Acetylated deoxycholic (DCA) and cholic (CA) acids are potent ligands of pregnane X (PXR) receptor

The Pregnane X (PXR), Vitamin D (VDR) and Farnesoid X (FXR) nuclear receptors have been shown to be receptors of bile acids controlling their detoxification or synthesis. Chenodeoxycholic (CDCA) and lithocholic (LCA) acids are ligands of FXR and VDR, respectively, whereas 3-keto and acetylated derivates of LCA have been described as ligands for all three receptors. In this study, we hypothesized that oxidation or acetylation at position 3, 7 and 12 of bile acids DCA (deoxycholic acid), LCA, CA (cholic acid), and CDCA by detoxification enzymes or microbiome may have an effect on the interactions with bile acid nuclear receptors. We employed reporter gene assays in HepG2 cells, the TR-FRET assay with recombinant PXR and RT-PCR to study the effects of acetylated and keto bile acids on the nuclear receptors activation and their target gene expression in differentiated hepatic HepaRG cells. We demonstrate that the DCA 3,12-diacetate and CA 3,7,12-triacetate derivatives are ligands of PXR and DCA 3,12-diacetate induces PXR target genes such as CYP3A4, CYP2B6 and ABCB1/MDR1. In conclusion, we found that acetylated DCA and CA are potent ligands of PXR. Whether the acetylated bile acid derivatives are novel endogenous ligands of PXR with detoxification or physiological functions should be further studied in ongoing experiments.

EPR studies of nitric oxide interactions of alkoxyl and peroxyl radicals in in vitro and ex vivo model systems

A model compound of lipid peroxidation, tert-butyl hydroperoxide (tBOOH), was used in vitro to investigate (i) the generation of tBOOH-derived free radicals by hematin or rat enterocytes and (ii) the modulation of cell-generated free radical production by a nitric oxide (NO) donor, or when these cells were primed to produce NO. In hematin-catalyzed decomposition of tBOOH, NO from nitrosoglutathione, or S-nitroso-N-acetylpenicillamine suppressed the generation of peroxyl radicals (measured by direct electron paramagnetic resonance) and tert-butylalkoxyl, methoxyl, and methyl radicals (measured by electron paramagnetic resonance spin trapping). Similarly, co-incubation of S-nitroso-N-acetylpenicillamine or nitrosoglutathione with tBOOH caused significant decreases in tBOOH-derived free radical generation catalyzed by enterocytes. Epithelial cells are the known source of the inducible form of NO synthase in the intestine of rats challenged with lipopolysaccharide (LPS). Enterocytes isolated from LPS-treated rats produced decreased levels of tBOOH-derived radicals. These decreases in free radical production were further decreased when these cells were treated with LPS in vitro. These findings demonstrated that exogenously added or endogenously produced NO could modulate the extent of tBOOH-derived free radical generation in enterocytes. These decreases in free radical production could, at least in part, describe the protective role of NO from hydroperoxide-induced injury.

Desulfonation of a colitis inducer 2,4,6-trinitrobenzene sulfonic acid produces sulfite radical

2,4,6-Trinitrobenzene sulfonic acid (TNBS) has been used in vivo to induce colitis. With the nitroreductase of intestinal cells, TNBS underwent redox cycling to produce TNBS-nitro and superoxide radical anions which are thought to be involved in initial oxidative reactions that lead to colonic injury. In this study, we demonstrated that the TNBS desulfonative reaction with tissue amino acids produces sulfite which is subsequently oxidized to sulfite radical. Sulfite radical was measured using a spin trapping methodology. Sulfite radical adducts of 5,5-dimethyl-1-pyrroline N-oxide (DMPO) or 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) were detected in a mixture of TNBS and lysine, xanthine oxidase, red blood cells, colonic mucosal or submucosal muscle tissues. TNBS alone did not produce sulfite radical, indicating that its formation required the presence of amino acids. Because sulfite radical is the precursor of highly reactive sulfiteperoxyl and sulfate radicals, our data imply that these sulfite-derived free radicals may also contribute to oxidative reactions leading to colonic injury in TNBS-induced colitis.

Activation of the superoxide-generating NADPH oxidase of intestinal lymphocytes produces highly reactive free radicals from sulfite

The objective of this study was to investigate the ability of immune cells of the small intestine to produce highly reactive free radicals from the food additive sulfites. These free radicals were characterized with a spin-trapping technique using the spin traps 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO) and 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In the presence of glucose, purified lymphocytes from intestinal Peyer's patches (PP) and mesenteric lymph nodes (MLN) were stimulated with phorbol 12-myristate 13-acetate (PMA) to produce superoxide and hydroxyl DEPMPO radical adducts. The formation of these adducts was inhibited by superoxide dismutase or diphenyleneiodonium chloride, indicating that these cells produced superoxide radical during reduced nicotinamide adenine dinucleotide phosphate (NADPH) oxidase activation. With the treatment of sodium sulfite, PMA-stimulated PP lymphocytes produced a DEPMPO-sulfite radical adduct and an unknown radical adduct. When DEPMPO was replaced with DMPO, DMPO-sulfite and hydroxyl radical adducts were detected. The latter adduct resulted from DMPO oxidation by sulfate radical, which was capable of oxidizing formate or ethanol. Oxygen consumption rates were further increased after the addition of sulfite to PMA-stimulated lymphocytes, suggesting the presence of sulfiteperoxyl radical. Taken together, oxidants generated by stimulated lymphocytes oxidized sulfite to sulfite radical, which subsequently formed sulfiteperoxyl and sulfate radicals. The latter two radicals are highly reactive, contributing to increased oxidative stress, which may lead to sulfite toxicity, altered functions in intestinal lymphocytes, or both.

