Chemical characterization of the structure of cholera toxin and its natural toxoid

Acrylamide gel electrophoresis demonstrated that the toxin of Vibrio cholerae is comprised of three polypeptide chains, α, β, and γ, of molecular weights 24,000, 9,700, and 9,700 daltons, respectively. Amino acid sequence analysis of intact toxin indicated a molecular composition of αγβ₄. Acrylamide gel electrophoresis and sequence analysis confirmed that the natural toxoid (choleragenoid) is identical to the toxin β chain. The α and γ chains of the toxin are disulfide linked (fragment A) but are noncovalently bound to the β chains. About 50% of the primary structure of the N terminal portion of the β chain has been identified, and a small segment of the C terminus has also been characterized. Twenty residues of the N terminal portions of the α and γ chains have been tentatively identified. The amino acid composition of the β chain was determined and compared to that of the natural toxoid.