Patients with vitiligo accumulate millimolar levels of H2O2 in their epidermis. The recycling process of (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin in these patients is disrupted due to deactivation of 4a-OH-BH4 dehydratase by H2O2. The H2O2 oxidation products 6-and 7-biopterin lead to the characteristic fluorescence of the affected skin upon Wood's light examination (UVA 351 nm). Here we report for the first time the presence and accumulation of pterin-6-carboxylic acid (P-6-COOH) in the epidermis of these patients. Exploring potential sources for P-6-COOH revealed that sepiapterin and 6-biopterin are readily photo-oxidised to P-6-COOH by UVA/UVB irradiation. Photolysis of sepiapterin and 6-biopterin produces stoichiometrie H2O2 under aerobic conditions, where O-2 is the electron acceptor, thus identifying an additional source for H2O2 generation in vitiligo. A detailed analysis utilising UV/visible spectrophotometry, HPLC, TLC, and mass spectroscopy showed for sepiapterin direct oxidation to P-6-COOH, whereas 6-biopterin formed the intermediate 6-formylpterin (P-6-CHO) which is then further photo-oxidised to P-6-COOH. (C) 2002 Elsevier Science (USA).