Cystinuria and Cystine Lithiasis

Calculi composed mostly of cystine (SCH2CH(COOH)NH2)2, an amino acid which is a part of the protein molecule, are among the rarer stones found in the kidney. The disease which we call cystinuria is an ailment believed to have been present from birth and often present in the siblings [47]. It is characterized by excretion in the urine of large quantities of the amino acids cystine, lysine, arginine, ornithine and also of the mixed disulphide of cysteine and homocysteine, and by the tendency to form calculi in the urinary tract composed usually almost entirely of cystine, but occasionally admixed with apatite or calcium oxalate monohydrate. Cysteine, a component of the protein molecule, can be readily oxidized to form cystine, which, since the reaction is readily reversible, can be readily reconverted into cysteine. Methionine, a third sulphur-containing amino acid which is indispensable to the body and a cystic precursor, can be irreversibly converted into cysteine.