Effect of Blood Nitrite and Nitrate Levels on Murine Platelet Function

Nitric oxide (NO) appears to play an important role in the regulation of thrombosis and hemostasis by inhibiting platelet function. The discovery of NO generation by reduction of nitrite (NO₂ ⁻) and nitrate (NO₃ ⁻) in mammals has led to increased attention to these anions with respect to potential beneficial effects in cardiovascular diseases. We have previously shown that nitrite anions at 0.1 µM inhibit aggregation and activation of human platelet preparations in vitro in the presence of red blood cells and this effect was enhanced by deoxygenation, an effect likely due to NO generation. In the present study, we hypothesized that nitrite and nitrate derived from the diet could also alter platelet function upon their conversion to NO in vivo. To manipulate the levels of nitrite and nitrate in mouse blood, we used antibiotics, NOS inhibitors, low nitrite/nitrate (NOx) diets, endothelial NOS knock-out mice and also supplementation with high levels of nitrite or nitrate in the drinking water. We found that all of these perturbations affected nitrite and nitrate levels but that the lowest whole blood values were obtained by dietary restriction. Platelet aggregation and ATP release were measured in whole blood and the results show an inverse correlation between nitrite/nitrate levels and platelet activity in aggregation and ATP release. Furthermore, we demonstrated that nitrite-supplemented group has a prolonged bleeding time compared with control or low NOx diet group. These results show that diet restriction contributes greatly to blood nitrite and nitrate levels and that platelet reactivity can be significantly affected by these manipulations. Our study suggests that endogenous levels of nitrite and nitrate may be used as a biomarker for predicting platelet function and that dietary manipulation may affect thrombotic processes.     

ReviewMethods of quantitative analysis of the nitric oxide metabolites nitrite and nitrate in human biological fluids

In human organism, the gaseous radical molecule nitric oxide (NO) is produced in various cells from l-arginine by the catalytic action of NO synthases (NOS). The metabolic fate of NO includes oxidation to nitrate by oxyhaemoglobin in red blood cells and autoxidation in haemoglobin-free media to nitrite. Nitrate and nitrite circulate in blood and are excreted in urine. The concentration of these NO metabolites in the circulation and in the urine can be used to measure NO synthesis in vivo under standardized low-nitrate diet. Circulating nitrite reflects consitutive endothelial NOS activity, whereas excretory nitrate indicates systemic NO production. Today, nitrite and nitrate can be measured in plasma, serum and urine of humans by various analytical methods based on different analytical principles, such as colorimetry, spectrophotometry, fluorescence, chemiluminescence, gas and liquid chromatography, electrophoresis and mass spectrometry. The aim of the present article is to give an overview of the most significant currently used quantitative methods of analysis of nitrite and nitrate in human biological fluids, namely plasma and urine. With minor exception, measurement of nitrite and nitrate by these methods requires method-dependent chemical conversion of these anions. Therefore, the underlying mechanisms and principles of these methods are also discussed. Despite the chemical simplicity of nitrite and nitrate, accurate and interference-free quantification of nitrite and nitrate in biological fluids as indicators of NO synthesis may be difficult. Thus, problems associated with dietary and laboratory ubiquity of these anions and other preanalytical and analytical factors are addressed. Eventually, the important issue of quality control, the use of commercially available assay kits, and the value of the mass spectrometry methodology in this area are outlined.     

Methods of quantitative analysis of the nitric oxide metabolites nitrite and nitrate in human biological fluids

