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A novel heat-stable enterotoxin (designated Y-STb) was isolated and purified to homogeneity from the culture supernatant of a pathogenic but yst gene-negative strain of Yersinia enterocolitica. The amino acid sequence of the toxin was determined to be Lys-Ala-Cys-Asp-Thr-Gln-Thr-Pro-Ser-Pro-Ser-Glu-Glu-Asn-Asp-Asp-Trp-Cys-CysGlu-Val-Cys-Cys-Asn-Pro-Ala-Cys-Ala-Gly-Cys. Y-STb was 20-fold more potent (minimum effective dose in the suckling mouse assay was 0.35 pmol) than the previously documented heat-stable enterotoxin (YoSTa) which is produced by yst gene-positive strains of Y. enteroeolitiea and has a minimum effective dose of 7.8 pmol. The sequence of Y-STb is different from that of Y-STa in the N-terminal half (1-I 7), but quite similar in the C-terminal half (l 8-30). To elucidate the effect of 13 amino acid substitutions in Y-STb on enhancing the toxicity, several short analogs of Y-STb were synthesized and their toxicities were compared in the suckling mouse assay. The enhanced enterotoxicity could be ascribed to the addition of a tryptophan residue at the N-terminus of the ST toxic domain which is the minimum structure essential for toxic activity; the presence of an aspartic acid residue at the same position caused a decrease in toxicity.

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.

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