Degradation of tryptophan and tyrosine in peptides photosensitized by pterin

CAROLINA CASTAÑO; MARIANA VIGNONI; ESTHER OIVEROS; PATRICIA VICENDO; ANDRÉS H. THOMAS

Congreso; 25th Inter-America Photochemistry Society (IAPS) Meeting; 2016

Most of the solar UV energy incident on the Earthsurface corresponds to UV-A radiation (320?400 nm), which is not absorbedsignificantly by DNA and proteins and therefore does not affect directly thebiomacromolecules. However, UV-A radiation acts indirectly by photosensitizedreactions and is now recognized as a class I carcinogen. The chemical changesin biological components resulting from photosensitized reactions can takeplace through different mechanisms. Currently, it is accepted that thephotosensitization of proteins occurs mainly through type II photooxidation,which involves the reaction of singlet oxygen (1O2) withtryptophan (Trp), tyrosine (Tyr), histidine, methionine and cysteineside-chains.[1]Oxidized pterins, efficient photosensitizers underUV-A irradiation,[2,3] accumulate in the skin of patients sufferingfrom vitiligo, a chronic depigmentation disorder. In this work, we haveinvestigated the ability of pterin (Ptr), the parent compound of oxidizedpterins, to photosensitize the oxidation of peptides containing Trp and Tyrresidues. As model peptide we chose the α-melanocyte-stimulating hormone(α-MSH), which stimulates the production and release of melanin by melanocytesin skin and hair and has the following amino acid sequence:Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val. In this chemicalstructure Trp and Tyr residues are most sensitive to oxidation. Therefore weprepared two peptides containing a modified α-MSH sequence, by replacing Tyr byglycine (Gly) in one peptide and Trp by Gly in the other one. We could thenanalyze separately the degradation of Trp and Tyr in the peptide chain, Glybeing an amino acid that does not undergo oxidation.Our results showed that Ptr is able to photoinducethe degradation of both amino acids upon UV-A irradiation and that the reactionis initiated by an electron transfer from the peptide to the triplet excitedstated of Ptr. As a consequence of the photosensitized process theincorporation of oxygen atoms takes place in both Trp and Tyr residues. Inaddition, the dimerization of the peptide containing Tyr was also observed anddimers of Tyr were detected, which indicates that Ptr may photoinducecrosslinking of peptides. Mass spectrometry, chromatography and fluorescence were used to followthe photochemical processes and analyze the chemical modifications. The presentfindings are analyzed in the context of the general behavior of pterins asphotosensitizers and the biological implications are discussed. [1] D. I. Pattison, et. al., Photochem. Photobiol. Sci. 11, 38 (2012).[2] A. H. Thomas, et., al., Free Radic.Biol. Med. 63, 467 (2013).[3]C. Castaño, et. al., Photochem.Photobiol. 89, 1448 (2013).