Cystin

Cystin, commonly written cystine in chemical usage, is the disulfide-linked dimer formed by two cysteine residues. Its chemical formula is C6H12N2O4S2, and its molecular mass is about 240.3 g/mol. Cystin arises when two thiol groups of cysteine are oxidized to form a disulfide bond (S–S), releasing two hydrogen ions in the process. In proteins, cystin cross-links help stabilize three-dimensional structure and contribute to the rigidity of keratin-rich tissues such as hair and nails. Within cells, these bonds can be reduced to regenerate free cysteine residues.

Natural cystin is the diastereomer formed from two L-cysteines, often referred to as L,L-cystine. The cystin/cysteine redox couple participates in cellular redox biology, and thiol-disulfide exchange reactions allow interconversion between cystine and cysteine under physiological conditions. In proteins, disulfide bonds are important for maintaining stability in extracellular environments where oxidative conditions prevail.

In human health, defective renal reabsorption of dibasic amino acids, including cystine, causes the inherited disorder cystinuria, leading to high urinary excretion of cystine and a predisposition to cystine kidney stones. Cystin is metabolically connected to cysteine; it can be interconverted and, upon digestion, cystine is hydrolyzed to yield cysteine, which is used in protein synthesis and various metabolic pathways. Overall, cystin’s role as a structural cross-link in proteins underpins the stability of many biological tissues.