In human organism, the gaseous radical molecule nitric oxide (NO) is produced in various cells from L-arginine by the catalytic action of NO synthases (NOS). The metabolic fate of NO includes oxidation to nitrate by oxyhaemoglobin in red blood cells and autoxidation in haemoglobin-free media to nitrite. Nitrate and nitrite circulate in blood and are excreted in urine. The concentration of these NO metabolites in the circulation and in the urine can be used to measure NO synthesis in vivo under standardized low-nitrate diet. Circulating nitrite reflects constitutive endothelial NOS activity, whereas excretory nitrate indicates systemic NO production. Today, nitrite and nitrate can be measured in plasma, serum and urine of humans by various analytical methods based on different analytical principles, such as colorimetry, spectrophotometry, fluorescence, chemiluminescence, gas and liquid chromatography, electrophoresis and mass spectrometry. The aim of the present article is to give an overview of the most significant currently used quantitative methods of analysis of nitrite and nitrate in human biological fluids, namely plasma and urine. With minor exception, measurement of nitrite and nitrate by these methods requires method-dependent chemical conversion of these anions. Therefore, the underlying mechanisms and principles of these methods are also discussed. Despite the chemical simplicity of nitrite and nitrate, accurate and interference-free quantification of nitrite and nitrate in biological fluids as indicators of NO synthesis may be difficult. Thus, problems associated with dietary and laboratory ubiquity of these anions and other preanalytical and analytical factors are addressed. Eventually, the important issue of quality control, the use of commercially available assay kits, and the value of the mass spectrometry methodology in this area are outlined.     

Intragastric nitration by dietary nitrite: implications for modulation of protein and lipid signaling

Inorganic nitrite, derived from the reduction of nitrate in saliva, has recently emerged as a protagonist in nitric oxide (^?NO) biology as it can be univalently reduced to ^?NO, in the healthy human stomach. Important physiological implications have been attributed to nitrite-derived ^?NO in the gastrointestinal tract, namely modulation of host defense, blood flow, mucus formation and motility. At acidic pH, nitrite generates different nitrogen oxides depending on the local microenvironment (redox status, gastric content, pH, inflammatory conditions), including ^?NO, nitrogen dioxide (^?NO₂), dinitrogen trioxide (N₂O₃), and peroxynitrite. Thus, the gastric environment is a significant source of nitrating and nitrosating agents, especially in individuals consuming a nitrate/nitrite-rich diet on a daily basis. Both, the gastric lumen and mucosa contain putative targets for nitration, not only proteins and lipids from ingested aliments but also endogenous proteins secreted by the oxyntic glands. The physiological and functional consequences of nitration of gastric mediators will impact on local processes including food digestion and ulcerogenesis. Additionally, gastric nitration products (such as nitrated lipids) may be absorbed and affect systemic pathways. Thus, dietary ingestion of nitrate will have direct consequences for endogenous protein nitration, as indicated by our preliminary data.     

Nitrite oxidation in the Namibian oxygen minimum zone

Nitrite oxidation is the second step of nitrification. It is the primary source of oceanic nitrate, the predominant form of bioavailable nitrogen in the ocean. Despite its obvious importance, nitrite oxidation has rarely been investigated in marine settings. We determined nitrite oxidation rates directly in ¹⁵N-incubation experiments and compared the rates with those of nitrate reduction to nitrite, ammonia oxidation, anammox, denitrification, as well as dissimilatory nitrate/nitrite reduction to ammonium in the Namibian oxygen minimum zone (OMZ). Nitrite oxidation (⩽372 nM NO₂⁻ d⁻¹) was detected throughout the OMZ even when in situ oxygen concentrations were low to non-detectable. Nitrite oxidation rates often exceeded ammonia oxidation rates, whereas nitrate reduction served as an alternative and significant source of nitrite. Nitrite oxidation and anammox co-occurred in these oxygen-deficient waters, suggesting that nitrite-oxidizing bacteria (NOB) likely compete with anammox bacteria for nitrite when substrate availability became low. Among all of the known NOB genera targeted via catalyzed reporter deposition fluorescence in situ hybridization, only Nitrospina and Nitrococcus were detectable in the Namibian OMZ samples investigated. These NOB were abundant throughout the OMZ and contributed up to ∼9% of total microbial community. Our combined results reveal that a considerable fraction of the recently recycled nitrogen or reduced NO₃⁻ was re-oxidized back to NO₃⁻ via nitrite oxidation, instead of being lost from the system through the anammox or denitrification pathways.     

