Nitrite reacts with deoxyhemoglobin to form nitric oxide (NO) and methemoglobin. Though this reaction is experimentally associated with NO generation and vasodilation, kinetic analysis suggests that NO should not be able to escape inactivation in the erythrocyte. We have discovered that products of the nitrite-hemoglobin reaction generate dinitrogen trioxide (N2O3) via a novel reaction of NO and nitrite-bound methemoglobin. The oxygen-bound form of nitrite-methemoglobin shows a degree of ferrous nitrogen dioxide (Fe(II)-NO2*) character, so it may rapidly react with NO to form N2O3. N2O3 partitions in lipid, homolyzes to NO and readily nitrosates thiols, all of which are common pathways for NO escape from the erythrocyte. These results reveal a fundamental heme globin- and nitrite-catalyzed chemical reaction pathway to N2O3, NO and S-nitrosothiol that could form the basis of in vivo nitrite-dependent signaling. Because the reaction redox-cycles (that is, regenerates ferrous heme) and the nitrite-methemoglobin intermediate is not observable by electron paramagnetic resonance spectroscopy, this reaction has been 'invisible' to experimentalists over the last 100 years.

Experimental evidence suggests that nitric oxide (NO) and hydrogen sulfide (H 2 S) signaling pathways are intimately intertwined, with mutual attenuation or potentiation of biological responses in the cardiovascular system and elsewhere. The chemical basis of this interaction is elusive. Moreover, polysulfides recently emerged as potential mediators of H 2 S/sulfide signaling, but their biosynthesis and relationship to NO remain enigmatic. We sought to characterize the nature, chemical biology, and bioactivity of key reaction products formed in the NO/sulfide system. At physiological pH, we find that NO and sulfide form a network of cascading chemical reactions that generate radical intermediates as well as anionic and uncharged solutes, with accumulation of three major products: nitrosopersulfide (SSNO − ), polysulfides, and dinitrososulfite [N-nitrosohydroxylamine-N-sulfonate (SULFI/NO)], each with a distinct chemical biology and in vitro and in vivo bioactivity. SSNO − is resistant to thiols and cyanolysis, efficiently donates both sulfane sulfur and NO, and potently lowers blood pressure. Polysulfides are both intermediates and products of SSNO − synthesis/decomposition, and they also decrease blood pressure and enhance arterial compliance. SULFI/NO is a weak combined NO/nitroxyl donor that releases mainly N 2 O on decomposition; although it affects blood pressure only mildly, it markedly increases cardiac contractility, and formation of its precursor sulfite likely contributes to NO scavenging. Our results unveil an unexpectedly rich network of coupled chemical reactions between NO and H 2 S/sulfide, suggesting that the bioactivity of either transmitter is governed by concomitant formation of polysulfides and anionic S/N-hybrid species. This conceptual framework would seem to offer ample opportunities for the modulation of fundamental biological processes governed by redox switching and sulfur trafficking.sulfide | nitric oxide | nitroxyl | redox | gasotransmitter N itrogen and sulfur are essential for all known forms of life on Earth. Our planet's earliest atmosphere is likely to have contained only traces of O 2 but rather large amounts of hydrogen sulfide (H 2 S) (1). Indeed, sulfide may have supported life long before the emergence of O 2 and NO (2, 3).* This notion is consistent with a number of observations: H 2 S is essential for efficient abiotic amino acid generation as evidenced by the recent reanalysis of samples of Stanley Miller's original spark discharge experiments (4), sulfide is an efficient reductant in protometabolic reactions forming RNA, protein, and lipid precursors (5), and sulfide is both a bacterial and mitochondrial substrate (6), enabling even multicellular lifeforms to exist and reproduce under conditions of permanent anoxia (7). Thus, although eukaryotic cells may have originated from the symbiosis of sulfurreducing and -oxidizing lifeforms within a self-contained sulfur redox metabolome (8), sulfide may have been essential even earlier by providing the basic building blocks of ...

