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PubMed2004

Serum nitrite and nitrate levels in epileptic children using valproic acid or carbamazepine.

In experimental epilepsy studies, nitric oxide was found to act as both proconvulsant and anticonvulsant. The objective of this study was to investigate the effects of valproic acid and carbamazepine on serum levels of nitrite and nitrate, which are the metabolites of nitric oxide. To achieve this goal, serum nitrite and nitrate levels were determined in active epileptic 34 children using valproic acid and 23 children using carbamazepine and in non-active epileptic 38 children (control group) not using any antiepileptic drug. In the valproic acid group serum nitrite and nitrate levels were 2.66 +/- 2.11 micromol/l and 69.35 +/- 23.20 micromol/l, 1.89 +/- 1.01 micromol/l and 49.39 +/- 10.61 micromol/l in the carbamazepine group, and 1.22 +/- 0.55 micromol/l, 29.53 +/- 10.05 micromol in the control group, respectively. Nitrite and nitrate levels were significantly high in both valproic acid and carbamazepine groups compared to the control group (P < 0.01). When valproic acid and carbamazepine groups were compared to each other, level of nitrate was found statistically higher in the valproic acid group in relation to the carbamazepine group (P < 0.01), however, there was no statistically significant difference in the levels of nitrite (P > 0.05). No relation could be found between serum drug levels and nitrite and nitrate levels. According to these results, it can be suggested that valproic acid and carbamazepine might have antiepileptic effects through nitric oxide.

Anticonvulsants↗

PubMed2000

Nitrite-induced deamination and hypochlorite-induced oxidation of DNA in intact human respiratory tract epithelial cells.

No modification of purine or pyrimidine bases was observed when isolated DNA was incubated with 1 mM nitrite at pH 7.4. However, exposure of human bronchial epithelial cells in culture medium at pH 7.4 to nitrite at concentrations of 100 microM or greater led to deamination of purine bases in cellular DNA. Deamination was more extensive in cells exposed to lower extracellular pH values and higher nitrite concentrations. Significant increases in the levels of xanthine and hypoxanthine, putative deamination products of guanine and adenine, respectively, were observed in DNA from nitrite-treated cells but no rise in any base oxidation products such as 8-hydroxyguanine. This pattern of damage suggests that exposure of cells to nitrite (even at pH 7.4) leads to intracellular generation of "reactive nitrogen species" capable of deaminating purines in DNA. In addition, significant DNA strand breakage occurred in nitrite-treated cells. The time course of base damage suggested that the repair of deaminated purine lesions in these cells is slow. By contrast, DNA isolated from cells exposed to hypochlorous acid (HOCl) has significant oxidation of pyrimidine bases and chlorination of cytosine but little oxidation of purines. Exposure of cells to both species (NO(2)(-) plus HOCl) potentiated the oxidative DNA base damage observed but decreased the extent of deamination. We hypothesize that this is due to the formation of nitryl chloride (NO(2)Cl) from reaction of HOCl with *NO(2)(-). The relevance of our observations to events in the stomach and respiratory tract, at sites of inflammation, and in ischemic tissues is discussed.

Bronchi↗

PubMed1999

Oxidation of biological electron donors and antioxidants by a reactive lactoperoxidase metabolite from nitrite (NO2-): an EPR and spin trapping study.

We report that a lactoperoxidase (LPO) metabolite derived from nitrite (NO2-) catalyses one-electron oxidation of biological electron donors and antioxidants such as NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C. The radical products of the reaction have been detected and identified using either direct EPR or EPR combined with spin trapping. While LPO/H2O2 alone generated only minute amounts of radicals from these compounds, the yield of radicals increased sharply when nitrite was also present. In aerated buffer (pH 7) the nitrite-dependent oxidation of NAD(P)H by LPO/H2O2 produced superoxide radical, O2*-, which was detected as a DMPO/*O2H adduct. We propose that in the LPO/H2O2/NO2-/biological electron donor systems the nitrite functions as a catalyst because of its preferential oxidation by LPO to a strongly oxidizing metabolite, most likely a nitrogen dioxide radical *NO2, which then reacts with the biological substrates more efficiently than does LPO/H2O2 alone. Because both nitrite and peroxidase enzymes are ubiquitous our observations point at a possible mechanism through which nitrite might exert its biological and cytotoxic action in vivo, and identify some of the physiological targets which might be affected by the peroxidase/H2O2/nitrite systems.

