Photochemical Oxidation and Dimerization of Dihydrobiopterin

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

Florida, USA

Conferencia; 19th Winter Conference -Inter-American Photochemical Society; 2009

Dihydrobiopterin (H2Bip) and tetrahydrobiopterin (H4Bip) are heterocycliccompounds that participate as enzymatic cofactors in the melanin biosynthesis. Biopterin(Bip), a good singlet oxygen sensitizer,1 and its photoproducts accumulate in the white skinpatches of patients affected by vitiligo,2,3 where hydrogen peroxide (H2O2) is present in highconcentrations. In this disease, the cells undergo oxidative stress, deactivation of enzymestakes place and the protection of the skin against UV radiation fails due to the lack ofmelanin. 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 notyield Bip4, as previously assumed.5 In this work, we study the UV-A photochemistry ofH2Bip and present an alternative source for in vivo Bip generation. Under aerobic conditionsH2Bip undergoes two reaction pathways: i) oxidation to yield Bip and H2O2, and ii) theformation of dimers without O2 consumption. The first process is accelerated when theproduct accumulates. In anaerobic conditions only dimers are produced. Products werecharacterized by HPLC and mass spectrometry. The corresponding quantum yields arereported 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)