Two beta-bungarotoxin isotoxins BM12 and BM13 were isolated from Bungarus multicinctus (Taiwan banded krait) venom by sequential chromatography on ion-exchange and reverse phase columns. The two toxins have the same A chain, but different B chains. Different phospholipase A2 activity and different potencies in inhibiting the spontaneous enhancement of spontaneous synaptic current frequency and muscle contraction were observed for BM12 and BM13. Nevertheless, modification of Lys-64 in the A chain of BM12 and BM13 similarly reduced in their phospholipase A2 activity and toxicity. The modified derivatives retained their affinity with Ca2+ and their conformation as deduced by CD. These results suggest that Lys-64 of the A chain is involved in the phospholipase A2 activity and in the neurotoxic effect of beta-bungarotoxin.
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