Structure-Activity Study of Guinea Pig Trachea Tachykinin NK-2 Receptor: Effect of Substitution at the Seventh Position from C-Terminus of Neurokinin A

Substance P (SP: Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-GlyLeu-Met-NH2), neurokinin A (NKA: His-Lys-Thr-Asp-ThrPhe-Val-Gly-Leu-Met-NH2) and neurokinin B (NKB: AspMet-His-Asp-Phe-Phe-Val-Gly-Leu-Met-NH2) are mammalian tachykinin peptides that have similar biological actions, such as smooth muscle contraction, salivation, vasodilatation and neuronal excitation. These mammalian tachykinin peptides are characterized by the C-terminal common sequence, Phe-Xaa-Gly-Leu-Met-NH2, where Xaa represents an aromatic (Phe, Tyr) or branched aliphatic (Val, Ile). Since SP, NKA and NKB are widely distributed throughout the central and peripheral nervous systems, they are believed to act as putative neurotransmitters. SP, NKA, and NKB interact with at least three receptor subtypes. The rank order of potency for tachykinin receptors is SP > NKA > NKB for the NK-1 receptor, NKA > NKB > SP for the NK-2 receptor, and NKB > NKA > SP for the NK-3 receptor. The receptors have been cloned and are members of the superfamily of G protein-coupled receptors with seven putative membranespanning segments. The carboxy-terminal heptapeptide of SP, NKA, and NKB was known to be at least the shortest length to display the contractile activity on various isolated muscles such as guinea pig ileum (GPI), guinea pig trachea (GPT), and rat vas deferens (RVD) containing tachykinin receptors. The difference between SP and neurokinin peptides (NKA and NKB) in their active sequence of C-terminal heptapeptide is the fourth and seventh positions from C-terminus. The fourth position from C-terminus of SP and neurokinin peptides is aromatic (Phe) and aliphatic (Val), respectively. The seventh position from C-terminus is acidic (Asp) and carboxyamide (Gln), respectively. In this study, in order to investigate the effects of the seventh residue C-terminus of NKA in the binding NK-2 receptor and design more selective agonist against NK2 receptor than NKA, we synthesized three analogues substituted with Glu, 2-aminobutyric acid (Abu), or Ala at this position of NKA. The biological activity of the peptides was examined by the contractile activity of the smooth muscle of guinea pig trachea (GPT) known as the NK-2 receptor-specific tissue.