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RG Sharkey, E Jover, F Couraud, DG Baden and WA Catterall
The effects of Ptychodiscus brevis toxin 2 (PbTx-2) on the binding of neurotoxins at four different neurotoxin receptor sites on voltage- sensitive sodium channels in rat brain synaptosomes were examined. Binding of saxitoxin at neurotoxin receptor site 1 and Leiurus quinquestriatus alpha-scorpion toxin (LqTx) at neurotoxin receptor site 3 was unaffected. PbTx-2 enhanced binding of batrachotoxinin A 20-alpha- benzoate (BTX-B) to neurotoxin receptor site 2 and Centruroides suffusus suffusus beta-scorpion toxin (CsTx II) to site 4 on sodium channels. These results support the proposal that PbTx-2 and related toxins act at a new receptor site (site 5) that has not been previously analyzed in binding experiments. Half-maximal effects of PbTx-2 were observed in the range of 20-50 nM PbTx-2. The enhancement of BTX-B binding was reduced by depolarization. Saturating concentrations of PbTx-2 reduced KD values for binding of BTX-B and CsTx-II 2.9-fold and 2.6-fold, respectively. The effects of PbTx-2 and LqTx in enhancing BTX- B binding were synergistic. A model involving both preferential binding of BTX-B, PbTx-2, LqTx, and CsTx II to active states of sodium channels and allosteric interactions among the four receptor sites at which these toxins act accommodates these and previous results.
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