1-O-alkyl-2-arachidonoyl-sn-glycero-3-phosphocholine. A common source of platelet-activating factor and arachidonate in human polymorphonuclear leukocytes

1-O-[³H]Alkyl-2-lyso-sn-glycero-3-phosphocholine (1-O-[³H]Alkyl-2-lyso-GPC) incubated with human polymorphonuclear leukocytes (PMN) for 30 min is metabolized to 1-O-alkyl-2-acyl-GPC containing greater than 80% arachidonate at the 2 position (Chilton, F.H., O'Flaherty, J.T., Ellis, J.M., Swendsen, C.L., and Wykle, R.L. (1983) J. Biol. Chem. 258, 7268-7271). PMN containing 1-O-[³H]Alkyl-2-arachidonoyl-GPC incorporated into their cellular phospholipids in this manner were stimulated with Ca²⁺ ionophore (A23187). Within 5 min after stimulation, 14%, 7%, and 7% of the total 1-O-[³H]Alkyl-2-arachidonoyl-GPC in the cells had been converted to 1-O-[³H]Alkyl-2-acetyl-GPC (platelet-activating factor), 1-O-[³H]Alkyl-2-lyso-GPC, and ³H-labeled neutral lipid, respectively. Stimulation by opsonized zymosan yielded similar results. In related studies, cells were labeled with 1-O-hexadecyl-2-arachidonoyl-GPC containing a [methyl-¹⁴C] choline moiety. The nature of the long-chain acyl residues in the sn-2 position of the labeled 1-O-hexadecyl-2-acyl-GPC remaining after stimulation with A23187 was examined. Analysis by high-performance liquid chromatography using synthetic 1-O-hexadecyl-2-acyl-GPC standards indicated there is a time-dependent loss of arachidonate from the 2 position of the labeled 1-O-hexadecyl-2-arachidonoyl-GPC followed by reacylation by other fatty acids (primarily linoleic and oleic). This shift in the acylation pattern exhibited after Ca²⁺ ionophore stimulation was further examined in PMN preincubated with A23187 and subsequently incubated with labeled 1-O-alkyl-2-lyso-GPC; the stimulated cells produced 1-O-[³H]alkyl-2-acetyl-GPC (> 15% of total label) and 1-O-[³H]Alkyl-2-acyl-GPC containing linoleic acid and oleic acid, rather than arachidonic acid in the sn-2 position. The findings demonstrate that upon stimulation of PMN, 1-O-alkyl-2-arachidonoyl-GPC can yield arachidonate and 1-O-alkyl-2-lyso-GPC; the 1-O-alkyl-2-lyso-GPC formed may be acetylated producing platelet-activating factor or reacylated with fatty acyl residues other than arachidonate.