Tyrosinase catalyzes in mammals the first and rate-limiting step in the biosynthesis of the
melanin, the main pigment of the skin.
Pterins,
heterocyclic compounds able to photoinduce oxidation of biomolecules, accumulate in the skin of patients suffering from
vitiligo, where there is a lack of
melanin.
Folic acid (PteGlu) is a conjugated
pterin widespread in
biological systems. Aqueous solutions of
tyrosinase were exposed to UV-A irradiation (350nm) in the presence of PteGlu and its photoproducts (6-formylpterin and 6-carboxypterin). The reactions were followed by UV-Vis spectrophotometry,
enzyme activity measurement, fluorescence spectroscopy and HPLC. In this work, we present data that demonstrate unequivocally that solutions of
tyrosinase exposed to UV-A irradiation in the presence of PteGlu, undergo
enzyme inactivation. However, PteGlu itself causes a negligible effect on the activity of the
enzyme. In contrast, PteGlu photoproducts are efficient
photosensitizers. The
tyrosinase inactivation involves two different pathways: (i) a
photosensitization process and (ii) the oxidation of the
enzyme by the
hydrogen peroxide produced during the photooxidation of PteGlu and its photoproduct. The former pathway affects both the active site and the
tryptophan residues, whereas the latter affects only the active site. The
biological implications of the results are discussed.