SIMILARITIES OF β‐BUNGAROTOXIN AND PHOSPHOLIPASE A2 AND THEIR MECHANISM OF ACTION

Abstract— β‐Bungarotoxin, a presynaptic neurotoxin isolated from the venom of Bungarus multicinctus, has been shown to initially cause an increase in the frequency of miniature endplate potentials and subsequently block neuromuscular transmission by inhibiting nerve impulse induced release of acetylcholine. In rat brain synaptosomes it causes a Ca2+‐dependent release of acetylcholine together, with a strong inhibition of the high affinity choline uptake system. In this report we demonstrate that β‐bungarotoxin acts as a phospholipase A2 (phosphatide 2‐acyl hydrolase, EC 3.1.1.4), liberating fatty acids from synaptic membrane phospholipids. It also exhibits a striking similarity in a number of neurochemical properties with that of a purified phospholipase A2 from Naja naja siamensis. In addition, both agents produce a marked depolarization of synaptosomal preparations as measured by a fluorescent dye. We propose that disruption of the membrane phospholipids by phospholipase activity can lead to depolarization of the synaptosomal preparation which promotes both transmitter release and inhibition of the energy‐dependent high affinity choline uptake system. With this decreased supply of choline, the acetylcholine content of the cell would be gradually depleted leading to a decrease in transmission.