Cyclic N-Oxides↗

PubMed2000

Pitfalls in the toxicological analysis of an isobutyl nitrite-adulterated coffee drink.

A forensic investigation was carried out on one poisoning case, where cyanide was first detected in an evidence sample of a canned coffee drink. A more complete study revealed that it had been adulterated with isobutyl nitrite (IBN) and not cyanide. We examined the detectivity of IBN and related compounds by headspace gas chromatography and capillary electrophoresis. IBN decomposes to isobutyl alcohol (iBuOH) and nitrite in aqueous solution, and under higher temperature and more acidic conditions, the rate of this reaction becomes more rapid. IBN was also produced by the esterification of iBuOH with nitrite below pH 5. Cyanide was produced in a coffee solution by the addition of nitrite below pH 6. An IBN-spiked canned coffee drink solution was stored at 4 degrees C and periodically analyzed for IBN, iBuOH, nitrite, nitrate, and cyanide. Since the IBN level decreased rapidly, iBuOH was produced in an almost 90% molar yield. Nitrite production reached a maximum of 40% molar recovery on the first day and then gradually disappeared. The nitrate level reached a plateau of approximately 60% molar recovery. Cyanide was also detected, and its level at the 14th day was approximately 0.26% molar recovery. These findings suggest that, in a coffee drink solution, IBN undergoes hydrolysis to produce iBuOH and nitric acid, which is oxidized to nitrate and also produces cyanide through the nonspecific oxidation of organic compounds under acidic conditions.

Chromatography, Gas↗

PubMed2002

Indirect measurement of nitric oxide production by monitoring nitrate and nitrite using microchip electrophoresis with electrochemical detection.

An indirect method for monitoring nitric oxide (NO) by determining nitrate and nitrite using microchip capillary electrophoresis (CE) with electrochemical (EC) detection has been developed. This method combines determination of nitrite by direct amperometric detection following a microchip-based CE separation and conversion of nitrate to nitrite by chemical reduction using Cu-coated Cd granules. The amount of nitrate is quantified by calculating the difference in the amount of nitrite in the sample before and after the reduction of nitrate. Optimization of the separation, injection, detection, and reduction reaction conditions, as well as studies involving integration of the reduction reaction onto the microchip, are described. It was found that nitrite can be separated and detected in approximately 45 s by microchip CEEC. The reduction reaction was successfully integrated on-chip and carried out in approximately 1 min following activation of the Cd granules. The usefulness of this device was demonstrated by monitoring the amount of nitrate and nitrite produced from 3-morpholinosydnonimine, a NO-releasing compound.

Electrochemistry↗

PubMed1989

Nitrite, a new substrate for nitrogenase.

We have examined the reactivity of the purified component proteins of Azotobacter vinelandii nitrogenase (Av1 and Av2) toward nitrate and nitrite. Nitrate has no effect on H2 evolution or C2H2 reduction by nitrogenase and thus is neither a substrate nor an inhibitor. Nitrite dramatically inhibits H2 evolution. This inhibition has two components, one irreversible and one reversible upon addition of CO. The irreversible inhibition is due to nitrite inactivation of the Fe protein. The rate of this inactivation is greatly enhanced by addition of MgATP, suggesting the [4Fe-4S] cluster is the site of nitrite attack. The reversible inhibition does not represent an inhibition of electron flow but rather a diversion of electrons away from H2 evolution and into the six-electron reduction of nitrite to ammonia. Thus, nitrogenase functions as a nitrite reductase.

Azotobacter↗

PubMed2005

Structure of spinach nitrite reductase: implications for multi-electron reactions by the iron-sulfur:siroheme cofactor.

