Nitric oxide-induced conversion of cellular chelatable iron into macromolecule-bound paramagnetic dinitrosyliron complexes

Insights into the diverse effects of nitric oxide on tumor biology

Among its many roles in cellular biology, nitric oxide (·NO) has long been associated with cancers both as a protumorigenic and as an antitumorigenic agent. The dual nature of this signaling molecule in varied settings is attributable to its temporal and concentration-dependent effects that produce different phenotypes. The steady-state ·NO concentration within the cell is a balance between its rate of enzymatic synthesis from the three nitric oxide synthase (NOS) isoforms and consumption via numerous metabolic pathways and demonstrates strong dependence on the tissue oxygen concentration. NOS expression and ·NO production are often deregulated and associated with numerous types of cancers with dissimilar prognostic outcomes. ·NO influences several facets of tumor initiation and progression including DNA damage, chronic inflammation, angiogenesis, epithelial-mesenchymal transition, and metastasis, to name a few. The role of ·NO as an epigenetic modulator has also recently emerged and has potentially important mechanistic implications in regulating transcription of oncogenes and tumor-suppressor genes. ·NO-derived cellular adducts such as dinitrosyliron complexes and the formation of higher nitrogen oxides further alter its cellular behavior. Among anticancer strategies, the use of NOS as a prognostic biomarker and modulation of ·NO production for therapeutic benefit have gained importance over the past decade. Numerous ·NO-releasing drugs and NOS inhibitors have been evaluated in preclinical and clinical settings to arrest tumor growth. Taken together, ·NO affects various arms of cancer signaling networks. An overview of this complex interplay is provided in this chapter.

Mechanisms and kinetic profiles of superoxide-stimulated nitrosative processes in cells using a diaminofluorescein probe

In this study, we examined the mechanisms and kinetic profiles of intracellular nitrosative processes using diaminofluorescein (DAF-2) as a target in RAW 264.7 cells. The intracellular formation of the fluorescent, nitrosated product diaminofluorescein triazol (DAFT) from both endogenous and exogenous nitric oxide (NO) was prevented by deoxygenation and by cell membrane-permeable superoxide (O2(-)) scavengers but not by extracellular bovine Cu,Zn-SOD. In addition, the DAFT formation rate decreased in the presence of cell membrane-permeable Mn porphyrins that are known to scavenge peroxynitrite (ONOO(-)) but was enhanced by HCO3(-)/CO2. Together, these results indicate that nitrosative processes in RAW 264.7 cells depend on endogenous intracellular O2(-) and are stimulated by ONOO(-)/CO2-derived radical oxidants. The N2O3 scavenger sodium azide (NaN3) only partially attenuated the DAFT formation rate and only with high NO (>120 nM), suggesting that DAFT formation occurs by nitrosation (azide-susceptible DAFT formation) and predominantly by oxidative nitrosylation (azide-resistant DAFT formation). Interestingly, the DAFT formation rate increased linearly with NO concentrations of up to 120-140 nM but thereafter underwent a sharp transition and became insensitive to NO. This behavior indicates the sudden exhaustion of an endogenous cell substrate that reacts rapidly with NO and induces nitrosative processes, consistent with the involvement of intracellular O2(-). On the other hand, intracellular DAFT formation stimulated by a fixed flux of xanthine oxidase-derived extracellular O2(-) that also occurs by nitrosation and oxidative nitrosylation increased, peaked, and then decreased with increasing NO, as previously observed. Thus, our findings complementarily show that intra- and extracellular O2(-)-dependent nitrosative processes occurring by the same chemical mechanisms do not necessarily depend on NO concentration and exhibit different unusual kinetic profiles with NO dynamics, depending on the biological compartment in which NO and O2(-) interact.

