Nitrite Consumption Research Articles

Dietary nitrate and nitrite intake and risk of non-Hodgkin lymphoma

Although established risk factors such as immunodeficiency and viral infections may be responsible for a portion of cases of non-Hodgkin lymphoma (NHL), the vast majority of cases of NHL remain unexplained. The role of dietary nitrate and nitrite in NHL risk is of interest since they are precursors of N-nitroso compounds, and nitrosoureas have been shown to induce B- and T-cell lymphomas in animal studies. However, few studies have evaluated the potential association between consumption of nitrate and nitrite and NHL by subtype or chromosomal translocation status, and the results of these studies have been inconsistent. We estimated the dietary intake of nitrate and nitrite using a food frequency questionnaire in a population-based, case–control study of 348 cases and 470 controls conducted in Nebraska in 1999–2002. A non-significant excess risk of NHL was found among women who reported an intake of nitrite in the highest quartile compared to the lowest quartile (odds ratio [OR] = 1.6; 95% confidence interval [CI]: 0.8–2.9), particularly nitrite from animal sources (OR = 1.9; 95% CI: 1.0–3.4). No significant associations were observed for nitrate or nitrite by NHL subtype. Although there were some increases in risk that support the N-nitroso hypothesis, they were not significant and do not confer strong evidence of an association.

Effect of storage on levels of nitric oxide metabolites in platelet preparations

Nitric oxide (NO), a potent signaling molecule, is known to inhibit platelet (PLT) function in vivo. We investigated how the levels of NO and its metabolites change during routine PLT storage. We also tested whether the material of PLT storage containers affects nitrite content since many plastic materials are known to contain and release nitrite. For nitrite and nitrate measurement, leukoreduced apheresis PLTs and concurrent plasma (CP) were collected from healthy donors using a cell separator. Sixty-milliliter aliquots of PLT or CP were stored in CLX or PL120 Teflon containers at 20 to 24°C with agitation and daily samples were processed to yield PLT pellet and supernatant. In a separate experiment, PLTs were stored in PL120 Teflon to measure NO generation using electron paramagnetic resonance (EPR). Nitrite level increased markedly in both PLT supernatant and CP stored in CLX containers at a rate of 58 and 31 nmol/L/day, respectively. However, there was a decrease in nitrite level in PLTs stored in PL120 Teflon containers. Nitrite was found to leach from CLX containers and this appears to compensate for nitrite consumption in these preparations. Nitrate level did not significantly change during storage. PLTs stored at 20 to 24°C maintain measurable levels of nitrite and nitrate. The nitrite decline in nonleachable Teflon containers in contrast to increases in CLX containers that leach nitrite suggests that it is consumed by PLTs, residual white blood cells, or red blood cells. These results suggest NO-related metabolic changes occur in PLT units during storage.

Enrichment of denitrifying methanotrophic bacteria for application after direct low-temperature anaerobic sewage treatment

Despite many advantages of anaerobic sewage treatment over conventional activated sludge treatment, it has not yet been applied in temperate zones. This is especially because effluent from low-temperature anaerobic treatment contains nitrogen and dissolved methane. The presence of nitrogen and methane offers the opportunity to develop a reactor in which methane is used as electron donor for denitrification. Such a reactor could be used in a new concept for low-temperature anaerobic sewage treatment, consisting of a UASB-digester system, a reactor for denitrification coupled to anaerobic methane oxidation, and a nitritation reactor. In the present study denitrifying methanotrophic bacteria similar to ‘Candidatus Methylomirabilis oxyfera’ were enriched. Maximum volumetric nitrite consumption rates were 33.5mgNO2−-N/Ld (using synthetic medium) and 37.8mgNO2−-N/Ld (using medium containing effluent from a sewage treatment plant), which are similar to the maximum rate reported so far. Though the goal was to increase the rates, in both reactors, after reaching these maximum rates, volumetric nitrite consumption rates decreased in time. Results indicate biomass washout may have significantly decelerated enrichment. Therefore, to obtain higher volumetric consumption rates, further research should focus on systems with complete biomass retention.

