STRUCTURAL DEFINITION OF EARLY LYSINE AND HISTIDINE ADDUCTION CHEMISTRY OF 4-HYDROXYNONENAL

The lipid peroxidation product trans-4-hydroxy-2-nonenal (HNE) has bee n implicated in the covalent modification of low-density lipoproteins (LDL) thought to contribute to the over-accumulation of LDL in the art erial wall in the initial stages of atherosclerosis. Proposals for the exact structures of ''early'' protein side-chain modifications until now have been based on indirect evidence. In this paper, the structure s of first-formed His- and Lys-based adducts were elucidated by correl ating NMR spectral properties with those obtained on models with reduc ed chiral center content, in some cases following hydride reduction. I n this manner, we could confirm unambiguously the structure of a HNE-H is imidazole(N-tau) Michael adduct, stabilized as a cyclic hemiacetal and isolated from a neutral aqueous 1:1 stoichiometry reaction mixture . In the case of Lys/amine reactivity, where an excess of amine is nee ded to avert HNE aldol condensation, the predominance of a 1:1 Michael adduct in homogeneous aqueous solution and a 1:2 Michael-Schiff base adduct under two-phase aqueous-organic conditions could be verified by isolation of the respective borohydride-reduced forms. The 1:2 adduct , shown to exist as the cyclic hemiaminal, could represent a stable ly sine-based cross-link in certain protein microenvironments.