The primary structures and myotropic activities of two tachykinins isolated from the African clawed frog, Xenopus laevis

Two peptides with limited structural similarity to mammalian substance P (SP) and neurokinin A (NKA) have been isolated from extracts of the intestine of the African clawed frog (Xenopus laevis). The primary structure of an SP-like peptide was established as: Lys-Pro-Arg-Pro-Asp-Gln-Phe-Tyr-Gly-Leu-Met·NH₂, which is identical to the previously characterized peptide, bufokinin isolated from the toad Bufo marinus. The primary structure of an NKA-related peptide was established as Thr-Leu-Thr-Thr-Gly-Lys-Asp-Phe-Val-Gly-Leu-Met·NH₂. Only the five amino acids at the C-terminal region of the peptide are identical to mammalian NKA whereas the N-terminal region shows no structural similarity to previously characterized tachykinins. Immunohistochemichal investigations of the gut wall revealed a dense network of nerve fibres and nerve cell bodies containing SP/NKA-like substances. The myotropic effects of the Xenopus tachykinins were compared with the contractile effect of mammalian SP and NKA on isolated strips of circular smooth muscle from Xenopus stomach. No significant differences in potencies (-log EC₅₀) or in intrinsic activities were observed between the Xenopus and mammalian peptides. The potencies for the Xenopus SP-like (8.49±0.15) and the NKA-like peptide (8.12±0.06) were similar suggesting that the amino acid sequence at the N-terminal region of the tachykinins is not important in activating the tachykinin receptors in Xenopus gastric smooth muscle. The maximum response to Xenopus SP (α=0.59±0.06) was significantly lower than to the NKA-like peptide (α=1.0) suggesting a more effective interaction of the NKA-like peptide with the tachykinin receptor(s) in Xenopus stomach.