PROTON MAGNETIC-RESONANCE STUDIES OF BRADYKININ ANTAGONISTS

Bradykinin (BK) is a peptide hormone with sequence Arg1-Pro2-Pro3-Gly4 -Phe5-Ser6-Pro7-Phe8-Arg9 and has been implicated in a multitude of pa thophysiological processes such as the ability to lower systemic blood pressure and stimulate pain. BK analogues having bulky, beta-branched D-aliphatic residues at position 7 combined with bulky L-aliphatic re sidues at position 8 have now been observed to be strong antagonists. Conformational studies based on two-dimensional nmr experiments in met hanol/water (80/20 v/v) were carried out on several such active antago nists in a polar solvent. Included in this study were the very active antagonists, [D-Arg0, Hyp3, Thi5, D-Cpg7, Cpg8]-BK [Cpg: alpha-cyclo-p entyl-glycine; Hyp: trans-4-hydroxy-L-proline; Thi: beta-(2-thienyl) - L-alanine] (I), [D-Arg0, Hyp3, D-Cpg7, Cpg8]-BK (II), as well as its v ariant with D-Cpg7 replaced by Cpg, namely [ D-Arg0, Hyp3, Cpg7, Cpg8] -BK (III). A turn-like structure, which coexists with the extended con formation, was observed between residues 2 and 5 for the most active a ntagonists I and II, in direct correlation with the peptide activities . No turn-like structure was found for residues 6-9. In peptide III, a turn-like structure was not identified. The existence of a turn at th e C-terminal end of bradykinin and its analogues has been predicted by empirical calculations and supported by nmr measurements. But the pre sent nmr study on the most active antagonists (I, II) does not support this hypothesis. Instead, the data suggest that a turn-like structure between residues 2 and 5 could be important for antagonist activity. Finally, one weak inhibitor [D-Cpg7]-BK (IV) showed no defined seconda ry structure.