Purification and sequence determination of heat‐stable enterotoxin elaborated by a cholera toxin‐producing strain of Vibrio cholerae O1

Four molecular species of heat‐stable enterotoxins elaborated by a cholera toxin‐producing strain of Vibrio cholerae O1 were isolated from its culture supernatant. The amino acid sequence of one of the enterotoxins was determined to be Phe‐Ile‐Lys‐Gln‐Val‐Asp‐Glu‐Asn‐Gly‐Asn‐Leu‐Ile‐Asp‐Cys‐Cys‐Glu‐Ile‐Cys‐Cys‐Asn‐Pro‐Ala‐Cys‐Phe‐Gly‐Cys‐Leu‐Asn with three intramolecular disulfide linkages. The other enterotoxins had shorter amino acid sequences in the N‐terminal regions, but possessed the same sequence in their C‐terminal regions including the three disulfide linkages. The enterotoxins with the shorter N‐terminal sequences showed more potent toxicities, and the minimum effective dose of the longest one with 28 amino acid residues was 10‐folds of that of the shortest one.