A fluorescence study of the binding of calcium and terbium ions to angiotensin

The interactions of angiotensin II and a synthetic analogue, [Asn¹, Val⁵] angiotensin II, with Ca²⁺ and Tb³⁺ have been monitored using the intrinsic fluorescence of the tyrosine residue at position 4 in both molecules. The data indicate that angiotensin II binds both metals with a dissociation constant of ∼1 × 10^-4 M^-1, while no significant binding was observed with the amide analogue. This suggests that the side chain carboxyl group of aspartic acid forms part of the binding site. Since the value of the dissociation constant suggests chelation of the metals by the hormone, the terminal carboxyl group of the peptide is also probably involved in metal binding. The fact that energy transfer was observed between Tb³⁺ and the tyrosine of angiotensin places the hydroxl or carbonyl group of the tyrosine close to the metal binding site.