Fig 2. Mass spectrometric identification of methionine residues oxidized by H₂O₂ during TmPOx inactivation.

MALDI MS spectra were measured for unaffected POx (A), for POx inactivated during D-glucose oxidation (B) or for POx inactivated by endogenous H₂O₂ (C). The selected MALDI spectra in the left panel illustrate that Met497 of the tryptic peptide ITDAYNMPQPTFDFR with a theoretical MH⁺ of 1815.8 was extensively oxidised in TmPOx inactivated either during substrate turnover (B, left panel) or by H₂O₂ treatment (C, left panel). In contrast, some methionine residues were found not to be oxidised during TmPOx inactivation as shown for Met74 of the peptide VAMFDIGEIDSGLK having a MH⁺ of 1494.8 (right panel). The small signals at m/z 1510.8 are related to the oxidized form of the peptide generated due to the presence of air oxygen.