Photochemical oxidation and dimerization of dihydrobiopterin

MARIANA VIGNONI; M. LAURA DÁNTOLA; FRANCO M. CABRERIZO; NATHALIE MARTINS-FROMENT; JEAN-CHRISTOPHE GARRIGUES; ESTHER OLIVEROS; ANDRÉS H. THOMAS; CAROLINA LORENTE

St.Petersburg, Florida, USA

Congreso; 19th I-APS (Inter-American Photochemical Society) Conference on Photochemistry; 2009

Dihydrobiopterin (H2Bip) and tetrahydrobiopterin (H4Bip) are heterocyclic compounds that participate as enzymatic cofactors in the melanin biosynthesis. Biopterin (Bip), a good singlet oxygen sensitizer,1 and its photoproducts accumulate in the white skin patches of patients affected by vitiligo,2,3 where hydrogen peroxide (H2O2) is present in high concentrations. In this disease, the cells undergo oxidative stress, deactivation of enzymes takes place and the protection of the skin against UV radiation fails due to the lack of melanin. Therefore, the photochemistry of pterins is of particular interest to this disease. It was demonstrated very recently that oxidation of H2Bip by O2 and H2O2 does not yield Bip4, as previously assumed.5 In this work, we study the UV-A photochemistry of H2Bip and present an alternative source for in vivo Bip generation. Under aerobic conditions H2Bip undergoes two reaction pathways: i) oxidation to yield Bip and H2O2, and ii) the formation of dimers without O2 consumption. The first process is accelerated when the product accumulates. In anaerobic conditions only dimers are produced. Products were characterized by HPLC and mass spectrometry. The corresponding quantum yields are reported and a possible mechanism is discussed. 1 Thomas, A. H.; Lorente, C.; Capparelli, A. L.; Martínez, C. G.; Braun, A. M.; Oliveros, E. Photochem. Photobiol. Sci. 2, 245-250 (2003). 2 K. U. Schallreuter, J. Moore, J. M. Wood, W. D. Beazley, E. M. Peters, L. K. Marles, S. C. Behrens-Williams, R. Dummer, N. Blau, B. Thöny J. Invest. Dermatol. 116, 167-174 (2001). 3 H. Rokos, W. D. Beazley, K. U. Schallreuter Biochem. Biophys. Res. Commun. 292, 805-811 (2002). 4 M. L. Dántola, M. Vignoni, A. L. Capparelli, C. Lorente, A. H. Thomas Helvetica Chimica Acta 91, 411-425 (2008) 5 Moore J., Wood J. M. and Schallreuter, K. U. J. Raman Spectrosc. 33, 610-617 (2002)