Differential regulation of inducible nitric oxide synthase gene expression by ethanol in the human intestinal epithelial cell line DLD-1

We have examined the regulation of inducible nitric oxide synthase (iNOS) gene expression by ethanol in monolayers of DLD-1 cells, an epithelial cell line derived from human intestinal adenocarcinoma. Optimum induction of iNOS mRNA in these cells was obtained with IFN-gamma and IL-1beta treatment, while further addition of TNF-alpha did not have significant effect. In a set of experiments to study ethanol effects, DLD-1 monolayers were pretreated with ethanol for 24 h and were then treated with IFN-gamma + IL-1beta for an additional 24 h. Cells pretreated with ethanol showed decreased iNOS mRNA levels, indicating that ethanol may inhibit cytokine-induced iNOS transcription or affect mRNA destabilization. The suppression was ethanol-dose dependent with an IC50 of 50 mM. In another set of experiments to study ethanol effects, DLD-1 monolayers were pretreated with 66 mM ethanol for 24 h. These cells showed significant upregulation of IL-1beta mRNA and protein as detected in the supernatants. Aliquoted supernatants from these cells (i.e., conditioned media) were added to naive DLD-1 monolayers together with IFN-gamma. Conditioned medium from ethanol-treated cells increased the IFN-gamma-induced iNOS mRNA of naive cells by threefold. Two different effects of ethanol are now reported: (a) ethanol inhibits IFN-gamma + IL-1beta-induced iNOS mRNA of the same DLD-1 cells and (b) ethanol induces cellular paracrine signals by releasing IL-1beta into the medium, which in combination with IFN-gamma increases iNOS mRNA levels of the recipient naive DLD-1 cells. Because IFN-gamma and IL-1beta are produced by intestinal immune cells, these findings may have implications for differential in vivo regulation of epithelial iNOS genes by ethanol, depending on the inflammatory and immune status of the host.

Characterization of reactive oxygen species induced effects on human spermatozoa movement and energy metabolism

Reactive oxygen species (ROS) inhibit sperm movement and have been implicated in male infertility. In this study, we determined the effects of specific ROS produced by activated leukocytes on human spermatozoa and investigated their metabolic site of action. We used chemiluminescence and electron paramagnetic resonance (EPR) to characterize the ROS generated by both blood and seminal leukocytes. We also determined the effects of these ROS on sperm energy metabolism using biochemical analyses and flow cytometry. Both blood and seminal leukocytes produced the same characteristic ROS which were determined to be hydrogen peroxide (H2O2) and superoxide radicals (O2*-). EPR using the spin trapping technique indicated that superoxide radical-dependent hydroxyl radicals (HO.) were also generated. ROS generated by PMA-stimulated blood leukocytes (2-5 x 10(6)/ml) caused inhibition of sperm movement in 2 h (p < .01). Using the hypoxanthine/ xanthine oxidase (0.5 U/ml) system to generate ROS, we determined that spermatozoa ATP levels, after ROS treatment, were reduced approximately eight-fold in 30 min (0.10 x 10(10) moles/10(6) sperm cells) compared to control (0.84 X 10(-10) moles/10(6) sperm cells) (p < .01). Sperm ATP reduction paralleled the inhibition of sperm forward progression. Neither superoxide dismutase (100 U/ml) nor dimethyl sulfoxide (100 mM) reversed these effects; however, protection was observed with catalase (4 X 10(3) U/ml). Flow cytometric analyses of sperm treated with various doses of H2O2 (0.3 mM-20.0 mM) showed a dose-dependent decrease in sperm mitochondrial membrane potential (MMP); however, at low concentrations of H2O2, sperm MMP was not significantly inhibited. Also, sperm MMP uncoupling with CCClP had no effect on either sperm ATP levels or forward progression. These results indicate that H2O2 is the toxic ROS produced by activated leukocytes causing the inhibition of both sperm movement and ATP production. O2*- and HO. do not play a significant role in these processes. Low concentrations of H2O2 causing complete inhibition of sperm movement and ATP levels inhibit sperm energy metabolism at a site independent of mitochondrial oxidative phosphorylation.

Priming of Peyer's patch lymphoid cells by hemorrhagic shock and resuscitation to produce superoxide radical

Hemorrhagic shock (HS) can cause whole body ischemia including the gastrointestinal tract. We investigated whether cells from small intestine Peyer's patches (PP) were capable of producing superoxide radical when animals underwent HS or HS followed by resuscitation (HS/RS). HS was initiated by removing 60% of the blood volume of surgically prepared guinea pigs. PP lymphoid cells were purified and stimulated with phorbol 12-myristate 13-acetate in the presence of spin trap 5-diethoxyphosphoryl-5-methyl-1-pyrroline-N-oxide (DEPMPO). Electron paramagnetic resonance spectra of PP lymphoid cells from sham-treated control, HS, and HS/RS animals produced DEPMPO radical adducts characterized as the adducts of superoxide (DEPMPO/*OOH) and hydroxyl (DEPMPO/*OH) radicals. The formation of both radical adducts was totally inhibited by superoxide dismutase or a nicotinamide adenine dinucleotide phosphate (reduced form) oxidase inhibitor, diphenyleneiodonium chloride. HS/RS increased radical adduct formation, expressed as a percentage control, by 160% and 225% for DEPMPO/*OOH, and DEPMPO/*OH, respectively. When animals were allowed to recover for 24 h post-HS/RS treatment, PP cells decreased the superoxide generation to the same level as controls. Thus, RS following HS may prime PP lymphoid cells for increased nicotinamide adenine dinucleotide phosphate (reduced form) oxidase-dependent superoxide generation, and this process may have cytotoxic and/or immunomodulatory effects on the host.

Nitric oxide inhibited peroxyl and alkoxyl radical formation with concomitant protection against oxidant injury in intestinal epithelial cells

A model compound of lipid peroxidation, tert-butyl hydroperoxide (tBOOH), was used in vitro to investigate (i) the generation of tBOOH-derived peroxyl and alkoxyl radicals by rat intestinal epithelial cells or enterocytes and (ii) the role of nitric oxide (NO) on cell-generated free radical formation and cellular cytotoxicity. Peroxyl, alkoxyl, and methyl radicals were detected and characterized by direct and spin-trapping electron paramagnetic resonance spectroscopy in incubations containing tBOOH and hematin, enterocytes, or intestinal epithelial cell line-6 cells. The direct interactions of tBOOH-derived radicals and NO from nitrosoglutathione (GSNO), nitrosoacetyl penicillamine (SNAP), or 1-¿b3-aminopropy-4-(3-aminopropylammonio)¿ butylamino-diazeniumdiolate (SpNONOate) were demonstrated as their levels were depleted in these incubations. SNAP, not GSNO or SpNONOate, was capable of trapping methyl radical produced during hematin-catalyzed decomposition of tBOOH. Cellular cytotoxicity expressed by percentage of dead cells and lactate dehydrogenase was increased with tBOOH treatment. Addition of GSNO, SNAP, or SpNONOate suppressed tBOOH-induced elevation of cell cytotoxicity. The NO donor precursor glutathione, acetylpenicillamine, or spermine did not have any effects on tBOOH-derived radical generation or cell cytotoxicity. These findings demonstrated free radical-free radical reactions between NO- and tBOOH-derived alkoxyl and peroxyl radicals generated by enterocytes. These reactions, at least in part, describe the protective role of NO from hydroperoxide-induced injury in intestinal epithelial cells.

