Properties and purification of an arachidonoyl-hydrolyzing phospholipase A2 from a macrophage cell line, RAW 264.7

The lipid mediators, platelet activating factor (PAF) and the eicosanoids, can be coordinately produced from the common phospholipid precursor, 1-O-alkyl-2-arachidonoylglycerophosphocholine (1-O-alkyl-2-arachidonoyl-GPC), through the initial action of a phospholipase A2 that cleaves arachidonic acid from the sn-2 position. The mouse macrophage cell line RAW 264.7, which was used as a model macrophage system to study the arachidonoyl-hydrolyzing phospholipase A2 enzyme(s), could be induced to release arachidonic acid in response to inflammatory stimuli. A phospholipase A2 that hydrolyzed 1-O-hexadecyl-2-[3H]arachidonoyl-GPC was identified in the cytosolic fraction of these macrophages. This phospholipase activity was optimal at pH 8 and dependent on calcium. Enzyme activity could be stimulated 3-fold by heparin, suggesting the presence of phospholipase inhibitory proteins in the macrophage cytosol. Compared to 1-alkyl-2-arachidonoyl-GPC, the enzyme hydrolyzed 1-acyl-2-arachidonoylglycerophosphoethanolamine (1-acyl-2-arachidonoyl-GPE) with similar activity but showed slightly greater activity against 1-acyl-2-arachidonoyl-GPC, suggesting no specificity for the sn-1 linkage or the phospholipid base group. Although comparable activity against 1-acyl-2-arachidonoylglycerophosphoinositol (1-acyl-2-arachidonoyl-GPI) could be achieved, the enzyme exhibited much lower affinity for the inositol-containing substrate. The enzyme did, however, show apparent specificity for arachidonic acid at the sn-2 position, since much lower activity was observed against choline-containing substrates with either linoleic or oleic acids at the sn-2 position. The cytosolic phospholipase A2 was purified by first precipitating the enzyme with ammonium sulfate followed by chromatography over Sephadex G150, where the phospholipase A2 eluted between molecular weight markers of 67,000 and 150,000. The active peak was then chromatographed over DEAE-cellulose, phenyl-Sepharose, Q-Sepharose, Sephadex G150 and finally hydroxylapatite. The purification scheme has resulted in over a 1000-fold increase in specific activity (2 mumol/min per mg protein). Under non-reducing conditions, a major band on SDS-polyacrylamide gels at 70 kDa was observed, which shifted to a lower molecular weight, 60,000, under reducing conditions. The properties of the purified enzyme including the specificity for sn-2-arachidonoyl-containing phospholipids was similar to that observed for the crude enzyme. The results demonstrate the presence of a phospholipase A2 in the macrophage cell line. RAW 264.7, that preferentially hydrolyzes arachidonoyl-containing phospholipid substrates.