Mass spectroscopic characterization of protein modification by 4-hydroxy-2-(E)-nonenal and 4-oxo-2-(E)-nonenal

The modification of proteins by 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) was investigated using mass spectroscopic approaches. Electrospray ionization MS analysis of HNE- and ONE-treated myoglobin and apomyoglobin revealed that the latter more "open" protein structure resulted in more extensive modification. Reductive methylation of Lys residues halved the extent of modification, implicating the importance of adduction of HNE and ONE to both His and Lys residues. HPLC-MS/MS analysis of tryptic and chymotryptic peptides of HNE- or ONE-adducted apomyoglobin was aided by the knowledge of structures previously elucidated through model reactions. In the case of HNE, the adducts detected were the HNE-His Michael adduct (on H24, H36, H64, and H113), its dehydrated form (on H36), and the HNE-Lys pyrrole adduct (on K16, K42, K45, K145, and K147). In the case of the more reactive ONE, the adducts detected were the ONE-His Michael adduct (on H24), the ONE-Lys pyrrolinone adduct (on K16 and K145), and the ONE-His-Lys pyrrole cross-link (linking K16 to H24 in the C(5) peptide). Although previous analyses of tryptic peptides yielded findings about the nature of His modification, the current chymotryptic peptide analysis produced the first structural characterization of Lys modification on intact proteins by HNE and ONE using mass spectrometry.