In vivo endotoxin enhances biliary ethanol-dependent free radical generation

Endotoxemia is associated with alcoholic liver diseases; however, the effect of endotoxin on the oxidation of ethanol is not known. We tested the hypothesis that endotoxin treatment enhances hepatic ethanol radical production. The generation of free radicals by the liver was studied with spin-trapping technique utilizing the primary trap ethanol (0.8 g/kg) and the secondary trap alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone (4-POBN; 500 mg/kg). Electron paramagnetic resonance (EPR) spectra of bile showed six-line signals, which were dependent on ethanol, indicating the trapping of ethanol-dependent radicals. Intravenous injections of Escherichia coli lipopolysaccharide (0.5 mg/kg) 0.5 h before 4-POBN plus ethanol treatment caused threefold increases of biliary radical adducts. EPR analyses of bile from [1-13C]ethanol-treated endotoxic rats showed the presence of species attributable to alpha-hydroxyethyl adduct, carbon-centered adducts, and ascorbate radical. The generation of endotoxin-induced increases of ethanol-dependent radicals was suppressed by 50% on GdCl3 (20 mg/kg i.v.) or desferrioxamine mesylate (1 g/kg i.p.) treatment. Our data show that in vivo endotoxin increases biliary ethanol-dependent free radical formation and that these processes are modulated by Kupffer cell activation and catalytic metals.

Nitrosylation of blood hemoglobin and renal nonheme proteins in autoimmune MRL-lpr/lpr mice

MRL-lpr/lpr mice spontaneously develop manifestations of autoimmunity including arthritis, vasculitis, and glomerulonephritis. The paramagnetic molecule nitric oxide has been implicated as an effector molecule in initiation and propagation of these inflammatory conditions. In this study, we utilized electron paramagnetic resonance spectroscopy to directly detect nitrosylated protein complexes as products of nitric oxide in whole blood and in kidneys of MRL-lpr/lpr mice. Electron paramagnetic resonance spectra of blood samples from MRL-lpr/lpr mice showed nitrosyl hemoglobin species. Amounts of blood nitrosyl hemoglobin in MRL-lpr/lpr mice were significantly increased as compared to age-matched control mice. Electron paramagnetic resonance spectra of MRL-lpr/lpr kidney tissue exhibited a signal characteristic of a dinitrosyl-iron-dithiolate complex at g approximately 2.04. Formation of nitrosylated nonheme protein in diseased kidneys is associated with development of glomerulonephritis in the autoimmune mice. The presence of nitrosylated nonheme protein indicates the formation of nitric oxide within the kidneys of the diseased mice signifying in situ renal nitric oxide formation.

Generation of nitro and superoxide radical anions from 2,4,6-trinitrobenzenesulfonic acid by rat gastrointestinal cells

Reactive oxygen and nitrogen species have been implicated in the inflammation of the gastrointestinal tract. The objective of this study was to investigate mechanisms of free radical formation from the colitis inducer 2,4,6-trinitrobenzene sulfonic acid (TNBS). We showed that TNBS was rapidly metabolized to TNBS nitro radical anion via metabolic reduction by flavinmononucleotide/NADPH, xanthine/xanthine oxidase as well as the rat small intestine and colon. TNBS nitro radical anion was directly detected with electron paramagnetic resonance (EPR) spectroscopy. EPR spectra of TNBS nitro radical anion showed hyperfine coupling constants from the proximal nitrogen, two hydrogens and the two distal nitrogens with respective magnitudes of a(N)(4) = 9.7 G; a(H)(3,5) = 3.2 G (2); and a(N)(2,6) = 0.25 G. EPR spin trapping using 5.5-dimethyl-1-pyrroline N-oxide in aerobic incubations of isolated enterocytes (or colonocytes, or red blood cells) and TNBS, in the presence or absence of NADPH, produced radical adducts characteristic of superoxide and hydroxyl radicals. Our EPR data showing generation of TNBS nitro and superoxide radical anions demonstrate that one-electron reduction of TNBS may be an initial step in the cascade of the in vivo inflammatory events in TNBS-induced colitis.

Nitric oxide interactions with cobalamins: biochemical and functional consequences

Nitric oxide (NO) is a paramagnetic gas that has been implicated in a wide range of biologic functions. The common pathway to evoke the functional response frequently involves the formation of an iron-nitrosyl complex in a target (heme) protein. In this study, we report on the interactions between NO and cobalt-containing vitamin B12 derivatives. Absorption spectroscopy showed that of the four Co(III) derivatives (cyanocobalamin [CN-Cbl], aquocobalamin [H2O-Cbl], adenosylcobalamin [Ado-Cbl], and methylcobalamin [MeCbl]), only the H2O-Cbl combined with NO. In addition, electron paramagnetic resonance spectroscopy of H2O-Cbl preparations showed the presence of a small amount of Cob-(II)alamin that was capable of combining with NO. The Co(III)-NO complex was very stable, but could transfer its NO moiety to hemoglobin (Hb). The transfer was accompanied by a reduction of the Co(III) to Co(II), indicating that NO+ (nitrosonium) was the leaving group. In accordance with this, the NO did not combine with the Hb Fe(II)-heme, but most likely with the Hb cysteine-thiolate. Similarly, the Co(III)-NO complex was capable of transferring its NO to glutathione. Ado-Cbl and Me-Cbl were susceptible to photolysis, but CN-Cbl and H2O-Cbl were not. The homolytic cleavage of the Co(III)-Ado or Co(III)-Me bond resulted in the reduction of the metal. When photolysis was performed in the presence of NO, formation of NO-Co(II) was observed. Co(II)-nitrosyl oxidized slowly to form Co(III)-nitrosyl. The capability of aquocobalamin to combine with NO had functional consequences. We found that nitrosylcobalamin had diminished ability to serve as a cofactor for the enzyme methionine synthase, and that aquocobalamin could quench NO-mediated inhibition of cell proliferation. Our in vitro studies therefore suggest that interactions between NO and cobalamins may have important consequences in vivo.

