Structure-activity relationships of trout bradykinin ([Arg(0),Trp(5),Leu(8)]-bradykinin])

Trout bradykinin ( [Arg(0),Trp(5),Leu(8)]-BK) produces sustained and concen tration-dependent contractions of isolated longitudinal smooth muscle from trout stomach, although mammalian BK is without effect. Circular dichroism studies have demonstrated that trout BK, unlike mammalian BK, does not adop t a stable beta -turn conformation, even in the presence of sodium dodecyl sulfate (SDS) or trifluoroethanol. The myotropic actions of a series of ana logs in which each amino acid in trout BK was replaced by either alanine or the corresponding D-isomer were investigated, The peptides with Ala(4), D- Pro(3), D-Trp(5), D-Ser(6), and D-Pro(7) substitutions were inactive and di d not act as antagonists of trout BK. The analog with [Ala(5)] was a weak p artial agonist. The substitution (Arg(0) --> Ala) led to >50-fold decrease in potency but, in contrast to the importance of Phe(8) in both BK and desA rg(9)-BK in activating the mammalian B-2 and B-1 receptors respectively, su bstitutions at Leu(8) in trout BK had only a minor effect on potency. Antag onists to the mammalian B-2 receptor generally contain a D-aromatic amino a cid at position 7 of BK but the analog [Arg(0),Trp(5),D-Phe(7),Leu(8)]-BK w as a weak agonist at the trout receptor. Similarly, the potent nonpeptide m ammalian B-2 receptor antagonist FR173657 was without effect on the action of trout BK. These data suggest the hypothesis that the receptor binding co nformation of trout BK is defined by the central region (residues 3-7) of t he peptide but is adopted only upon interaction with the receptor. The bioa ctive conformation is probably stabilized by an ionic interaction between A rg(0) in the peptide and an acidic residue in the receptor. (C) 2000 Elsevi er Science Inc. All rights reserved.