Glutathione S-transferase and MRP1 form an integrated system involved in the storage and transport of dinitrosyl-dithiolato iron complexes in cells

Nitrogen monoxide (NO) is vital for many essential biological processes as a messenger and effector molecule. The physiological importance of NO is the result of its high affinity for iron in the active sites of proteins such as guanylate cyclase. Indeed, NO possesses a rich coordination chemistry with iron and the formation of dinitrosyl-dithiolato iron complexes (DNICs) is well documented. In mammals, NO generated by cytotoxic activated macrophages has been reported to play a role as a cytotoxic effector against tumor cells by binding and releasing intracellular iron. Studies from our laboratory have shown that two proteins traditionally involved in drug resistance, namely multidrug-resistance protein 1 and glutathione S-transferase, play critical roles in intracellular NO transport and storage through their interaction with DNICs (R.N. Watts et al., Proc. Natl. Acad. Sci. USA 103:7670-7675, 2006; H. Lok et al., J. Biol. Chem. 287:607-618, 2012). Notably, DNICs are present at high concentrations in cells and are biologically available. These complexes have a markedly longer half-life than free NO, making them an ideal "common currency" for this messenger molecule. Considering the many critical roles NO plays in health and disease, a better understanding of its intracellular trafficking mechanisms will be vital for the development of new therapeutics.

Redox activities of mono- and binuclear forms of low-molecular and protein-bound dinitrosyl iron complexes with thiol-containing ligands

EPR, optical, electrochemical and stopped-flow methods were used to demonstrate that Fe(NO)2 fragments in paramagnetic mononuclear and diamagnetic binuclear forms of dinitrosyl iron complexes with glutathione are reversibly reduced by a two-electron mechanism to be further transformed from the initial state with d(7) configuration into states with the d(8) and d(9) electronic configurations of the iron atom. Under these conditions, both forms of DNIC display identical optical and EPR characteristics in state d(9) suggesting that reduction of the binuclear form of DNIC initiates their reversible decomposition into two mononuclear dinitrosyl iron fragments, one of which is EPR-silent (d(8)) and the other one is EPR-active (d(9)). Both forms of DNIC produce EPR signals with the following values of the g-factor: g⊥=2.01, g||=1.97, gaver.=2.0. M-DNIC with glutathione manifest an ability to pass into state d(9), however, only in solutions with a low content of free glutathione. Similar transitions were established for protein-bound М- and B-DNIC with thiol-containing ligands.

Nitrosothiol formation and protection against Fenton chemistry by nitric oxide-induced dinitrosyliron complex formation from anoxia-initiated cellular chelatable iron increase

Dinitrosyliron complexes (DNIC) have been found in a variety of pathological settings associated with (•)NO. However, the iron source of cellular DNIC is unknown. Previous studies on this question using prolonged (•)NO exposure could be misleading due to the movement of intracellular iron among different sources. We here report that brief (•)NO exposure results in only barely detectable DNIC, but levels increase dramatically after 1-2 h of anoxia. This increase is similar quantitatively and temporally with increases in the chelatable iron, and brief (•)NO treatment prevents detection of this anoxia-induced increased chelatable iron by deferoxamine. DNIC formation is so rapid that it is limited by the availability of (•)NO and chelatable iron. We utilize this ability to selectively manipulate cellular chelatable iron levels and provide evidence for two cellular functions of endogenous DNIC formation, protection against anoxia-induced reactive oxygen chemistry from the Fenton reaction and formation by transnitrosation of protein nitrosothiols (RSNO). The levels of RSNO under these high chelatable iron levels are comparable with DNIC levels and suggest that under these conditions, both DNIC and RSNO are the most abundant cellular adducts of (•)NO.

Catalase (KatA) plays a role in protection against anaerobic nitric oxide in Pseudomonas aeruginosa