Nitric oxide and liver injury in alcohol-fed rats after lipopolysaccharide administration

Earlier studies showed that alcohol-fed animals were more susceptible than controls to injurious effects of endotoxin. Increased superoxide radical production by hepatocyte organelles, Kupffer cells, and neutrophils from alcohol-fed animals has been well documented. In this study, electron paramagnetic resonance spectroscopy was used to detect nitrosyl protein complexes indicating nitric oxide (.NO) production. We showed that the concentrations of nitrosyl complexes in whole blood and in liver tissues of alcohol-fed rats treated with lipopolysaccharide (alc + LPS), increased 3-fold, compared with those from rats on control diet treated with LPS (con+LPS). Electron paramagnetic resonance spectra of whole blood and liver tissues from the alc + LPS-treated group exhibited features characteristic of hemoglobin nitrosyl complexes. Plasma levels of the hepatic ASTs and ALTs from the alc + LPS-treated group were increased 2- to 3-fold, compared with those from the con+LPS-treated group. Inhibition of .NO production of aminoguanidine treatment attenuated plasma hepatic enzyme levels in the alc + LPS-treated group. Thus, under the conditions of elevated inflammatory oxidative states caused by chronic alcohol feeding, endotoxin treatment enhanced liver injury as a result of the actions of .NO, and/or the cytotoxic species derived from .NO.

Endotoxin-induced oxidative stress in the rat small intestine: role of nitric oxide

Reactive oxygen species have been implicated in the gastrointestinal pathogenesis of septic and endotoxic shock. The objective of this study was to investigate the role of inducible nitric oxide synthase during endotoxin-induced formation of oxidants by cells of the small intestine. After intravenous Escherichia coli lipopolysaccharide (LPS) (1 mg/kg) injection, nitric oxide production was measured as nitrosyl complex formation in the ileum using electron paramagnetic resonance spectroscopy. Oxidative stress biomarkers were determined as duodenal mucosal-reduced thiols, the ileal lipid peroxidation and luminal free radical production using spin trapping methodology. Demonstration of nitrosyl complex formation commenced at 3 h and diminished 24 h post-LPS. Mucosal thiol levels were decreased at 3, 6, 12, and 18 h post-LPS treatment. At these time point, the ileal lipid peroxidation also increased as did luminal formation of hydroxyl radical adduct. Nitric oxide synthase inhibitors reversed the elevation of hydroxyl radical formation and reversed the decrease in mucosal-reduced thiol levels in the LPS-treated rats. Our data indicate that nitric oxide or its oxidant product(s), such as peroxynitrite, contribute to oxidative injury in the small intestine of rats treated with endotoxin.

Nitrosyl complex formation during endotoxin-induced injury in the rat small intestine

The objective of this study was to demonstrate nitric oxide (NO) production and determine its role in the rat small intestine following endotoxin treatment. By using electron paramagnetic resonance (EPR) spectroscopy, we were able to detect high concentrations of nitrosylated proteins in the small intestines of rats administered 1 mg/kg lipopolysaccharide (LPS) and sacrificed 6 h later. EPR spectra of non-heme and heme nitrosyl complexes were detected in the epithelium layer and intestinal wall. Only EPR spectra characteristic of nitrosyl hemoprotein complexes were detected in the luminal contents of these rats. LPS administration elevated the concentrations of intestinal lipid peroxidation biomarkers, thiobarbituric acid-reactive substances, and conjugated dienes. These changes were attenuated by NO synthase inhibitor treatment. We conclude that oxidants associated with NO formation were at least in part involved in the oxidation of tissue lipids. This process may be one of the mechanisms of intestinal injury induced by LPS.

Tumor necrosis factor-alpha and nitric oxide production in endotoxin-primed rats administered carbon tetrachloride

Tumor necrosis factor-alpha (TNF alpha) is elevated in the sera of rats administered non-lethal doses of carbon tetrachloride (CCl4) followed by endotoxin. Elevated TNF alpha levels are correlated with the increased release of hepatic enzymes indicating hepatic damage. Under these conditions, nitric oxide (NO) was also produced in the liver as evidenced by the formation of nitrosyl complexes which were measured by electron paramagnetic resonance (EPR) spectroscopy. Decreased nitrosyl complex formation occurred in livers following treatment with either an inhibitor or macrophage activation (gadolinium trichloride; GdCl3), an inhibitor of cytokine responses (dexamethasone) or a NO synthase inhibitor (NG-monomethyl-L-arginine; 1-NMA), GdCl3 or dexamethasone treatment decreased, while 1-NMA treatment increased, TNF alpha serum level. Taken together, these data suggest that TNF alpha and NO are induced following CCl4 and LPS exposure and may be important regulators in the hepatotoxicity of this liver injury model.

Phenyl N-tert-butyl nitrone forms nitric oxide as a result of its FE(III)-catalyzed hydrolysis or hydroxyl radical adduct formation

Phenyl N-tert-butyl nitrone (PBN) is commonly employed in spin-trapping studies. We report here evidence that PBN in aqueous solutions is decomposed by two pathways leading to the generation of nitric oxide (.NO). The first pathway is by hydrolysis of PBN, which is strongly catalyzed by ferric iron. The second pathway is via PBN-hydroxyl radical adduct formation. .NO was trapped in the presence of cysteine and ferrous iron to form a [(cys)2Fe(NO)2]-3 complex, which was measured by use of electron paramagnetic resonance (EPR) spectroscopy. A concomitant metabolite, benzaldehyde, was detected from both reaction mixtures. We propose that PBN is hydrolyzed by Fe3+ or attacked by hydroxyl radical, leading eventually to a common transient species, tert-butyl hydronitroxide [t-BuN(O.)H], which is further oxidized to a .NO source, t-BuNO. Our data imply that PBN may decompose to .NO when used in biological models with oxidative stress conditions.

