Chemical changes in bovine serum albumin photoinduced by pterin.

The exposure to UV-A radiation of bovine serum albumin (BSA) in aerated aqueous solution in the presence of pterin (Ptr), results in chemical and conformational modifications of the protein. Ptr belongs to a family of heterocyclic compounds that are well-known type I (electron-transfer) and type II (singlet oxygen) photosensitizers. The evolution of the photosensitized processes was followed by UV/vis spectrophotometry and fluorescence spectroscopy indicating that tryptophan (Trp) and tyrosine (Tyr) residues were affected. Additionally, conformational changes were evaluated by electrophoresis (SDS-PAGE) and size exclusion chromatography coupled with dynamic light scattering detection, showing that BSA undergoes dimerization, via the formation of Tyr radicals. The degradation of Trp residues takes place faster than the oligomerization of the protein. The photosensitized process is initiated by an electron transfer from BSA to the triplet excited stated of Ptr, being a purely dynamic mechanism.

Inactivation of tyrosinase photoinduced by pterin.

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 DNA and its components, accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails due to the lack of melanin. Aqueous solutions of tyrosinase were exposed to UV-A irradiation (350 nm) in the presence of pterin, the parent compound of oxidized pterins, under different experimental conditions. The enzyme activity in the irradiated solutions was determined by spectrophotometry and HPLC. In this work, we present data that demonstrate unequivocally that the enzyme is photoinactivated by pterin. The mechanism of the photosensitized process involves an electron transfer from tyrosinase to the triplet excited state of pterin, formed after UV-A excitation of pterin. The biological implications of the results are discussed.