Pseudomonas aeruginosa (PA) is a common bacterial pathogen, responsible for a high incidence of nosocomial and respiratory infections. KatA is the major catalase of PA that detoxifies hydrogen peroxide (H₂O₂), a reactive oxygen intermediate generated during aerobic respiration. Paradoxically, PA displays elevated KatA activity under anaerobic growth conditions where the substrate of KatA, H₂O₂, is not produced. The aim of the present study is to elucidate the mechanism underlying this phenomenon and define the role of KatA in PA during anaerobiosis using genetic, biochemical and biophysical approaches. We demonstrated that anaerobic wild-type PAO1 cells yielded higher levels of katA transcription and expression than aerobic cells, whereas a nitrite reductase mutant ΔnirS produced ∼50% the KatA activity of PAO1, suggesting that a basal NO level was required for the increased KatA activity. We also found that transcription of the katA gene was controlled, in part, by the master anaerobic regulator, ANR. A ΔkatA mutant and a mucoid mucA22 ΔkatA bacteria demonstrated increased sensitivity to acidified nitrite (an NO generator) in anaerobic planktonic and biofilm cultures. EPR spectra of anaerobic bacteria showed that levels of dinitrosyl iron complexes (DNIC), indicators of NO stress, were increased significantly in the ΔkatA mutant, and dramatically in a ΔnorCB mutant compared to basal levels of DNIC in PAO1 and ΔnirS mutant. Expression of KatA dramatically reduced the DNIC levels in ΔnorCB mutant. We further revealed direct NO-KatA interactions in vitro using EPR, optical spectroscopy and X-ray crystallography. KatA has a 5-coordinate high spin ferric heme that binds NO without prior reduction of the heme iron (K_d ∼6 μM). Collectively, we conclude that KatA is expressed to protect PA against NO generated during anaerobic respiration. We proposed that such protective effects of KatA may involve buffering of free NO when potentially toxic concentrations of NO are approached.

Hammett correlations as test of mechanism of CO-induced disulfide elimination from dinitrosyl iron complexes

The use of Hammett correlations provide experimental evidence for an unusual role of the frontier molecular orbitals of an iron dinitrosyl unit in CO induced reductive elimination of disulfide.

An introduction to nitric oxide sensing and response in bacteria

Nitric oxide (NO) is a radical gas that has been intensively studied for its role as a bacteriostatic agent. NO reacts in complex ways with biological molecules, especially metal centers and other radicals, to generate other bioactive compounds that inhibit enzymes, oxidize macromolecules, and arrest bacterial growth. Bacteria encounter not only NO derived from the host during infection but also NO derived from other bacteria and inorganic sources. The transcriptional responses used by bacteria to respond to NO are diverse but usually involve an iron-containing transcription factor that binds NO and alters its affinity for either DNA or factors involved in transcription, leading to the production of enzymatic tolerance systems. Some of these systems, such as flavohemoglobin and flavorubredoxin, directly remove NO. Some do not but are still important for NO tolerance through other mechanisms. The targets of NO that are protected by these systems include many metabolic pathways such as the tricarboxylic acid cycle and branched chain amino acid synthesis. This chapter discusses these topics and others and serves as a general introduction to microbial NO biology.

A novel protein protects bacterial iron-dependent metabolism from nitric oxide

Reactive nitrogen species (RNS), in particular nitric oxide (NO), are toxic to bacteria, and bacteria have mechanisms to allow growth despite this stress. Understanding how bacteria interact with NO is essential to understanding bacterial physiology in many habitats, including pathogenesis; however, many targets of NO and enzymes involved in NO resistance remain uncharacterized. We performed for the first time a metabolomic screen on NO-treated and -untreated bacteria to define broadly the effects of NO on bacterial physiology, as well as to identify the function of NnrS, a previously uncharacterized enzyme involved in defense against NO. We found many known and novel targets of NO. We also found that iron-sulfur cluster enzymes were preferentially inhibited in a strain lacking NnrS due to the formation of iron-NO complexes. We then demonstrated that NnrS is particularly important for resistance to nitrosative stress under anaerobic conditions. Our data thus reveal the breadth of the toxic effects of NO on metabolism and identify the function of an important enzyme in alleviating this stress.

