Influence of a glycine or proline substitution on the functional properties of a 14-amino-acid analog of Escherichia coli heat-stable enterotoxin.

Analogs of Escherichia coli heat-stable enterotoxin (ST) differing in chain length or the presence of turn-forming residues were assessed for binding to receptors, activation of particulate guanylate cyclase, and stimulation of secretion in suckling mice. These analogs included the native 18-amino-acid peptide (ST), the 14-amino-acid carboxy terminus of this native peptide with a proline at position 12 (ST[5-18]proline), and the 14-amino-acid carboxy terminus in which the proline at position 12 was substituted with glycine (ST[5-18]glycine). Each analog bound to the receptor in a dose-dependent fashion, completely displacing [125I]ST in competitive binding assays. However, their potencies differed significantly: ST demonstrated the highest affinity (inhibition constant [Ki], 10(-9) M), followed by ST[5-18]proline (Ki, 10(-7) M) and ST[5-18]glycine (Ki, 10(-6) M). Similarly, these peptides maximally activated particulate guanylate cyclase and stimulated intestinal secretion in suckling mice. Their rank order of potency in these assays was similar to that described for receptor binding: ST greater than ST[5-18]proline greater than ST[5-18]glycine. These data demonstrate that the full peptide structure is not absolutely required for pharmacological, biochemical, or biological activity. However, the four amino-terminal residues contribute significantly to the potency of these peptides. In addition, the turn imposed by the proline residue at position 12 is not absolutely required for receptor occupancy or activation of the biochemical cascade that results in intestinal secretion. However, it significantly increases the potency of the toxin. These data illustrate the importance of primary and secondary structures to the biochemical, pharmacological, and physiological activities of the ST produced by E. coli.