ABSTRACT
Four sets of angiotensin II (AngII) analogues with position 5 modifications, two agonist series with either Asp or Sar in position 1 and L-Phe in position 8, and two antagonist series with again Asp or Sar in position 1 and Leu in position 8 were synthesized. Modifications in positions 5 were introduced successively: Ile, Nle, Met, S-ethyl Cys, S-n-propyl-Cys, S-n-butyl Cys, S-t-butyl Cys and S-benzyl Cys in all four series. The study was undertaken in order to investigate the 5-position residue of AngII by replacing the hydrophobic side-chain by another containing an electrophilic moiety. The analogues were synthesised by solid phase synthesis using the Boc/Bzl or Fmoc/But strategy. All analogues were evaluated by their binding properties to the AT1 receptor on bovine adrenocortical membranes (bAT1). The results indicate that AngII analogues bind, irrespective of their agonistic or antagonistic nature or of their position 1 modification, in a similar manner and that position 5 modifications without beta-branching behave in an additive manner towards their affinity.
Subject(s)
Angiotensin II/analogs & derivatives , Angiotensin II/pharmacology , Adrenal Cortex/drug effects , Adrenal Cortex/metabolism , Angiotensin I/metabolism , Angiotensin II/chemical synthesis , Animals , Cattle , Chromatography, Gel , In Vitro Techniques , Membranes/drug effects , Membranes/metabolism , Rabbits , Receptors, Angiotensin/drug effects , Receptors, Angiotensin/metabolism , Structure-Activity RelationshipABSTRACT
Angiotensin II (AII) analogs bearing n-Leu, Met or S-substituted groups for cysteine at position 5 were studied regarding their agonistic and tachyphylactic properties. It was shown that these analogs lowered the relative affinity towards the AT1 receptor as determined by contractile responses, which could be due to the removal of the beta-branching residue at position 5. Insertion of a sulfur atom in a different position away from the attached backbone carbon atom presented no significant difference in EC50 values for these analogs. Interestingly, the S-bearing analogs at position 5 were full agonists but the tachyphylactic property was lost, in contrast to [n-Leu5]AII, which still induced reduction of the contractile responses. Nevertheless after replacing the Asp with Sar in position 1 (Sar1) tachyphylaxis was again established. It is concluded that the insertion of Met or an S-substituted cysteine into the side chain at position 5 of AII may promote interactions with its receptor due to the slight electronegative character of the sulfur atom and changes in the restricted conformational freedom of the Ile5 residue in the AII molecule. This was overcome by Sar1, probably through interactions due to its fully protonated N-terminal amino group and favoring the conformation responsible for the tachyphylaxis phenomenon.
Subject(s)
Angiotensin II/chemistry , Angiotensin II/pharmacology , Sulfur/chemistry , Tachyphylaxis , Vasoconstrictor Agents/chemistry , Vasoconstrictor Agents/pharmacology , Amino Acid Substitution , Angiotensin II/analogs & derivatives , Animals , Female , Guinea Pigs , Ileum/drug effects , Male , Sarcosine/chemistry , Structure-Activity RelationshipABSTRACT
Six analogues of angiotensin II (Ang) were synthesized with modifications in positions 1 and 7. The study was undertaken in order to learn more about the influence of alkylations in positions 1 and 7 and their interdependence. Previous studies have shown that alpha, alpha-dimethylation of Gly (aminoisobutyric acid, Aib) in position 1 produces quite potent analogues, as does N-methylation of Gly (sarcosine). Combination of both C alpha- and N alpha-methylations to N-Me-Aib1, however, did not produce an affinity increase. Decyclisation of the Pro7-residue produced moderately active analogues with position 7 N-methylation and inactive analogues if the N-alkylation was suppressed. In order to investigate a possible stereochemical interdependence of positions 1 and 7, a group of peptides with combinations of position 1 and 7 alkylations were investigated. The following analogues were prepared: [Sar1,Aib7]Ang, [Sar1,Aib,Leu8]Ang, [Aib1,7,Leu8]Ang, [Aib1,7,Leu8]Ang, [N-Me-Aib1,Aib7]Ang, [N-Me-Aib1,Aib7,Leu8]Ang. They were synthesized by classical solid phase synthesis using the BOC-TFA-HF scheme. The biological properties of these peptides were assessed on the rabbit aorta preparation and their binding potencies were measured on bovine adrenal membranes. Both on agonistic and antagonistic [Leu8]Ang analogues single Aib substitutions in position 1 or 7 induced affinity reduction in both bioassays. Simultaneous Aib modifications in positions 1 and 7 induced more important affinity loss in a synergic manner in both bioassays and as well for agonists and antagonists. The N-Me-Aib1 modifications induce similar affinity loss with or without concomitant Aib7 modification.(ABSTRACT TRUNCATED AT 250 WORDS)
