Cystine

Cystine, C₆H₁₂N₂S₂O₄, the disulfide of cysteine [α-amino-β-mercapto-propionic acid HS CH₂ CH(NH₂) COOH]; obtained by acid and enzymatic hydrolysis of various proteins, also occurs in free form in urinary stones and urine. Specific rotation of natural L-cystine [α]_D²⁰ = -224.3 (solution in HCl). Decomposition temperature 258-261°. Cystine is poorly soluble in cold water (1:9,000 at 17°), better in hot water, soluble in alkalis, mineral acids, NH₄OH, C₂H₂O₄. It crystallizes in the form of 6-sided tablets, rhombohedrons, or short rectangular prisms, in a contaminated state - in the form of balls. C. is precipitated by phosphotungstic acid from a sulfuric acid solution; by tin in an acidic medium it is reduced to cysteine; bacterial decomposition of C. is accompanied by the release of H₂S. The discovery of C.-see Urine.

Quantitative determination of C. by Folin and Looney is based on the reduction of C. to cysteine by sodium sulfite; cysteine gives a blue color with phosphotungstic acid (Folin's and Denis' reagent), which is measured colorimetrically. The biological significance of C. is very great. The absence of C. in food is accompanied by a delay in growth, insufficient development of fur, hair, and other disorders. The absence of C. causes the nutritional incompleteness of protein (see Nutrition). In some proteins (casein) instead of C. there is presumably methionine, which replaces it, CH₃S CH₂ CH₂ CH(NH₂) COOH (α-amino-γ-methyl-thiobutyric acid). C. is part of insulin.

An essential role in the processes of tissue respiration belongs to the cystine-cysteine systems, oxidized and reduced glutathione (see) (glutathione-tripeptide-γ-glutamyl-cysteinyl-glycine; previously incorrectly considered as a dipeptide - cysteinyl-glutamic acid). In the processes of autolysis, cysteine and the reduced form of glutathion activate cathepsin. C. serves as a source for the formation of taurine in the body (NH₂-CH₂-CH₂-SO₃H).