cystinen

Cystine is the dimeric form of the amino acid cysteine, created when two cysteine molecules are linked by a disulfide bond. Its chemical formula is C6H12N2O4S2. It is the oxidized form of cysteine, with the reduced form being cysteine.

In proteins, cystine residues form disulfide bridges that help stabilize protein structure, particularly in extracellular environments. These disulfide bonds are important for the stability of many secreted proteins and for the structural integrity of keratin, which is abundant in hair, nails, and skin.

Cystine is not considered an essential amino acid for humans; the body can generate cysteine from other nutrients, and cystine can be reduced to cysteine when needed. Cystine and cysteine also participate in cellular redox chemistry, with the cysteine/cystine redox couple contributing to the maintenance of redox balance in cells and tissues.

Clinical relevance includes cystinuria, a hereditary disorder in which the renal transporter for dibasic amino acids fails to reabsorb cystine, leading to high urinary cystine levels and a propensity for kidney stone formation. Management typically involves high fluid intake and urinary alkalinization to improve cystine solubility; in some cases, thiol-containing drugs may be used to increase cystine solubility.

Cystine commonly occurs in structural proteins, contributing to their stability, and its plasma and urinary levels can be of interest in studies of oxidative stress and metabolic disorders.