Human safety controversies surrounding nitrate and nitrite in the diet

Nitrate and nitrite are part of the human diet as nutrients in many vegetables and part of food preservation systems. In the 1950s and 1960s the potential for formation of nitrosamines in food was discovered and it ignited a debate about the safety of ingested nitrite which ultimately focused on cured meats. Nitrate impurities in salt used in the drying of meat in ancient times resulted in improved protection from spoilage during storage. This evolved into their deliberate modern use as curing ingredient responsible for ‘fixing’ the characteristic color associated with cured meats, creating a unique flavor profile, controlling the oxidation of lipids, and serving as an effective antimicrobial. Several critical reports and comprehensive reviews reporting weak associations and equivocal evidence of nitrite human health safety have fostered concerns and debate among scientists, regulators, press, consumer groups, and consumers. Despite periodic controversy regarding human health concerns from nitrite consumption, a building base of scientific evidence about nitrate, nitrite, heme chemistry, and the overall metabolism of nitrogen oxides in humans has and continues to affirm the general safety of nitrate/nitrite in human health. As nitrite based therapeutics emerge, it is important to consider the past controversies and also understand the beneficial role in the human diet.

Oxygen Regulates Tissue Nitrite Metabolism

Once dismissed as an inert byproduct of nitric oxide (NO) auto-oxidation, nitrite (NO(2)(-)) is now accepted as an endocrine reservoir of NO that elicits biological responses in major organs. While it is known that tissue nitrite is derived from NO oxidation and the diet, little is known about how nitrite is metabolized by tissue, particularly at intermediate oxygen tensions. We investigated the rates and mechanisms of tissue nitrite metabolism over a range of oxygen concentrations. We show that the rate of nitrite consumption differs in each organ. Further, oxygen regulates the rate and products of nitrite metabolism. In anoxia, nitrite is reduced to NO, with significant formation of iron-nitrosyl proteins and S-nitrosothiols. This hypoxic nitrite metabolism is mediated by different nitrite reductases in each tissue. In contrast, low concentrations (∼3.5 μM) of oxygen increase the rate of nitrite consumption by shifting nitrite metabolism to oxidative pathways, yielding nitrate. While cytochrome P(450) and myoglobin contribute in the liver and heart, respectively, mitochondrial cytochrome c oxidase plays a significant role in nitrite oxidation, which is inhibited by cyanide. Using cyanide to prevent artifactual nitrite decay, we measure metabolism of oral and intraperitoneally administered nitrite in mice. These data provide insight into the fate of nitrite in tissue, the enzymes involved in nitrite metabolism, and the role of oxygen in regulating these processes. We demonstrate that even at low concentrations, oxygen is a potent regulator of the rate and products of tissue nitrite metabolism.

