Selective modification of tyrosine-68 in β1-bungarotoxin from the venom ofBungarus multicinctus (Taiwan banded krait)

Abstract

β1-Bungarotoxin (β1-Bgt) fromBungarus multicinctus (Taiwan banded krait) snake venom was subjected to tyrosine modification withp-nitrobenzenesulfonyl fluoride (NBSF) atpH 8.0 and the NBS derivatives were separated by high-performance liquid chromatography. The results of amino acid analysis revealed that only one Tyr residue out of 14 was modified, and the modified residue was identified to be Tyr-68 in the A chain of β1-Bgt. Spectrophotometric titration indicated that the phenolic group of Tyr-68 has apK of 10.1. Modification of Tyr-68 in the A chain caused a selective loss in lethal toxicity, but had no effect on either enzymatic or antigenic activities. The Ca2+-induced difference spectra and fluorescence study indicated that β1-Bgt possesses at least two different types of Ca2+-binding sites. However, modification of Tyr-68 in β1-Bgt did not cause any change of the Ca2+-induced difference spectra and fluorescence spectra in native toxin and the two types of Ca2+-binding sites were retained. Moreover, the affinity of Tyr-68-modified β1-Bgt for 8-anilinonaphthalene sulfonate was also unaffected in both the presence and absence of Ca2+. All of the results indicated that Tyr-68 is not involved in the Ca2+ and substrate bindings in the A chain of β1-Bgt. It is concluded that lethal toxicity is not necessarily associated with enzymatic, antigenic, and Ca2+-binding activities in β1-Bgt.