title: Simultaneous ultraviolet B-induced photo-oxidation of tryptophan/tyrosine and racemization of neighboring aspartyl residues in peptides.
abstract: Although proteins consist exclusively of l-amino acids, it is well known that d-isomers of aspartyl (Asp) residues occur at specific sites in lens crystallins of elderly people with cataracts. The presence of d-isomers is thought to result from the racemization of Asp residues in the crystallins during aging. It has been reported that this racemization progresses owing to UV-B exposure; however, the underlying mechanism remains unknown because Asp is not a photosensitive residue because there is no aromatic group in its chemical structure. In this study, we synthesized peptides in which the residue neighboring the Asp was the photosensitive residue tryptophan (Trp) or tyrosine (Tyr). After exposing these peptides to UV-B, we used RP-HPLC to confirm that racemization of Asp residues occurred in peptides in which a Trp or Tyr residue was inserted near the Asp; simultaneously, several varieties of photoproducts derived from Trp and Tyr were detected by mass spectroscopy. Promotion of the racemization of Asp residues in peptides with a neighboring Trp was much more significant than in those with Tyr. In particular, when Trp was next to an Asp residue on the C-terminal side of the peptide, the racemization reaction was accelerated.
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan.
authors.forename: Simin
authors.initials: S
authors.lastname: Cai
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan.
authors.forename: Norihiko
authors.initials: N
authors.lastname: Fujii
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Saito
authors.affiliation: Research Reactor Institute, Kyoto University, Kumatori, Osaka 590-0494, Japan. Electronic address: nfujii@rri.kyoto-u.ac.jp.
authors.forename: Noriko
authors.initials: N
authors.lastname: Fujii
mesh_terms: D000595:Amino Acid Sequence; D003459:Crystallins / Q000737:chemistry; D006801:Humans; D008969:Molecular Sequence Data; D009842:Oligopeptides / Q000737:chemistry*; D010084:Oxidation-Reduction; D055668:Photochemical Processes; D013237:Stereoisomerism; D014364:Tryptophan / Q000737:chemistry*; D014443:Tyrosine / Q000737:chemistry*; D014466:Ultraviolet Rays
chemical_list: D003459:Crystallins; D009842:Oligopeptides; D014443:Tyrosine; D014364:Tryptophan
keywords: Cataract; D-Amino acid; Free radicals; Photo-oxidation; Racemization; UV damage