The amino acid methionine (Met) is highly susceptible to oxidation via one- and two-electron pathways. When Met is embedded in a peptide or protein sequence, the respective mechanisms and product formation are controlled by peptide/protein sequence and conformation. Here, we report on the formation of novel photo-oxidation products, and cross-links between Met oxidation product(s) and a neighboring histidine residue. Such cross-links may represent neo-epitopes, which could contribute to the immunogenicity of oxidized proteins, especially because the sequence Met-His is a common motif present in human proteins. Experimentally, we have subjected a series of model peptides derived from various protein sequences, LPLMHAL, LPLMH(15N3)AL, LMHAL, LPIMHQD, PEAILTPLMHNGK and SVSEIQLMHNGK, to photo-oxidation by 4-carboxybenzophenone (4-CBP). The photo-oxidation products were analyzed by UPLC-MS and MS/MS, where the nature of characteristic products and mass spectrometric fragments was confirmed with synthetic N15-isotope labeled peptides such as LPLMH(15N3)AL. Mechanisms for the formation of these products will be proposed. Specifically, the formation of cross-links is hypothesized to involve photo-oxidation of Met to an aldehyde, followed by reaction with the imidazole side chain of His, and elimination of water.

Methionine oxidation leads to novel cross-links in peptide sequences containing methionine and histidine

255th National Meeting (2018)

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