Reaction of N-Acetylglycyllysine Methyl Ester with 2-Alkenals: An Alternative Model for Covalent Modification of Proteins

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Abstract
Among the various reactions of lipid peroxidation products with proteins, 2-alkenals have been shown to react extensively with the ε-amino group of lysine residues [Žídek et al. (1997) Chem. Res. Toxicol. 10, 702−710]. To obtain additional information about the kinetic and mechanistic aspects of this modification, a model peptide (N-acetylglycyllysine O-methyl ester) was reacted with 2-hexenal. The reaction products were characterized through a combination of NMR and MS techniques. The structural elucidation efforts have shown the formation of pyridinium salts through the reaction of two or more alkenals with one amino group. Kinetic data were obtained using a continuous infusion of the reaction mixture into an electrospray ionization mass spectrometer. A mechanism is proposed that offers an alternative model for the formation of stable protein cross-links. The reaction progresses through a Schiff base intermediate to form a dihydropyridine species which can be alternatively reduced to form various 3,4- or 2,5-substituted pyridinium species or react with another Schiff base to form a trialkyl-substituted pyridinium structure. The stoichiometry of this structure (aldehyde/amine) is 3:2, in contrast to the widely accepted 1:2. Therefore, it represents another possible cross-linking mechanism for bifunctional products of lipid peroxidation.