What does cystine mean?
Cystine is a sulfur-containing compound of central importance in biochemistry, best understood as two cysteine molecules oxidatively joined by a disulfide bond. Although often listed among the amino acids, cystine is not directly encoded in DNA; instead it forms spontaneously when cysteine residues within or between proteins are oxidized. These disulfide linkages are crucial to the structure and stability of many proteins, most famously keratin — the tough fibrous protein of hair, wool, horns, and nails — where the density of cystine cross-links largely determines hardness and elasticity. In medicine, cystine is familiar as the building block of cystine kidney stones, which occur in people with the inherited disorder cystinuria. The word derives from the Greek kystis, meaning bladder or sac, reflecting the compound's early discovery in bladder calculi. For chemists and clinicians alike, cystine remains a key term for discussing protein architecture and metabolic disease.
nounA crystalline, sulfur-containing amino acid formed by the oxidation of two molecules of cysteine, joined by a disulfide bond; it occurs notably in keratin, insulin, and urinary calculi.
- An oxidized dimer of the amino acid cysteine, in which two cysteine residues are linked by a disulfide bond; abundant in structural proteins such as keratin.
"The strength of human hair comes largely from cystine disulfide bonds linking adjacent keratin strands."
"Wool owes its resilience to the high proportion of cystine cross-links within its keratin fibers."
"Patients with cystinuria may develop recurrent cystine stones in the kidneys."
Rarely pluralized outside technical contexts; when counting multiple cystine molecules, scientists usually refer simply to 'molecules of cystine'.
"Each cystine residue contributes a covalent cross-link to the protein's folded structure."
Permanent waves work by breaking and re-forming cystine bonds in your hair — chemistry you can literally style.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of cystine
Cystine was named in the early 19th century after being isolated from bladder stones; the name derives ultimately from the Greek 'kystis', meaning bladder or sac, via the Latin 'cystis'. Its sibling term cysteine, discovered shortly afterward as a constituent of cystine, shares the same root. The Greek root also gives us the words cyst and cystic.
How cystine is actually used
Cystine is a technical term used chiefly in biochemistry, medicine, and nutrition. It is technically not one of the standard proteinogenic amino acids encoded by DNA, since it arises from the post-translational oxidation of cysteine residues.
Easily confused with cystine
Cysteine is the single reduced amino acid containing a thiol (-SH) group, whereas cystine is its oxidized dimer made of two cysteine units joined by a disulfide bond.