indicative of [[cystinosis]], a rare genetic disease. Cystine stones account for about 1-2% of [[kidney stone disease]] in adults.<ref name=Frassetto2011>{{cite journal| author=Frassetto L, Kohlstadt I| title=Treatment and prevention of kidney stones: an update. | journal=Am Fam Physician | year= 2011 | volume= 84 | issue= 11 | pages= 1234–42 | pmid=22150656 | doi= | pmc= | url=https://pubmed.ncbi.nlm.nih.gov/22150656  }}</ref><ref>{{cite web|title=Cystine stones|url=http://www.uptodate.com/contents/cystine-stones|work=[[UpToDate]]|access-date=20 February 2014|url-status=live|archive-url=https://web.archive.org/web/20140226110022/http://www.uptodate.com/contents/cystine-stones|archive-date=26 February 2014|df=dmy-all}}</ref>