Nitrite Oxidase Activities of Cytochrome P450 and Mitochondria

Abstract 5310Once dismissed as an inert byproduct of nitric oxide (NO) auto-oxidation, nitrite (NO2−) is now accepted as an endocrine reserve of NO that elicits a number of fundamental biological responses in all major organ systems. While it is known that tissue nitrite is derived from both oxidation of NO and from dietary nitrite and nitrate, much less is known about how nitrite is metabolized by tissue or about the factors that influence this metabolism. Here we investigate the rates and mechanisms by which nitrite is metabolized by tissue over a range of oxygen tensions in rats and mice. We show that the rate of nitrite metabolism differs in heart, liver, lung and brain tissue. Further, oxygen regulates the rate and products of nitrite metabolism in each of these tissues. In hypoxic tissue, nitrite is predominantly reduced to NO, with significant formation of iron-nitrosyl heme proteins and S-nitrosothiols. Interestingly, this hypoxic nitrite metabolism is mediated by different sets of nitrite reductase enzymes in each tissue. In contrast, tissue consumption of nitrite is more rapid in normoxia and the major end product is nitrate. While cytochrome P450s and myoglobin contributed in the liver and heart respectively, mitochondrial cytochrome c oxidase played a significant role in this normoxic nitrite oxidation, which could be completely inhibited by cyanide in all tissues. We used cyanide-based nitrite preservation solution to measure the pharmacokinetics of oral and intraperitoneally administered nitrite in vivo. Using this methodology, we measured basal levels of nitrite in the major tissues and confirm that the heart contains the greatest concentration of nitrite, followed by the liver and finally the lung. We demonstrate that intraperitoneal administration of nitrite to mice increases nitrite levels most significantly in the liver and heart, where nitrite uptake is rapid (5–10 min) and steadily decreases thereafter, such that levels are back to baseline by 30 min. Little to no increase was observed in the lung. In these studies, changes in nitrate were difficult to detect due to the high levels of basal nitrate present in vivo and low concentration of nitrite administered. However, the rapid metabolism of nitrite in the tissue suggests that oxidation is at least partially responsible. In contrast to intraperitoneal nitrite, oral nitrite increased nitrite levels in all organs of mice, an effect which peaked in all organs except liver at 3 days. Collectively, these data provide insight into the fate of nitrite in tissue, the enzymes involved in hypoxic and normoxic nitrite metabolism and the role of oxygen in regulating these processes. Disclosures:No relevant conflicts of interest to declare.

Effect of nitrate and nitrite on the selection of microorganisms in the denitrifying anaerobic methane oxidation process

Two cultures were inoculated with sludges taken from a parent culture containing archaea distantly related to anaerobic methanotrophic archaea (ANME) and bacteria related to Candidatus Methylomirabilis oxyfera, both of which have previously been found in cultures performing denitrifying anaerobic methane oxidation process. The cultures were fed with nitrate and nitrite, respectively, along with methane. The nitrate-fed culture, Culture B, showed a stable microbial community composition and denitrifying anaerobic methane oxidation activity. In contrast, the nitrite-fed culture, Culture A, achieved a substantial increase in the nitrite consumption rate, from 1.1 to 7.3 mmol NO2 (-) -N (g VSS)(-1) day(-1) in 90 days. Concurrent with this activity increase, the archaeal population in Culture A decreased with time, and became undetectable after 100 days, while bacteria related to M. oxyfera increasingly dominated the culture. This observation suggests that the bacteria related to M. oxyfera are more competitive for nitrite reduction compared with the archaea related to ANME. This study showed that nitrate or nitrite feeding has a significant impact on the ecology and activities of microorganisms in the denitrifying anaerobic methane oxidation process. This study also revealed that nitrite overloading may have a toxic effect on the bacteria related to M. oxyfera.

2-Chlorophenol consumption and its effect on the nitrifying sludge

The kinetic behavior of a nitrifying sludge exposed to 2-chlorophenol (2-CP) was evaluated in batch culture. The assays were performed using a stabilized nitrifying sludge. In control assays with (mg L −1): NH 4 +-N (100) and NaHCO 3 −-C (250), the substrates were consumed in 8 h, the ammonium consumption efficiency was 99% and the NO 3 − yield higher than 0.9. When 5 mg 2-CP-C L −1 was added, it was transformed into an unidentified intermediate and the nitrifying efficiency decreased to 10%. Ammonium specific consumption rate diminished 95%, but the NO 3 − yield remained higher than 0.9. The biomass previously exposed to 2-CP was newly suspended with NH 4 +-N or NO 2 −-N in order to evaluate the ammonium and nitrite oxidizing processes. The consumption efficiencies and NO 3 − yields were similar to those obtained in control assays. However, the total time required for ammonium and nitrite consumption increased to 120 and 42 h, respectively. Specific consumption rates for NH 4 +-N and NO 2 −-N decreased by 95% and 83% respectively, compared to control assays. Thus, the previous contact to 2-CP had more influence on ammonium oxidizing process than the nitrite oxidizing process. These are the first evidences where a nitrifying sludge exposed to 2-CP are reported.

