Free radical oxidation of plasmalogen glycerophosphocholine containing esterified docosahexaenoic acid: Structure determination by mass spectrometry

Plasmalogen phospholipids have a vinyl ether substituent at the sn-1 position that is susceptible to oxidative reactions that occur at cell membranes. However, the mechanism by which this oxidation occurs and the effect of the polyunsaturated fatty acid at the sn-2 position have not been established. To gain insight into these mechanisms, the oxidized phospholipid products resulting from the exposure of 1-O-hexadec-1'-enyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (16:0p/22:6-GPCho) to the free radical initiator 2, 2'-azobis (2-amidinopropane) hydrochloride were examined. Electrospray ionization tandem mass spectrometry, UV spectroscopy, and electron ionization-gas chromatography/mass spectrometry were used to structurally characterize the oxidized glycerophosphocholine (GPCho) products. The radical-induced peroxidation of 16:0p/22:6-GPCho revealed two major classes of oxidized phospholipids. The first class of products was formed by oxidation at the sn-1 position and included 1-lyso-2-docosahexaenoyl-GPCho and 1-formyl-2-docosahexaenoyl-GPCho. Additionally, the second class of oxidized products where oxidation occurred at the sn-2 position was classified into three categories that included chain-shortened omega-aldehydes, terminal gamma-hydroxy-alpha,beta-unsaturated aldehydes, and the addition of one or two oxygen atoms onto the sn-2 position of 16:0p/22:6-GPCho. These results clearly indicate that free radical-induced oxidation of plasmalogen phospholipids with esterified docosahexaenoic acid at the sn-2 position underwent oxidation at both the sn-1 and sn-2 positions.