CHEMICAL MODIFICATIONS OF THE VASOCONSTRICTOR PEPTIDE ANGIOTENSIN-II BY NITROGEN-OXIDES (NO, HNO2, HOONO) - EVALUATION BY MASS-SPECTROMETRY

Nitric oxide (NO) and angiotensin II an natural regulators of blood pr essure. Under aerobic conditions, NO is transformed into its higher ox ides (N2O4, NO2, NO/NO2 or N2O3) and oxoperoxonitrate (currently named peroxynitrite) by coupling with superoxide. Previous studies have sho wn that these reactive nitrogen species should be involved in vivo in the transformation of cysteine and tyrosine into the corresponding nit rosothiol and 3-nitrotyrosine. In the present study, attention has bee n focused on the relative reactivities of HNO2, peroxynitrite, and NO in the presence of dioxygen, towards the arginine and tyrosine residue s of the peptide angiotensin II. Nitration of the tyrosine residue is clearly the main reaction with peroxynitrite. By contrast, besides 20% of nitration of the tyrosine residue, NO in the presence of dioxygen leads to nitrosation reactions with the arginine residue similar to th ose observed with HNO2 at pH 5, possibly through the intermediate N2O3 reactive species. angiotensin II is converted for the most part to pe ptides having lost tither a terminal amine function or the whole guani do group, leading respectively to citrulline-containing angiotensin II or to a diene derivative. Identification established mainly by tandem mass spectrometry of peptidic by-products allows us to propose a casc ade of nitrosations of all the amine functions of the arginine residue . Further in vivo studies show that transformations of the arginine re sidue in angiotensin II do not alter its vasoconstrictive properties, whereas nitration of the tyrosine residue totally inhibits them.