Photochemical degradation of dihydrobiopterin in the presence and in the absence of O2

MARIANA VIGNONI; CAROLINA LORENTE; FRANCO M. CABRERIZO; ANDRÉS H. THOMAS

Cubatao/Santos, SP., Brasil

Congreso; IX Encuentro Latinoamericano de Fotoquímica y Fotobiología (IX ELAFOT); 2008

Biopterin (Bip) and its reduced forms dihydrobiopterin (H2Bip) and tetrahydrobiopterin (H4Bip) are heterocyclic compounds that participate as enzymatic cofactors in important redox reactions. These compounds accumulate in the white skin patches of patients affected by vitiligo,[i] where hydrogen peroxide (H2O2) is present in high concentrations. In this disease, the cells undergo oxidative stress, deactivation of enzymes of the melanin biosynthesis 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. Moreover, photooxidation of pterins in vivo under these pathological conditions has recently been proven.[ii]             In contrast to Bip,[iii],[iv] no studies on photochemistry of H2Bip have been reported. In this contribution, we present a first approach to photodegradation of H2Bip in neutral aqueous solution under UV-A irradiation. In O2-free solutions, H2Bip is converted into several non-pterinic compounds. On the other hand, in the presence of O2 photolysis of H2Bip yields the same compounds, but, in addition, Bip and H2O2 are formed. The rate of this latter process increased after some Bip is accumulated. [i]       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). [ii]       H. Rokos, W. D. Beazley, K. U. Schallreuter Biochem. Biophys. Res. Commun. 292, 805-811 (2002). [iii]      R. Mengel, W. Pfleiderer, W. R. Knappe Tetrahedron Lett. 32, 2817-2820 (1977). [iv]      M. Vignoni, F. M. Cabrerizo, C. Lorente, A. H. Thomas Photochem. Photobiol., in press.