The structure of nitrite reductase, a key enzyme in the process of nitrogen assimilation, has been determined using X-ray diffraction to a resolution limit of 2.8 A. The protein has a globular fold consisting of 3 alpha/beta domains with the siroheme-iron sulfur cofactor at the interface of the three domains. The Fe(4)S(4) cluster is coordinated by cysteines 441, 447, 482, and 486. The siroheme is located at a distance of 4.2 A from the cluster, and the central iron atom is coordinated to Cys 486. The siroheme is surrounded by several ionizable amino acid residues that facilitate the binding and subsequent reduction of nitrite. A model for the ferredoxin:nitrite reductase complex is proposed in which the binding of ferredoxin to a positively charged region of nitrite reductase results in elimination of exposure of the cofactors to the solvent. The structure of nitrite reductase shows a broad similarity to the hemoprotein subunit of sulfite reductase but has many significant differences in the backbone positions that could reflect sequence differences or could arise from alterations of the sulfite reductase structure that arise from the isolation of this subunit from the native complex. The implications of the nitrite reductase structure for understanding multi-electron processes are discussed in terms of differences in the protein environments of the cofactors.

Coenzymes↗

PubMed2003

New processes in the environmental chemistry of nitrite. 2. The role of hydrogen peroxide.

The oxidation of nitrite and nitrous acid to *NO2 upon irradiation of dissolved Fe(III), ferric (hydr)oxides, and nitrate has previously been shown to enhance phenol nitration. This allowed the proposal of a new role for nitrite and nitrous acid in natural waters and atmospheric aerosols. This paper deals with the interaction between hydrogen peroxide, a key environmental factor in atmospheric oxidative chemistry, and nitrite/nitrous acid. The reaction between nitrous acid and hydrogen peroxide yields peroxynitrous acid, a powerful nitrating agent and an important intermediate in atmospheric chemistry. The kinetics of this reaction is compatible with a rate-determining step involving either H3O2+ and HNO2 or H2O2 and protonated nitrous acid. In the former case the rate constant between the two species would be 179.6 +/- 1.4 M(-1) s(-1), in the latter case it would be as high as (1.68 +/- 0.01) x 10(10) M(-1) s(-1) (diffusion-controlled reaction). Due to the more reasonable value of the rate constant, the reaction between H3O2+ and HNO2 seems more likely. In the presence of HNO2 + H2O2 the nitration of phenol is strongly enhanced when compared with HNO2 alone. The nitration rate of phenol in the presence of peroxynitrous acid decreases as pH increases, thus HOONO is a potential source of atmospheric nitroaromatic compounds in acidic water droplets. The mixture Fe(II) + H2O2 (Fenton reagent) can oxidize nitrite and nitrous acid to nitrogen dioxide, which results in phenol nitration. The nitration in the presence of Fe(II) + H2O2 + NO2-/HNO2 occurs more rapidly than the one with H2O2 + NO2-/HNO2 at pH 5, where little HNO2 is available to directly react with hydrogen peroxide. Both systems, however, are more effective than NO2-/HNO2 alone in producing nitrophenols from phenol. Another process leading to the oxidation of nitrite to nitrogen dioxide is the photo-Fenton one. It can be relevant at pH > or = 6, as nitrite does not react with H2O2 at room temperature. Under such conditions the source of Fe(II) is the photolysis of ferric (hydr)oxides (heterogeneous photo-Fenton reaction). In the presence of nitrite this reaction induces very effective nitrophenol formation from phenol.

Aerosols↗

PubMed2005

Chemical nitrite oxidation in acid solutions as a consequence of microbial ammonium oxidation.

