Peroxidation of arachidonate containing plasmenyl glycerophosphocholine: facile oxidation of esterified arachidonate at carbon-5

Oxidation of 1-O-hexadec-1′-enyl-arachidonoyl glycerophosphocholine (16:0p/20:4-GPC) by hydroxyl radical generated from Cu(II)/H2O2 was found to yield major products corresponding to free carboxylic acids of 5-hydroxyeicosatetraenoic acid and several 5,12-dihydroxyeicosatetraenoic acid. These products were characterized by electrospray tandem mass spectrometry based upon characteristic product ion spectra, as well as HPLC retention time. Several products were found to be biologically active in terms of elevating neutrophil intracellular calcium ion concentration. When mixed micelles of 16:0p/20:4-GPC were treated with Cu(II)/H2O2, oxidation of the arachidonate esterified to the plasmalogen glycerophosphocholine lipid resulted in the most abundant products oxidized at carbon-5 of esterified arachidonate, but free carboxylic acid products were not formed. The mechanism of formation of these oxidized products is suggested to involve a cooperation between the sn-1 vinyl ether substituent and the arachidonoyl substituent at sn-2 of the glycerophospholipid to direct oxidation of the arachidonate ester at carbon-5. Since arachidonic acid is found in high abundance within most plasmalogen glycerophospholipids, the susceptibility of plasmalogens to free radical oxidation likely involves concomitant oxidation of the arachidonyl radyl group esterified at the sn-2 position.