Cationic factors affecting phospholipase activities from human lung

1. The effects of varying H+ and other cation concentrations on phospholipase activity were investigated on two particulate fractions from human lung, corresponding to the mitochondrial and microsomal fractions. 2. Three 14C-labelled substrates, arachidonyl-phosphatidylcholine (PC), -phosphatidylethanolamine (PE) and -phosphatidylinositol (PI) were used. 3. For two substrates, PE and PI, hydrolysis was maximal at pH 6, with either subcellular fraction. 4. Hydrolysis of all three substrates was strongly inhibited by EDTA and EGTA (10-25 mM). Addition of 2,2-dipyridyl, o-phenanthroline, 8-hydroxyquinoline or desferrioxamine (10 microM-1 mM) did not inhibit but often increased hydrolysis of all substrates. 5. Addition of Zn2+, as ZnCl2, (10 microM-1 mM) inhibited PE and PI, but not PC, hydrolysis. 6. The phospholipase activities from human lung appear to be dependent on Ca2+ for maximal activity and to be inhibited by other metal ions including Zn2+.