Photoinactivation of tyrosinase sensitized by folic acid photoproducts

DANTOLA, MARIA LAURA; ZURBANO,BEATRIZ N.; THOMAS, ANDRÉS H.

JOURNAL OF PHOTOCHEMISTRY AND PHOTOBIOLOGY B-BIOLOGY

ELSEVIER SCIENCE SA

Lugar: Amsterdam; Año: 2015 vol. 149 p. 172 - 179

1011-1344

Tyrosinase catalyzes in mammals the first and rate-limiting stepin 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 lackof melanin. Folic acid (PteGlu) is a conjugated pterin widespread in biologicalsystems. Aqueous solutions of tyrosinase were exposed to UV-A irradiation (350 nm) in the presence of PteGlu and its photoproducts (6-formylpterin and6-carboxypterin). The reactions were followed by UV-Visspectrophotometry, enzyme activitymeasurement, fluorescence spectroscopy and HPLC. In this work, wepresent data that demonstrate unequivocally that solutions of tyrosinaseexposed to UV-A irradiation in the presence of PteGlu, undergo enzymeinactivation. However, PteGlu itself causes a negligible effect on the activityof the enzyme. In contrast, PteGlu photoproducts are efficient photosensitizers.The tyrosinase inactivation involves twodifferent pathways: i) a photosensitization process and ii) the oxidation ofthe enzyme by the hydrogen peroxide produced during the photooxidation of PteGlu andits photoproduct. The formerpathway affects both the active site and the tryptophan residues, whereas the latteraffects only the active site. The biological implications of the results arediscussed.