5 S Cysteinyldopa
L-Tyrosine, 3-((2-amino-2-carboxyethyl)thio)-5-hydroxy-, (R)-
Found in large amounts in the plasma and urine of patients with malignant melanoma. It is therefore used in the diagnosis of melanoma and for the detection of postoperative metastases. Cysteinyldopa is believed to be formed by the rapid enzymatic hydrolysis of 5-S-glutathionedopa found in melanin-producing cells.