Quantitative structure-activity analyses of bufokinin and other tachykinins at bufokinin (bNK₁) receptors of the small intestine of the cane toad, Bufo marinus

The toad tachykinin, bufokinin (Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Leu-Met amide; BUF), acts via tachykinin NK₁-like receptors to contract the intestine of the cane toad, Bufo marinus. In this structure-activity study, we used isolated segments of toad small intestine and performed binding studies with [¹²⁵I] Bolton-Hunter BUF in intestinal membranes to compare the contribution of individual amino acid residues to the potencies of 18 naturally occurring tachykinins and 13 BUF analogs. Potencies were similar (r = 0.94) in functional and binding studies, with BUF and ranakinin being most potent. Ranatachykinin A, physalaemin, hylambatin and cod, trout and mammalian SPs exhibited 10-60% of the potency of BUF. The Ala-substituted BUF analogs were 11-60% as potent as BUF in functional studies, with [Ala²]-BUF and [Ala⁴]-BUF the least efficacious, indicating the importance of both proline residues. QSAR equations were developed using 12 connectivity, shape and steric parameters for each of the 7 hypervariable amino acid residues in these peptides. For the binding data, the optimal regression equation explained 81% of the variance, and indicated the importance of the steric function at [Pro ²] and simple connectivity functions at [Gln⁶] and [Tyr⁸]. The optimal functional regression equation (80% of variance) confirmed the importance of connectivity functions at [Gln⁶] and [Tyr⁸], as well as the shape of residues [Lys¹] and [Pro⁴]. The potencies of most full-length peptides were well predicted using the leave-one-out procedure, as were the potencies of a series of model Ala-substituted BUFs, thus emphasising the potential utility of these equations in the design of new ligands interacting with tachykinin receptors.