Identification and analysis of products formed from phospholipids in the free radical oxidation of human low density lipoproteins

Ginger L. Milne, Jennifer R. Seal, Christine M. Havrilla, Maikel Wijtmans, and Ned A. Porter

Department of Chemistry, Vanderbilt University, Nashville, TN 37235

Corresponding Author: n.porter{at}vanderbilt.edu

Phospholipids reside in the surface layer of low density lipoproteins (LDL) and comprise approximately 20-25% of the particle by weight. We report a study of the primary products generated from the most abundant molecular species of phosphatidylcholines present in LDL during in vitro free radical oxidations. The 13-hydroperoxides of 1-palmitoyl-2-linoleoyl-sn-glycero-3-phosphocholine (PLPC), 1-stearoyl-2-linoleoyl-sn-glycero-phosphocholine (SLPC) and the 15-hydroperoxides of 1-palmitoyl-2-arachidonyl-sn-glycero-3-phosphocholine (PAPC) and 1-stearoyl-2-arachidonoyl-sn-glycero-phosphocholine (SAPC) were found to increase in a time dependent manner and in significant amounts even in the presence of $alpha$ -tocopherol. Phospholipid alcohols also formed during the course of the oxidations. Early in the LDL oxidations while $alpha$ -tocopherol was still present, the thermodynamically favored trans,trans products of PLPC and SLPC were found to form in significantly larger quantities than those formed from cholesteryl linoleate. Additionally, quantities of PAPC 11-OOH decreased over time relative to PAPC 15-OOH even while $alpha$ -tocopherol was still present in the oxidation, presumably due to further oxidation of PAPC 11-OOH to form cyclic peroxide oxidation products. These results, together, suggest that $alpha$ -tocopherol is more closely associated with the inner cholesteryl ester-rich hydrophobic core of an LDL particle and is not as effective as an antioxidant in the outer phospholipid layer as it is in the lipid core.

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