Nitration of angiotensin II by · NO 2 radicals and peroxynitrite

Abstract

To react with peptides, nitric oxide · NO has to be activated by oxidation, or by coupling with superoxide (O ·− 2 ) thereby producing peroxynitrite. In the course of · NO oxidation, · NO 2 free radicals and N 2 O 3 may be formed. Using γ‐irradiation methods, we characterized the products formed by these nitrogen oxides with angiotensin II. Angiotensin II is specifically nitrated at its tyrosinyl residue by · NO 2 or peroxynitrite. Equimolecular amounts of each reagent in K + /P i solutions at pH 7.4 led to 56% and 5% nitration yields, respectively. Nitrogen oxides produced by autoxidation of · NO, as well as · NO 2 under · NO, reacted only with the arginine residue, giving a mixture of peptides containing citrulline, a N ‐(hydroxylamino‐cyanamido‐) instead of guanido group, and a conjugated diene derived from an arginine side‐chain. However, nitrosation reactions by N 2 O 3 occurred only when the initial concentration of · NO 2 was 10 times that able to react with angiotensin II. Thus, in this case · NO appears to protect against · NO 2 action.