Structure–activity relationships of neurokinin A (4–10) at the human tachykinin NK 2 receptor: the role of natural residues and their chirality

Abstract

A structure–activity study of neurokinin A (NKA) (4–10) was performed to investigate the importance of residue and chirality for affinity and efficacy at the NK 2 receptor in human colon circular muscle. Two series of NKA(4–10) analogues were produced with either l-alanine or the d-enantiomer substituted. Their activities were determined in vitro by means of radioligand binding and isolated smooth muscle pharmacology. NKA was more potent than NKA(4–10) at the human, unlike the rabbit, NK 2 receptor. The contractile response of NKA(4–10) was unaffected by N-terminal acetylation. l-Ala substitution of Asp 4, Val 7, Leu 9, and Met 10 caused an 8- to 80-fold decrease, and substitution of Phe 6 caused a 5000-fold decrease in binding affinity ( P < 0.01). Positions Ser 5 and Gly 8 were not significantly affected. In functional studies, a similar pattern was observed. The replacement of residues with their respective d-enantiomer drastically reduced binding affinity and functional potency, particularly at positions 6 and 7 ( P < 0.05). NKA(4–10) analogues l-Ala 6, l-Ala 8, d-Phe 6, d-Val 7, and d-Met 10 were partial agonists. An excellent correlation was observed between binding and functional data ( r = 0.95). A retro-inverso analogue of NKA(4–10) was inactive. In conclusion, the side chains of Asp 4, Phe 6, Val 7, Leu 9, and Met 10 are structurally important features of NKA(4–10) for agonist activity, and changes in amino acid chirality are detrimental to binding affinity and functional activity. Overall, our data are broadly similar to those of previous studies in the rat. However, at the human NK 2 receptor, unlike the rat, [Ala 8]NKA(4–10) was an antagonist.