Erythrocyte-dependent regulation of human skeletal muscle blood flow: role of varied oxyhemoglobin and exercise on nitrite, S-nitrosohemoglobin, and ATP

The erythrocyte is proposed to play a key role in the control of local tissue perfusion via three O(2)-dependent signaling mechanisms: 1) reduction of circulating nitrite to vasoactive NO, 2) S-nitrosohemoglobin (SNO-Hb)-dependent vasodilatation, and 3) release of the vasodilator and sympatholytic ATP; however, their relative roles in vivo remain unclear. Here we evaluated each mechanism to gain insight into their roles in the regulation of human skeletal muscle blood flow during hypoxia and hyperoxia at rest and during exercise. Arterial and femoral venous hemoglobin O(2) saturation (O(2)Hb), plasma and erythrocyte NO and ATP metabolites, and leg and systemic hemodynamics were measured in 10 healthy males exposed to graded hypoxia, normoxia, and graded hyperoxia both at rest and during submaximal one-legged knee-extensor exercise. At rest, leg blood flow and NO and ATP metabolites in plasma and erythrocytes remained unchanged despite large alterations in O(2)Hb. During exercise, however, leg and systemic perfusion and vascular conductance increased in direct proportion to decreases in arterial and venous O(2)Hb (r(2) = 0.86-0.98; P = 0.01), decreases in venous plasma nitrite (r(2) = 0.93; P < 0.01), increases in venous erythrocyte nitroso species (r(2) = 0.74; P < 0.05), and to a lesser extent increases in erythrocyte SNO (r(2) = 0.59; P = 0.07). No relationship was observed with plasma ATP (r(2) = 0.01; P = 0.99) or its degradation compounds. These in vivo data indicate that, during low-intensity exercise and hypoxic stress, but not hypoxic stress alone, plasma nitrite consumption and formation of erythrocyte nitroso species are associated with limb vasodilatation and increased blood flow in the human skeletal muscle vasculature.

Syntrophy of aerobic and anaerobic ammonia oxidisers

Deammonification is known as an efficient and resource saving sidestream process option to remove the nitrogen load from sludge liquors. The transfer of the intermediate product nitrite between both syntrophic groups of organisms - aerobic and anaerobic ammonia oxidizers (AOB) - appears very sensitive to process conditions such as temperature, dissolved oxygen (DO) and operating nitrite level. Growth kinetics for aerobic and anaerobic AOBs differ by one order of magnitude and require an adequate selection of sludge retention time. This paper provides measurement- and model-based results on how selected sludge wasting impacts population dynamics in a suspended growth deammonification system. Anammox enrichment up to a doubled portion in mixed liquor solids can substantially improve process stability in difficult conditions. A case-study on low temperature operations outlines two possible strategies to balance syntrophic consumption of ammonium and nitrite.

