Fluorescence properties of angiotensin II analogues in receptor-simulating environments: relationship between tyrosinate fluorescence lifetime and biological activity.

Nanosecond time-resolved decays from excited-state tyrosinate fluorescence of angiotensin II analogues were measured from the emission at 350 nm. Fluorescence lifetimes were determined in several different solvents using N-acetyltyrosinamide as the reference standard. Long-lifetime tyrosinate fluorescence (LTF) of angiotensin II (ANG II) was observed in propylene glycol, trifluoroethanol and isopropanol but not in DMSO or water. The addition of SDS at a concentration sufficient to induce micelle formation in water resulted in LTF for ANG II. LTF for ANG II was longer in propylene glycol (21 ns) than in isopropanol (16 ns) whereas the % conformer(s) producing LTF was higher in isopropanol (79%) than in propylene glycol (19%). For a series of ten angiotensin analogues, LTF values determined in propylene glycol and isopropanol were a reflection of the contractile activities of these analogues in the rat uterus assay. In propylene glycol, with the notable exception of ANG III, biologically active analogues had longer LTFs (13-21 ns) than inactive analogues (0-11 ns). In isopropanol, strong agonists had longer LTFs (13-16 ns) than weak agonists/inactive analogues (0-11 ns). Structure-fluorescence relationships suggest that the primary TyrOH acceptor in ANG II is the His6 imidazole group, and that the C-terminal carboxylate has an essential auxiliary role in generating long-lived tyrosinate fluorescence. The present findings appear to support the proposition that the receptor conformation of ANG II contains a tripartite interaction of Tyr, His and carboxylate groups which is analogous to that found at the active site of serine proteinases, and that the tyrosinate nucleophile may activate angiotensin receptors. Solvents of intermediate polarity such as propylene glycol and isopropanol appear to induce conformations for small peptides such as angiotensin which resemble those present at membrane receptors.