Metabolic fate of platelet-activating factor in neutrophils

1-O-[3H]Alkyl-2-acetyl-sn-glycero-3-phosphocholine (1-O-[3H]alkyl-2-acetyl-GPC) incubated with rabbit polymorphonuclear leukocytes was metabolized to 1-O-alkyl-2-acyl-GPC containing long chain groups at the 2 position. Within 5 min, 60% of the total label added to the cell suspension was found in the 1-O-[3H]alkyl-2-acyl-GPC. At earlier time points, 1-O-[3H]alkyl-2-lyso-GPC was present. No metabolites were detected that would indicate the ether bond had been cleaved or the polar head group had been altered by the cells. At subaggregating and subdegranulating concentrations of labeled 1-O-[3H]alkyl-2-acyl-GPC, the same pattern of product formation was observed. Furthermore, when 1-O-[3H]alkyl-2-lyso-GPC was added to the cells, it was taken up and acylated in the same manner as that from 1-O-alkyl-2-acetyl-GPC. The nature of the long chain acyl residues incorporated into the 2 position was then examined by argentation chromatography and high performance liquid chromatography. Argentation chromatography of 1-O-[3H]alkyl-2-acyl-3-acetylglycerols obtained from 1-O-[3H]alkyl-2-acyl-GPC after acetolysis indicated that mono-, di-, and tetraenoic fatty acids were the predominant molecular species being incorporated into the 2 position of the molecule (18, 55, and 18%, respectively). Furthermore, high performance liquid chromatographic analysis using synthetic 1-O-alkyl-2-acyl-GPC standards indicated that three fatty acids, linoleic, arachidonic, and oleic (50, 15, and 12%, respectively), were the major chains being incorporated into the 1-O-hexadecyl-linked species. Analysis of 1-O-[3H]alkyl-2-acyl-GPC derived from exogenously added 1-O-[3H]alkyl-2-lyso-GPC revealed the same distribution of acyl groups linked to the 2 position. The findings are consistent with a pathway in which two enzymatic activities are responsible for the metabolism of exogenous platelet-activating factor in the rabbit neutrophils: one that hydrolyzes the acetyl residue (acetylhydrolase) and another that transfers a fatty acyl chain to the 1-O-alkyl-2-lyso-GPC formed (possibly acyl-CoA:1-O-alkyl-2-lyso-GPC acyltransferase). These events appear to play an important role in inactivating this potentially lethal phospholipid.