Reaction of N(alpha)-hippuryllysine with 2-hydroxyheptanal: a model for lysine-directed protein modifications by lipid peroxidation

2-Hydroxyheptanal (2-HH) is one of the major aldehydes derived from peroxidation of polyunsaturated fatty acids. In the present study, to obtain an insight into the contributions of 2-HH to protein modifications during lipid peroxidation, a lysine-containing dipeptide, N(alpha)-hippuryllysine (N-benzoylglycyl-L-lysine, BGL), was reacted with 2-HH at neutral pH. The products were characterized on the basis of LC/MS and NMR spectroscopy. The reaction afforded a 2:1 2-HH-lysine adduct, 1-[5-(N-benzoylglycylamino)-5-carboxypentyl]-4-butyl-5-pentyl-1,2,6-trihydropyridin-3-one (I). In addition, we obtained a 1:1 2-HH-lysine adduct, N-[5-(N-benzoylglycylamino)-5-carboxypentyl]-1-amino-2-heptanone (III). The treatment of the purified III with 2-HH produced I. On the other hand, when the reaction mixture was allowed prolonged standing, I was slowly oxidized to 1-[5-(N-benzoylglycylamino)-5-carboxypentyl]-4-butyl-5-pentyl-3-hydroxypyridinium (V). This conversion was strongly accelerated by the addition of copper(II) ion and 2,2'-bipyridyl. We propose here that the above series of conversions is the main pathway for the modification of lysine residues of proteins by 2-HH.