A cytosolic phospholipase in human neutrophils that hydrolyzes arachidonoyl-containing phosphatidylcholine

In stimulated neutrophils the production of eicosinoids and the lipid mediator, platelet-activating factor, is thought to be initiated by the activation of a phospholipase A2 which cleaves arachidonic acid from choline-containing glycerophospholipids. Accordingly, studies were undertaken in human neutrophils to characterize phospholipase enzymes that can hydrolyze 1-acyl- and 1-alkyl-linked arachidonoyl-containing phosphatidylcholine (PC). Cellular homogenates were incubated with sonicated dispersions of the arachidonoyl-labeled phospholipid substrates and the hydrolysis of radiolabeled arachidonate was measured. The phospholipase activity was cytosolic, optimal at pH 8.0, and calcium dependent. The homogenization conditions used were important in determining the amount of recoverable enzymatic activity. Vigorous sonication and the presence of calcium during homogenization were strongly inhibitory, whereas the presence of EGTA, heparin and proteinase inhibitors during homogenization increased the activity. Competitive experiments with unlabeled substrates suggested that the phospholipase hydrolyzed arachidonic acid equally well from either 1-acyl- or 1-alkyl-linked PC. However, the phospholipase did show specificity for arachidonic acid, compared to oleic or linoleic acids, at the sn-2 position of 1-acyl-linked PC. When neutrophils were first stimulated with the ionophore A23187, the phospholipase activity against 1-O-hexadecyl-2-[3H]arachidonoylglycerophosphocholine (GPC) increased in a time-dependent fashion up to 3.5-fold over the unstimulated level. The activity against 1-palmitoyl-2-[3H]arachidonoyl-GPC also increased after ionophore stimulation but to a lesser extent. The results demonstrate the presence of a cytosolic, activatable phospholipase that may be involved in PC turnover, arachidonic acid release, and platelet-activating factor production in human neutrophils.