Inhibition of lysosomal phospholipases C and A in rabbit alveolar macrophages, polymorphonuclear leukocytes and rat liver by sodium bisulfite.

Phospholipase C activity was measured in disrupted rat liver lysosomes, Bacillus Calmette Guérin-induced rabbit alveolar macrophages and glycogen-induced rabbit polymorphonuclear leukocytes, using [N-methyl-14C]sphingomyelin and the phospholipids of human myelin. Phospholipase C activity with both substrates was maximal at pH 4.5 and was unaffected by monovalent or divalent cations or EDTA. The enzyme(s) had little or no substrate specificity, since most of the phospholipids of human myelin were degraded to the same extent. Solutions of gentamycin and tobramycin which contained preservatives inhibited activity, whereas similar preparations of streptomycin had no effect. On the other hand, preservative-free solutions of gentamycin and tobramycin were not inhibitory. Sodium bisulfite, an antioxidant, used in commercial preparations of gentamycin, tobramycin, streptomycin and other drugs, produced dose-dependent, reversible inhibition of the sphingomyelinase and myelin-degrading phospholipase C. Moreover, bisulfite-antibiotic interactions protected enzymatic activity from inactivation by the antioxidant. Other preservatives in commercial aminoglycosides, methyl- and propylparaben, phenol and EDTA had no effect on enzymic activity. Sodium bisulfite also inhibited acid-active phospholipase(s) A in rat liver lysosomes and alveolar macrophages, but had no effect on neutral-active, calcium-dependent phospholipases A2 from various cell types. Equimolar concentrations of streptomycin instantaneously relieved most of the inhibition by bisulfite, whereas molar ratios of gentamycin/bisulfite of 10 restored only 50% of the activity.