Inhibitory effect of EDTA.Ca2+ on the hydrolysis of synaptosomal phospholipids by phospholipase A2 toxins and enzymes.

Phospholipases A2 (PLA2) are Ca2(+)-dependent enzymes that are inhibited by EDTA; this inhibition would be expected to be reversed by restoring the Ca2+ concentration. By examining the hydrolysis of synaptosomal phospholipids by PLA2 enzymes, Naja naja atra and Naja nigricollis, and by toxins with PLA2 activity, beta-bungarotoxin (beta-BuTX) and notexin, we demonstrated a novel inhibitory action of EDTA manifested in the presence of excess Ca2+. We postulate the formation of an EDTA.Ca2+ complex which inhibits PLA2 activity in a concentration-dependent manner. Synaptosomes in which phospholipids are hydrolyzed by PLA2 have membranal damage expressed by increased acetylcholine (ACh) release and decreased osmotic activity. Addition of EDTA.Ca2+, which inhibits phospholipid hydrolysis, also reversed the PLA2 effect on ACh release, but not its effect on osmotic activity. The inhibition of PLA2 was observed on membranal phospholipids as well as on an artificial substrate of phospholipid-Triton mixed micelles. Moreover, we found that another enzyme, lactate dehydrogenase, was also inhibited. Our results indicate a non-specific inhibition exerted on the enzyme rather than on the substrate.