Isolation and characterization of a phospholipase A2 from an inflammatory exudate

Sterile peritoneal exudates produced in rabbits injected with 1% glycogen contain a phospholipase A activity in a cell-free supernatant fraction that hydrolyzed a synthetic phospholipid (1,2-diacyl-sn-glycero-3-phospho-ethanolamine) and phospholipids of autoclaved Escherichia coli. This phospholipase activity (phosphatidylacylhydrolase EC 3.1.1.4) exhibited an apparent bimodal pH optimum (pH 6.0 and pH 7.5) and was Ca(2+)-dependent; Mg(2+) and monovalent cations (Na(+) and K(+)) did not substitute for Ca(2+) in the reaction; EDTA was a potent inhibitor. The phospholipase hydrolyzed 1-[1-(14)C]palmitoyl-2-acyl-sn-glycero-3-phosphoethanolamine to form only radio-active lysophosphatidylethanolamine as the product, indicating that the enzyme had phospholipase A(2) specificity. The phospholipase A(2) was purified 302-fold by two successive chromatographic steps on carboxymethyl Sephadex. Gel filtration (Sephadex G75) of the purified enzyme resulted in a single peak of biological activity with a molecular weight of approximately 14,800. The same estimate of molecular weight was obtained by SDS-polyacrylamide gel electrophoresis, which yielded a single band. Polyacrylamide gel electrophoresis of this fraction at pH 4.3 revealed a single protein band migrating beyond lysozyme, with the dye front, suggesting that this protein was more basic than lysozyme (pI 10.5). The enzymatic and physical-chemical characteristics of this soluble enzyme were remarkably similar to a recently described phospholipase A(2) of rabbit polymorphonuclear leukocytes derived from glycogen-induced peritoneal exudates. The possible origin and physiological role of this soluble enzyme are discussed.