Binding characteristics of BmK I, an α‐like scorpion neurotoxic polypeptide, on cockroach nerve cord synaptosomes

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Abstract
In this study, the binding characteristics of BmK I, an α‐like neurotoxic polypeptide purified from the venom of the Chinese scorpion Buthus martensi Karsch, were investigated on rat brain and cockroach nerve cord synaptosomes. The results showed that BmK I can bind to a single class of noninteracting binding sites on cockroach nerve cord synaptosomes with medium affinity (Kd = 16.5 ± 4.4 nm) and low binding capacity (Bmax= 1.05 ± 0.23 pmol/mg protein), but lacks specific binding on rat brain synaptosomes. BmK AS, BmK AS‐1 (two novel sodium channel‐blocking ligands), BmK IT (an excitatory insect‐selective toxin) and BmK IT2 (a depressant insect‐selective toxin) from the same venom were found to be capable of depressing BmK I binding in cockroach nerve cord synaptosomes, which might be attributed to either allosteric modulation of voltage‐gated Na+ channels by these toxic polypeptides or partial overlapping between the receptor binding sites of BmK I and these toxins. This thus supported the notion that α‐like scorpion neurotoxic polypeptides bind to a distinct receptor site on sodium channels, which might be similar to the binding receptor site of α‐type insect toxins, and also related to those of BmK AS type and insect‐selective scorpion toxins on insect sodium channels.