Electron paramagnetic resonance investigations of nitrosyl complex formation during endotoxin tolerance

The prior administration of low dose endotoxin induces a state of hyporesponsiveness or tolerance to the lethal effects of endotoxin. It is generally accepted that macrophages are main cellular components in the development of tolerance, hence, nitric oxide (.NO) as one of the macrophage mediators may play a role in host defense mechanisms during tolerance. In this study, we utilized EPR spectroscopy to directly detect nitrosyl complexes as products of .NO in whole blood, livers and intestines of lipopolysaccharide (LPS)-tolerant rats. Male Sprague-Dawley rats were injected with a "low dose" LPS (0.5 mg/kg) 12-168 h prior to a "high dose" LPS (3 mg/kg), then sacrificed 6 h later. EPR signals of nitrosyl hemoprotein complexes were detected in specimens after high dose LPS. The post-LPS EPR signals of nitrosyl complexes from all samples were attenuated by a prior injection of low dose LPS. The signals of dinitrosyl-iron-dithiolate became apparent in samples from tolerant rats as signals of nitrosyl hemoprotein decreased. The maximal tolerance in terms of diminished .NO production was observed when low dose LPS was given 48-96 h prior to high dose LPS. Hemoglobin concentrations in the intestine used as biomarkers of hemorrhagic damage, were concomitantly attenuated in the jejunum of tolerant rats. These results together with our previous studies indicate that suppression of .NO production may contribute to the amelioration of hepatic and intestinal injury during endotoxin tolerance.

Targets of nitric oxide in a mouse model of liver inflammation by Corynebacterium parvum

Treatment of mice with Corynebacterium parvum induces chronic inflammation. This treatment followed by an injection of lipopolysaccharide (LPS) produces hepatic necrosis and death. We examined liver tissue by using electron paramagnetic resonance (EPR) spectroscopy and found that, in addition to the previously reported nonheme nitrosyl complexes, heme nitrosyl complexes were also formed. Hemoglobin nitrosyl complexes measured in the whole blood of mice treated with C. parvum were not increased after additional LPS treatment. However, this treatment significantly increased the heme nitrosyl complexes in the liver, whereas the nonheme nitrosyl complex concentration was unaffected. EPR signals from whole blood and liver tissues from mice treated with C. parvum and C. parvum + LPS were inhibited by prolonged treatment with NG-monomethyl-L-arginine (L-NMA). Nitric oxide (.NO) is known to bind to cytochrome P450 heme, and we consistently found a suppression of EPR signals attributable to ferric low-spin cytochrome P450/P420 peaks in the livers of mice treated with C. parvum and C. parvum + LPS. By performing analyses of EPR spectra obtained from hepatocytes exposed to .NO, we were able to unambiguously identify EPR signals attributable to cytochrome P420 and nonheme nitrosyl complexes in the livers of both treatments. Deconvolution of the composite in vivo EPR spectra indicated that hemoglobin nitrosyl complexes contributed weakly in the C. parvum livers, but threefold more in the C. parvum + LPS livers, suggesting that hemorrhage may have occurred. Experiments with L-NMA treatment revealed that this additional .NO production did not correlate with hepatic necrosis and onset of death. Immunoprecipitation of liver cytosols from C. parvum- and (C. parvum + LPS)-treated mice using an antibody against mouse inducible nitric oxide synthase showed that this enzyme was indeed present in the cytosolic fractions and was absent in those from control livers. Our novel detection of cytochrome P420 nitrosyl complex in vivo may be linked to any role of hepatic P450's functions during liver inflammation.

Nitric oxide production during endotoxic shock in carbon tetrachloride-treated rats

Earlier studies showed that hepatotoxicant-treated experimental animals were more susceptible than controls to the lethal effects of bacterial endotoxin. The exact mechanisms of this effect were not understood. In this paper we showed that nitric oxide (.NO) was produced in whole blood and in liver tissues of rats that had been treated with a nonlethal dose of CCl4 (1.3 g/kg) followed by a low dose of lipopolysaccharide (LPS) (100 micrograms/kg). EPR spectroscopy was used in this study to detect nitrosyl-protein complexes. Hemoglobin-nitrosyl complexes were detected in both whole blood and liver. By performing analyses of EPR spectra obtained from hepatocytes exposed to .NO, we were able to identify EPR signals attributable to nitrosyl-cytochrome P420 in rat liver. We found that nitrosyl complex formation in red blood cells and liver was inhibited by treatment with NG-mono-methyl-L-arginine, suggesting enzymatic biosynthesis of .NO. A small but significant inhibition of nitrosyl complex formation by gadolinium trichloride pretreatment was found in the liver, suggesting that Kupffer cells were also involved in .NO biosynthesis, because this treatment decreased Kupffer cells. There was a synergistic effect of CCl4 and LPS on the serum levels of the hepatic enzymes aspartate aminotransferase, alanine amino-transferase, lactate dehydrogenase, and sorbitol dehydrogenase, which are indices of parenchymal cell damage. NG-Mono-methyl-L-arginine treatment increased these hepatic enzyme activities, suggesting a protective role for .NO. EPR resonances at g approximately 2.48, 2.29, and 1.91, due to low-spin cytochromes P450/P420 (FE3+), were decreased in the livers of LPS-induced rats that had been previously treated with CCl4, indicating cytochrome P450/P420 destruction or at least a change in the valence state of the cytochrome P450/P420 heme groups to Fe2+ in the presence of .NO. Because nitrosyl-cytochrome P450 is not stable, the concomitant detection of nitrosyl-cytochrome P420 (Fe2+) could account, at least in part, for the decrease of the ferric low-spin heme groups. Our novel observations of hepatic nitrosyl species suggest that .NO plays an important role during hepatic injury caused by CCl4 in hosts exposed to endotoxin.