Artery-to-vein differences in nitric oxide metabolites are diminished in sepsis*

Nitric oxide deficiency may contribute to microvascular dysfunction in sepsis. Current physiologic paradigms contend that nitrite and/or S-nitrosohemoglobin mediate intravascular delivery of nitric oxide. These nitric oxide metabolites are purportedly consumed during hemoglobin deoxygenation, producing nitric oxide and coupling intravascular nitric oxide delivery with metabolic demand. Systemic nitrite and S-nitrosohemoglobin consumption can be assessed by comparing their concentrations in arterial vs. venous blood. We hypothesized that arterial vs. venous differences in nitrite and S-nitrosohemoglobin are diminished in sepsis and associated with mortality. Case-control and prospective cohort study. Adult intensive care units of an academic medical center. Eighty-seven critically ill septic patients and 52 control subjects. None. Nitrite and S-nitrosohemoglobin were measured using tri-iodide-based reductive chemiluminescence. In control subjects, arterial plasma, whole blood, and red blood cell nitrite levels were higher than the corresponding venous levels. In contrast, S-nitrosohemoglobin was higher in venous compared to arterial blood. In septic patients, arterial vs. venous red blood cell nitrite and S-nitrosohemoglobin differences were absent. Furthermore, the plasma nitrite arterial vs. venous difference was absent in nonsurvivors. In health, nitrite levels are higher in arterial vs. venous blood (suggesting systemic nitrite consumption), whereas S-nitrosohemoglobin levels are higher in venous vs. arterial blood (suggesting systemic S-nitrosohemoglobin production). These arterial vs. venous differences are diminished in sepsis, and diminished arterial vs. venous plasma nitrite differences are associated with mortality. These data suggest pathologic disruption of systemic nitrite utilization in sepsis.

Comparative analysis of nitrite uptake and hemoglobin-nitrite reactions in erythrocytes: sorting out uptake mechanisms and oxygenation dependencies

Nitrite uptake into red blood cells (RBCs) precedes its intracellular reactions with hemoglobin (Hb) that forms nitric oxide (NO) during hypoxia. We investigated the uptake of nitrite and its reactions with Hb at different oxygen saturations (So(2)), using RBCs with (carp and rabbit) and without (hagfish and lamprey) anion exchanger-1 (AE1) in the membrane, with the aim to unravel the mechanisms and oxygenation dependencies of nitrite transport. Added nitrite rapidly diffused into the RBCs until equilibrium. The distribution ratio of nitrite across the membrane agreed with that expected from HNO(2) diffusion and AE1-mediated facilitated NO(2)(-) diffusion. Participation of HNO(2) diffusion was emphasized by rapid transmembrane nitrite equilibration also in the natural AE1 knockouts. Following the equilibration, nitrite was consumed by reacting with Hb, which created a continued inward diffusion controlled by intracellular reaction rates. Changes in nitrite uptake with So(2), pH, or species were accordingly explained by corresponding changes in reaction rates. In carp, nitrite uptake rates increased linearly with decreasing So(2) over the entire So(2) range. In rabbit, nitrite uptake rates were highest at intermediate So(2), producing a bell-shaped relationship with So(2). Nitrite consumption increased approximately 10-fold with a 1 unit decrease in pH, as expected from the involvement of protons in the reactions with Hb. The reaction of nitrite with deoxyhemoglobin was favored over that with oxyhemoglobin at intermediate So(2). We propose a model for RBC nitrite uptake that involves both HNO(2) diffusion and AE1-mediated transport and that explains both the present and previous (sometimes puzzling) results.

Diffusion of nitric oxide through the gastric wall upon reduction of nitrite by red wine: Physiological impact

In this work we showed that nitric oxide produced via red wine- and ascorbate-dependent reduction of nitrite diffuses through the rat stomach, inducing smooth muscle relaxation. The studies encompassed ex vivo and in vivo models of diffusion. Regarding the former, luminal NO generated from a mixture of physiologic nitrite and ascorbate or wine diffuses across the stomach wall, being 8–20% of that produced in the mucosal side detected at high μM range (>100 μM) in the serosal side. In order to evaluate whether cellular dysfunction was associated with NO diffusion at the μM range, the gastric tissue exposed to NO was evaluated in terms of carbachol-induced muscle contraction in fundal strips and mitochondrial respiration and showed to remain functional and metabolically active. Moreover, pre-contracted gastric strips were shown to relax 86.5 ± 5.5% (control) and 75.0 ± 4.0% (nitrite/ascorbate-exposed tissue) when challenged with acidified nitrite. The studies in the living animal support the diffusion of luminal NO to the gastric vasculature as, following addition of nitrite/ascorbate to rat stomach in vivo, NO was not detected in the serosal environment but concentrations as high as 31 μM of NO were detected outside the stomach after cardiac arrest. Collectively, the results establish a link between the consumption of nitrite and dietary reductants (e.g., wine polyphenols) and stomach muscle relaxation via the local chemical generation of NO.

