Nitric oxide formed by nitrite reductase of Paracoccus denitrificans is sufficiently stable to inhibit cytochrome oxidase activity and is reduced by its reductase under aerobic conditions

Nitric oxide and Nitrous oxide accumulation, oxygen production during nitrite denitrification in an anaerobic/anoxic sequencing batch reactor: exploring characteristics and mechanism

Nitrite denitrification has received increasing attention due to its high efficiency, low energy consumption, and sludge yield. However, the nitric oxide (NO) and nitrous oxide (N₂O) which are harmful to the environment, microorganisms, and humans are produced in this process. In order to mitigate NO and N₂O production, the biological mechanisms of NO and N₂O accumulation, as well as NO detoxification during nitrite denitrification in a sequencing batch reactor were studied. Results showed that the peak of NO accumulation increased from 0.29 [Formula: see text] 0.01 to 3.12 [Formula: see text] 0.34 mg L^-1 with the increase of carbon to nitrogen ratio (COD/N), which is caused by the sufficient electron donor supply for NO₂^--N reduction process at high COD/N. Furthermore, the result suggested that NO accumulation with no pH adjustment was 12 times higher than that with pH adjustment. It is related to the inhibition on NO reductase caused by the high free nitrous acid (FNA) and NO concentration with no pH adjustment. The pathways of NO detoxification included NO emission, reduction, and dismutation, and the more NO produced, the high proportion of NO dismutation pathway. Result showed that the maximum of oxygen production during NO dismutation reached to 1.39 mg L^-1. N₂O accumulation was mainly associated with FNA and NO inhibition, COD/N. The peak of N₂O accumulation presented a completely opposite trend at pH adjustment and no pH adjustment, it is because that the higher FNA and NO concentration at high COD/N without pH adjustment will inhibit the N₂O reductase activity, resulting in the N₂O reduction was hindered during nitrite denitrification.

Deciphering the biodesulfurization potential of two novel Rhodococcus isolates from a unique Greek environment

Sustainable biodesulfurization (BDS) processes require the use of microbial biocatalysts that display high activity against the recalcitrant heterocyclic sulfur compounds and can simultaneously withstand the harsh conditions of contact with petroleum products, inherent to any industrial biphasic BDS system. In this framework, the functional microbial BDS-related diversity in a naturally oil-exposed ecosystem, was examined through a 4,6-dimethyl-dibenzothiophene based enrichment process. Two new Rhodococcus sp. strains were isolated, which during a medium optimization process revealed a significantly enhanced BDS activity profile when compared to the model strain R. qingshengii IGTS8. In biocatalyst stability studies conducted in biphasic mode using partially hydrodesulfurized diesel under various process conditions, the new strains also presented an enhanced stability phenotype. In these studies, it was also demonstrated for all strains, that the BDS activity losses were decoupled from the overall cells' viability, in addition to the fact that the use of whole-broth biocatalyst positively affected BDS performance.

Flocs are the main source of nitrous oxide in a high-rate anammox granular sludge reactor: insights from metagenomics and fed-batch experiments

Nitrous oxide (N₂O) emissions from anammox-based processes are well documented but insight into source of the N₂O emission in high-rate anammox granular sludge reactors (AGSR) is limited. In this study, metagenomics and fed-batch experiments were applied to investigate the relative contributions of anammox granules and flocs to N₂O production in a high-rate AGSR. Flocs, which constitute only ~10% of total biomass contributed about 60% of the total N₂O production. Granules, the main contributor of nitrogen removal (~95%), were responsible for the remaining ~40% of N₂O production. This result is inconsistent with reads-based analysis that found the gene encoding clade II type nitrous oxide reductase (nosZ_II) had similar abundances in both granules and flocs. Another notable trend observed was the relatively higher abundance of the gene for NO-producing nitrite reductase (nir) in comparison to the gene for the nitric oxide reductase gene (nor) in both granules and flocs, indicating nitric oxide (NO) may accumulate in the AGSR. This is significant since NO and N₂O pulse assays demonstrated that NO could lead to N₂O production from both granules and flocs. However, since anammox bacteria, which were shown to be in higher abundance in granules than in flocs, have the capacity to scavenge NO this provides a mechanism by which its inhibitory effects can be mitigated, limiting N₂O release from the granules, consistent with experimental observation. These results demonstrate flocs are the main source of N₂O emission in AGSR and provide lab-scale evidence that NO-dependent anammox can mitigate N₂O emission.

