- An amino acid, one of the 20 amino acids which serve as the building blocks of protein. Cystine is not an essential amino acid, that is, it is not essential to the diet since the body can synthesize (make) it. Cystine is particularly notable because it is the least soluble of all of the naturally occurring amino acids and it precipitates out of solution in the genetic disease of amino acid transport called cystinuria. In cystinuria, there is excessive excretion in the urine of four amino acids — arginine, cystine, lysine, and ornithine — but it is cystine, being the least soluble, that precipitates out and forms stones and these stones cause the signs and symptoms of the disease. In 1810 Wollaston found a yellow bladder stone composed of an unusual substance which he named cystic oxide since it came from the bladder (Greek kystis = bladder). Analysis showed it to be an amino acid. Thus, the bladder gave its name not only to the amino acid cystine (and another amino acid called cysteine) but also to the disease cystinuria. Cystine is abbreviated Cys.
* * *3,3′-Dithiobis(2-aminopropionic acid); the disulfide product of two cysteines in which two —SH groups become one —S—S— group; if two cysteinyl residues in polypeptide chains form a disulfide linkage, then the two polymers are cross-linked; sometimes occurs as a deposit in the urine, or forming a vesical calculus. Cf.:meso-c.. SYN: dicysteine.- c. desulfhydrase SYN: cystathionine γ-lyase.
* * *cys·tine 'sis-.tēn n an amino acid C6H12N2O4S2 that is a dimer of cysteine, is widespread in proteins (as keratins), and is a major metabolic sulfur source
* * *n.
* * *cys·tine (sisґtēn) (sisґtin) chemical name: 3,3′-dithiobis(2-aminopropanoic acid). An amino acid produced by the digestion or acid hydrolysis of proteins. It is sometimes found in the urine and in the kidneys in the form of minute hexagonal crystals, frequently forming a cystine calculus in the bladder. Cystine is the chief sulfur-containing compound of the protein molecule and is readily reduced to two molecules of cysteine (hence, also called dicysteine).
Medical dictionary. 2011.