Methionine oxidation in the tetradecapeptide CSF114(8-21). A case of study.

Methionine, one of the eight essential amino acids, is characterized by a thioether function on the side chain that is particularly sensitive to oxidation. Sulfoxide and sulfone forms can both be formed, being only the latter irreversible [1]. Herein is reported the unexpected Met-oxidation of the CSF114(8-21) sequence, GHSVFLAPYGWMVK, a fragment of the original 21-mer type I’ β-turn peptide structure, originally developed to be used as a synthetic antigenic probe for diagnostics of circulating autoantibodies in Multiple Sclerosis [2]. Recent studies by Liang et al. [3] reported that oxidation of Met residues on Met Rich Proteins (MRP) was strongly dependent on the protein used. Among those, calmodulin (from Mus Musculus), FeNos (Nostoc sp.), DZS18 (Zea Mays) or LegP (Legionella pneumophilia) were used. As an example, oxidation of some specific residues was possible only using H2O2 up to 5 times more concentrated, although the exact position of oxidized Met residues remain unidentified. This highlights the possible role played by consensus sequences and/or conformational effects in the oxidation process. However, up to date, short and precise consensus sequences for methionine oxidation have never been described and remain to be clarified.