Separation and characterization of the A chain and B chain in β1-bungarotoxin fromBungarus multicinctus (Taiwan banded krait) venom

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Abstract
The interchain disulfide bond between A chain and B chain of β1-bungarotoxin (β1-Bgt) was selectively cleaved by dithiothreitol, and the A and B chains were separated by HPLC. The separated A and B chains did not show detectable enzymatic activity and lethal toxicity, but exhibited an immunoreactivity with anti-β1-Bgt antibody. Analytical isoelectrofocusing revealed that the A chain is a neutral subunit with pI=7.4, and the B chain is a basic one with pI=9.6. The A chain exhibited a Ca2+-binding ability as revealed by fluorescence measurement. Moreover, fluorescence studies showed that the intact interchain disulfide bond is essential for maintaining the hydrophobic character of substrate binding site in β1-Bgt and stabilizing the architectural environment of Trp-19 in the A chain. However, combination of the A chain and B chain failed to restore the biological activities and physicochemical properties which the intact β1-Bgt possessed. These, together with our previous result that the Trp-19 of the A chain is involved in substrate binding, suggest that the integrity of the interchain disulfide bond favors the maintenance of the active conformation of β1-Bgt.