Nitrite to nitrate molar ratio is inversely proportional to oxidative cell damages and granulocytic apoptosis at the wound site following cutaneous injury in rats

Nitric oxide (NO) metabolism in response to the inflammatory cell infiltration and their apoptosis at the wound site, using a model of subcutaneously implanted sponges in Albino Oxford rats, were examined. The injured animals were sacrificed at days 1, 2 and 3 after the injury. Nitrites, nitrates (final products of NO metabolism), malondialdehyde (an indicator of oxidative cell damages), urea (product of arginase activity) and other parameters were measured both in plasma and wound fluid samples. Nitrite to nitrate molar ratio and sum of nitrites and nitrates (NO_x) were calculated. The total cell numbers were at similar level throughout the examined period, but a gradual decrease of viable granulocytes, mainly due to the increased apoptosis, and the increase of monocyte-macrophage number occurred after the second day. A gradual increase of wound fluid nitrates, NO_x and malondialdehyde suggested the increases of both NO and free oxygen radicals production. Interestingly, wound fluid nitrites peaked at the first day decreasing to the corresponding plasma levels thereafter. Wound fluid nitrite to nitrate molar ratio gradually decreased and negatively correlated both with the number of apoptotic cells (r = −0.752, p < 0.05) and malondialdehyde (r = −0.694, p < 0.05) levels. In conclusion, the inversely proportional relation between nitrite to nitrate molar ratio and both malondialdehyde and apoptotic cell number indicated a mutual relationship between NO metabolism, oxidative cell damages and cell apoptosis at the wound site early after the cutaneous wound. Moreover, the obtained findings suggest that measurement of both nitrites and nitrates contribute to better insight into overall wound NO metabolism.     

Analysis of nitrate,nitrite, and [15N]nitrate in biological fluids

A new automated system for the analysis of nitrate via reduction with a high-pressure cadmium column is described. Samples of urine, saliva, deproteinized plasma, gastric juice, and milk can be analyzed for nitrate, nitrite, or both with a lower limit of detection of 1.0 nmol NO₃^? or NO₂^?/ml. The system allows quantitative reduction of nitrate and automatically eliminates interference from other compounds normally present in urine and other biological fluids. Analysis rate is 30 samples per hour, with preparation for most samples limited to simple dilution with distilled water. The application of gas chromatography/mass spectrometry for the analysis of ¹⁵NO₃^? in urine after derivatization to ¹⁵NO₂-benzene is also described.     

Decreased leukocyte recruitment by inorganic nitrate and nitrite in microvascular inflammation and NSAID-induced intestinal injury

Nitric oxide (NO) generated by vascular NO synthases can exert anti-inflammatory effects, partly through its ability to decrease leukocyte recruitment. Inorganic nitrate and nitrite, from endogenous or dietary sources, have emerged as alternative substrates for NO formation in mammals. Bioactivation of nitrate is believed to require initial reduction to nitrite by oral commensal bacteria. Here we investigated the effects of inorganic nitrate and nitrite on leukocyte recruitment in microvascular inflammation and in NSAID-induced small-intestinal injury. We show that leukocyte emigration in response to the proinflammatory chemokine MIP-2 is reduced by 70% after 7 days of dietary nitrate supplementation as well as by acute intravenous nitrite administration. Nitrite also reduced leukocyte adhesion to a similar extent and this effect was inhibited by the soluble guanylyl cyclase inhibitor ODQ, whereas the effect on emigrated leukocytes was not altered by this treatment. Further studies in TNF-α-stimulated endothelial cells revealed that nitrite dose-dependently reduced the expression of ICAM-1. In rats and mice subjected to a challenge with diclofenac, dietary nitrate prevented the increase in myeloperoxidase and P-selectin levels in small-intestinal tissue. Antiseptic mouthwash, which eliminates oral nitrate reduction, markedly blunted the protective effect of dietary nitrate on P-selectin levels. Despite attenuation of the acute immune response, the overall ability to clear an infection with Staphylococcus aureus was not suppressed by dietary nitrate as revealed by noninvasive IVIS imaging. We conclude that dietary nitrate markedly reduces leukocyte recruitment to inflammation in a process involving attenuation of P-selectin and ICAM-1 upregulation. Bioactivation of dietary nitrate requires intermediate formation of nitrite by oral nitrate-reducing bacteria and then probably further reduction to NO and other bioactive nitrogen oxides in the tissues.     