Modulation of mitochondria and NADPH oxidase function by the nitrate-nitrite-NO pathway in metabolic disease with focus on type 2 diabetes

Mitochondria play fundamental role in maintaining cellular metabolic homeostasis, and metabolic disorders including type 2 diabetes (T2D) have been associated with mitochondrial dysfunction. Pathophysiological mechanisms are coupled to increased production of reactive oxygen species and oxidative stress, together with reduced bioactivity/signaling of nitric oxide (NO). Novel strategies restoring these abnormalities may have therapeutic potential in order to prevent or even treat T2D and associated cardiovascular and renal co-morbidities. A diet rich in green leafy vegetables, which contains high concentrations of inorganic nitrate, has been shown to reduce the risk of T2D. To this regard research has shown that in addition to the classical NO synthase (NOS) dependent pathway, nitrate from our diet can work as an alternative precursor for NO and other bioactive nitrogen oxide species via serial reductions of nitrate (i.e. nitrate-nitrite-NO pathway). This non-conventional pathway may act as an efficient back-up system during various pathological conditions when the endogenous NOS system is compromised (e.g. acidemia, hypoxia, ischemia, aging, oxidative stress). A number of experimental studies have demonstrated protective effects of nitrate supplementation in models of obesity, metabolic syndrome and T2D. Recently, attention has been directed towards the effects of nitrate/nitrite on mitochondrial functions including beiging/browning of white adipose tissue, PGC-1α and SIRT3 dependent AMPK activation, GLUT4 translocation and mitochondrial fusion-dependent improvements in glucose homeostasis, as well as dampening of NADPH oxidase activity. In this review, we examine recent research related to the effects of bioactive nitrogen oxide species on mitochondrial function with emphasis on T2D.

The effect of nitric oxide on mitochondrial respiration

This article reviews the interactions between nitric oxide (NO) and mitochondrial respiration. Mitochondrial ATP synthesis is responsible for virtually all energy production in mammals, and every other process in living organisms ultimately depends on that energy production. Furthermore, both necrosis and apoptosis, that summarize the main forms of cell death, are intimately linked to mitochondrial integrity. Endogenous and exogenous •NO inhibits mitochondrial respiration by different well-studied mechanisms and several nitrogen derivatives. Instantaneously, low concentrations of •NO, specifically and reversibly inhibit cytochrome c oxidase in competition with oxygen, in several tissues and cells in culture. Higher concentrations of •NO and its derivatives (peroxynitrite, nitrogen dioxide or nitrosothiols) can cause irreversible inhibition of the respiratory chain, uncoupling, permeability transition, and/or cell death. Peroxynitrite can cause opening of the permeability transition pore and opening of this pore causes loss of cytochrome c, which in turn might contribute to peroxynitrite-induced inhibition of respiration. Therefore, the inhibition of cytochrome c oxidase by •NO may be involved in the physiological and/or pathological regulation of respiration rate, and its affinity for oxygen, which depend on reactive nitrogen species formation, pH, proton motriz force and oxygen supply to tissues.

The effects of two different doses of ultraviolet-A light exposure on nitric oxide metabolites and cardiorespiratory outcomes

Purpose

The present study investigated different doses of ultraviolet-A (UV-A) light on plasma nitric oxide metabolites and cardiorespiratory variables.

Methods

Ten healthy male participants completed three experimental conditions, 7 days apart. Participants were exposed to no light (CON); 10 J cm² (15 min) of UV-A light (UVA10) and 20 J cm² (30 min) of UV-A light (UVA20) in a randomized order. Plasma nitrite [NO₂⁻] and nitrate [NO₃⁻] concentrations, blood pressure (BP), and heart rate (HR) were recorded before, immediately after exposure and 30 min post-exposure. Whole body oxygen utilization (\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\dot{V}}}{\rm O}_{2}$$\end{document}V˙O2), resting metabolic rate (RMR) and skin temperature were recorded continuously.

Results

None of the measured parameters changed significantly during CON (all P > 0.05). \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\dot{V}}}{\rm O}_{2}$$\end{document}V˙O2 and RMR were significantly reduced immediately after UVA10 (P < 0.05) despite no change in plasma [NO₂⁻] (P > 0.05). Immediately after exposure to UVA20, plasma [NO₂⁻] was higher (P = 0.014) and \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\dot{V}}}{\rm O}_{2}$$\end{document}V˙O2 and RMR tended to be lower compared to baseline (P = 0.06). There were no differences in [NO₂⁻] or \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\dot{V}}}{\rm O}_{2}$$\end{document}V˙O2 at the 30 min time point in any condition. UV-A exposure did not alter systolic BP, diastolic BP or MAP (all P > 0.05). UV-A light did not alter plasma [NO₃⁻] at any time point (all P > 0.05).

