UV-A radiation (320-400 nm) induces damage to the
DNA molecule and its components through photosensitized reactions.
Biopterin (Bip) and its photoproducts
6-formylpterin (Fop) and
6-carboxypterin (Cap) accumulate in the skin of human beings suffering from
vitiligo, a depigmentation disorder where the protection against UV radiation fails because of the lack of
melanin. This study was aimed to evaluate the photosensitizing properties of oxidized
pterins present in the skin and to elucidate the mechanisms involved in the photosensitized oxidation of
purine nucleotides by
pterins in vitro. For this purpose, steady-state and time-resolved experiments in acidic (pH 5.0-5.8) aqueous
solution were performed using Bip, Fop and Cap as
photosensitizers and the
nucleotide 2'-deoxyguanosine 5'-monophosphate (
dGMP) as an oxidizable target. The three
pterin derivatives are able to photosensitize
dGMP, being Fop the most efficient sensitizer. The reactions proceed through two competing pathways: (1) electron transfer from
dGMP to triplet excited-state of
pterins (type I mechanism) and (2) reaction of
dGMP with (1)O(2) produced by
pterins (type II mechanism). Kinetic analysis revealed that the electron transfer pathway is the main mechanism and the interaction of
dGMP with the triplet excited-state of
pterins and the formation of the corresponding
dGMP radicals were demonstrated by
laser flash photolysis experiments. The
biological implications of the results obtained are also discussed.