Structure-activity studies at the rat tachykinin NK2 receptor: effect of substitution at position 5 of neurokinin A

A series of analogues of neurokinin A(4-10) was synthesized using solid pha se techniques with Chiron pins, and purified by HPLC. The potencies of 10 p eptides with substitution at Ser(5) were assessed at rat fundus NK2 recepto rs. In membrane binding studies with [I-125)-[Lys(5),Tyr(I-2)(7),MeLeu(9),N le(10)]-NKA(4-10), all compounds except [Asp(5)]INKA(4-10) showed reasonabl e affinity, and analogues with Lys and Arg substitutions were fivefold more potent than NKA(4-10). In functional studies, all peptides were able to co ntract the rat isolated fundus strips. Analogues with Phe, His and Asn subs titutions were substantially weaker in functional than in binding studies, whereas there was an excellent correlation (r = 0.95) between binding and f unctional potency for the remaining seven peptides. [Phe(5)]NKA(4-10) is in fact neurokinin B(4-10) and this residue may be cri tical in determining selectivity between NK2 and NK3 receptors. Analogues w ith a basic residue (Lys, Arg) at position 5 showed both increased affinity and functional potency, whereas the neutral [Asn(5)]NKA(4-10) was equally as weak in contractile studies as the acidic [Asp(5)]NKA(4-10). However, [G lu(5)]NKA(4-10) and [Gln(5)]NKA(4-10) were no different from NKA(4-10). Our results could indicate the presence of a negative charge on the NK2 recept or, close to position 5 of NKA. This would facilitate interaction with posi tively charged side chains and impede interaction with negatively charged s ide chains, particularly the inflexible side chain of aspartic acid. Thus, not only the charge, but also the length of the side chain of the residue a t position 5, seems to be important for interaction with the rat NK2 recept or.