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Journal of Lipid Research, Vol. 15, 380-388, July 1974
Copyright © 1974 by Lipid Research, Inc.

Phospholipid metabolism by phagocytic cells. Phospholipases A₂ associated with rabbit polymorphonuclear leukocyte granules

R. Franson , P. Patriarca , and P. Elsbach

Department of Medicine, New York University School of Medicine, New York 10016, and the Department of General Pathology, University of Trieste, Trieste, Italy

Polymorphonuclear leukocytes obtained from sterile peritoneal exudates in rabbits contain two phospholipid-splitting activities (phosphatidylacylhydrolases EC 3.1.1.4), one most active at pH 5.5 and the other between pH 7.2 and 9.0. Hydrolysis of phospholipid was demonstrated using Escherichia coli labeled during growth with [1-¹⁴C]oleate and then autoclaved to inactivate E. coli phospholipases and to increase the accessibility of the microbial phospholipid substrates.

The acid and alkaline phospholipase activities are both membrane bound, calcium dependent, and heat stable, and they appear to be specific for the 2-acyl position of phospholipids. Evidence was also obtained suggesting that the E. coli envelope phospholipids with oleate in position 2 are more readily degraded than those with palmitate. The two activities are associated with azurophilic as well as specific granules (obtained by zonal centrifugation) and with phagosomes (isolated after ingestion of paraffin particles by the granulocytes).

Phospholipase A activities at pH 5.5 and pH 7.5 degrade the two major phospholipids of E. coli, phosphatidylethanolamine and phosphatidylglycerol, to the same extent, but the phospholipase activity at acid pH does not hydrolyze micellar dispersions of phosphatidylethanolamine. By contrast, phospholipase A₂ activity at pH 7.5 degrades both types of phosphatidylethanolamine substrates. Heparin and chondroitin sulfate inhibit phospholipase activity at pH 5.5 but have little effect on actvity at pH 7.5.

All detergents tested inhibited phospholipase activity, and both activities are inhibited by reaction products, free fatty acid and lysophosphatidylethanolamine. This product inhibition is only partially prevented by addition of albumin.

Supernatant fractions of granulocyte homogenates contain a heat-labile inhibitor of granule phospholipase activity at pH 7.5. Boiling the fraction not only removes the inhibition but actually results in stimulation of hydrolysis at pH 7.5 as well as pH 5.5.

These granule-associated phospholipase A activities of polymorphonuclear leukocytes differ in several of their properties from granule or lysosomal phospholipases of other phagocytic cells.

Supplementary key words hydrolysis of Escherichia coli phospholipids • granules • phagosomes • inhibition by heparin • product inhibition • heat-labile inhibitor • zonal centrifugation

Submitted on November 13, 1973
Accepted on April 5, 1974

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