Hydrogen sulfide chemical biology: pathophysiological roles and detection

Hydrogen sulfide (H2S) is the most recent endogenous gasotransmitter that has been reported to serve many physiological and pathological functions in different tissues. Studies over the past decade have revealed that H2S can be synthesized through numerous pathways and its bioavailability regulated through its conversion into different biochemical forms. H2S exerts its biological effects in various manners including redox regulation of protein and small molecular weight thiols, polysulfides, thiosulfate/sulfite, iron-sulfur cluster proteins, and anti-oxidant properties that affect multiple cellular and molecular responses. However, precise measurement of H2S bioavailability and its associated biochemical and pathophysiological roles remains less well understood. In this review, we discuss recent understanding of H2S chemical biology, its relationship to tissue pathophysiological responses and possible therapeutic uses.

Nitric oxide modifies global histone methylation by inhibiting Jumonji C domain-containing demethylases

Methylation of lysine residues on histone tails is an important epigenetic modification that is dynamically regulated through the combined effects of methyltransferases and demethylases. The Jumonji C domain Fe(II) α-ketoglutarate family of proteins performs the majority of histone demethylation. We demonstrate that nitric oxide ((•)NO) directly inhibits the activity of the demethylase KDM3A by forming a nitrosyliron complex in the catalytic pocket. Exposing cells to either chemical or cellular sources of (•)NO resulted in a significant increase in dimethyl Lys-9 on histone 3 (H3K9me2), the preferred substrate for KDM3A. G9a, the primary methyltransferase acting on H3K9me2, was down-regulated in response to (•)NO, and changes in methylation state could not be accounted for by methylation in general. Furthermore, cellular iron sequestration via dinitrosyliron complex formation correlated with increased methylation. The mRNA of several histone demethylases and methyltransferases was also differentially regulated in response to (•)NO. Taken together, these data reveal three novel and distinct mechanisms whereby (•)NO can affect histone methylation as follows: direct inhibition of Jumonji C demethylase activity, reduction in iron cofactor availability, and regulation of expression of methyl-modifying enzymes. This model of (•)NO as an epigenetic modulator provides a novel explanation for nonclassical gene regulation by (•)NO.

Is S-nitrosocysteine a true surrogate for nitric oxide?

S-Nitrosothiol (RSNO) formation is one manner by which nitric oxide (•NO) exerts its biological effects. There are several proposed mechanisms of formation of RSNO in vivo: auto-oxidation of •NO, transnitrosation, oxidative nitrosylation, and from dinitrosyliron complexes (DNIC). Both free •NO, generated by •NO donors, and S-nitrosocysteine (CysNO) are widely used to study •NO biology and signaling, including protein S-nitrosation. It is assumed that the cellular effects of both compounds are analogous and indicative of in vivo •NO biology. A quantitative comparison was made of formation of DNIC and RSNO, the major •NO-derived cellular products. In RAW 264.7 cells, both •NO and CysNO were metabolized, leading to rapid intracellular RSNO and DNIC formation. DNIC were the dominant products formed from physiologic •NO concentrations, however, and RSNO were the major product from CysNO treatment. Chelatable iron was necessary for DNIC assembly from either •NO or CysNO, but not for RSNO formation. These profound differences in RSNO and DNIC formation from •NO and CysNO question the use of CysNO as a surrogate for physiologic •NO. Researchers designing experiments intended to elucidate the biological signaling mechanisms of •NO should be aware of these differences and should consider the biological relevance of the use of exogenous CysNO.

Exposure to nitric oxide increases the nitrosyl-iron complexes content in sorghum embryonic axes

This work was aimed to investigate nitrosyl-Fe complexes formation by reaction of endogenous ligands and Fe, in sorghum embryonic axes exposed to NO-donors. Electron paramagnetic resonance (EPR) was employed to detect the presence of nitrosyl-Fe complexes in plant embryos, as well as changes in labile iron pool (LIP). Nitrosyl-Fe complexes formation was detected in sorghum embryonic axes homogenates incubated in vitro in the presence of 1 mM of NO donors: diethylenetriamine NONOate (DETA NONOate), S-nitrosoglutathione (GSNO) and sodium nitroprusside (SNP). In axes isolated from seeds incubated in vivo in the presence of 1 mM SNP for 24 h, the content of NO was increased by 2-fold, and the EPR spectrum from mononitrosyl-Fe complexes (MNIC) was observed with a concomitant increase in the fresh weight of sorghum axes. The simultaneous exposure to deferoxamine and the NO donor precluded the increase in fresh weight observed in the presence of excess NO. While total Fe content in the axes isolated from seeds exposed to 1mM SNP was not significantly affected as compared to control axes, the LIP was increased by over 2-fold.The data reported suggest a critical role for the generation of complexes between Fe and NO when cells faced a situation leading to a significant increase in NO content. Moreover, demonstrate the presence of MNICs as one of the important components of the LIP, which could actively participate in Fe cellular mobilization.

