ABSTRACT
Glycine-9 and leucine-10 of substance P (SP) are critical for (NK)-1 receptor recognition and agonist activity. Propsi(Z)-CH=CH(CH3)-CONH)Leu (or Met) and Propsi((E)-CH=CH(CH3)-CONH)Leu (or Met) have been introduced in the sequence of SP, in order to restrict the conformational flexibility of the C-terminal tripeptide, Gly-Leu-Met-NH2, of SP. Propsi((Z)-CH=C(CH2CH(CH3)2)-CONH)Met-NH2, with an isobutyl substituent to mimic the Leu side-chain, was also incorporated in place of the C-terminal tripeptide. The substituted-SP analogs were tested for their affinity to human NK-1 receptor specific binding sites (NK-1M and NK-1m) and their potency to stimulate adenylate cyclase and phospholipase C in Chinese Hamster ovary (CHO) cells transfected with the human NK-1 receptor. The most potent SP analogs [Pro9psi((Z)CH=C(CH3)CONH)Leu10]SP and [Pro9psi ((E)CH=C(CH3)CONH)Leu10]SP, are about 100-fold less potent than SP on both binding sites and second messenger pathways. These vinylogous (Z)- or (E)-CH=C(CH3)- or (Z)-CH=C(CH2CH(CH3)2) moieties hamper the correct positioning of the C-terminal tripeptide of SP within both the NK-1M- and NK-1m-specific binding sites. The origin of these lower potencies is related either to an incorrect peptidic backbone conformation and/or an unfavorable receptor interaction of the methyl or isobutyl group.
