Photooxidation of Tetrahydrobiopterin under UV Irradiation: Possible Pathways and Mechanisms

• Published in: Photochemistry and photobiology 2014-09, Vol.90 (5), p.1017-1026
• Main Authors: Buglak, Andrey A., Telegina, Taisiya A., Lyudnikova, Tamara A., Vechtomova, Yulia L., Kritsky, Mikhail S.
• Format: Article
• Language: English
• Subjects: Amino acids; Biopterins - analogs & derivatives; Biopterins - chemistry; Buffers; Electron transfer; Electron Transport; Electrons; Energy Transfer; Hydrogen-Ion Concentration; Oxidation; Oxidation-Reduction; Oxidative stress; Oxygen; Photochemical Processes; Photosensitizing Agents - chemistry; Proteins; Quantum Theory; Singlet Oxygen - chemistry; Solutions - radiation effects; Ultraviolet Rays
• ISSN: 0031-8655; 1751-1097
• DOI: 10.1111/php.12285

Tetrahydrobiopterin (H4Bip) is a cofactor for several key enzymes, including NO synthases and aromatic amino acid hydroxylases (AAHs). Normal functioning of the H4Bip regeneration cycle is extremely important for the work of AAHs. Oxidized pterins may accumulate if the H4Bip regeneration cycle is disrupted or if H4Bip autoxidation occurs. These oxidized pterins can photosensitize the production of singlet molecular oxygen 1O2 and thus cause oxidative stress. In this context, we studied the photooxidation of H4Bip in phosphate buffer at pH 7.2. We found that UV irradiation of H4Bip affected its oxidation rate (quantum yield Φ300 = (2.7 ± 0.4) × 10−3). The effect of UV irradiation at λ = 350 nm on H4Bip oxidation was stronger, especially in the presence of biopterin (Bip) (Φ350 = (9.7 ± 1.5) × 10−3). We showed that the rate of H4Bip oxidation linearly depends on Bip concentration. Experiments with KI, a selective quencher of triplet pterins at micromolar concentrations, demonstrated that the oxidation is sensitized by the triplet state biopterin 3Bip. Apparently, electron transfer sensitization (Type‐I mechanism) is dominant. Energy transfer (Type‐II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H4Bip photooxidation and their biological meaning are discussed. The photochemistry of tetrahydrobiopterin (H4Bip) has never been studied, probably because it is pretty unstable. This cofactor is essential for higher organisms as it participates in several important biocatalytic reactions. We studied the photooxidation of H4Bip using HPLC and absorption spectroscopy. The process of H4Bip oxidation is induced by triplet state biopterin 3Bip. Apparently, electron transfer sensitization (Type‐I mechanism) is dominant. Energy transfer (Type‐II mechanism) and singlet oxygen generation play only a secondary role. The mechanisms of H4Bip photooxidation and their biological application are discussed in the context of vitiligo etiology.