In long-term experiments with membrane aerated biofilm reactors we observed complete nitrite oxidation in highly concentrated ammonium nitrite solutions with a contaminant pH decrease to values below 3. The maximum initial concentration for ammonium was 42 mM and for nitrite was 41 mM. We hypothesized that (1) acid-tolerant ammonium oxidizing bacteria were responsible for the pH decrease, and (2) chemical processes caused complete nitrite oxidation at low pH values. To test this hypothesis we set up a mechanistic computer model based on kinetic data from literature and we validated the model with additional experiments. The simulations fitted the measurements very well. Additionally, an experiment with the inhibitor allylthiourea showed that ammonium-oxidizing bacteria were active at pH values far below 5.5. Experiments in a sterile reactor confirmed the chemical nitrite oxidation to nitrate. Nitrogen balances revealed that 8 +/- 4% of the initial nitrogen (ammonium, nitrite, and nitrate) were lost during the cycles. On the basis of measurements and simulations we concluded that volatilization was responsible for the significant nitrogen loss. We estimated that about half of the lost nitrogen volatilized as nitrous acid HNO2. The rest mainly volatilized as dinitrogen N2 and nitrous oxide N2O.

Bacteria↗

PubMed2004

Electrocatalytic reductions of nitrite, nitric oxide, and nitrous oxide by thermophilic cytochrome P450 CYP119 in film-modified electrodes and an analytical comparison of its catalytic activities with myoglobin.

Previous investigations of nitrite and nitric oxide reduction by myoglobin in surfactant film modified electrodes characterized several distinct steps in the denitrification pathway, including isolation of a nitroxyl adduct similar to that proposed in the P450nor catalytic cycle. To investigate the effect of the axial ligand on these biomimetic reductions, we report here a comparison of the electrocatalytic activity of myoglobin (Mb) with a thermophilic cytochrome P450 CYP119. Electrocatalytic nitrite reduction by CYP119 is very similar to that by Mb: two catalytic waves at analogous potentials are observed, the first corresponding to the reduction of nitric oxide, the second to the production of ammonia. CYP119 is a much more selective catalyst, giving almost exclusively ammonia during the initial half-hour of reductive electrolysis of nitrite. More careful investigations of specific steps in the catalytic cycle show comparable rates of nitrite dehydration and almost identical potentials and lifetimes for ferrous nitroxyl intermediate (Fe(II)-NO(-)) in CYP119 and Mb. The catalytic efficiency of nitric oxide reduction is reduced for CYP119 as compared to Mb, attributable to both a lower affinity of the protein for NO and a decreased rate of N-N coupling. Isotopic labeling studies show ammonia incorporation into nitrous oxide produced during nitrite reduction, as has been termed co-denitrification for certain bacterial and fungal nitrite reductases. Mb has a much higher co-denitrification activity than CYP119. Conversely, CYP119 is shown to be slightly more efficient at the two-electron reduction of N(2)O to N(2). These results suggest that thiolate ligation does not significantly alter the catalytic reactivity, but the dramatic difference in product distribution may suggest an important role for protein stability in the selectivity of biocatalysts.

Ammonia↗

PubMed1997

Direct measurement of nitrite transport across erythrocyte membrane vesicles using the fluorescent probe, 6-methoxy-N-(3-sulfopropyl) quinolinium.

Nitrite was shown to quench the fluorescence of 6-methoxy-N-(3-sulfopropyl) quinolinium (SPQ) almost twofold more than chloride. SPQ loaded inside vesicles prepared from asolectin and isolated erythrocyte ghosts allowed for the direct measurement of nitrite movement across these membranes. Movement of nitrite across asolectin occurred by diffusion as HNO2 in a pH-dependent manner. By contrast, erythrocyte ghosts had very low diffusion rates for nitrous acid. Erythrocyte ghosts preloaded with 50 mM nitrite to quench SPQ fluorescence were utilized to study heteroexchange with externally added anions. SPQ fluorescence increases (becomes unquenched) with added bicarbonate and nitrate, indicating that nitrite is moving out of the preloaded vesicles. The pH optimum for this exchange was approximately 7.6 and exchange was inhibited by N-ethylmaleimide (NEM) and dihydro-4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). These data indicate that nitrite moves across erythrocyte plasma membranes as NO2- by a heteroexchange mechanism with other monovalent anions.

Anions↗

PubMed2005

Nitrite is a signaling molecule and regulator of gene expression in mammalian tissues.

