Role of phospholipase A activity in the neuromuscular paralysis produced by some components isolated from the venom of the seasnake, Laticauda semifasciata

Three purified phospholipase A components from the venom of the seasnake, Laticauda semifasciata, were tested for neuromuscular blocking potency under conditions that would alter their enzyme activities. The non-enzymic neurotoxin from the same venom, erabutoxin b, was also tested to show that effects of changing the physiological salt solution resulted from changes in enzyme activity and not to any nonspecific effects at the neuromuscular junction. Phospholipase A I had about ten times the enzyme activity but only half the blocking potency of phospholipases A III and IV. In conditions that increased the enzyme activity of phospholipase A I (increasing the calcium concentration to 5 and 20 mM; pH 8.2) there were parallel increases in blocking potency; after oxidation with N-bromosuccinimide, phospholipase A I lost both enzyme and blocking actions. In contrast, there was no relationship between the blocking potencies and enzyme activities with phospholipases A III and IV. Phospholipase A III was also shown to compete with α-bungarotoxin for binding to purified acetylcholine receptors, suggesting that these phospholipases block transmission by occupying the acetylcholine-binding site and not by uncoupling or blocking receptor ion channels. It is concluded that enzyme activity is required for blockade by phospholipase A I but is not important for phospholipases A III and IV which bind directly to cholinoceptors.