Studies on the Role of Phospholipids in Phagocytosis

The increased incorporation of 32P into phosphatidic acid and phosphatidylinositol in polymorphonuclear leukocytes undergoing phagocytosis is not secondary to an elevated specific activity of adenosine triphosphate during phagocytosis. The incorporation of labeled glycerol, oleic acid, and linolenic acid into the above phosphatides is not increased on stimulation of phagocytosis. Phosphatidic acid is synthesized directly from α-glycerophosphate added to the incubation medium, but this synthesis is not increased on stimulation of phagocytosis. If it is assumed that the α-glycerophosphate which enters the cell is accessible to all of the intracellular α-glycerophosphate this result argues against the increased synthesis of phosphatidic acid being catalyzed by α-glycerophosphate acylase. The formation of phosphatidic acid in homogenates of leukocytes from ATP and diglyceride or monoglyceride was, respectively, 20 and 12 times that from α-glycerophosphate and palmityl coenzyme A plus oleyl coenzyme A. These results suggest that the increased synthesis of phosphatidic acid on induction of phagocytosis may be brought about by diglyceride kinase or possibly monoglyceride kinase plus lysophosphatidic acid acylase. The incorporation of labeled inositol into phosphatidylinositol is increased in cells undergoing phagocytosis, indicating that the increased incorporation of 32P into this phosphatide is not solely a result of a higher specific activity of phosphatidic acid, which may be the precursor for phosphatidylinositol in this system.