Cysteine sulfenic acid formation in proteins results from the oxidative modification of susceptible cysteine residues by hydrogen peroxide, alkyl hydroperoxides and peroxynitrite. This species represents a biologically-significant modification occurring during oxidant signaling or oxidative stress and it can modulate protein function. Most methods to identify such oxidatively-modified proteins rely on monitoring the loss of one or more thiol group(s) or on selective labeling of nascent thiol groups following reduction of oxidized proteins. Our previous work reported the direct labeling of these chemically distinct modifications with a dimedone analogue, 1,3-cyclohexadione, to which a linker and functional group (an alcohol) had been added; further addition of a fluorescent isatoic acid or methoxycoumarin reporter allowed detection of the incorporated tag by fluorescence techniques [Poole, L. B., Zeng, B. B., Knaggs, S. A., Yakubu, M., and King, S. B. (2005) Synthesis of chemical probes to map sulfenic acid modifications on proteins. Bioconjug Chem 16, 1624Chem 16, -1628. We have now expanded our arsenal of tagging reagents to include two fluorescein-, two rhodamineand three biotin-conjugated probes based on the original approach. The new tools provide readily detectable fluorescent and affinity probes to identify sulfenic acid modifications in proteins and have been used in subsequent mass spectrometric analyses to confirm covalent attachment of the conjugates and directly determine the site of modification. Keywordscysteine sulfenic acid; reactive oxygen species; oxidized cysteine; papain; peroxiredoxins; peroxide; redox sensor; redox signaling Given the significant role played by formation of cysteine sulfenic acid (S-hydroxycysteine, R-SOH) in the redox regulation of enzymes and transcription regulators (1-3) and its general instability toward protein analytical methods (4), there is a critical need for better reagents to trap and identify these modifications in proteins. Based on a known alkylator of R-SOH, *To whom correspondence should be addressed: Department of Biochemistry, Center for Structural Biology, Medical Center Boulevard, Winston-Salem, NC 27157; Telephone: 336-716-6711 (Poole), 336-758-5774 (King); Fax: 336-777-3242 (Poole), 336-758-4656 (King); E-mail: lbpoole@wfubmc.edu, kingsb@wfu.edu. NIH Public Access Author ManuscriptBioconjug Chem. Author manuscript; available in PMC 2008 November 1. Published in final edited form as:Bioconjug Chem. 2007 ; 18(6): 2004-2017. doi:10.1021/bc700257a. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript dimedone (5,5-dimethyl-1,3-cyclohexanedione), we previously designed, synthesized and validated the use of two fluorescent reagents linked to the reactive core of dimedone, 1,3-cyclohexadione, as detectable markers of R-SOH formation in proteins (5). These reagents were shown to specifically trap only the R-SOH modification in a test protein, AhpC (a cysteine-based peroxidase from bacteria), leaving underivatized the other protein...

Small increases in physiological nitrite concentrations have now been shown to mediate a number of biological responses, including hypoxic vasodilation, cytoprotection after ischemia/ reperfusion, and regulation of gene and protein expression. Thus, while nitrite was until recently believed to be biologically inert, it is now recognized as a potentially important hypoxic signaling molecule and therapeutic agent. Nitrite mediates signaling through its reduction to nitric oxide, via reactions with several heme-containing proteins. In this report, we show for the first time that the mitochondrial electron carrier cytochrome c can also effectively reduce nitrite to NO. This nitrite reductase activity is highly regulated as it is dependent on pentacoordination of the heme iron in the protein and occurs under anoxic and acidic conditions. Further, we demonstrate that in the presence of nitrite, pentacoordinate cytochrome c generates bioavailable NO that is able to inhibit mitochondrial respiration. These data suggest an additional role for cytochrome c as a nitrite reductase that may play an important role in regulating mitochondrial function and contributing to hypoxic, redox, and apoptotic signaling within the cell.

Aims Poor blood flow and hypoxia/ischemia contribute to many disease states and may also be a factor in the decline of physical and cognitive function in aging. Nitrite has been discovered to be a vasodilator that is preferentially harnessed in hypoxia. Thus, both infused and inhaled nitrite are being studied as therapeutic agents for a variety of diseases. In addition, nitrite derived from nitrate in the diet has been shown to decrease blood pressure and improve exercise performance. Thus, dietary nitrate may also be important when increased blood flow in hypoxic or ischemic areas is indicated. These conditions could include age-associated dementia and cognitive decline. The goal of this study was to determine if dietary nitrate would increase cerebral blood flow in older adults. Methods and Results In this investigation we administered a high vs. low nitrate diet to older adults (74.7 ± 6.9 years) and measured cerebral perfusion using arterial spin labeling magnetic resonance imaging. We found that the high nitrate diet did not alter global cerebral perfusion, but did lead to increased regional cerebral perfusion in frontal lobe white matter, especially between the dorsolateral prefrontal cortex and anterior cingulate cortex. Conclusion These results suggest that dietary nitrate may be useful in improving regional brain perfusion in older adults in critical brain areas known to be involved in executive functioning.

