Elucidation of reaction scheme describing malondialdehyde-acetaldehyde-protein adduct formation

Malondialdehyde and acetaldehyde react together with proteins and form hybr id protein conjugates designated as MAA adducts, which have been detected i n livers of ethanol-fed animals. Our previous studies have shown that MAA a dducts are comprised of two distinct products. One adduct is composed of tw o molecules of malondialdehyde and one molecule of acetaldehyde and was ide ntified as the 4-methpl-1,4-dihydropyridine-3,5-dicarbaldehyde derivative o f an amino group (MHHDC adduct). The other adduct is a 1:1 adduct of malond ialdehyde and acetaldehyde and was identified as the 2-formyl-3-(alkylamino )butanal derivative of an amino group (FAAB adduct). In this study, informa tion on the mechanism of MAA adduct formation was obtained, focusing on whe ther the FAAB adduct serves as a precursor for the MDHDC adduct. Upon the b asis of chemical analysis and NMR spectroscopy, two initial reaction steps appear to be a prerequisite for MDHDC formation. One step involves the reac tion of one molecule of malondialdehyde and one of acetaldehyde with an ami no group of a protein to form the FAAB product, while the other step involv es the generation of a malondialdehyde-enamine. It appears that generation of the MDHDC adduct requires the FAAB moiety to be transferred to the nitro gen of the MDA-enamine. For efficient reaction of FAAB with the enamine to take place, additional experiments indicated that these two intermediates l ikely must be in positions on the protein of close proximity to each other. Further studies showed that the incubation of liver proteins from ethanol- fed rats with MDA resulted in a marked generation of MDHDC adducts, indicat ing the presence of a pool of FAAB adducts in the liver of ethanol-fed anim als. Overall, these findings show that MDHDC-protein adduct formation occur s via the reaction of the FAAB moiety with a malondialdehyde-enamine, and f urther suggest that a similar mechanism may be operative in vivo in the liv er during prolonged ethanol consumption.