Evidence that malondialdehyde-derived aminoenimine is not a fluorescent age pigment.

It has been suggested that protein modifications by malondialdehyde (MDA), a major product of lipid peroxidation, contribute to the fluorescence formation of lipofuscin. Although early studies proposed an aminoenimine structure (RNHCH=CHCH=NR) formed from MDA and the epsilon-amino groups of the lysine residues for the fluorophores, there has been considerable doubt as to whether the aminoenimine is fluorescent. To date, however, there is no conclusive evidence that the aminoenimine is nonfluorescent. This is because that it has not yet been isolated. In this study, we succeeded in isolating an aminoenimine, N,N'-bis[5-(tert-butoxycarboxamido)-5-carboxypentyl]-1-amino-3-iminopropene [(Boc-Lys)(2)MDA], formed from the reaction of MDA with a lysine derivative, N(alpha)-tert-butoxycarbonyl-L-lysine (Boc-Lys), at neutral pH, and confirmed that the purified (Boc-Lys)(2)MDA exhibited no fluorescence. This result demonstrates that aminoenimines formed from MDA and lysine residues do not contribute to the fluorescence formation of lipofuscin.