Study on the influence of tyrosine deprotonation on the myotropic action of angiotensin II.

The influence of phenolic tyrosine ionization in angiotensin II on the myotropic action of this peptide has been investigated in vitro on rabbit aortic strips. [Sar1, Tyr4]angiotensin II and [Sar1, (4'-amino) Phe4]angiotensin II (as a reference which cannot undergo the same ionization) were tested over a pH range from 6.8 to 9.0 and their activities compared. The results clearly indicate that angiotensin II with a deprotonated phenolic hydroxyl group on Tyr in position 4 is not the most active or only active form of angiotensin II.