Nitrogen monoxide (NO) storage and transport by dinitrosyl-dithiol-iron complexes: long-lived NO that is trafficked by interacting proteins

Nitrogen monoxide (NO) markedly affects intracellular iron metabolism, and recent studies have shown that molecules traditionally involved in drug resistance, namely GST and MRP1 (multidrug resistance-associated protein 1), are critical molecular players in this process. This is mediated by interaction of these proteins with dinitrosyl-dithiol-iron complexes (Watts, R. N., Hawkins, C., Ponka, P., and Richardson, D. R. (2006) Proc. Natl. Acad. Sci. U.S.A. 103, 7670-7675; Lok, H. C., Suryo Rahmanto, Y., Hawkins, C. L., Kalinowski, D. S., Morrow, C. S., Townsend, A. J., Ponka, P., and Richardson, D. R. (2012) J. Biol. Chem. 287, 607-618). These complexes are bioavailable, have a markedly longer half-life compared with free NO, and form in cells after an interaction between iron, NO, and glutathione. The generation of dinitrosyl-dithiol-iron complexes acts as a common currency for NO transport and storage by MRP1 and GST P1-1, respectively. Understanding the biological trafficking mechanisms involved in the metabolism of NO is vital for elucidating its many roles in cellular signaling and cytotoxicity and for development of new therapeutic targets.

A Conspectus of Cellular Mechanisms of Nitrosothiol Formation from Nitric Oxide

Although chemical mechanisms for the formation of nitrosothiol from •NO have been studied extensively "in the test tube", surprisingly little is known regarding the mechanism(s) of how nitrosothiols are formed in vivo. This lack of understanding has hampered more general acceptance of the concept of cysteine nitrosothiol formation as a generally applicable, regulated, and functionally significant protein posttranslational modification (as opposed to multiple other •NO-induced thiol modifications). Here we provide a brief overview/summary of the cellular formation of nitrosothiols from •NO via two possible mechanisms involving oxygen or transition metals.

Dinitrosyliron complexes are the most abundant nitric oxide-derived cellular adduct: biological parameters of assembly and disappearance

It is well established that nitric oxide ((•)NO) reacts with cellular iron and thiols to form dinitrosyliron complexes (DNIC). Little is known, however, regarding their formation and biological fate. Our quantitative measurements reveal that cellular concentrations of DNIC are proportionally the largest of all (•)NO-derived adducts (900 pmol/mg protein, or 45-90 μM). Using murine macrophages (RAW 264.7), we measured the amounts, and kinetics, of DNIC assembly and disappearance from endogenous and exogenous sources of (•)NO in relation to iron and O(2) concentration. Amounts of DNIC were equal to or greater than measured amounts of chelatable iron and depended on the dose and duration of (•)NO exposure. DNIC formation paralleled the upregulation of iNOS and occurred at low physiologic (•)NO concentrations (50-500 nM). Decreasing the O(2) concentration reduced the rate of enzymatic (•)NO synthesis without affecting the amount of DNIC formed. Temporal measurements revealed that DNIC disappeared in an oxygen-independent manner (t(1/2)=80 min) and remained detectable long after the (•)NO source was removed (>24 h). These results demonstrate that DNIC will be formed under all cellular settings of (•)NO production and that the contribution of DNIC to the multitude of observed effects of (•)NO must always be considered.

