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, alpha, beta, and gamma, of molecular weights 24,000, 9,700, and 9,700 daltons, respectively. Amino acid sequence analysis of intact toxin indicated a molecular composition of alpha gamma beta4. Acrylamide gel electrophoresis and sequence analysis confirmed that the natural toxoid (choleragenoid) is identical to the toxin beta-chain. The alpha- and gamma-chains of the toxin are disulfide-linked (fragment A) but are noncovalently bound to the beta-chains. About 50% of the primary structure of the N-terminal portion of the beta-chain has been identified and a small segment of the D-terminus has also been characterized. Twenty residues of the N-terminal portions of the alpha- and gamma-chains have been tentativly identified. The amino acid composition of the beta-chain was determined and compared to that of the natural toxoid.