The nitrosation of cellular thiols has attracted much interest as a regulatory mechanism that mediates some of the pathophysiological effects of nitric oxide (NO). In cells, virtually all enzymes contain cysteine residues that can be subjected to S-nitrosation, whereby this process often acts as an activity switch. Nitrosation of biological thiols is believed to be mediated by N2O3, metal-nitrosyl complexes, and peroxynitrite. To date, however, enzymatic pathways for S-denitrosation of proteins have not been identified. Herein, we present experimental evidence that two ubiquitous cellular dithiols, thioredoxin and dihydrolipoic acid, catalyze the denitrosation of S-nitrosoglutathione, S-nitrosocaspase 3, S-nitrosoalbumin, and S-nitrosometallothionenin to their reduced state with concomitant generation of nitroxyl (HNO), the one-electron reduction product of NO. In these reactions, formation of NO and HNO was assessed by ESR spectrometry, potentiometric measurements, and quantification of hydroxylamine and sodium nitrite as end reaction products. Nitrosation and denitrosation of caspase 3 was correlated with its proteolytic activity. We also report that thioredoxin-deficient HeLa cells with mutated thioredoxin reductase denitrosate S-nitrosothiols less efficiently. We conclude that both thioredoxin and dihydrolipoic acid may be involved in the regulation of cellular S-nitrosothiols.

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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...

A comparative evaluation of the applicability of different radionuclide sources for the determination of toxic elements in plants by ED-XRF is presented. 23Spu or l~ are suggested as most suitable single excitation sources in ED-XRF for monitoring investigations. More elements are determined with a combination of 55Fe/l~The results obtained by radionuclide ED-XRF analysis of different plants show that the method permits the reliable determination of Br, Ca, Fe, K, Mn, Rb, Sr and Zn in plant bioindicators. For toxic elements like As, Cd, Cu, Cr, Hg, Ni, Se and Pb the detection limits of the method are not low enough.

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Abstract:© Versita Sp. z o.o. The monensic acid reacts with Hg(II) in a tetradentate coordination manner via both oxygen atoms of the carboxylate function and oxygens of two hydroxyl groups. The twofold negative charge of the ligand is achieved by deprotonation of carboxylic and secondary hydroxyl groups located at the opposite ends of the molecule. Hg(II) is surrounded by five oxygen atoms in a distorted square pyramidal molecular geometry. Keywords: Monovalent polyether ionophore • Heavy metal complexes