Characterization of oxidized phospholipids in oxidatively modified low density lipoproteins by nanoflow liquid chromatography–tandem mass spectrometry

Abstract

Oxidized low density lipoproteins (Ox-LDLs) have an important role in the development of age-related vascular disease, such as atherosclerosis. Ox-LDLs are defined as oxidatively modified LDLs in the blood by enzymatic or non-enzymatic oxidation of phospholipids (PLs). For the characterization of Ox-LDLs at molecular level, oxidation patterns of oxidized PL (Ox-PL) products were systematically examined with standard PL molecules (16:0/22:6-PC, 18:0/22:6-PA, and 18:0/22:6-PG), by the formation of bilayer vesicles of each standard, followed by oxidation of PL vesicles using a Cu2+ solution. This oxidative modification was applied to LDL standard materials. Nanoflow liquid chromatography–electrospray ionization–tandem mass spectrometry (nLC–ESI–MS/MS) analysis of the extracted PL mixtures resulted in the identification of 276 PLs both in the modified and non-modified LDL, including 139 Ox-PL species. Examination of the identified PL species from the standard LDL before and after oxidation supported the postulate that the specific location of an acyl chain of LPL can be exchanged between the sn-1 and -2 positions. This exchange occurs when the neighboring acyl chain is cleaved during oxidation and the polar head group of PL molecules can be dissociated to form PA molecules that result in the formation of various Ox-PA products. This study demonstrates that nLC–ESI–MSn can be utilized for the separation and structural characterization of complicated Ox-PL mixtures, including long chain products with hydroxylation or hydroperoxylation at an unsaturated acyl chain, and short chain products from the cleavage of unsaturated fatty acyl chains to form lysophospholipids (LPLs) or the truncation of an acyl chain into a shorter chain terminated with aldehyde or carboxylic acid.