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 concentration-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 adopt a stable beta-turn conformation, even in the presence of sodium dodecyl sulfate (SDS) or trifluoroethanol. The myotropic actions of a series of analogs 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 did not act as antagonists of trout BK. The analog with [Ala(5)] was a weak partial 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 desArg(9)-BK in activating the mammalian B(2) and B(1) receptors respectively, substitutions at Leu(8) in trout BK had only a minor effect on potency. Antagonists to the mammalian B(2) receptor generally contain a D-aromatic amino acid at position 7 of BK but the analog [Arg(0),Trp(5),D-Phe(7),Leu(8)]-BK was a weak agonist at the trout receptor. Similarly, the potent nonpeptide mammalian B(2) receptor antagonist FR173657 was without effect on the action of trout BK. These data suggest the hypothesis that the receptor binding conformation of trout BK is defined by the central region (residues 3-7) of the peptide but is adopted only upon interaction with the receptor. The bioactive conformation is probably stabilized by an ionic interaction between Arg(0) in the peptide and an acidic residue in the receptor.