Nitric oxide suppresses tumor cell migration through N-Myc downstream-regulated gene-1 (NDRG1) expression: role of chelatable iron

N-Myc downstream-regulated gene 1 (NDRG1) is a ubiquitous cellular protein that is up-regulated under a multitude of stress and growth-regulatory conditions. Although the exact cellular functions of this protein have not been elucidated, mutations in this gene or aberrant expression of this protein have been linked to both tumor suppressive and oncogenic phenotypes. Previous reports have demonstrated that NDRG1 is strongly up-regulated by chemical iron chelators and hypoxia, yet its regulation by the free radical nitric oxide ((•)NO) has never been demonstrated. Herein, we examine the chemical biology that confers NDRG1 responsiveness at the mRNA and protein levels to (•)NO. We demonstrate that the interaction of (•)NO with the chelatable iron pool (CIP) and the appearance of dinitrosyliron complexes (DNIC) are key determinants. Using HCC 1806 triple negative breast cancer cells, we find that NDRG1 is up-regulated by physiological (•)NO concentrations in a dose- and time-dependant manner. Tumor cell migration was suppressed by NDRG1 expression and we excluded the involvement of HIF-1α, sGC, N-Myc, and c-Myc as upstream regulatory targets of (•)NO. Augmenting the chelatable iron pool abolished (•)NO-mediated NDRG1 expression and the associated phenotypic effects. These data, in summary, reveal a link between (•)NO, chelatable iron, and regulation of NDRG1 expression and signaling in tumor cells.

Nitrosyl iron complexes--synthesis, structure and biology

Nitrosyl complexes of iron are formed in living species in the presence of nitric oxide. They are considered a form in which NO can be stored and stabilized within a living cell. Upon entering a topic in bioinorganic chemistry the researcher faces a wide spectrum of issues concerning synthetic methods, the structure and chemical properties of the complex on the one hand, and its biological implications on the other. The aim of this review is to present the newest knowledge on nitrosyl iron complexes, summarizing the issues that are important for understanding the nature of nitrosyl iron complexes, their possible interactions, behavior in vitro and in vivo, handling of the preparations etc. in response to the growing interest in these compounds. Herein we focus mostly on the dinitrosyl iron complexes (DNICs) due to their prevailing occurrence in NO-treated biological samples. This article reviews recent knowledge on the structure, chemical properties and biological action of DNICs and some mononitrosyls of heme proteins. Synthetic methods are also briefly reviewed.

Iron-sulfur proteins are the major source of protein-bound dinitrosyl iron complexes formed in Escherichia coli cells under nitric oxide stress

Protein-bound dinitrosyl iron complexes (DNICs) have been observed in prokaryotic and eukaryotic cells under nitric oxide (NO) stress. The identity of proteins that bind DNICs, however, still remains elusive. Here we demonstrate that iron-sulfur proteins are the major source of protein-bound DNICs formed in Escherichia coli cells under NO stress. Expression of recombinant iron-sulfur proteins, but not proteins without iron-sulfur clusters, almost doubles the amount of protein-bound DNICs formed in E. coli cells after NO exposure. Purification of recombinant proteins from the NO-exposed E. coli cells further confirms that iron-sulfur proteins, but not proteins without iron-sulfur clusters, are modified, forming protein-bound DNICs. Deletion of the iron-sulfur cluster assembly proteins IscA and SufA to block the [4Fe-4S] cluster biogenesis in E. coli cells largely eliminates the NO-mediated formation of protein-bound DNICs, suggesting that iron-sulfur clusters are mainly responsible for the NO-mediated formation of protein-bound DNICs in cells. Furthermore, depletion of the "chelatable iron pool" in wild-type E. coli cells effectively removes iron-sulfur clusters from proteins and concomitantly diminishes the NO-mediated formation of protein-bound DNICs, indicating that iron-sulfur clusters in proteins constitute at least part of the chelatable iron pool in cells.

