Formation of Nε-(succinyl)lysine in vivo:: a novel marker for docosahexaenoic acid-derived protein modification

Free radical-catalyzed peroxidation of docosahexaenoic acid (DHA, C22:6/omega-3) generates various lipid peroxidation products that covalently modify biomolecules such as proteins. Under a free radical-generating system, DHA significantly modified lysine residues in bovine serum albumin. Upon incubation of oxidized DHA with an amino-compound pyridoxamine or a lysine-containing peptide, N-propanoyl and N-succinyl adducts were determined to be the major modification products. The hydroperoxide levels in the oxidized DHA closely reflected the formation of the N-epsilon-(succinyl) lysine (SUL) upon reaction with the peptide, indicating that the hydroperoxides of DHA represent a potential pathway for the formation of SUL. To detect the DHA-derived protein modification in vivo, we developed a monoclonal antibody (mAb2B12) specific to SUL and found that the antibody specifically reacts with the SUL moiety. The formation of SUL was then immunochemically demonstrated in the liver of mice fed with DHA followed by intraperitoneal injection of carbon tetrachloride (CCl4), a hepatic lipid peroxidation model. Immunoreactive materials with mAb2B12 were observed in the DHA + CCl4 group, but were not significant in the control, DHA-alone, and CCl4-alone groups. These data suggest that the formation of DHA-derived adducts such as SUL may be implicated in the oxidative damage observed in DHA-enriched tissues.