Aggregate Size and Architecture Determine Microbial Activity Balance for One-Stage Partial Nitritation and Anammox

Aerobic ammonium-oxidizing bacteria (AerAOB) and anoxic ammonium-oxidizing bacteria (AnAOB) cooperate in partial nitritation/anammox systems to remove ammonium from wastewater. In this process, large granular microbial aggregates enhance the performance, but little is known about granulation so far. In this study, three suspended-growth oxygen-limited autotrophic nitrification-denitrification (OLAND) reactors with different inoculation and operation (mixing and aeration) conditions, designated reactors A, B, and C, were used. The test objectives were (i) to quantify the AerAOB and AnAOB abundance and the activity balance for the different aggregate sizes and (ii) to relate aggregate morphology, size distribution, and architecture putatively to the inoculation and operation of the three reactors. A nitrite accumulation rate ratio (NARR) was defined as the net aerobic nitrite production rate divided by the anoxic nitrite consumption rate. The smallest reactor A, B, and C aggregates were nitrite sources (NARR, >1.7). Large reactor A and C aggregates were granules capable of autonomous nitrogen removal (NARR, 0.6 to 1.1) with internal AnAOB zones surrounded by an AerAOB rim. Around 50% of the autotrophic space in these granules consisted of AerAOB- and AnAOB-specific extracellular polymeric substances. Large reactor B aggregates were thin film-like nitrite sinks (NARR, <0.5) in which AnAOB were not shielded by an AerAOB layer. Voids and channels occupied 13 to 17% of the anoxic zone of AnAOB-rich aggregates (reactors B and C). The hypothesized granulation pathways include granule replication by division and budding and are driven by growth and/or decay based on species-specific physiology and by hydrodynamic shear and mixing.

Food sources of nitrates and nitrites: the physiologic context for potential health benefits

Thepresence of nitrates and nitrites in food is associated with an increased risk of gastrointestinal cancer and, in infants, methemoglobinemia. Despite the physiologic roles for nitrate and nitrite in vascular and immune function, consideration of food sources of nitrates and nitrites as healthful dietary components has received little attention. Approximately 80% of dietary nitrates are derived from vegetable consumption; sources of nitrites include vegetables, fruit, and processed meats. Nitrites are produced endogenously through the oxidation of nitric oxide and through a reduction of nitrate by commensal bacteria in the mouth and gastrointestinal tract. As such, the dietary provision of nitrates and nitrites from vegetables and fruit may contribute to the blood pressure–lowering effects of the Dietary Approaches to Stop Hypertension (DASH) diet. We quantified nitrate and nitrite concentrations by HPLC in a convenience sample of foods. Incorporating these values into 2 hypothetical dietary patterns that emphasize high-nitrate or low-nitrate vegetable and fruit choices based on the DASH diet, we found that nitrate concentrations in these 2 patterns vary from 174 to 1222 mg. The hypothetical high-nitrate DASH diet pattern exceeds the World Health Organization’s Acceptable Daily Intake for nitrate by 550% for a 60-kg adult. These data call into question the rationale for recommendations to limit nitrate and nitrite consumption from plant foods; a comprehensive reevaluation of the health effects of food sources of nitrates and nitrites is appropriate. The strength of the evidence linking the consumption of nitrate- and nitrite-containing plant foods to beneficial health effects supports the consideration of these compounds as nutrients.