Oxidation and radical intermediates associated with the glutathione conjugation of mucochloric acid

The inactivation of the drinking water mutagen mucochloric acid (MCA) by reduced glutathione (GSH) was linked to the formation of an MCA-GSH conjugate, a nonmutagen in the Salmonella typhimurium (TA100) plate incorporation assay. Anaerobic formation of MCA-GSH is found now to be associated with oxidized glutathione (GSSG) and unconverted MCA. The anaerobic reaction of GSH with MCA in the presence of the radical trap 2-methyl-2-nitrosopropane (tNB; "tert-nitrosobutane") gives rise to an electron paramagnetic resonance (EPR) resulting from the overlapping spectra of two radical adducts. The first species exhibited hyperfine coupling constants of aN = 13.65 G and aH beta = 0.73 G. The second radical adduct exhibited a three-line signal of aN = 12.8 G. The first species is assigned to an adduct of the MCA radical because deuteration of MCA (5-deuterio-MCA) caused the beta-hydrogen hyperfine coupling to collapse. The second radical adduct is unaffected by the deuteration of MCA. Thus, the involvement of both GSSG and a carbon-centered MCA radical in the action of MCA on GSH is indicated.

Nitric oxide formation during light-induced decomposition of phenyl N-tert-butylnitrone

Phenyl N-tert-butylnitrone (PBN) is a spin trap commonly employed in free radical research. PBN has been shown to have adverse and beneficial effects on various biological systems. We report here evidence that photolysis (or even ambient light) decomposes PBN to nitric oxide in aqueous solutions. Non-heme and heme proteins have been employed to form nitrosyl complexes, which were detected using EPR spectroscopy. Concomitantly, nitrite formation was detected after light-induced decomposition of PBN. In addition, we found that tert-nitrosobutane and decomposed PBN caused an activation of guanylate cyclase. We propose a mechanism where PBN is decomposed by light to tert-nitrosobutane. The latter compound is, in turn, decomposed to nitric oxide. This study suggests the possibility that PBN or PBN radical adducts may be sources of nitric oxide in biological environments. When using PBN as a spin trap in biological samples, not only is the trapping of reactive free radicals operative, but nitric oxide produced from PBN decomposition may play an important role in altering biological functions.

Fatty acid radical formation in rats administered oxidized fatty acids: in vivo spin trapping investigation

We report in vivo evidence for fatty acid-derived free radical metabolite formation in bile of rats dosed with spin traps and oxidized polyunsaturated fatty acids (PUFA). When rats were dosed with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and oxidized PUFA, the DMPO thiyl radical adduct was formed due to a reaction between oxidized PUFA and/or its metabolites with biliary glutathione. In vitro experiments were performed to determine the conditions necessary for the elimination of radical adduct formation by ex vivo reactions. Fatty acid-derived radical adducts of alpha-(4-pyridyl-1-oxide)-N-tert-butylnitrone (4-POBN) were detected in vivo in bile samples collected into a mixture of iodoacetamide, desferrioxamine, and glutathione peroxidase. Upon the administration of oxidized 13C-algal fatty acids and 4-POBN, the EPR spectrum of the radical adducts present in the bile exhibited hyperfine couplings due to 13C. Our data demonstrate that the carbon-centered radical adducts observed in in vivo experiments are unequivocally derived from oxidized PUFA. This in vivo evidence for PUFA-derived free radical formation supports the proposal that processes involving free radicals may be the molecular basis for the previously described cytotoxicity of dietary oxidized PUFA.

When are metal ion-dependent hydroxyl and alkoxyl radical adducts of 5,5-dimethyl-1-pyrroline N-oxide artifacts?

The formation of the 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/.OH adduct of the spin trap DMPO has been reported to occur through nucleophilic addition of water in the presence of aqueous ferric chloride (K. Makino, T. Hagiwara, A. Hagi, M. Nishi, and A. Murakami, 1990, Biochem. Biophys. Res. Commun. 172, 1073-1080). Due to the serious implications of these findings with respect to many spin trapping studies, the suitability of DMPO as a hydroxyl radical spin trap was studied in typical Fenton systems. Using 17O-enriched water, we show conclusively that nucleophilic addition of water occurs at the nitrone carbon (or C-2 position) of DMPO in the presence of either Fe or Cu ions. Furthermore, our results demonstrate that this nucleophilic reaction is a major pathway to the DMPO/.OH adduct, even during the reaction of Fe(II) or Cu(I) with hydrogen peroxide. Primary alkoxyl adducts of DMPO also form in aqueous solution through nucleophilic addition in the presence of both Fe(III) and Cu(II). Attempts to obtain secondary and tertiary alkoxyl adducts by this mechanism were unsuccessful, possibly due to steric effects. When the reaction is carried out in various buffers, however, or in the presence of metal ion chelators, nucleophilic addition to DMPO from Fe(III) is effectively suppressed. Chelators also suppress the reaction with Cu(II). Hence, under most common experimental conditions in biochemical free radical research, nucleophilic addition to DMPO should not be of major concern.

Evidence against the 1:2:2:1 quartet DMPO spectrum as the radical adduct of the lipid alkoxyl radical

It was reported that the electron paramagnetic resonance (EPR) spectrum of 5,5-dimethyl-1-pyrroline N-oxide (DMPO)/lipid alkoxyl radical exhibited a quartet with 1:2:2:1 relative intensity that is identical to that of DMPO/hydroxyl radical (K. M. Schaich and D. C. Borg, 1990, Free Radicals Res. Commun. 9, 267-278). We repeated these EPR experiments using HPLC separation of radical adducts and isotope substitution. We found that the HPLC/EPR chromatogram of the radical adduct with a 1:2:2:1 quartet obtained by the reduction of methyl linoleate hydroperoxide (MLOOH) with Fe2+ exhibited identical retention time to that of the DMPO/OH radical adduct obtained from the Fenton reaction in two different solvent systems. Upon performing the same reaction in 17O-enriched water, the 17O-hyperfine coupling constants due to DMPO/17OH were identified. Ultimately, approximately 80-90% of the total DMPO/OH is derived from water by an iron-dependent nucleophilic addition reaction. Initially, a water-independent mechanism also significantly contributes to DMPO/OH formation. Regardless of its mechanism of formation, the 1:2:2:1 quartet radical adduct of DMPO formed during the reduction of MLOOH by Fe2+ is in fact DMPO/OH.