Mammalian tissues produce nitric oxide (NO) to modify proteins at heme and sulfhydryl sites, thereby regulating vital cell functions. The majority of NO produced is widely assumed to be neutralized into supposedly inert oxidation products including nitrite (NO2(-)). Here we show that nitrite, also ubiquitous in dietary sources, is remarkably efficient at modifying the same protein sites, and that physiological nitrite concentrations account for the basal levels of these modifications in vivo. We further find that nitrite readily affects cyclic GMP production, cytochrome P450 activities, and heat shock protein 70 and heme oxygenase-1 expression in a variety of tissues. These cellular activities of nitrite, combined with its stability and abundance in vivo, suggest that this anion has a distinct and important signaling role in mammalian biology, perhaps by serving as an endocrine messenger and synchronizing agent. Thus, nitrite homeostasis may be of great importance to NO biology.

Animals↗

PubMed2004

Inhaled nebulized nitrite is a hypoxia-sensitive NO-dependent selective pulmonary vasodilator.

The blood anion nitrite contributes to hypoxic vasodilation through a heme-based, nitric oxide (NO)-generating reaction with deoxyhemoglobin and potentially other heme proteins. We hypothesized that this biochemical reaction could be harnessed for the treatment of neonatal pulmonary hypertension, an NO-deficient state characterized by pulmonary vasoconstriction, right-to-left shunt pathophysiology and systemic hypoxemia. To test this, we delivered inhaled sodium nitrite by aerosol to newborn lambs with hypoxic and normoxic pulmonary hypertension. Inhaled nitrite elicited a rapid and sustained reduction ( approximately 65%) in hypoxia-induced pulmonary hypertension, with a magnitude approaching that of the effects of 20 p.p.m. NO gas inhalation. This reduction was associated with the immediate appearance of NO in expiratory gas. Pulmonary vasodilation elicited by aerosolized nitrite was deoxyhemoglobin- and pH-dependent and was associated with increased blood levels of iron-nitrosyl-hemoglobin. Notably, from a therapeutic standpoint, short-term delivery of nitrite dissolved in saline through nebulization produced selective, sustained pulmonary vasodilation with no clinically significant increase in blood methemoglobin levels. These data support the concept that nitrite is a vasodilator acting through conversion to NO, a process coupled to hemoglobin deoxygenation and protonation, and evince a new, simple and inexpensive potential therapy for neonatal pulmonary hypertension.

15-Hydroxy-11 alpha,9 alpha-(epoxymethano)prosta-5↗

PubMed1993

Method for group determination of total N-nitroso compounds and nitrite in fresh gastric juice by chemical denitrosation and thermal energy analysis.

A method for the determination of total N-nitroso compounds (NOC) and nitrite in fresh human gastric juice is described, which is based on earlier methods. A freshly obtained gastric juice sample is injected directly into refluxing ethyl acetate containing glacial acetic acid, HCl and HBr to determine the total concentration (A) of nitrite, total NOC and thermo- and acid-labile thermal energy analyser (TEA)-responsive compounds (TAC). Another fresh sample of the same juice (with or without the addition of sulfamic acid) is injected directly into refluxing ethyl acetate containing glacial acetic acid and HCl for determining the TAC level (B) and the total level (C) of nitrite and TAC. The NO released from nitrite, TAC and NOC is detected by TEA acting as a chemiluminescence detector. The differences between A and C and between C and B represent the concentrations of total NOC and nitrite, respectively. The method is rapid, reproducible (relative standard deviation 1-6%) and sensitive (detection limit 1.0 pmol). The behaviour of nitrite and NOC in the analytical system was studied and relevant techniques for the direct analysis of fresh gastric juice samples and for stabilization of fresh samples have been developed.

Differential Thermal Analysis↗

PubMed1996

Simultaneous assay of nitrite, nitrate and chloride in meat products by flow injection.

