Tachykinins with unusual structural features from a urodele, the amphiuma, an elasmobranch, the hammerhead shark, and an agnathan, the river lamprey

Tachykinins were purified from extracts of gastrointestinal tissues of the urodele, Amphiuma tridactylum (three-toed amphiuma), and the elasmobranch Sphyrna lewini (hammerhead shark), and from the brain of the agnathan Lampetra fluviatilis (river lamprey). The amphiuma substance P (SP) (DNPSVGQFYGLM-NH₂) contains 12 amino residues compared with 11 for mammalian SP and lacks the Arg/Lys-Pro-Xaa-Pro motif that is characteristic of NK₁ receptor-selective agonists. Lampetra SP (RKPHPKEFVGLM-NH₂) is identical to SP from the sea lamprey and the shark SP-related peptide (AKFDKFYGLM-NH₂) is identical to dogfish scyliorhinin I. Amphiuma neurokinin A (NKA) (HKDAFIGLM-NH₂) and lamprey NKA (HFDEFVGLM-NH₂) contain 9 amino acid residues compared with 10 for mammalian NKA. The shark NKA-related peptide (ASGPTQAGIV¹⁰GRKRQKGEMF²⁰VGLM-NH₂) shows limited structural similarity to mammalian neuropeptide γ and the teleost tachykinin, carassin but contains 24-rather than 21 amino acid residues. The data show that the primary structures of the tachykinins have been very poorly conserved during vertebrate evolution and that pressure has acted only to maintain the functionally important sequence -Phe-Xaa-Gly Leu-Met-NH₂ at the COOH-termini of the peptides.