Dietary nitrite restores NO homeostasis and is cardioprotective in endothelial nitric oxide synthase-deficient mice

Endothelial production of nitric oxide (NO) is critical for vascular homeostasis. Nitrite and nitrate are formed endogenously by the stepwise oxidation of NO and have, for years, been regarded as inactive degradation products. As a result, both anions are routinely used as surrogate markers of NO production, with nitrite as a more sensitive marker. However, both nitrite and nitrate are derived from dietary sources. We sought to determine how exogenous nitrite affects steady-state concentrations of NO metabolites thought to originate from nitric oxide synthase (NOS)-derived NO as well as blood pressure and myocardial ischemia-reperfusion (I/R) injury. Mice deficient in endothelial nitric oxide synthase (eNOS-/-) demonstrated decreased blood and tissue nitrite, nitrate, and nitroso proteins, which were further reduced by low-nitrite (NOx) diet for 1 week. Nitrite supplementation (50 mg/L) in the drinking water for 1 week restored NO homeostasis in eNOS-/- mice and protected against I/R injury. Nitrite failed to alter heart rate or mean arterial blood pressure at the protective dose. These data demonstrate the significant influence of dietary nitrite intake on the maintenance of steady-state NO levels. Dietary nitrite and nitrate may serve as essential nutrients for optimal cardiovascular health and may provide a novel prevention/treatment modality for disease associated with NO insufficiency.     

Rapid determination of nitrite by reversed-phase high-performance liquid chromatography with fluorescence detection

Measurement of nitrite and nitrate, the stable oxidation products of nitric oxide (NO), provides a useful tool to study NO synthesis in vivo and in cell cultures. A simple and rapid fluorometric HPLC method was developed for determination of nitrite through its derivatization with 2,3-diaminonaphthalene (DAN). Nitrite, in standard solution, cell culture medium, or biological samples, readily reacted with DAN under acidic conditions to yield the highly fluorescent 2,3-naphthotriazole (NAT). For analysis of nitrate, it was converted to nitrite by nitrate reductase, followed by the derivatization of nitrite with DAN to form NAT. NAT was separated on a 5-μm reversed-phase C₈ column (150×4.6 mm, I.D.) guarded by a 40-μm reversed-phase C₁₈ column (50×4.6 mm, I.D.), and eluted with 15 mM sodium phosphate buffer (pH 7.5) containing 50% methanol (flow-rate, 1.3 ml/min). Fluorescence was monitored with excitation at 375 nm and emission at 415 nm. Mean retention time for NAT was 4.4 min. The fluorescence intensity of NAT was linear with nitrite or nitrate concentrations ranging from 12.5 to 2000 nM in water, cell culture media, plasma and urine. The detection limit for nitrite and nitrate was 10 pmol/ml. Because NAT is well separated from DAN and other fluorescent components present in biological samples, our HPLC method offers the advantages of high sensitivity and specificity as well as easy automation for quantifying picomole levels of nitrite and nitrate in cell culture medium and biological samples.     

Quantification of nitrite and nitrate in human urine and plasma as pentafluorobenzyl derivatives by gas chromatography—mass spectrometry using their N-labelled analogs

For the quantification of nitrite and nitrate, the stable metabolites of -arginine-derived nitric oxide (NO) in human urine and plasma, we developed a gas chromatographic—mass spectrometric (GC—MS) method in which [¹⁵N]nitrite and [¹⁵N]nitrate were used as internal standards. Endogenous nitrite and [¹⁵N]nitrite added to acetone-treated plasma and urine samples were converted into their pentafluorobenzyl (PFB) derivatives using PFB bromide as the alkylating agent. For the analysis of endogenous nitrate and [¹⁵N]nitrate they were reduced to nitrite and [¹⁵N]nitrite, respectively, by cadmium in acidified plasma and urine samples prior to PFB alkylation. Reaction products were extracted with toluene and 1-μl aliquots were analyzed by selected-ion monitoring at m/z 46 for endogenous nitrite (nitrate) and m/z 47 for [¹⁵N]nitrite ([¹⁵N]nitrate). The intra- and inter-assay relative standard deviations for the determination of nitrite and nitrate in urine and plasma were below 3.8%. The detection limit of the method was 22 fmol of nitrite. Healthy subjects (n = 12) excreted into urine 0.49 ± 0.25 of nitrite and 109.5 ± 61.7 of nitrate (mean ± S.D., μmol/mmol creatinine) with a mean 24-h output of 5.7 μmol for nitrite and 1226 μmol for nitrate. The concentrations of nitrite and nitrate in the plasma of these volunteers were determined to be (mean ± S.D., μmol/l) 3.6 ± 0.8 and 68 ± 17, respectively.     

