The nitroxyl anion (NO. ), respectively, can lead to oxidation, hydroxylation, nitration, and nitrosation of biomolecules (1). Although a substantial literature exists on the putative biological effects of other RNOS, few studies have focused on nitroxyl (NO Ϫ ), the one electron reduction product of NO. Several reports suggest that NO Ϫ (or its conjugate acid, HNO) can be generated from chemical reactions that occur in vivo (2, 3) including oxidation of L-arginine by tetrahydrobiopterin-free nitric oxide synthase (NOS) (4 -6) and decomposition of Snitrosothiols (7, 8). Taken together, these studies indicate that the chemistry of NO Ϫ is an essential component of the redox chemistry of NO in biological systems.Angeli's salt (AS) is the most commonly used synthetic donor in the study of NO Ϫ effects under biological conditions (9). At physiological pH and temperature, AS spontaneously decomposes to HNO and nitrite with a half-life of 2.5 min,The cytotoxic effects of AS are several orders of magnitude greater than those of other RNOS and are comparable to alkylhydroperoxides (10), suggesting that NO Ϫ formation in vivo could have deleterious consequences. In a myocardial ischemiareperfusion model, treatment with AS markedly increased infarct area (11). In contrast, NO, either from a donor or from oxidation of NO Ϫ in the presence of an electron acceptor, afforded protection in the same model. In the present report, the chemistry of AS is compared with that of ONOO Ϫ and NO/N 2 O 3 to gain insight into the biological mechanisms in which the chemistry of NO Ϫ could be involved. MATERIALS AND METHODSAngeli's salt (Na 2 N 2 O 3 ) was synthesized as described previously (10). The NONOate, DEA/NO (NaEt 2 NN(O)NO), was a generous gift from Dr. Joseph Saavedra (National Cancer Institute, Frederick, MD). Stock solutions (ϳ10 mM) of AS and DEA/NO were prepared in 10 mM NaOH and stored at Ϫ20°C (12). Peroxynitrite was synthesized by mixing solutions of 0.5 M NO 2 Ϫ in 0.5 M HCl and 0.5 M hydrogen peroxide (H 2 O 2 ) followed by rapid quenching in 1 M NaOH, as described previously (13). The resulting basic solution was exposed to MnO 2 to remove excess H 2 O 2 , which was reduced to Ͻ1% per mol of ONOO Ϫ . After filtering, aliquots were stored at Ϫ20°C for less than 2 weeks. Directly prior to use, the concentrations of these RNOS donors in 10 mM NaOH were