Observation of a New Nonfluorescent Malondialdehyde-Acetaldehyde-Protein Adduct by (13)C NMR Spectroscopy

It has been shown that malondialdehyde (MDA) and acetaldehyde react with proteins via the (epsilon)-amino group of a lysine residue to yield hybrid MDA-acetaldehyde (MAA)-protein adducts. The structure of one MAA adduct has been confirmed to be 4-methyl-l,4-dihydropyridine-3,5-dicarbaldehyde (3). In this study, (13)C NMR spectroscopy was used to identify the structure of a second MAA adduct as 2-formyl-3-(alkylamino)butanal (4). Isotopically labeled [l-(13)C]acetaldehyde was reacted with MDA and the protein, bovine serum albumin, under a variety of conditions, and the reactions were monitored at various time intervals by (13)C NMR. In each experiment, new signals grew in at 50 and 22 ppm. By comparison to model compounds, the signals at 50 ppm correspond to a 2-formyl-3-(alkylamino)butanal adduct and the signals at 22 ppm correspond to the known l,4-dihydropyridine-3,5-dicarbaldehyde adduct. Similar results were found when the BSA was replaced with polylysine. Overall, it appears that MAAprotein adducts are composed of two major products, 3 and 4.