cystine

from WordNet (r) 3.0 (2006)

cystine
    n 1: a crystalline amino acid found in proteins (especially
         keratin); discovered in bladder stones

from The Collaborative International Dictionary of English v.0.48

Cystine \Cyst"ine\ (s?s"t?n; 104), n. [See {Cyst}.] (Physiol.
   Chem.)
   A white crystalline substance, {C3H7NSO2}, containing
   sulphur, occuring as a constituent of certain rare urinary
   calculi, and occasionally found as a sediment in urine.
   [1913 Webster]