Nitroxide metabolites from alkylhydroxylamines and N-hydroxyurea derivatives resulting from reductive inhibition of soybean lipoxygenase

One proposed mechanism of the inactivation of lipoxygenase by inhibitors is the reduction of the catalytically active ferric form of the enzyme to its ferrous form. Recent studies have shown that compounds containing the hydroxamate moiety are potent inhibitors of lipoxygenase. The hydroxamate portion of the inhibitor is thought to bind to iron at the catalytic site of the enzyme. We now report evidence that the NOH of the hydroxamate group of N-(4-chlorophenyl)-N-hydroxy-N'-(3-chlorophenyl)urea, N-[(E)-3-(3-phenoxyphenyl)prop-2-enyl]acetohydroxamic acid (BW A4C), and N-(1-benzo(b)thien-2-ylethyl)-N-hydroxyurea (Zileuton) is oxidized by lipoxygenase to form their corresponding nitroxides, which are directly detected by electron paramagnetic resonance spectroscopy. It is consistently found that the selected NOH-containing compounds, e.g. alkylhydroxylamines or N-hydroxyureas, are also oxidized by lipoxygenase to form their corresponding nitroxides.

Superoxide and peroxyl radical generation from the reduction of polyunsaturated fatty acid hydroperoxides by soybean lipoxygenase

Soybean lipoxygenase is shown to catalyze the breakdown of polyunsaturated fatty acid hydroperoxides to produce superoxide radical anion as detected by spin trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO). In addition to the DMPO/superoxide radical adduct, the adducts of peroxyl, acyl, carbon-centered, and hydroxyl radicals were identified in incubations containing linoleic acid and lipoxygenase. These DMPO radical adducts were observed just prior to the system becoming anaerobic. Only a carbon-centered radical adduct was observed under anaerobic conditions. The superoxide radical production required the presence of fatty acid substrates, fatty acid hydroperoxides, active lipoxygenase, and molecular oxygen. Superoxide radical production was inhibited when nordihydroguaiaretic acid, butylated hydroxytoluene, or butylated hydroxyanisole was added to the incubation mixtures. We propose that polyunsaturated fatty acid hydroperoxides are reduced to form alkoxyl radicals and that after an intramolecular rearrangement, the resulting hydroxyalkyl radical reacts with oxygen, forming a peroxyl radical which subsequently eliminates superoxide radical anion.

Free radical formation from organic hydroperoxides in isolated human polymorphonuclear neutrophils

We have demonstrated with electron paramagnetic resonance (EPR) that organic hydroperoxides are decomposed to free radicals by both human polymorphonuclear leukocytes (PMNs) and purified myeloperoxidase. When tert-butyl hydroperoxide was incubated with either PMNs or purified myeloperoxidase, peroxyl, alkoxyl, and alkyl radicals were trapped by the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In the case of ethyl hydroperoxide, DMPO radical adducts of peroxyl and alkyl (identified as alpha-hydroxyethyl when trapped by tert-nitrosobutane) radicals were detected. Radical adduct formation was inhibited when azide was added to the incubation mixture. Myeloperoxidase-deficient PMNs produced DMPO radical adduct intensities at only about 20-30% of that of normal PMNs. Our studies suggest that myeloperoxidase in PMNs is primarily responsible for the decomposition of organic hydroperoxides to free radicals. The finding of the free radical formation derived from organic hydroperoxides by PMNs may be related to the cytotoxicity of this class of compounds.

Alkyl free radicals from the beta-scission of fatty acid alkoxyl radicals as detected by spin trapping in a lipoxygenase system

2-Methyl-2-nitrosopropane (tNB)-radical adducts from incubation mixtures of fatty acids and soybean lipoxygenase in borate buffer (pH 9.0) were measured by electron paramagnetic resonance (EPR). In addition to the previously reported six-line signal of secondary carbon-centered radicals (RCHR'), a weak signal submerged in the baseline was detected after the peroxidation phase was finished. We propose that this radical is a decomposition product formed via beta-scission of fatty acid alkoxyl radicals. EPR spectra of tNB-radical adducts formed in mixtures of either linoleic acid, arachidonic acid, or 15-hydroperoxyeicosatetraenoic acid with lipoxygenase exhibited hyperfine structure characteristic of tNB/.CH2CH2-R with hyperfine coupling constants: aN = 17.1 G; aH beta = 11.2 G (2H); and aH gamma = 0.6 G (2H). In the case of linolenic acid, this radical tNB/.CH=CH-R' with hyperfine coupling constants: aN = 17.1 G; aH beta = 10.9 G (2H); aH gamma = 1.1 G; and aH delta = 0.5 G. In accord with the decomposition scheme of hydroperoxides derived from unsaturated fatty acids, the radical adducts tNB/.CH2CH2-R and tNB/.CH2-CH=CH-R' were assigned as the pentyl and 2-pentenyl radicals, respectively.

The catalytic activity of iron in synovial fluid as monitored by the ascorbate free radical

Human synovial fluid, from a patient with synovitis disease, was examined by electron spin resonance spectroscopy for evidence of free radicals. The ascorbate free radical was observed and its intensity was affected by iron chelating agents, demonstrating that the iron in the synovial fluid is indeed available for oxidative catalysis.

