Acetaldehyde and malondialdehyde react together to generate distinct protein adducts in the liver during long-term ethanol administration

Acetaldehyde and the lipid peroxidation-derived aldehyde malondialdehyde (MDA), are reactive compounds that are generated during ethanol metabolism in the liver, and both aldehydes have been shown to be capable of binding to proteins and forming stable adducts. Because similar concentrations of MDA and acetaldehyde can coexist in the liver during ethanol oxidation, protein adduct formation in the presence of both of these aldehydes was studied under both in vitro and in vivo conditions. When proteins were incubated in the presence of both MDA and acetaldehyde, MDA caused a marked and concentration-dependent increase in the stable binding of acetaldehyde to proteins. Maximum stimulation of binding occurred at approximately a fourfold molar excess of MDA relative to acetaldehyde when concentrations of 1.0 mmol/L and 0.1 mmol/L were tested. The formation of highly fluorescent product or products was associated with the MDA stimulation of acetaldehyde binding, indicating that new and distinct products were being generated. These hybrid adducts of MDA and acetaldehyde have been designated as MAA adducts. An affinity-purified polyclonal antibody was produced that specifically recognized MAA epitopes on proteins and did not cross-react with carrier proteins or proteins modified with either acetaldehyde or MDA alone. A quantitative competitive enzyme-linked immunosorbent assay (ELISA) was developed and detected the presence of MAA-modified proteins in liver cytosol from ethanol-fed rats but not in pair-fed controls. Quantification of the data from the competitive ELISA indicated the presence of approximately 75 pmoles protein-bound MAA per milligram liver cytosol proteins of the ethanol-fed animals. These results indicate that acetaldehyde and MDA can react together in a synergistic manner and generate hybrid adducts (MAA-adducts) and further suggest that MAA adducts may represent a major species of adducts formed in the liver during ethanol metabolism in vivo.