Synthesis and biological activities of new side chain and backbone cyclic bradykinin analogues

Abstract

A series of conformationally constrained cyclic analogues of the peptide hormone bradykinin (BK, Arg-Pro-Pro-Gly-Phe-Ser-Pro-Phe-Arg) was synthesized to check different turned structures proposed for the bioactive conformation of BK agonists and antagonists. Cycles differing in the size and direction of the lactam bridge were performed at the C- and N-terminal sequences of the molecule. Glutamic acid and lysine were introduced into the native BK sequence at different positions for cyclization through their side chains. Backbone cyclic analogues were synthesized by incorporation of N-carboxy alkylated and N-amino alkylated amino acids into the peptide chain. Although the coupling of Fmoc-glycine to the N-alkylated phenylalanine derivatives was effected with DIC/HOAt in SPPS, the dipeptide building units with more bulky amino acids were pre-built in solution. For backbone cyclization at the C-terminus an alternative building unit with an acylated reduced peptide bond was preformed in solution. Both types of building units were handled in the SPPS in the same manner as amino acids. The agonistic and antagonistic activities of the cyclic BK analogues were determined in rat uterus (RUT) and guinea-pig ileum (GPI) assays. Additionally, the potentiation of the BK-induced effects was examined. Among the series of cyclic BK agonists only compound 3 with backbone cyclization between positions 2 and 5 shows a significant agonistic activity on RUT. To study the influence of intramolecular ring closure we used an antagonistic analogue with weak activity, [D-Phe7]-BK. Side chain as well as backbone cyclization in the N-terminus of [D-Phe7]-BK resulted in analogues with moderate antagonistic activity on RUT. Also, compound 18 in which a lactam bridge between positions 6 and 9 was achieved via an acylated reduced peptide bond has moderate antagonistic activity on RUT. These results support the hypothesis of turn structures in both parts of the molecule as a requirement for BK antagonism. Certain active and inactive agonists and antagonists are able to potentiate the bradykinin-induced contraction of guinea-pig ileum.

Development of Conformationally Restricted Analogues of Bradykinin and Somatostatin Using Constrained Amino Acids and Different Types of Cyclization

“…Among the series of cyclic agonists, only the compound 3 (Table 2) with backbone cyclization at the N-terminus between positions 2 and 5 shows a significant activity on RUT [28]. Our results with the series of cyclic agonists could not confirm the assumption of a C-terminal turn structure as a requirement for agonistic activity.…”

“…[28]. Side chain as well as backbone cyclization in the N-terminus of [DPhe 7 ]-BK ( Table 2, compounds 4 and 5) in some cases resulted in analogues with moderate antagonistic activity on RUT, whereas all analogues with side chain cyclization at the C-terminus were inactive.…”

“…Since we found incomplete condensation of hydrophobic amino acids onto N α -functionalised building units we prefer the utilisation of preformed dipeptide building units. Both, the solution syntheses of suitable building units as well as the assembly of backbone cyclic peptide hormones on the solid phase were described by different authors and published in corresponding chemical journals [for a selection see [26][27][28].…”

Backbone-methylated Analogues of the Principle Receptor Binding Region of Human Parathyroid Hormone

“…Fig. 3 31 , no loss of helix was observed, but ϳ34% of the helix was lost when methylation was at Asp 30 . Similarly, when the methylation was at the N terminus of the helix, the apparent loss of helix was ϳ15%, based on the [] 222 values.…”

“…These methylations have been used successfully to modulate binding specificity of an agonist for the C5A anaphylatoxin receptors (29), and to create subtype-specific somatostatin receptor agonists and antagonists (30,31). Methylation of a backbone NH not only prevents formation of a H-bond at that point, but it also has an effect on secondary structures, such as ␣-helices or ␤-turns, that may have existed in the region of the modification.…”