Transformation of 6-tetrahydrobiopterin in aqueous solutions under UV-irradiation

Abstract

© 2017 Elsevier B.V. Melanogenesis disturbance leads to several pathologies, including vitiligo disease. Ultraviolet (UV) narrowband phototherapy (308 or 311nm) is used in treating vitiligo; however, the mechanism of phototherapy is not yet understood. Vitiligo is accompanied by three-fivefold increased de-novo synthesis of (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin (H 4 Bip), its excess and its further oxidation can be considered as significant factors in the pathogenesis of vitiligo. (H 4 Bip), as the phenylalanine 4-hydroxylase coenzyme, catalyzes the oxidation of phenylalanine to tyrosine (a melanin precursor). In this context, photo-transformation of H 4 Bip in aqueous buffer solutions has been studied. HPLC-MS/MS has demonstrated that pterin products of H 4 Bip autoxidation (7,8-dihydropterin (H 2 Ptr), dihydroxanthopterin and pterin) predominate over biopterin products (7,8-dihydrobiopterin (H 2 Bip) and biopterin). We have shown that UV irradiation accelerates the autoxidation while the products of oxidative degradation of H 4 Bip act as photosensitizers. The distinctive feature of photooxidation of H 4 Bip from autoxidation is the formation of dihydropterin (N 2 Ptr) 2 and dihydrobiopterin (N 2 Bip) 2 dimers. By means of HPLC-MS/MS it was found that formation of dihydropterin dimers is the predominant process. The signal of molecular ion of the dimer (N 2 Ptr) 2 (m/z =331) was almost a thousand times higher than the signal of (N 2 Bip) 2 (m/z =479). The key point of the dimerization is photoexcitation (at 310-320nm) of the intermolecular complex (qH 2 Ptr-N 2 Ptr) generated in dark. As a result of the photoreaction azacyclobutane dimers have been formed. In the case of alternation of dark and light intervals H 4 Bip converted into dimers with 96 % yield. The data obtained are discussed in the context of UV-B narrowband vitiligo phototherapy.