A long-acting heat-stable enterotoxin analog of enterotoxigenic Escherichia coli with a single D-amino acid.

A heat-stable enterotoxin (STp) consisting of 18 amino acid residues including 6 half-cystine residues is produced by a porcine strain of enterotoxigenic Escherichia coli. Analogs of STp with replacements of single residues at each from positions 5 to 17 by the corresponding D-amino acid residue were synthesized by a solid-phase method. Of these analogs, [D-Cys5]-STp[5-17] showed the same biological properties as STp[5-17]. Moreover, its activity to cause fluid accumulation in suckling mouse lasts more than 24 hours, whereas the activity of STp[5-17] decreases after 6-10 hours. These results indicate that the action of the analog [D-Cys5]-STp[5-17] is strongly agonistic to that of the native ST.