Evidence for different mechanisms involved in the formation of lyso platelet-activating factor and the calcium-dependent release of arachidonic acid from human neutrophils

Recent studies suggest that the first step in platelet-activating factor (PAF) biosynthesis, 1-alkyl-2-lyso-GPC (lyso PAF) formation, may be initiated by the selective transfer of arachidonate from 1-alkyl-2-arachidonoyl-GPC to an acceptor lyso phospholipid by a CoA-independent transacylase activity (CoA-IT). The present study was designed to determine whether the formation of 1-alkyl-2-lyso-GPC and the release of arachidonic acid can occur by different mechanisms. These experiments examined both the formation of 1-[³H]alkyl-2-lyso-GPC from 1-[³H]alkyl-2-acyI-GPC and the release of arachidonic acid from membrane phospholipids as determined by GC/MS in neutrophil homogenates under various conditions. The addition of unlabelled lyso phospholipids to neutrophil homogenates stimulated the time-dependent formation of 1-[³H]alkyl-2-lyso-GPC from 1-[³H]alkyl-2-acyl-GPC. Without exogenous lyso phospholipids, little 1-[³H]alkyl-2-lyso-GPC was formed in this reaction. The activity which catalyzed the formation of 1-[³H]alkyl-2-lyso-GPC had characteristics identical to CoA-IT as indicated by the fact that both reactions were: independent of Ca²⁺, Mg²⁺, CoA and CoA fatty acids, located in microsomal fractions, and stable in 10 mM dithiothreitol. In sharp contrast to the aforementioned reaction, addition of lyso phospholipids did not affect the quantity of arachidonic acid released from membrane phospholipids. Furthermore, there was a Ca²⁺-independent release of arachidonic acid from membrane phospholipid that was increased 4 to 5-fold after the addition of 5 mM Ca²⁺. Finally, Ca²⁺-dependent arachidonic acid release was inhibited by putative phospholipase A₂ inhibitors, aristolochic acid and scalaradial, at concentrations where neither the production of 1-[³H]alkyl-2-lyso-GPC nor Ca²⁺-independent arachidonic acid release was altered. Together these data imply that there may be different mechanisms involved in the formation of 1-alkyl-2-lyso-GPC and arachidonic acid from membrane phospholipids.