TsISTYN — 3,3' - dithiobis - 2 - amino - propionic acid, sulfur-containing replaceable amino acid, is a part of many proteins and peptides. Cystine joins in a polipeitidny chain not directly, and is formed as a result of oxidation of the remains of cysteine which are contained in it (see). The order of «short circuit» of the disulfide bridges which are formed at the same time is defined by tertiary structure of protein (see Proteins). Believe that these bonds stabilize spatial configuration of protein, providing preservation to them fiziol. activities. Genetically caused disturbances of exchange of C. lead to development of a hereditary disease of accumulation — to a cystinosis (see) and to the hereditary disease which is characterized by disturbance of transport of C. and other amino acids (see) in epithelial cells of renal tubules and intestines (see the Cystinuria) and formation of tsistinovy urinary stones (see).
In small amounts of C. contains in all cells augo-and heterotrophic organisms, the C especially is a lot of. in keratins (see). In a blood plasma of the person the maintenance of C. makes 0,8 — 3 MGIOO ml.
Pier. the weight (weight) of cystine is equal to 240,3. The dissociation constant for SOON-and NH2 rpynn 1,7 and 7,48 respectively, an isoelectric point (see) is at pH 5,0. Cystine exists in the form of L-, D - an izome-ditch, DL forms and zhezo-forms. Allocated from biol. objects L-cystine represents colourless crystals; specific rotation of the plane of the polarized light at 25 ° — [a]2^ = — 213 ° (concentration of 1 g of L - tsi-stina in i of 00 ml 1 N salt to - you). Crystals of D-cystine represent colourless plates, [and] e — + 221 °, g°pl 247 — 249 °, solubility of D-cystine same, as well as at L-cystine.
At a temperature of 258 — 261 ° cystine decays. It we will badly dissolve in water, we will dissolve in mineral to-takh and water solutions of alkalis, it is insoluble in ethanol, ether and benzene. At recovery of C. passes into cysteine, during the heating turns into Cystaminum (see). C. possesses weak absorption at 240 nanometers at the expense of the - S — S-communication.
Methods of determination of C. are based on its recovery in cysteine, in a look to-rogo it it can be identified by means of Sallie vein reaction or reaction with Sodium nitroprussidum. If the summary contents of cystine and cysteine is known, the number of SH-group of cysteine is defined, then it is possible to calculate contents Ts.
Biol. value C. identically to value of cysteine since their metabolic equivalence is established. However in tissues of mammals, including and the person, the fermental system providing interconversions of C. and cysteine, it is not found. Free C. in cells is available or in insignificant quantities, or it in general is absent. In the presence of oxygen and such cations as iron and copper, C. the recovered glutathione can be formed of cysteine in the non-enzymatic way, (see) can recover C also not enzymatically.
in cysteine, to-ry it is used by a cell. Daily excretion of C. at the person makes 75 — 125 mg on 1 g of creatinine a day, and part C. it is removed in the form of taurine (see).
See also Amino acids. Bibliography: Metsler D. Biochemistry, the lane with English, t. 1 — 3, M., 1980; At and y t And. and d river. Fundamentals of biochemistry, the lane with English, t. 1 — 3, M., 1981. A. M. Shaposhnikov.