Iron regulatory protein 1 outcompetes iron regulatory protein 2 in regulating cellular iron homeostasis in response to nitric oxide

In mammals, iron regulatory proteins (IRPs) 1 and 2 posttranscriptionally regulate expression of genes involved in iron metabolism, including transferrin receptor 1, the ferritin (Ft) H and L subunits, and ferroportin by binding mRNA motifs called iron responsive elements (IREs). IRP1 is a bifunctional protein that mostly exists in a non-IRE-binding, [4Fe-4S] cluster aconitase form, whereas IRP2, which does not assemble an Fe-S cluster, spontaneously binds IREs. Although both IRPs fulfill a trans-regulatory function, only mice lacking IRP2 misregulate iron metabolism. NO stimulates the IRE-binding activity of IRP1 by targeting its Fe-S cluster. IRP2 has also been reported to sense NO, but the intrinsic function of IRP1 and IRP2 in NO-mediated regulation of cellular iron metabolism is controversial. In this study, we exposed bone marrow macrophages from Irp1(-/-) and Irp2(-/-) mice to NO and showed that the generated apo-IRP1 was entirely responsible for the posttranscriptional regulation of transferrin receptor 1, H-Ft, L-Ft, and ferroportin. The powerful action of NO on IRP1 also remedies the defects of iron storage found in IRP2-null bone marrow macrophages by efficiently reducing Ft overexpression. We also found that NO-dependent IRP1 activation, resulting in increased iron uptake and reduced iron sequestration and export, maintains enough intracellular iron to fuel the Fe-S cluster biosynthetic pathway for efficient restoration of the citric acid cycle aconitase in mitochondria. Thus, IRP1 is the dominant sensor and transducer of NO for posttranscriptional regulation of iron metabolism and participates in Fe-S cluster repair after exposure to NO.

Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6-diisopropylphenyl)NC(Me)](2)CH). The dinitrosyliron complexes [Fe(NO)(2)(Ar-nacnac)] (1) and (Bu(4)N)[Fe(NO)(2)(Ar-nacnac)] (2) react with [Fe(III)(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [Fe(II)(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO)(2)(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported.

Detecting and understanding the roles of nitric oxide in biology

We are pursuing a dual strategy for investigating the chemistry of nitric oxide as a biological signaling agent. In one approach, metal-based fluorescent sensors for the detection of NO in living cells are evaluated, and a sensor based on a copper fluorescein complex has proved to be a valuable lead compound. Sensors of this class permit identification of NO from both inducible and constitutive forms of nitric oxide synthase and facilitate investigation of different NO functions in response to external stimuli. In the other approach, we employ synthetic model complexes of iron-sulfur clusters to probe their reactivity toward nitric oxide as biomimics of the active sites of iron-sulfur proteins. Our studies reveal that NO disassembles the Fe-S clusters to form dinitrosyl iron complexes.

Detection of nitric oxide by electron paramagnetic resonance spectroscopy

Electron paramagnetic resonance (EPR) spectroscopy has been used in a number of ways to study nitric oxide chemistry and biology. As an intrinsically stable and relatively unreactive diatomic free radical, the challenges of detecting this species by EPR are somewhat different from those of transient radical species. This review gives a basic introduction to EPR spectroscopy and discusses its uses to assess and quantify nitric oxide formation in biological systems.

Oxygen is required for the L-cysteine-mediated decomposition of protein-bound dinitrosyl-iron complexes

Increasing evidence suggests that iron-sulfur proteins are the primary targets of nitric oxide (NO). Exposure of Escherichia coli cells to NO readily converts iron-sulfur proteins to protein-bound dinitrosyl-iron complexes (DNICs). Although the protein-bound DNICs are stable in vitro under aerobic or anaerobic conditions, they are efficiently repaired in aerobically growing E. coli cells even without new protein synthesis. The cellular repair mechanism for the NO-modified iron-sulfur proteins remains largely elusive. Here we report that, unlike aerobically growing E. coli cells, starved E. coli cells fail to reactivate the NO-modified iron-sulfur proteins. Significantly, the addition of L-cysteine, but not other related biological thiols, results in decomposition of the protein-bound DNICs in starved E. coli cells and in cell extracts under aerobic conditions. However, under anaerobic conditions, L-cysteine has little or no effect on the protein-bound DNICs in starved E. coli cells or in vitro, suggesting that oxygen is required for the L-cysteine-mediated decomposition of the protein-bound DNICs. Additional studies reveal that L-cysteine is able to release the DNIC from the protein and bind to it, and the L-cysteine-bound DNICs are rapidly disrupted by oxygen, resulting in the eventual decomposition of the protein-bound DNICs under aerobic conditions.

