Abstract 389: Covalent Modifications of Apolipoproteins in High Density Lipoprotein by Oxidized Phospholipids Impair the Cholesterol Efflux Function of High Density Lipoprotein and are Detected in Hyperlipidemic Plasma

Abstract

Oxidized phospholipids (oxPLs) accumulate at sites of oxidative stress and contribute significantly to atherosclerosis and thrombosis. Most oxPLs have electrophilic substituents and are highly likely to form covalent adducts with proteins, thus compromising protein function. Detection of covalent interactions between oxPLs and proteins could provide important information regarding the type of proteins preferentially modified by oxPLs. However, to date, such studies are extremely limited due to significant technical challenges. We now carry out systematic studies on the protein adduction by oxPLs formed in murine and human plasma. Plasma samples were exposed to a physiologically relevant myeloperoxidase/H 2 O 2 /NO 2 – oxidizing system of phagocytes. Protein adduction by the oxPLs generated in plasma was assessed using LC-MS/MS after tryptic digestion and peptide enrichment using a novel method that we developed. We found that HDL apolipoproteins are the major targets of modification by oxPLs in both murine and human plasma. For apoA-I, the most abundant apolipoprotein in HDL, the major modification sites of oxPLs were located in the region (AA144-186), which is critical for the ABCA1 mediated cholesterol efflux to HDL. We further demonstrated that human apoA-I was also heavily crosslinked by specific oxPLs via histidine and lysine residues located in the region (AA144-186) with apoA-I, or other apolipoproteins, including apoA-II, apoA-IV and apoC-I. In vitro experiments demonstrated that oxPLs modification on lipid free apoA-I or nascent HDL (HDL3) dramatically impairs their function as cholesterol efflux mediators. Using hyperlipidemic LDLr-/- mice, we detected a crosslink adduct of apoA-II with apoE by oxPL in murine plasma. To the best of our knowledge, this is the first report for the detection of endogenous protein adducts with oxPLs.