MYOTOXIC ACTIVITY OF THE TOXIC PHOSPHOLIPASE, NOTEXIN, FROM THE VENOMOF THE AUSTRALIAN TIGER SNAKE

Notexin is a neurotoxic and myotoxic phospholipase A, derived from the venom of the Australian tiger snake, Notechis scutatus. Though the ph ospholipase activity has been implicated in the toxicity of notexin, l ittle is understood of its site and mode of action. In this study we i nvestigated the myotoxicity of notexin on the skeletal muscle of the r at. Using immunogold labeling both in vivo and in vitro, we demonstrat ed that notexin bound exclusively to the sarcolemma. At the early stag es when notexin was injected into the muscle there was no evidence of internalization, though at more advanced degeneration when muscle fibe rs were necrotic, the toxin was able to penetrate the interior of the fibers to exhibit nonspecific labeling. We also showed the toxin was a ble to bind to glycolytic muscle fibers, which are known to be resista nt to the myotoxic effects of notexin. Electron microscopy confirmed t he localization of the binding site to the sarcolemma. Scanning electr on microscopy showed that the primary pathological changes associated with exposure to notexin were the appearance of areas of hypercontract ion along the muscle fibers associated with small lesions in the sarco lemma. At more advanced stages large tears appeared in the sarcolemma between adjacent areas of hypercontraction, revealing apparently intac t myofibrils below. We conclude that the sarcolemma is the binding sit e for the toxin. We propose that the hydrolytic activity causes the ap pearance of small lesions in the membrane, the loss of ion gradients, and hypercontraction. The weakened area between sites of hypercontract ion is then ruptured, leading ultimately to the degeneration of the mu scle fibers.