Optical activity and conformation of beta-bungarotoxin in solution.

beta-Bungarotoxin, which consists of two polypeptide chains (A- and B-chain), in the venom of Formosan banded krait is stable in 7.5 M urea but can be denatured in 6.0 M guanidine hydrochloride. Its conformation remains virtually the same in solvents of lower polarity than water such as a mixture of 1,2-ethanediol-water (4:1 by volume). The circular dichroism spectrum in water shows a double minima at 222 and 209 nm, which is characteristic of the helical structure. The ellipticities at these two wavelengths indicate that the helical content of this toxin is not high. Comparing how guanidine hydrochloride effects the helix-coil transition of the toxin with that of phospholipase A2's which are structurally homologous to A-chain implicates that the two polypeptide chains should be coexisted and interacted with each other in order to maintain the active conformation of beta-bungarotoxin. Removal of eight amino acid residues from the N-terminus of the A-chain by action of CNBr on beta-bungarotoxin does not disrupt the polypeptide folding but abolishes the neurotoxicity.