Chemical Synthesis of a Heat-stable Enterotoxin Produced by Enterotoxigenic Escherichia coli Strain 18D

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Abstract
Two peptides with the two primary structures of 18 amino acid residues proposed for a heat-stable enterotoxin from enterotoxigenic Escherichia coli strain 18D were synthesized by solution methods and their physicochemical and biological properties were compared with those of native toxin. One of these peptides (Asn–Thr–Phe–Tyr–Cys–Cys–Glu–Leu–Cys–Cys–Asn–Pro–Ala–Cys–Ala–Gly–Cys–Tyr) showed the same heat-stability and 1H-NMR spectrum as those of the native toxin and evoked fluid secretion in suckling mice at a dose of 1.5–2.0 ng, which is similar to the effective dose of native toxin. Moreover, its toxicity was neutralized by antisera against the native toxin. The other peptide (Asn–Thr–Phe–Tyr–Cys–Cys–Glu–Leu–Cys–Cys–Tyr–Pro–Ala–Cys–Ala–Gly–Cys–Asn) showed different physicochemical and biological behaviors from those of the native toxin and exhibited toxic activity at a dose of 50–80 ng, which is about thirty times the effective dose of the native toxin. Moreover, its toxicity was not neutralized by antisera against the native toxin. These observations indicate that the former peptide is identical to native toxin but that the latter peptide differs in properties from the native toxin.

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