Review of the genotoxicity of nitrogen oxides

Nitrogen oxides (NO_x) are formed in combustion processes and are major pollutants in urban air. Relatively few studies on the genotoxicity of NO₂ and NO have been performed. These studies indicate that NO₂ is genotoxic in vitro, but the effect of NO seems to be very slight.One in vivo study showed chromosome aberrations and mutations in lung cells after inhalation of NO₂ (and NO), but tests for chromosome aberrations in lymphocytes and spermatocytes or micronuclei in bone marrow were negative after inhalation of NO₂. Based on present studies, there is no clear evidence of a carcinogenic potential of NO₂, although lung adenomas were induced in the susceptible strain A/J mouse.The primary metabolites of NO_x are nitrite and nitrate. Nitrate seems to be devoid of genotoxic properties, but nitrite is genotoxic in vitro, and there are also positive in vivo results. Cancer studies have been mainly negative. However, carcinogenic nitrosamines have been shown to be formed in vivo after inhalation of NO₂.Nitrogen oxides are key components in atomospheric smog formation, which may lead to secondary effects. Strongly mutagenic nitro-PAH compounds are easily formed, and mutagenic reaction products may be formed photochemically from alkenes.     

Ethyl nitrite is produced in the human stomach from dietary nitrate and ethanol,releasing nitric oxide at physiological pH: potential impact on gastric motility

Background. Nitric oxide (^∙NO), a ubiquitous molecule involved in a plethora of signaling pathways, is produced from dietary nitrate in the gut through the so-called nitrate–nitrite–NO pathway. In the stomach, nitrite derived from dietary nitrate triggers a network of chemical reactions targeting endogenous and exogenous biomolecules, thereby producing new compounds with physiological activity.Objective. The aim of this study was to ascertain whether compounds with physiological relevance are produced in the stomach upon consumption of nitrate- and ethanol-rich foods.Design. Human volunteers consumed a serving of lettuce (source of nitrate) and alcoholic beverages (source of ethanol). After 15 min, samples of the gastric headspace were collected and ethyl nitrite was identified by GC–MS. Wistar rats were used to study the impact of ethyl nitrite on gastric smooth muscle relaxation at physiological pH.Result. Nitrogen oxides, produced from nitrite in the stomach, induce nitrosation of ethanol from alcoholic beverages in the human stomach yielding ethyl nitrite. Ethyl nitrite, a potent vasodilator, is produced in vivo upon the consumption of lettuce with either red wine or whisky. Moreover, at physiological pH, ethyl nitrite induces gastric smooth muscle relaxation through a cGMP-dependent pathway. Overall, these results suggest that ethyl nitrite is produced in the gastric lumen and releases ^∙NO at physiological pH, which ultimately may have an impact on gastric motility. Systemic effects may also be expected if ethyl nitrite diffuses through the gastric mucosa reaching blood vessels, therefore operating as a ^∙NO carrier throughout the body.Conclusion. These data pinpoint posttranslational modifications as an underappreciated mechanism for the production of novel molecules with physiological impact locally in the gut and highlight the notion that diet may fuel compounds with the potential to modulate gastrointestinal welfare.     

