Structure–activity studies of bufokinin, substance P and their C-terminal fragments at bufokinin receptors in the small intestine of the cane toad, Bufo marinus

Abstract

Bufokinin is a substance P-related tachykinin peptide with potent spasmogenic actions, isolated from the intestine of the cane toad, Bufo marinus. Bufokinin acts via a tachykinin receptor with similarities to the mammalian NK 1 receptor. In this structure–activity study of bufokinin, substance P (SP) and their C-terminal fragments, we have used isolated segments and homogenates of toad small intestine to compare the contractile potencies and abilities to compete for the binding of [ 125 I ]-Bolton–Hunter bufokinin. In general, potency was very similar in both studies ( r=0.956) and was primarily related to peptide length, with the natural undecapeptide tachykinins bufokinin≈ranakinin>SP≈cod SP≈trout SP being most potent. The weakest peptides were [Pro 9]SP, BUF(7–11) and SP(7–11). Bufokinin fragments (BUF) were approximately equipotent to the corresponding SP fragments, with only BUF(5–11) showing unexpectedly low binding affinity. Data obtained with SP, bufokinin and fragments were subjected to quantitative structure–activity (QSAR) analysis which demonstrated that molecular connectivity and shape descriptors yielded significant regression equations ( r≈0.90). The predictive capacity of the equations was confirmed using ranakinin, trout SP and cod SP, but not using the synthetic analogs [Pro 9]SP and [Sar 9]SP. The study suggests that the full undecapeptide sequence of bufokinin is required for optimal activity, with high potency conferred by Lys 1, Pro 2, Gly 9 and probably Tyr 8. The finding that receptor–ligand interactions were correlated with the shape descriptor 2 κ α and favored by basic and rigid residues at position 1–3 is consistent with an important role of conformation at the N-terminus of bufokinin.