Metabolism of platelet activating factor (PAF) and related ether lipids by neonatal rat myocytes

Suspensions of neonatal rat myocytes were used to investigate the metabolism of [3H]PAF (1-alkyl-2-acetyl-(sn-glycero-3-phosphocholine) (GPC] and [3H]alkyllyso-GPC. [3H]Alkylacyl-GPC consisting of molecular species with four or more double bonds (87%) was the major metabolite formed when either [3H]PAF (4 x 10(-6) - 4 x 10(-9) M) or [3H]alkyllyso-GPC (2 x 10(-7) M) was the precursor. However, substantial amounts of [3H]alkyllyso-GPC (mostly in the media) were also generated from [3H]PAF during the early periods of the incubations. At 2 x 10(-7) M or higher concentrations of either [3H]PAF or [3H]alkyllyso-GPC, [3H]alkylglycerols were also formed (4-12% of the total radioactivity). The [3H]alkylglycerols appear to be produced from [3H]alkyllyso-GPC through the combined actions of lysophospholipase D and a phosphatase. Pretreatment of neonatal rat myocytes with phenylmethylsulfonyl fluoride partially blocked the deacetylation of [3H]PAF and decreased both the formation and subsequent acylation of [3H]alkyllyso-GPC in intact cells. Phenylmethylsulfonyl fluoride also significantly inhibited the activity of cytosolic acetylhydrolase, but it had no direct effect on the microsomal transacylase in vitro. Our data demonstrate that the metabolism of PAF and alkyllyso-GPC by neonatal rat myocytes is qualitatively, but not quantitatively, similar to what occurs in other cell types. In addition, when [3H]alkylacetylglycerol was the precursor, 75% of the label appeared as alkylglycerols in the neonatal rat myocytes, but [3H]PAF was not detected. Therefore, it appears that the de novo pathway via the dithiothreitol-insensitive cholinephosphotransferase step does not contribute to the biosynthesis of PAF in neonatal rat myocytes.