EFFECT OF DIACYLGLYCEROLS ON THE ACTIVITY OF COBRA VENOM, BEE VENOM, AND PIG PANCREATIC PHOSPHOLIPASES-A2

The effects of a series of diacylglycerols (DAGs) with varying acyl chain lengths and degree of unsaturation on the activity of cobra venom, bee venom, and pig pancreatic phospholipases A2 (PL-A2s) were studied using two lipid substrates: dipalmitoylphosphatidylcholine (DPPC) or bovine liver phosphatidylcholine (BL-PC). The activities of the phospholipases critically depended on the chain length and degree of unsaturation of the added DAGs and on the chemical composition of the substrate. The effects of DAGs on cobra or bee venom PL-A2s were similar, but significantly different from the pig pancreatic PL-A2. The data, taken together with our previous NMR studies on physicochemical effects of these DAGs on lipid bilayer structure [De Boeck, H., & Zidovetzki, R. (1989) Biochemistry 28, 7439; (1992) Biochemistry 31, 623], allowed detailed correlation of the type of a bilayer perturbation induced by DAG with the activation or inhibition of the phospholipase on the same system. In general, the activation of the phospholipases correlated with the DAG-induced defects of the lipid bilayer structure. The results, however, argue against general designation of DAGs as "activators" or "inhibitors" of PL-A2s. Thus, for example, diolein activated phospholipases with the BL-PC lipid substrate, but inhibited them with the DPPC substrate. Dihexanoylglycerol and dioctanoylglycerol inhibited pig pancreatic PL-A2 with both lipid substrates and inhibited cobra or bee venom PL-A2 with the DPPC substrate, but activated the latter two enzymes with the BL-PC substrate. Longer-chain DAGs (C > 12), which induce lateral phase separation of the bilayers into the regions of different fluidities, activated all PL-A2s with both lipid substrates. The effect on pig pancreatic PL-A2 activity was generally an order of magnitude higher than on cobra or bee venom enzymes. The effects of longer-chain DAGs and diolein on activity of pig pancreatic PL-A2 were significant at 2.5-5 mol % of these DAGs relative to the phospholipids, which is within physiological concentrations of endogenous diacylglycerols in the biological membranes.