Résumé : Atherosclerosis is an inflammatory disease characterized by the accumulation of lipids in the subendothelial space. Among the proatherogenic factors, the oxidative modifications of low-density lipoproteins (LDLs) are frequently mentioned. Apolipoprotein-B-100 (ApoB100) is the major protein of LDLs which stabilizes the particle, binds to the LDL receptor and plays a key role in lipoprotein metabolism. It is also a huge protein (550kDa) containing 4536 amino acids. Although many experiments have been done on copper-oxidized LDLs, other more physiological pathways of oxidation are known and myeloperoxidase (MPO) is one of them (1). MPO is indeed able to oxidize ApoB100 by catalysing the synthesis of hypochlorous acid (HOCl), a powerful oxidant, in the presence of hydrogen peroxide (H2O2) and chloride ions (2). However, the oxidative modifications of ApoB100 are still unclear. In order to determine characteristics of the specific MPO-dependent oxidation, we analyzed in vitro modifications of ApoB100 under 2 oxidative conditions: the oxidation of native-LDLs with (i) HOCl (to mimic MPO-oxidation), and (ii) the MPO/H2O2/Cl- system. ApoB100 was isolated from LDLs, digested using trypsin and the resulting peptides were analyzed by LC-autoMS/MS. As ApoB100 is a large protein, we optimized numbers of parameters involved in the analytical method and the data analyses, in order to increase the sequence coverage (79% instead of 50% in the literature) and to detect a maximum of modifications(3). The three major residues targeted by oxidative species were methionine, tryptophan and tyrosine, forming methionine sulfoxide, hydroxy-tryptophan and chloro-tyrosine, respectively. Although similar oxidized residues were detected in both conditions, several amino acids were specifically oxidized by MPO: 2 tyrosine residues (Y76 and Y1901) and some methionine residues (M723, M727, M1080, M1715, M1716 and M3986). ApoB100 modifications are therefore different under both experimental conditions, revealing important characteristics of the specific MPO-dependent oxidation, a physiologically relevant process. The MPO site-specific oxidations will be further examined in patients who present different clinical situations. (1) Daugherty and Roselaar, Cardiovascular Research 29 (1995). (2) Klebanoff, J Leukoc Biol 77 (2005). (3) Delporte et al., J. Anal. Biochem. 411 (2011).