Lipid peroxyl radical intermediates in the peroxidation of polyunsaturated fatty acids by lipoxygenase. Direct electron spin resonance investigations

Lipid peroxyl radicals resulting from the peroxidation of polyunsaturated fatty acids by soybean lipoxygenase were directly detected by the method of rapid mixing, continuous-flow electron spin resonance spectroscopy. When air-saturated borate buffer (pH 9.0) containing linoleic acid or arachidonate acid was mixed with lipoxygenase, fatty acid-derived peroxyl free radicals were readily detected; these radicals have a characteristic g-value of 2.014. An organic free radical (g = 2.004) was also detected; this may be the carbon-centered fatty acid free radical that is the precursor of the peroxyl free radical. The ESR spectrum of this species was not resolved, so the identification of this free radical was not possible. Fatty acids without at least two double bonds (e.g. stearic acid and oleic acid) did not give the corresponding peroxyl free radicals, suggesting that the formation of bisallylic carbon-centered radicals precedes peroxyl radical formation. The 3.8-G doublet feature of the fatty acid peroxyl spectrum was proven (by selective deuteration) to be a hyperfine coupling due to a gamma-hydrogen that originated as a vinylic hydrogen of arachidonate. Arachidonate peroxyl radical formation was shown to be dependent on the substrate, active lipoxygenase, and molecular oxygen. Antioxidants are known to protect polyunsaturated fatty acids from peroxidation by scavenging peroxyl radicals and thus breaking the free radical chain reaction. Therefore, the peroxyl signal intensity from micellar arachidonate solutions was monitored as a function of the antioxidant concentration. The reaction of the peroxyl free radical with Trolox C was shown to be 10 times slower than that with vitamin E. The vitamin E and Trolox C phenoxyl radicals that resulted from scavenging the peroxyl radical were also detected.

Epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene via a hydroperoxide-dependent mechanism catalyzed by lipoxygenases

The lipoxygenase catalyzed epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene (BP-7,8-diol) was examined. Epoxidation of the BP-7,8-diol was catalyzed by 5- and 15-lipoxygenase in the presence of either arachidonic acid, gamma-linolenic acid, or 15-hydroperoxyeicosatetraenoic acid (15-HPETE). The anti-9,10-epoxy-7,8-dihydroxy-7,8-dihydrobenzo[a]pyrene isomer was formed in greater quantities than the syn isomer, indicative of peroxyl radical mediated epoxidation. Epoxidation was dependent on time, enzyme and fatty acid concentration. There was no difference in the time course of epoxidation with either arachidonic acid or 15-HPETE, although the initial rate of oxygen consumption was approximately 55-fold greater with arachidonic acid. The lipoxygenase inhibitor and anti-oxidant nordihydroguaiaretic acid inhibited epoxidation in a dose-dependent manner in incubations initiated with either arachidonic acid or 15-HPETE. The anti-oxidant butylated hydroxyanisole also inhibited the epoxidation. Incubations conducted under anaerobic conditions with 15-lipoxygenase and either arachidonic acid or 15-HPETE significantly decreased epoxidation. This suggests that the oxygen inserted into BP-7,8-diol is derived from the atmosphere. The epoxidizing peroxyl radicals could not be detected but their precursors, carbon-centered radicals, were detected by using the ESR spin trapping technique in incubations of 15-lipoxygenase with 15-HPETE. This radical, formed by reduction and rearrangement of the hydroperoxide, may trap oxygen to form a peroxyl radical. We propose that the epoxidizing species is a peroxyl radical derived from 15-HPETE rather than from arachidonic acid. This proposal is based on the similar amounts of epoxidation, but dissimilar amount of oxygen consumed with both fatty acids. Since lipoxygenases are widely distributed in vivo, especially in areas where tumors arise such as the pulmonary epithelium, peroxyl radical formation by these enzymes may have an important role in chemical carcinogenesis.

Peroxyl, alkoxyl, and carbon-centered radical formation from organic hydroperoxides by chloroperoxidase

The decomposition of organic hydroperoxides as catalyzed by chloroperoxidase was investigated with electron spin resonance (ESR) spectroscopy. Tertiary peroxyl radicals were directly detected by ESR from incubations of tert-butyl hydroperoxide or cumene hydroperoxide with chloroperoxidase at pH 6.4. Peroxyl, alkoxyl, and carbon-centered free radicals from tertiary hydroperoxide/chloroperoxidase systems were successfully trapped by the spin trap 5,5-dimethyl-1-pyrroline N-oxide, whereas alkoxyl radicals were not detected in the ethyl hydroperoxide/chloroperoxidase system. The carbon-centered free radicals were further characterized by spin-trapping studies with tert-nitrosobutane. Oxygen evolution measured by a Clark oxygen electrode was detected for all the hydroperoxide/chloroperoxidase systems. The classical peroxidase mechanism is proposed to describe the formation of peroxyl radicals. In the case of tertiary peroxyl radicals, their subsequent self-reactions result in the formation of alkoxyl free radicals and molecular oxygen. beta-Scission and internal hydrogen atom transfer reactions of the alkoxyl free radicals lead to the formation of various carbon-centered free radicals. In the case of the primary ethyl peroxyl radicals, decay through the Russell pathway forms molecular oxygen.

Detection of singlet (1O2) oxygen phosphorescence during chloroperoxidase-catalyzed decomposition of ethyl hydroperoxide

Evidence for the production of singlet molecular oxygen (1O2) during the chloroperoxidase-catalyzed decomposition of ethyl hydroperoxide has been obtained through the use of optical spectroscopy, oxygen electrode experiments, and electron spin resonance (ESR). ESR spin-trapping experiments with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) demonstrate the production of the ethyl peroxyl free radical during the chloroperoxidase/ethyl hydroperoxide reaction. Oxygen and acetaldehyde concentrations suggest that the production of ethyl peroxyl radicals constitutes less than 2% of the decomposition of ethyl hydroperoxide at the concentrations of reactants used. The phosphorescence of 1O2 at 1268 nm was observed during the chloroperoxidase-catalyzed decomposition of ethyl hydroperoxide in deuterium oxide buffer. Chloroperoxidase also catalyzes the decomposition of tert-butyl hydroperoxide to its corresponding peroxyl radical. Alkoxyl and alkyl-DMPO spin adducts were also detected. A much lower yield of 1O2 phosphorescence was observed during the chloroperoxidase-catalyzed decomposition of tert-butyl hydroperoxide. This phosphorescence probably arises through secondary production of alkyl peroxyl radicals. These results suggest that the initial enzyme-dependent production of ethyl peroxyl radicals is followed by enzyme-independent reaction of two peroxyl radicals through the tetroxide intermediate, as originally proposed by Russell (Russell, G. A. (1957) J. Am. Chem. Soc. 79, 3871-3877), to form acetaldehyde, ethyl alcohol, and molecular oxygen.