Insertion of an electronegative sulfur atom in the side chain of position 5 of angiotensin II: changes in the tachyphylactic properties of the peptide.

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.