Mass spectroscopic characterization of protein modification by malondialdehyde

Malondialdehyde (MDA), a naturally occurring dialdehyde produced in the membrane by lipid peroxidation, is a strong alkylating agent of primary amino groups. We recently raised a monoclonal antibody (mAb1F83) directed to the lipofuscin-like MDA--lysine adduct and demonstrated the presence of immunoreactivity to the antibody in the atherosclerotic lesions, in which intense positivity was associated primarily with macrophage-derived foam cells (Yamada et al., (2001) J. Lipid Res. 42, 1187-1196). To identify the structure of the epitope in the protein recognized by mAb1F83, in the present study, we exposed chain B from bovine insulin (insulin B chain) to MDA and characterized the MDA adducts by mass spectrometry. The MDA-modified insulin B chain was digested with V8 protease, and the resulting peptides were subjected to liquid chromatography--electrospray ionization--mass spectrometry (LC--ESI--MS/MS). The MS/MS analyses confirmed the formation of N-propenal- (+54 Da) and dihydropyridine-type (DHP, +134 Da) adducts in both Lys29 and the N-terminus of insulin B chain. The ELISA analysis of HPLC fractions of peptides, including the DHP adducts using mAb1F83, showed that the immunoreactivity of the DHP--lysine adduct was more significant than the DHP--N-terminus adduct. The results of this study chemically characterized that the MDA adducts such as DHP-type adducts generated in the epsilon-amino group of lysine and N-terminal amino acid residues in the protein and the structure of the epitope recognized by mAb1F83 were DHP--lysine adducts in protein.