Structural elucidation of a 2:2 4-ketoaldehyde-amine adduct as a model for lysine-directed cross-linking of proteins by 4-ketoaldehydes.

4-Ketoaldehydes react with lysine-based amines to form the same pyrroles that form in low yield by treatment of proteins with 4-hydroxy-2-alkenals generated endogenously during lipid peroxidation. Since pyrroles are susceptible to oxidative coupling, it has been presumed that the cross-linking of proteins observed for 4-ketoaldehydes involves the intermediacy of the corresponding lysine-derived pyrroles. However, the monoalkylpyrroles generated from simple 4-ketoaldehydes do not exhibit oxidative coupling within the time frame that 4-ketoaldehydes induce substantial protein cross-linking. Model studies have revealed that a side reaction competes with the formation of pyrroles from 4-ketoaldehydes and amines. In this paper, we show that this side reaction involves a non-redox aldol-like condensation process resulting in 2:2 adducts, which have been structurally characterized as isomeric pyrrolo[3.2. 1]azabicyclooctanes. Formation of these 2:2 adducts in competition with pyrrole formation is proposed to represent the major pathway for rapid lysine-dependent cross-linking of proteins by simple 4-ketoaldehydes.