Control of microbial souring by nitrate,nitrite or glutaraldehyde injection in a sandstone column

Microbial souring (production of hydrogen sulfide by sulfate-reducing bacteria, SRB) in crushed Berea sandstone columns with oil field-produced water consortia incubated at 60°C was inhibited by the addition of nitrate (NO₃) or nitrite (NO ₂ ^– ). Added nitrate (as nitrogen) at a concentration of 0.71 mM resulted in the production of 0.57–0.71 mM nitrite by the native microbial population present during souring and suppressed sulfate reduction to below detection limits. Nitrate added at 0.36 mM did not inhibit active souring but was enough to maintain inhibition if the column had been previously treated with 0.71 mM or greater. Continuous addition of 0.71–0.86 mM nitrite also completely inhibited souring in the column. Pulses of nitrite were more effective than the same amount of nitrite added continuously. Nitrite was more effective at inhibiting souring than was glutaraldehyde, and SRB recovery was delayed longer with nitrite than with glutaraldehyde. It was hypothesized that glutaraldehyde killed SRB while nitrite provided a long-term inhibition without cell death. Removal of nitrate after as long as 3 months of continuous addition allowed SRB in a biofilm to return to their previous level of activity. Inhibition was achieved with much lower levels of nitrate and nitrite, and at higher temperatures, than noted by other researchers.     

Uptake pathway and continuous removal of nitric oxide from flue gas using microalgae

Nitric oxide (NO), a major constituent of NO_x in fossil fuel flue gas, can be removed by the microalga, Dunaliella tertiolecta, in a bubble-column-type bioreactor. The uptake pathway of NO was investigated, and it was found that little NO was oxidized in the medium before its uptake by algal cells and that NO mostly permeated directly into the cells by diffusion based on the mass balance of nitrogen and the change in nitrate and nitrite concentration in the medium in batch culture. For further application of this system, it is necessary to remove NO over a long duration, and the stability of NO removal is important. NO removal rate of about 50–60% could be maintained stably for 15 days in continuous culture under the light condition. Because the consumption of nitrate was reduced by the amount of taken NO, NO rather than nitrate is preferentially utilized as a nitrogen source for cell growth. Therefore, this algal system is useful for continuous NO removal and production of algal biomass using NO as a nitrogen source.     

Measurement of nitrite and nitrate levels in biological samples by capillary electrophoresis

Nitrite is one of the products of NO-synthase in biological media. It is slowly oxidized in animals to nitrate. We developed a simple and rapid method to determine simultaneously nitrite and nitrate in biological samples. Capillary ion electrophoresis with direct UV detection at 214 nm was used employing a carrier electrolyte consisting of 10 mM sodium sulfate and an electroosmotic flow modifier. The detection limit in ultrafilates of plasma, urine and brain tissue extracts was 25 ng/ml for both compounds. Nitrate levels in human plasma and urine were in the μg/ml range. Nitrite could not be detected. Rat brain tissue extracts contained detectable amounts of nitrite and nitrate.     

Nitrite-induced methemoglobinemia in Nile tilapia, Oreochromis niloticus

Exposure of Nile tilapia, Oreochromis niloticus (mean weight, 55.72 ± 4.30 g), to two sublethal NO₂–N concentrations was studied for 24 and 48 h in a static test. In nitrite exposure tests, the percentages of methemoglobin, external nitrite, plasma nitrite, hemoglobin and hematocrit were assessed. Nitrite exposure in the range of 0.50 and 1.38 mg l⁻¹ NO₂–N caused an increase in methemoglobin levels; however, methemoglobin percentages ranging from 16% to 42% represented a mild methemoglobinemia. Levels of methemoglobin were unrelated to environmental and plasmatic nitrite concentrations. The nitrite concentration in the blood did not seem to be linked to time of exposure. Nitrite exposure in Nile tilapia was associated with a marked reduction in hemoglobin and hematocrit.     