Increased Oxidative-nitrative Stress In The Hypoxic Human Brain

Oxidative stress may be responsible for the reduction in systemic nitric oxide (NO) bioavailability and impaired neurovascular reactivity recently observed in hypoxia (DM Bailey et al., J. Physiol. in-the-press). However, the brain's contribution to these systemic changes in redox homeostasis remains unknown. PURPOSE: The present study determined the trans-cerebral exchange kinetics of oxidative-nitrative stress biomarkers in response to hypoxia. Specifically, we hypothesized that hypoxia would increase the net cerebral output of free radicals that would inactivate NO as indicated by a decrease in the net uptake or consumption of nitrite (NO2) and increased output of 3-nitrotyrosine (3-NT). METHODS: Ten healthy males aged 27 (mean) ± 4 (SD) years were examined in normoxia (N: 20.9%O2) and following 9h passive exposure to hypoxia (H: 12.9%O2). Global cerebral blood flow (CBF) was measured using the Kety-Schmidt technique with paired samples obtained from the radial artery and jugular venous bulb. Global cerebral plasma flow (CPF) was determined as CBF × (1-hematocrit). The serum concentration of spin-trapped a-phenyl-tert-butylnitrone (PBN)-adducts was assessed via X-band electron paramagnetic resonance spectroscopy. Plasma NO2 was determined by ozone-based chemiluminescence using modified tri-iodide reagent and 3-NT via ELISA. Trans-cerebral net exchange was calculated as the arterio-jugular venous concentration difference x CPF. Data were analyzed with a two-way repeated measures ANOVA and post-hoc Bonferroni-corrected paired samples t-tests. RESULTS: Despite a marked reduction in PaO2 (N: 107 ± 6 to H: 46 ± 3 mmHg, P < 0.05), the cerebral metabolic rate for O2 was preserved (N: 2.4 ± 0.5 vs.H: 2.5 ± 0.3 μmol/g/min) due to a trend towards increased CBF (N: 85 ± 15 to H: 94 ± 17 mL/100g/min). Hypoxia increased the net cerebral output of PBN-adducts from -73 ± 192 to -360 ± 253 AU/min/g (P < 0.05). This was associated with an attenuation in the net uptake of NO2 (N: 126.4 ± 93.9 to H: 16.0 ± 46.7 nmol/min/g, P < 0.05) and increased output of 3-NT (N: -3.1 ± 9.0 to H: -10.7 ± 18.2 nmol/min/g, P < 0.05). CONCLUSION: These findings provide direct evidence for increased oxidative-nitrative stress across the hypoxic human brain and suggest that neurovascular NO bioavailability may be subject to redox-regulation.

Diet, Helicobacter pylori infection, food preservation and gastric cancer risk: are there new roles for preventative factors?

Risks for gastric cancer in relation to diet and other environmental factors are receiving renewed attention. New developments include the emerging relationship between Helicobacter pylori infection and an increased risk for gastric cancer, and positive results of chemoprevention trials in decreasing gastric cancer risk with the use of beta-carotene and vitamin E supplements. Factors that may enhance risk include consumption of nitrites, nitrates, alcohol, and highly salted, pickled, fermented, or smoked foods. Other environmental factors which may promote cancer are H. pylori infection, inappropriate food storage, metal and cement dust exposure, and cigarette smoking. High intakes of fruits and vegetables or of antioxidants, such as beta-carotene, vitamin E, and vitamin C may decrease risk.

Regulation of nitrite transport in red blood cells by hemoglobin oxygen fractional saturation

