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Basic polypeptides producing a variety of effects on animals and cells have been isolated from snake venoms. Many possess common structural features and also produce similar pharmacological effects. This has led to doubt as to the specificity of each polypeptide. Study of toxin gamma (cardiotoxin from Naja nigricollis), cytotoxin P6 (from naja naja, preferentially cytotoxic to certain cells) and neurotoxin alpha (Naja nigricollis) under identical conditions shows that they are separate entities though having some common structural properties. The amino acid composition shows certain resemblance between the nontoxic polypeptides, P6 and toxin gamma, as compared to the neurotoxin alpha. Their molecular weights are of the same order. Sulphydryl groups are absent in all but they possess a high proportion of disulphide linkages. The behavior of toxin gamma, cytotoxin P6 and neurotoxin alpha on Yoshida sarcoma cells and human erythrocytes demonstrate that whereas cytotoxin P6 was more active in lysing Yoshida sarcoma cells the order of activity was reversed in the human erythrocytes. Apparently these two cell systems respond differently to the action of the two polypeptides suggesting that they bind to different membrane receptors. The selectively displayed in changing the membrane permeability of different cells is probably dependent not ony on their basic charge but on the specificity of their protein structure.