Formation of Nitroxyl and Hydroxyl Radical in Solutions of Sodium Trioxodinitrate

Abstract

Despite its negative redox potential, nitroxyl (HNO) can trigger reactions of oxidation. Mechanistically, these reactions were suggested to occur with the intermediate formation of either hydroxyl radical ( ⅐ OH) or peroxynitrite (ONOO ؊ ). In this work, we present further experimental evidence that HNO can generate ⅐ OH. Sodium trioxodinitrate (Na 2 N 2 O 3 ), a commonly used donor of HNO, oxidized phenol and Me 2 SO to benzene diols and ⅐ CH 3 , respectively. The oxidation of Me 2 SO was O 2 -independent, suggesting that this process reflected neither the intermediate formation of ONOO ؊ nor a redox cycling of transition metal ions that could initiate Fenton-like reactions. In solutions of phenol, Na 2 N 2 O 3 yielded benzene-1,2-diol and benzene-1,4-diol at a ratio of 2:1, which is consistent with the generation of free ⅐ OH. Ethanol and Me 2 SO, which are efficient scavengers of ⅐ OH, impeded the hydroxylation of phenol. A mechanism for the hydrolysis of Na 2 N 2 O 3 is proposed that includes dimerization of HNO to cis-hyponitrous acid (HO-N‫؍‬N-OH) with a concomitant azo-type homolytic fission of the latter to N 2 and ⅐ OH. The HNO-dependent production of ⅐ OH was with 1 order of magnitude higher at pH 6.0 than at pH 7.4. Hence, we hypothesized that HNO can exert selective toxicity to cells subjected to acidosis. In support of this thesis, Na 2 N 2 O 3 was markedly more toxic to human fibroblasts and SK-N-SH neuroblastoma cells at pH 6.2 than at pH 7.4. Scavengers of ⅐ OH impeded the cytotoxicity of Na 2 N 2 O 3 . These results suggest that the formation of HNO may be viewed as a toxicological event in tissues subjected to acidosis.The biochemistry of nitroxyl (HNO) has attracted considerable interest in recent years. In cells, the biosynthesis of HNO is believed to proceed via reduction of NO ⅐ by superoxide dismutase (1) and cytochrome c (2), and reduction of S-nitrosoglutathione by low molecular weight and protein thiols (3-5). It has been suggested that HNO can affect the etiology of various pathophysiological conditions such as inflammation and neurodegenerative diseases, especially when H 2 O 2 and transition metal ions are present (6, 7). Similar to NO ⅐ and NO ϩ , HNO is a potent inducer of the antioxidant protein heme oxygenase 1 (8), exhibits vasorelaxant properties (9), and modulates the activity of thiol-containing proteins, such as aldehyde dehydrogenase (10, 11) and the N-methyl-D-aspartate receptor (12, 13). In in vivo experiments, Paolocci et al. (14) observed that HNO exerts positive inotropic and lusitropic action, which unlike NO ⅐ and nitrates is independent and additive to ␤-adrenergic stimulation and increases the release of plasma calcitonin gene-related peptide; these results suggest that donors of HNO are potential prodrugs for the treatment of heart failure (14). At high doses, HNO has been shown to induce DNA singlestrand breakage (15, 16) and a concentration-dependent cytotoxicity in murine thymocytes (16). This cytotoxicity was associated with activation of the nuclear nick sen...

Nitroxyl enhances myocyte Ca2 transients by exclusively targeting SR Ca2 -cycling

“…The possibility exists for AS to generate the hydroxyl radical ( · OH) under certain extreme biochemical conditions (9, 10). Therefore, we examined the effect of acute hydroxyl radical exposure on isolated rat cardiomyocyte function.…”

“…Studies have demonstrated hydroxyl radical production using very high concentrations of AS (>1 mmol/L) under conditions of very low pH (pH 4–6) (9, 10). Although hydroxyl radical production is minimal under our experimental conditions at pH 7.4, hydroxyl radical production may occur in certain intracellular compartments of low pH (i.e., mitochondria).…”

“…However, previous reports have not examined the role of extracellular Ca 2+ in the positive inotropic action of HNO. Further, under certain extreme biochemical conditions, the breakdown of AS may lead to the formation the hydroxyl radical ( · OH) (9, 10), which is a well-known negative modulator of myocardial function (11–13). Another potential breakdown product of AS includes nitrite (NO 2 − ).…”

A comparison of the chemistry associated with the biological signaling and actions of nitroxyl (HNO) and nitric oxide (NO)

“…The chemistry responsible for these oxidation processes is not established but has been proposed to be due to several possible reactions. For example, Angeli's salt-derived HNO is proposed to be toxic to cells via the dimerization to hyponitrous acid (HONNOH) followed by a proton-dependent decomposition to N 2 and HO• (Reactions 34 and 35) [74,75].…”

The Chemistry and Biology of Nitroxyl (HNO): A Chemically Unique Species with Novel and Important Biological Activity

“…Interestingly, HOC, spin trapped by using a pyrroline N-oxide, was detected in a decomposing solution of Angeli's salt. [84,85] The formation of HOC from HNO was proposed to occur via an azo-type homolytic fission of cis-hyponitrous acid [Eq. (16)], a reaction initially proposed almost 40 years ago to explain a small amount of radical chain chemistry associated with the decomposition of hyponitrous acid.…”