Allosteric regulation of nitrite reduction by deoxyhemoglobin has been proposed to mediate nitric oxide (NO) formation during hypoxia. Nitrite is predominantly an anion at physiological pH, raising questions about the mechanism by which it enters the red blood cell (RBC) and whether this is regulated and coupled to deoxyhemoglobin-mediated reduction. We tested the hypothesis that nitrite transport by RBCs is regulated by fractional saturation. Using human RBCs, nitrite consumption was faster at lower fractional saturations, consistent with faster reactions with deoxyheme. A membrane-based regulation was suggested by slower nitrite consumption with intact versus lysed RBCs. Interestingly, upon nitrite addition, intracellular nitrite concentrations attained a steady state that, despite increased rates of consumption, did not change with decreasing oxygen tensions, suggesting a deoxygenation-sensitive step that either increases nitrite import or decreases the rate of nitrite export. A role for anion exchanger (AE)-1 in the control of nitrite export was suggested by increased intracellular nitrite concentrations in RBCs treated with DIDS. Moreover, deoxygenation decreased steady-state levels of intracellular nitrite in AE-1-inhibited RBCs. Based on these data, we propose a model in which deoxyhemoglobin binding to AE-1 inhibits nitrite export under low oxygen tensions allowing for the coupling between deoxygenation and nitrite reduction to NO along the arterial-to-venous gradient.

Transcription factor NtcB specifically controls the nitrate assimilation genes in the marine cyanobacterium Synechococcus sp. strain PCC 7002

SUMMARYThe ntcA and ntcB genes, encoding cAMP‐receptor‐protein (CRP) type and LysR type transcriptional regulators, respectively, were isolated from the transformable marine cyanobacterium Synechococcus sp. strain PCC 7002 to study the inactivation of these genes by interposon mutagenesis. The ntcA gene could not be disrupted in this cyanobacterial strain, indicating that the ntcA gene is essential for cell growth on either urea or nitrate as the sole nitrogen source. The ntcB mutant cells grew very slowly on nitrate or nitrite as the sole nitrogen source but could grow on urea at the same rate as wild‐type cells. The consumption of nitrate and nitrite was barely detectable in the ntcB mutant cells, indicating that nitrate and nitrite use was impaired in the ntcB mutant. In wild‐type cells, mRNA levels for the three genes required for nitrate assimilation, nrtP (nitrate permease), narB (nitrate reductase) and nirA (nitrite reductase), increased when cells were shifted to nitrate growth conditions, but no increase in transcription of these genes occurred in the ntcB mutant. The mRNA level of ntcA was nearly identical in the wild‐type and ntcB mutant cells. These results demonstrate that NtcB specifically controls the transcription of the three genes for nitrate assimilation in this marine cyanobacterium.

Nitrite consumption in ischemic rat heart catalyzed by distinct blood-borne and tissue factors

Nitric oxide (NO) may limit myocardial ischemia-reperfusion injury by slowing the mitochondrial metabolism. We examined whether rat heart contains catalysts potentially capable of reducing nitrite to NO during an episode of regional myocardial ischemia produced by temporary coronary artery occlusion. In intact Sprague-Dawley rats, a 15-min coronary occlusion lowered the nitrite concentration of the myocardial regions exhibiting ischemic glucose metabolism to approximately 50% that of nonischemic regions (185 +/- 223 vs. 420 +/- 203 nmol/l). Nitrite was rapidly repleted during subsequent reperfusion. The heart tissue tested in vitro acquired a substantial ability to consume nitrite when made hypoxic at neutral pH, and this ability was slightly enhanced by simultaneously lowering the pH to 5.5. More than 70% of this activity could be abolished by flushing the coronary circulation with crystalloid to remove trapped erythrocytes. Correspondingly, erythrocytes demonstrated the ability to reduce exogenous nitrite to NO under hypoxic conditions in vitro. In erythrocyte-free heart tissue, the nitrite consumption increased fivefold when the pH was lowered to 5.5. Approximately 40% of this pH-sensitive increase in nitrite consumption could be blocked by the xanthine oxidoreductase inhibitor allopurinol, whereas lowering the Po(2) sufficiently to desaturate myoglobin accelerated it further. We conclude that rat heart contains several factors capable of catalyzing ischemic nitrite reduction; the most potent is contained within erythrocytes and activated by hypoxia, whereas the remainder includes xanthine oxidoreductase and other pH-sensitive factors endogenous to heart tissue, including deoxymyoglobin.