Adductome-Based Identification of Novel Oxidized Phospholipid-Lysine Adducts Formed in Low-Density Lipoprotein

The oxidative modification of low-density lipoprotein (LDL) is involved in the generation of lipid peroxidation-derived electrophilic aldehydes, which covalently react with apolipoprotein B-100 to form various adducts. Although the oxidative modification of LDL is critical in the pathogenesis of atherosclerosis, the structural properties of adducts in oxidized LDL (oxLDL) remain unclear. Thus, we aimed to characterize oxLDL using the adductome approach, a comprehensive mass spectrometry-based analysis designed to detect specific product ions from positively ionized oxidized phosphatidylcholine (oxPC) adducts. The amounts of several adducts, including four major oxPC-lysine (oxPC-Lys) adducts, prominently increased when LDL was oxidized with Cu2+. Analysis of the synthetic adduct candidates through mass spectrometry revealed two oxPC-Lys Schiff base adducts and two novel oxPC-Lys amide-type adducts as the major lysine adducts in oxLDL. The oxPC-Lys Schiff base adducts could be transformed into stable amide-type adducts during LDL oxidation. The amounts of oxPC-Lys adducts were significantly higher in the sera of hyperlipidemic mice than in the sera of control mice. The recognition of oxPC-Lys adducts by macrophages suggested that these adducts were involved in the phagocytosis of oxLDL by macrophages. These findings provide insights into the structural properties of lipoproteins modified under oxidative stress and their biological implications. Moreover, this study is the first to identify and quantify oxPC-Lys amide-type adducts in vitro and in vivo.