Structural identification by mass spectrometry of oxidized phospholipids in minimally oxidized low density lipoprotein that induce monocyte/endothelial interactions and evidence for their presence in vivo

Entry of monocytes into the vessel wall is an important event in atherogenesis. Previous studies from our laboratory suggest that oxidized arachidonic acid-containing phospholipids present in mildly oxidized low density lipoproteins (MM-LDL) can activate endothelial cells to bind monocytes. In this study, biologically active oxidized arachidonic acid-containing phospholipids were produced by autoxidation of 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (Ox-PAPC) and analyzed by liquid chromatography and electrospray ionization mass spectrometry in conjuction with biochemical derivatization techniques. We have now determined the molecular structure of two of three molecules present in MM-LDL and Ox-PAPC that induce monocyte-endothelial interactions. These lipids were identified as 1-palmitoyl-2-(5-oxovaleryl)-sn-glycero-3-phosphocholine (m/z 594.3) and 1-palmitoyl-2-glutaryl-sn-glycero-3-phosphocholine (m/z 610.2). These two molecules were produced by unambiguous total synthesis and found to be identical by analytical techniques and bioactivity assays to those present in MM-LDL and Ox-PAPC. Evidence for the importance of all three oxidized phospholipids in vivo was suggested by their presence in fatty streak lesions from cholesterol-fed rabbits and by their immunoreactivity with natural antibodies present in ApoE null mice. Overall, these studies suggest that specific oxidized derivatives of arachidonic acid-containing phospholipids may be important initiators of atherogenesis.