AA-induced unstable and stable protein modifications in CAII were succesfully characterized by mass measurements of intact protein samples with ESI FT-ICR mass spectrometry. Due to unprecedented mass resolving power and mass accuracy inherent for the FT-ICR technique, unequivocal differentiation between the "Schiff base" (+26.02 Da) and the substituted amine (N-ethyllysine, +28.03 Da), i.e. approx2-Da mass difference at 29 kDa, was possible. SDS-PAGE and IEF results were difficult to interpret in terms of the protein structure. Further localization of the modification site for the single Schiff base formation in nonreducing solution conditions is currently underway by combination of on-line digestion and tandem mass spectrometry.
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