A flow injection (FI) analytical method for the simultaneous assay of nitrite, nitrate and chloride in meat products is reported. The method is based on the potentiometric determination of chloride using a tubular ISE and on the spectrophotometric determination of nitrite. The FI system consisted in splitting the flow after potentiometric detection using a tubular detector and the subsequent confluence of the flow before reaching the spectrophotometric detector. This allowed the reduction of nitrate to nitrite in part of the sample plug on an on-line copper cadmium reductor column. Since each channel had a different residence time, two peaks were obtained for nitrite and nitrite plus nitrate. Spectrophotometric determination was made after a diazotization-coupling reaction. The results obtained were in good agreement with reference procedures and showed adequate precision (RSDs less than 6% for chloride and nitrite and 2% for nitrate). A high sampling rate was obtained (120 determinations per hour corresponding to 40 samples per hour).

Chlorides↗

PubMed2001

Determination of nitrite by inhibition of the chemiluminescence of acriflavine in a flow-injection assembly.

The indirect determination of nitrite was performed with a flow-injection assembly on the basis of the inhibition of the analytical output obtained in a luminometer by oxidation of acriflavine. The acriflavine solution merged with the nitrite and the resulting mixture was injected into a pure water stream. This solution merged with the oxidant solution (potassium permanganate in sulfuric acid medium) and the resulting chemiluminiscence was affected (inhibited) by the presence of nitrite after reaction with the aminoacridine. The method was applicable over the range 10-800 microg l(-1) of nitrite with a correlation coefficient of 0.9960. The relative standard deviation was 1.4% and the throughput was 76 samples h(-1). The influence of foreign substances was also tested. A solid-phase reactor, filled with Amberlite IRA-900, was inserted in the assembly for the on-line preconcentration of nitrite; the analytical output resulted in an increase of up to 11.5-fold. The method was applied to the determination of nitrites in residual waters, industrial formulations and soil samples.

Acriflavine↗

PubMed1978

Purification and properties of nitrite reductase from Escherichia coli K12.

NADH-nitrite oxidoreductase (EC 1.6.4) was purified to better than 95% homogeneity from batch cultures of Escherichia coli strain OR75Ch15, which is partially constitutive for nitrite reductase synthesis. Yields of purified enzyme were low, mainly because of a large loss of activity during chromatography on DEAE-cellulose. The quantitative separation of cytochrome c-552 from nitrite reductase activity resulted in an increase in the specific activity of the enzyme: this cytochrome is not therefore an integral part of nitrite reductase. The subunit molecular weights of nitrite reductase and of a haemoprotein contaminant, as determined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis, were 88000 and 80000 respectively. The sedimentation coefficient was calculated to be in the range 8.5-9.5S, consistent with a mol.wt. of 190000. It is suggested therefore that the native enzyme is a dimer with two identical or similar-sized subunits. Purest samples contained 0.4 mol of flavin/mol of enzyme, but no detectable haem. Catalytic activity was totally inhibited by 20 micron-p-chloromercuribenzoate and 1 mM-cyanide, slightly inhibited by 1 micron-sulphite and 10mM-arsenite, but insensitive to 1 mM-2,2'-bipyridine, 4mM-1,10-phenanthroline and 10mM-NaN3. Three molecules of NADH were oxidized for each NO2-ion reduced: the product of the reaction is therefore assumed to be NH4+. The specific activity of hydroxylamine reductase increased at each step in the purification of nitrite reductase, and the elution profiles for these two activities during chromatography on DEAE-Sephadex were coincident. It is likely that a single enzyme is responsible for both activities.

Centrifugation, Density Gradient↗

PubMed2005

Function of the Rhizobium etli CFN42 nirK gene in nitrite metabolism.

Rhizobium etli CFN42 is not capable of growing anaerobically with nitrate but it grows with nitrite as a terminal electron acceptor. This bacterium contains the nirK gene encoding the copper-containing Nir (nitrite reductase), which is located on the cryptic plasmid pCFN42f. Mutational analysis has demonstrated that a nirK deficient mutant was not capable of growing under nitrite-respiring conditions. Moreover, microaerobic growth of this mutant was inhibited by the presence of nitrite. Nir activity and nitrite uptake were highly diminished in a nirK mutant, compared with the wild-type levels after incubation under anaerobic conditions. Our results suggest that the copper-containing Nir may have both a respiratory and a nitrite-detoxifying role in R. etli.

Genes, Bacterial↗