Importance of the leucine side‐chain to the spasmogenic activity and binding of Substance P analogues

Previous studies with Substance P (SP) antagonists (GR 71251, [dPro9, Pro10, Trp11]SP and dPro9, MeLeu10, Trp11]SP) have suggested the existence in the guinea-pig ileum (GPI) of two distinct tachykinin receptors associated with the contractile responses of [Pro9]SP and septide. In addition [Apa9-10]SP, a glycine-substituted analogue of SP with a carba bond between residues 9 and 10, [Gly9-ψ(CH2-CH2)-Gly10SP = [Apa9-10]SP, was shown to belong to the ‘septide family’ (low affinity for NK-1 specific binding sites and high potency in the GPI). In order to establish the importance of the isopropyl side-chain in position 10, the binding potencies and activities of [Gly9-ψ(CH2-CH2)-Gly10]SP, [Ala10]SP, [Gly9-ψ(CH2-CH2)-Leu10]SP and [Gly9-ψ(CH2-CH2)-dLeu10]SP were compared. Conformational behaviour of active peptides with a carba bond was analyzed by NMR and modelisation studies. This study with agonists demonstrated that undecapeptides substituted in position 10 in the SP sequence also enabled discrimination of NK-1 receptors from receptors responsible for the spasmogenic activities of peptides belonging to the ‘septide family’. [Gly9-ψ(CH2-CH2)- Leu10]SP is ahighly potent NK-1 agonist, [Gly9-ψ(CH2-CH2)-Gly10]SP acts on the septide-sensitive receptor, and [Ala10]SP is a mixed agonist.