title: Tryptophan oxidation photosensitized by pterin.
abstract: Pterins are normal components of cells and they have been previously identified as good photosensitizers under UV-A irradiation, inducing DNA damage and oxidation of nucleotides. In this work, we have investigated the ability of pterin (Ptr), the parent compound of oxidized pterins, to photosensitize the oxidation of another class of biomolecules, amino acids, using tryptophan (Trp) as a model compound. Irradiation of Ptr in the UV-A spectral range (350 nm) in aerated aqueous solutions containing Trp led to the consumption of the latter, whereas the Ptr concentration remained unchanged. Concomitantly, hydrogen peroxide (H₂O₂) was produced. Although Ptr is a singlet oxygen ((1)O₂) sensitizer, the degradation of Trp was inhibited in O₂-saturated solutions, indicating that a (1)O₂-mediated process (type II oxidation) was not an important pathway leading to Trp oxidation. By combining different analytical techniques, we could establish that a type I photooxidation was the prevailing mechanism, initiated by an electron transfer from the Trp molecule to the Ptr triplet excited state, yielding the corresponding radical ions (Trp(·+)/Trp(-H)· and Ptr(·-)). The Trp reaction products that could be identified by UPLC-mass spectrometry are in agreement with this conclusion.
authors.affiliation: Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas, Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, CCT La Plata-CONICET, Boulevard 113 y 64, 1900 La Plata, Argentina.
authors.forename: Andrés H
authors.initials: AH
authors.lastname: Thomas
authors.forename: Mariana P
authors.initials: MP
authors.lastname: Serrano
authors.forename: Virginie
authors.initials: V
authors.lastname: Rahal
authors.forename: Patricia
authors.initials: P
authors.lastname: Vicendo
authors.forename: Catherine
authors.initials: C
authors.lastname: Claparols
authors.forename: Esther
authors.initials: E
authors.lastname: Oliveros
authors.forename: Carolina
authors.initials: C
authors.lastname: Lorente
mesh_terms: D004578:Electron Spin Resonance Spectroscopy; D006861:Hydrogen Peroxide / Q000378:metabolism / Q000528:radiation effects; D009711:Nucleotides / Q000378:metabolism; D010084:Oxidation-Reduction; D017319:Photosensitizing Agents / Q000378:metabolism* / Q000494:pharmacology; D011622:Pterins / Q000378:metabolism* / Q000494:pharmacology; D026082:Singlet Oxygen / Q000378:metabolism; D014364:Tryptophan / Q000187:drug effects / Q000378:metabolism* / Q000528:radiation effects; D014466:Ultraviolet Rays
chemical_list: D009711:Nucleotides; D017319:Photosensitizing Agents; D011622:Pterins; D026082:Singlet Oxygen; D014364:Tryptophan; D006861:Hydrogen Peroxide
keywords: 5,5-dimethyl-1-pyrroline-N-oxide; DMPO; Electron transfer; NH(4)OAc; PBN; Photosensitization; Pterin; ROS; SOD; Tryptophan; Type I/type II photooxidation; ammonium acetate Ptr, pterin; reactive oxygen species; superoxide dismutase; α-(4-pyridyl-1-oxide) N-t-butylnitrone