Cystine(redirected from L-cystine)
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cysteine (sĭsˈtēn), organic compound, one of the 20 amino acids commonly found in animal proteins. Only the L-stereoisomer participates in the biosynthesis of mammalian protein. It is particularly abundant in the proteins of hair, hooves, and the keratin of the skin. Cysteine's importance is related to the presence of a sulfur-containing thiol group in its side chain. This group participates in the catalytic reactions of certain enzymes, such as that of papain, the enzyme from papaya latex used to make commercial meat tenderizers. The thiol group of one cysteine residue is capable of combining with the thiol group of another to form a disulfide bridge, either linking two peptide chains together, as in the case of insulin, or causing a single peptide chain to fold back on itself, making a loop. This latter effect on the secondary structure of proteins is evidently of great importance in maintaining the proper configuration of both structural proteins and enzymes. Two cysteine molecules linked together by a disulfide linkage make up the amino acid cystine, often occurring as a separate entry in lists of common amino acids. A major complication of cystinuria, an inherited metabolic disease, one of whose symptoms is a twentyfold to thirtyfold increase in urinary excretion of cystine, is the precipitation of this relatively insoluble amino acid in the kidney, impairing its function. A similar sort of renal failure often accompanies cystinosis, another inherited disease. Cystine was isolated from a urinary calculus in 1810 and from horn tissue in 1899. The reduction of cystine to cysteine was reported in 1884, and the structures of the two amino acids were proved by chemical synthesis in 1903–4. Neither cysteine nor cystine is essential to the diet of man; cystine and cysteine are interconvertible, and cysteine is made in the body from serine and methionine.
(also known as β,β’-dithio-di-α-aminopropionic acid), [HOOCCH(NH2)CH2S]2, a sulfur-containing amino acid; a disulfide of cysteine. Cystine exists as two optically active forms— D-cystine and L-cystine—and two optically inactive forms— DL-cystine and wieso-cystine. L-cystine is a component of almost all natural proteins and peptides. Hair and wool keratin contain up to 18 percent cystine (together with cysteine). The covalent disulfide (—S—S—) bonds formed by cystine residues between and within individual polypeptide chains maintain a definite three-dimensional molecular structure for proteins and biologically active peptides. The retention of disulfide bonds is a requisite for the characteristic properties of fibrous proteins, such as keratins, as well as for the normal activity of various hormones, such as oxytocin, vasopressin, and insulin, and for various enzymes, such as ribonuclease and chymotrypsin. Cystine is a non-essential amino acid. Its biosynthesis and metabolism in organisms are closely linked to cysteine, since cystine and cysteine readily interconvert in living organisms.
A hereditary disturbance of cystine metabolism leads to the development in childhood of cystinosis, a disease in which cystine crystals are deposited in the tissues, causing various problems. The increased excretion of cystine in urine—cystinuria—leads in severe cases to the formation of cystine calculi, from which cystine was first isolated in 1810.
E. N. SAFONOVA