Dietary inorganic nitrate mobilizes circulating angiogenic cells

Nitric oxide (NO) was implicated in the regulation of mobilization and function of circulating angiogenic cells (CACs). The supposedly inert anion nitrate, abundant in vegetables, can be stepwise reduced in vivo to form nitrite, and consecutively NO, representing an alternative to endogenous NO formation by NO synthases. This study investigated whether inorganic dietary nitrate influences mobilization of CACs. In a randomized double-blind fashion, healthy volunteers ingested 150 ml water with 0.15 mmol/kg (12.7 mg/kg) of sodium nitrate, an amount corresponding to 100-300 g of a nitrate-rich vegetable, or water alone as control. Mobilization of CACs was determined by the number of CD34(+)/KDR(+) and CD133(+)/KDR(+) cells using flow cytometry and the mobilization markers stem cell factor (SCF) and stromal cell-derived factor-1a (SDF-1α) were determined in plasma via ELISA. Nitrite and nitrate were measured using high-performance liquid chromatography and reductive gas-phase chemiluminescence, respectively. NOS-dependent vasodilation was measured as flow-mediated vasodilation. Further mechanistic studies were performed in mice after intravenous application of nitrite together with an NO scavenger to identify the role of nitrite and NO in CAC mobilization. Nitrate ingestion led to a rise in plasma nitrite together with an acute increase in CD34(+)/KDR(+) and CD133(+)/KDR(+)-CACs along with increased NOS-dependent vasodilation. This was paralleled by an increase in SCF and SDF-1α and the maximal increase in plasma nitrite correlated with CD133(+)/KDR(+)-CACs (r=0.73, P=0.016). In mice, nitrate given per gavage and direct intravenous injection of nitrite led to CAC mobilization, which was abolished by the NO scavenger cPTIO, suggesting that nitrite mediated its effect via formation of NO. Dietary inorganic nitrate acutely mobilizes CACs via serial reduction to nitrite and NO. The nitrate-nitrite-NO pathway could offer a novel nutritional approach for regulation of vascular regenerative processes.     

Nitrate (NO3) and nitrite (NO2) are endocrine disruptors to downregulate expression of tyrosine hydroxylase and motor behavior through conversion to nitric oxide in early development of zebrafish

With a view to consider the increasing concern over nitrogen pollution in the aquatic environment, we investigated effects of nitrate (NO₃⁻) and nitrite (NO₂⁻) on the activity of dopaminergic neuron in zebrafish embryos and larvae. Both nitrate and nitrite exposure decreased the expression of tyrosine hydroxylase (TH) in dopaminergic neurons at 48 hpf. Only nitrite decreased the response to tactile stimulation at 72 hpf, whereas both nitrate and nitrite decreased the swimming activity at 6 dpf. When the embryos were exposed to nitrate or nitrite together with an estrogen receptor blocker (ICI 182,780), the decreases in TH expression and motor behavior caused by nitrate or nitrite alone were reversed suggesting the effects of nitrate and nitrite were mediated through estrogen receptor (ER). The result of co-incubation with an oxidoreductase inhibitor, diphenyleneiodonium, indicated the conversion to nitric oxide (NO) is likely to be responsible for the effects of nitrate and nitrite, which was further supported by the increased staining for NO after exposure. The present study demonstrates that nitrate and nitrite are neurotoxicants acting as an endocrine disruptor possibly through conversion to NO to downregulate the activity of dopaminergic neuron in early development of zebrafish.     

Nitrite and nitrate-dependent generation of anti-inflammatory fatty acid nitroalkenes

A gap in our understanding of the beneficial systemic responses to dietary constituents nitrate (NO₃⁻), nitrite (NO₂⁻) and conjugated linoleic acid (cLA) is the identification of the downstream metabolites that mediate their actions. To examine these reactions in a clinical context, investigational drug preparations of ¹⁵N-labeled NO₃⁻ and NO₂⁻ were orally administered to healthy humans with and without cLA. Mass spectrometry analysis of plasma and urine indicated that the nitrating species nitrogen dioxide was formed and reacted with the olefinic carbons of unsaturated fatty acids to yield the electrophilic fatty acid, nitro-cLA (NO₂-cLA). These species mediate the post-translational modification (PTM) of proteins via reversible Michael addition with nucleophilic amino acids. The PTM of critical target proteins by electrophilic lipids has been described as a sensing mechanism that regulates adaptive cellular responses, but little is known about the endogenous generation of fatty acid nitroalkenes and their metabolites. We report that healthy humans consuming ¹⁵N-labeled NO₃⁻ or NO₂⁻, with and without cLA supplementation, produce ¹⁵NO₂-cLA and corresponding metabolites that are detected in plasma and urine. These data support that the dietary constituents NO₃⁻, NO₂^- and cLA promote the further generation of secondary electrophilic lipid products that are absorbed into the circulation at concentrations sufficient to exert systemic effects before being catabolized or excreted.