Reactive oxygen species and small-conductance calcium-dependent potassium channels are key mediators of inflammation-induced hypotension and shock

Septic shock is associated with life-threatening vasodilation and hypotension. To cause vasodilation, vascular endothelium may release nitric oxide (NO), prostacyclin (PGI2), and the elusive endothelium-derived hyperpolarizing factor (EDHF). Although NO is critical in controlling vascular tone, inhibiting NO in septic shock does not improve outcome, on the contrary, precipitating the search for alternative therapeutic targets. Using a hyperacute tumor necrosis factor (TNF)-induced shock model in mice, we found that shock can develop independently of the known vasodilators NO, cGMP, PGI2, or epoxyeicosatrienoic acids. However, the antioxidant tempol efficiently prevented hypotension, bradycardia, hypothermia, and mortality, indicating the decisive involvement of reactive oxygen species (ROS) in these phenomena. Also, in classical TNF or lipopolysaccharide-induced shock models, tempol protected significantly. Experiments with (cell-permeable) superoxide dismutase or catalase, N-acetylcysteine and apocynin suggest that the ROS-dependent shock depends on intracellular (*)OH radicals. Potassium channels activated by ATP (K(ATP)) or calcium (K(Ca)) are important mediators of vascular relaxation. While NO and PGI2-induced vasodilation involves K(ATP) and large-conductance BK(Ca) channels, small-conductance SK(Ca) channels mediate vasodilation induced by EDHF. Interestingly, also SK(Ca) inhibition completely prevented the ROS-dependent shock. Our data thus indicate that intracellular (*)OH and SK(Ca) channels represent interesting new therapeutic targets for inflammatory shock. Moreover, they may also explain why antioxidants other than tempol fail to provide survival benefit during shock.

Metallomic EPR spectroscopy

Based on explicit definitions of biomolecular EPR spectroscopy and of the metallome, this tutorial review positions EPR in the field of metallomics as a unique method to study native, integrated systems of metallobiomolecular coordination complexes subject to external stimuli. The specific techniques of whole-system bioEPR spectroscopy are described and their historic, recent, and anticipated applications are discussed.

Dinitrosyliron complexes and the mechanism(s) of cellular protein nitrosothiol formation from nitric oxide

Nitrosothiols (RSNO), formed from thiols and metabolites of nitric oxide (*NO), have been implicated in a diverse set of physiological and pathophysiological processes, although the exact mechanisms by which they are formed biologically are unknown. Several candidate nitrosative pathways involve the reaction of *NO with O(2), reactive oxygen species (ROS), and transition metals. We developed a strategy using extracellular ferrocyanide to determine that under our conditions intracellular protein RSNO formation occurs from reaction of *NO inside the cell, as opposed to cellular entry of nitrosative reactants from the extracellular compartment. Using this method we found that in RAW 264.7 cells RSNO formation occurs only at very low (<8 microM) O(2) concentrations and exhibits zero-order dependence on *NO concentration. Indeed, RSNO formation is not inhibited even at O(2) levels <1 microM. Additionally, chelation of intracellular chelatable iron pool (CIP) reduces RSNO formation by >50%. One possible metal-dependent, O(2)-independent nitrosative pathway is the reaction of thiols with dinitrosyliron complexes (DNIC), which are formed in cells from the reaction of *NO with the CIP. Under our conditions, DNIC formation, like RSNO formation, is inhibited by approximately 50% after chelation of labile iron. Both DNIC and RSNO are also increased during overproduction of ROS by the redox cycler 5,8-dimethoxy-1,4-naphthoquinone. Taken together, these data strongly suggest that cellular RSNO are formed from free *NO via transnitrosation from DNIC derived from the CIP. We have examined in detail the kinetics and mechanism of RSNO formation inside cells.