Photoaffinity labeling of the angiotensin II receptor. 3. Receptor inactivation with photolabile hormone analogs

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Abstract
The receptor of angiotensin II (AT) in rabbit aorta strips, rat aorta and rat stomach can be blocked specifically and irreversibly by several photolabile analogs of Sar-Arg-Val-Tyr-Val-His-Pro-Phe ([Sar1]AT) with irradiation. The effectiveness of a photolabel with light of wavelength 365 nm depends on the labeling amino acid (L-4''-nitrophenylalanine, L-4''-diazoniumphenylalanine, or L-4''-azidophenylalanine) and on its position in the peptide (replacing Tyr4 and/or Phe8). The (4''-azido)Phe-containing analogs are all good to fair photoinactivators. Their decreasing order of effectiveness is as follows: [Sar1,(4''-azido)Phe8]AT, [Sar1,(4''-azido)Phe4,8]AT and [Sar1,(4''-azido)Phe4]AT. The (4''-nitro)Phe analogs show the opposite relation; the good ligand [Sar1,(4''-nitro)Phe8]AT is almost ineffective, but the nonligand [Sar1,(4-nitro)Phe4]AT exhibits good, specific photoinactivation. This can be explained by the existence of a different photolysis pathway for (4''-nitro)Phe; this analog probably undergoes a multiphoton decay with a long-lived first excited state. A peptide in this state may differ in its pharmacological properties from the ground state and become a ligand.

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