Melanin-related metabolites as markers of the skin pigmentary system.

Three different groups of chemical intermediates are known to be formed during the synthesis of melanin in melanocytes: phenolic compounds, phenolic thio-conjugates, and indolic compounds. All these substances and their metabolites can be detected in urine. We measured the urinary excretion of 3,4-dihydroxyphenylalanine (dopa), 5-S-cysteinyldopa (5-S-CD), and 2 indolic compounds, namely 5-hydroxy-6-methoxyindole (5H6MI) and 5-hydroxy-6-methoxyindole-2-carboxylic acid (5H6MI2C) in urine samples of 4 groups of people with different contents of cutaneous melanin: Asian group, white group, and 2 groups of whites 1 with vitiligo and 1 with tyrosinase-negative oculocutaneous albinism. Dopa and 5-S-CD were determined with the method using high-performance liquid chromatography with an electrochemical detection. Indolic substances were measured by mass fragmentography with deuterium-labeled internal standards. Comparison of the melanin-related metabolites excreted in urine of people with different capacities for melanin biosynthesis indicates that, of all measured substances, 5H6MI2C is the best urinary marker of melanin formation in the skin pigmentary system.