Nitroxyl (HNO/NO(-)), the reduced form of nitric oxide, has gained attention based on its separate chemistry and biology from nitric oxide. The inherent reactivity of HNO requires new and mechanistically unique donors for the detailed study of HNO chemistry and biology. Oxidation of cyclohexanone oxime with lead tetraacetate yields 1-nitrosocyclohexyl acetate, whereas oxidation of oximes in the presence of excess carboxylic acid gives various acyloxy nitroso compounds. These bright blue compounds exist as monomers as indicated by their infrared, proton, and carbon NMR spectra, and X-ray crystallographic analysis reveals the nitroso groups possess a "nitroxyl-like" bent configuration. Hydrolysis of these compounds produces nitrous oxide, the dimerization and dehydration product of HNO, and provides evidence for the intermediacy of HNO. Both thiols and oxidative metal complexes inhibit nitrous oxide formation. Hydrolysis of these compounds in the presence of ferric heme complexes forms ferrous nitrosyl complexes providing further evidence for the intermediacy of HNO. Kinetic analysis shows that the rate of hydrolysis depends on pH and the structure of the acyl group of the acyloxy nitroso compound. These compounds relax pre-constricted rat aortic rings similar to known HNO donors. Together, these results identify acyloxy nitroso compounds as a new class of HNO donors.

Protein sulfenic acids are formed by the reaction of biologically relevant reactive oxygen species with protein thiols. Sulfenic acid formation modulates the function of enzymes and transcription factors either directly or through the subsequent formation of protein disulfide bonds. Identifying the site, timing, and conditions of protein sulfenic acid formation remains crucial to understanding cellular redox regulation. Current methods for trapping and analyzing sulfenic acids involve the use of dimedone and other nucleophilic 1,3-dicarbonyl probes that form covalent adducts with cysteine-derived protein sulfenic acids. As a mechanistic alternative, the present study describes highly strained bicyclo[6.1.0]nonyne (BCN) derivatives as concerted traps of sulfenic acids. These strained cycloalkynes react efficiently with sulfenic acids in proteins and small molecules yielding stable alkenyl sulfoxide products at rates more than 100× greater than 1,3-dicarbonyl reagents enabling kinetic competition with physiological sulfur chemistry. Similar to the 1,3-dicarbonyl reagents, the BCN compounds distinguish the sulfenic acid oxoform from the thiol, disulfide, sulfinic acid, and S-nitrosated forms of cysteine while displaying an acceptable cell toxicity profile. The enhanced rates demonstrated by these strained alkynes identify them as new bioorthogonal probes that should facilitate the discovery of previously unknown sulfenic acid sites and their parent proteins.

Isoform-specific signaling of Akt, a major signaling hub and a prominent therapeutic target, remained poorly defined until recently. Subcellular distribution, tissue-specific expression, substrate specificity, and posttranslational modifications are believed to underlie isoform-specific signaling of Akt. The studies reported here show inhibition of Akt2 activity under physiologically relevant conditions of oxidation created by PDGF-induced reactive oxygen species. Combined MS and functional assays identified Cys124 located in the linker region between the N-terminal pleckstrin homology domain and the catalytic kinase domain as one of the unique regulatory redox sites in Akt2 with functional consequence on PDGF-stimulated glucose uptake. A model is proposed describing the consequence of increased endogenous oxidation induced by extracellular cues such as PDGF on Akt2 activity.disulfide | receptor tyrosine kinase | DCF | DCP-Bio1 P KB/Akt is a major signaling hub between cytokine, growth factor, and integrin signaling pathways of consequence to many biological processes. Energy storage, protein synthesis, cell survival and growth, cell cycle progression, and cell death are differentially regulated by the three known isoforms of Akt kinase: Akt1/PKBα, Akt2/PKBβ, and Akt3/PKBγ (1). Although the molecular features underlying isoform-specific functional predominance are largely unknown, hypotheses to explain Akt isoform specificity include selective interactions with substrates and/ or binding partners, tissue specificity, subcellular location, and temporal changes in activation profiles of Akt isoforms (2). Posttranslational modifications may contribute to this isoform-specific signaling, but reports of modifications are lacking.Reactive oxygen species (ROS) are integral to cytokine and growth factor signaling; ROS generation in response to these extracellular cues is well documented (3). Earlier reports, in particular from Sundaresan et al. (4), showed ROS generation in response to PDGF stimulation of vascular smooth muscle cells and suggested that H 2 O 2 can relay redox signals to regulate physiological signaling in response to growth factors. More recently, it has been shown that PrxI phosphorylation at Y194 by Src family tyrosine kinases inhibited PrxI and resulted in H 2 O 2 accumulation at the cellular membrane where receptor tyrosine kinase activation occurs (5). Unanswered questions include which proteins are oxidized by receptor tyrosine kinase-induced ROS, which specific cysteine site(s) undergo oxidation, and the consequence of oxidation on activity of these proteins and propagation of receptor tyrosine kinase signaling. Recently, the synthesis of several dimedone-based chemoselective reagents capable of specific labeling of sulfenic acid oxidized proteins was reported; these reagents allow for specific enrichment and identification of oxidized proteins (6-10). In this study, we used a biotin-tagged 1,3-cyclohexadione derivative, DCP-Bio1 (Fig. 1A), to investigate isoform-specific effects of PDGF-induced ...