Mitochondria and sarcoplasmic reticulum as model targets for neurotoxic and myotoxic phospholipases A2

Certain neurotoxins and myotoxins from snake venoms have phospholipase A(2) activity (phosphatide 2-acylhydrolase, EC 3.1.1.4), which appears to be necessary for their toxicity. Several of these toxins inhibit the net uptake of Ca(2+) into sarcoplasmic reticulum vesicles and brain mitochondria. We have obtained evidence that the ability to inhibit this Ca(2+) uptake is a mechanistically relevant correlate of the toxicity of these proteins rather than being just a nonspecific consequence of their phospholipase A(2) activity. Two of the toxins, beta-bungarotoxin and notexin, had 5% and 50%, respectively, of the phospholipase A(2) activity of IVa phospholipase A(2)(a nontoxic enzyme), but beta-bungarotoxin was as effective as IVa in inhibiting Ca(2+) uptake into brain mitochondria and notexin was more effective. Each of the myotoxic enzymes substantially inhibited Ca(2+) uptake into sarcoplasmic reticulum, notexin being the most effective in this regard. This ability correlated better with their myotoxic potency than with their phospholipase A(2) activity. beta-Bungarotoxin lost its toxicity but not its measurable phospholipase A(2) activity after modification with ethoxyformic anhydride in the presence of dihexanoylphosphatidylcholine. The modified toxin also lost most of its ability to inhibit Ca(2+) uptake into sarcoplasmic reticulum and brain mitochondria. Sarcoplasmic reticulum vesicles reconstituted from solubilized sarcoplasmic reticulum retained their sensitivity to notexin.