Observation of a new nonfluorescent malondialdehyde-acetaldehyde-protein adduct by 13C 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-1,4-dihydropyridine-3,5-dicarbaldehyde (3). In this study, C-13 NMR spectroscopy was used to identify the structure of a second MAA adduct as 2-formyl-3-(alkylamino)butanal (4). Isotopically labeled [1-C-13]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 C-13 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 1,4-dihydropyridine-3,5-dicarbaldehyde adduct. Similar results were found when the BSA was replaced with polylysine. Overall, it appears that MAA- protein adducts are composed of two major products, 3 and 4.