Conclusions

This study demonstrates that a UV-A dose of 20 J cm² is necessary to increase plasma [NO₂⁻] although a smaller dose is capable of reducing \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\dot{V}}}{\rm O}_{2}$$\end{document}V˙O2 and RMR at rest. Exposure to UV-A did not significantly reduce BP in this cohort of healthy adults. These data suggest that exposure to sunlight has a meaningful acute impact on metabolic function.

Analysis of multiple haloarchaeal genomes suggests that the quinone-dependent respiratory nitric oxide reductase is an important source of nitrous oxide in hypersaline environments

Microorganisms, including Bacteria and Archaea, play a key role in denitrification, which is the major mechanism by which fixed nitrogen returns to the atmosphere from soil and water. While the enzymology of denitrification is well understood in Bacteria, the details of the last two reactions in this pathway, which catalyse the reduction of nitric oxide (NO) via nitrous oxide (N₂ O) to nitrogen (N₂ ), are little studied in Archaea, and hardly at all in haloarchaea. This work describes an extensive interspecies analysis of both complete and draft haloarchaeal genomes aimed at identifying the genes that encode respiratory nitric oxide reductases (Nors). The study revealed that the only nor gene found in haloarchaea is one that encodes a single subunit quinone dependent Nor homologous to the qNor found in bacteria. This surprising discovery is considered in terms of our emerging understanding of haloarchaeal bioenergetics and NO management.

Potential application of aerobic denitrifying bacterium Pseudomonas aeruginosa PCN-2 in nitrogen oxides (NOx) removal from flue gas

Conventional biological removal of nitrogen oxides (NOx) from flue gas has been severely restricted by the presence of oxygen. This paper presents an efficient alternative for NOx removal at varying oxygen levels using the newly isolated bacterial strain Pseudomonas aeruginosa PCN-2 which was capable of aerobic and anoxic denitrification. Interestingly, nitric oxide (NO), as the obligatory intermediate, was negligibly accumulated during nitrate and nitrite reduction. Moreover, normal nitrate reduction with decreasing NO accumulation was realized under O2 concentration ranging from 0 to 100%. Reverse transcription and real-time quantitative polymerase chain reaction (RT-qPCR) analysis revealed that high efficient NO removal was attributed to the coordinate regulation of gene expressions including napA (for periplasmic nitrate reductase), nirS (for cytochrome cd1 nitrite reductase) and cnorB (for NO reductase). Further batch experiments demonstrated the immobilized strain PCN-2 possessed high capability of removing NO and nitrogen dioxide (NO2) at O2 concentration of 0-10%. A biotrickling filter established with present strain achieved high NOx removal efficiencies of 91.94-96.74% at inlet NO concentration of 100-500ppm and O2 concentration of 0-10%, which implied promising potential applications in purifying NOx contaminated flue gas.

Microbial ecology of denitrification in biological wastewater treatment

Globally, denitrification is commonly employed in biological nitrogen removal processes to enhance water quality. However, substantial knowledge gaps remain concerning the overall community structure, population dynamics and metabolism of different organic carbon sources. This systematic review provides a summary of current findings pertaining to the microbial ecology of denitrification in biological wastewater treatment processes. DNA fingerprinting-based analysis has revealed a high level of microbial diversity in denitrification reactors and highlighted the impacts of carbon sources in determining overall denitrifying community composition. Stable isotope probing, fluorescence in situ hybridization, microarrays and meta-omics further link community structure with function by identifying the functional populations and their gene regulatory patterns at the transcriptional and translational levels. This review stresses the need to integrate microbial ecology information into conventional denitrification design and operation at full-scale. Some emerging questions, from physiological mechanisms to practical solutions, for example, eliminating nitrous oxide emissions and supplementing more sustainable carbon sources than methanol, are also discussed. A combination of high-throughput approaches is next in line for thorough assessment of wastewater denitrifying community structure and function. Though denitrification is used as an example here, this synergy between microbial ecology and process engineering is applicable to other biological wastewater treatment processes.

The significance of early accumulation of nanomolar concentrations of NO as an inducer of denitrification

Denitrifying bacteria have variable ability to perform efficient and balanced denitrification during oxygen depletion. NO is often assumed to exert a positive feedback in the transcription of denitrification genes, because NO-dependent activators have been identified. The regulatory network of denitrification is complex, however, and the significance of NO signalling needs to be studied in vivo. We utilized acetylene-catalysed NO oxidation to scavenge NO produced by batch cultures of denitrifying bacteria during transition from oxic to anoxic respiration, to explore the effects on the kinetics of NO, N(2) O and N(2) production. The results demonstrated that nanomolar concentrations of NO accumulating prior to complete depletion of oxygen exert a significant positive feedback on the initiation of denitrification in Paracoccus denitrificans. The early NO signal appeared essential to minimize the transient accumulation of NO during the subsequent anoxic phase for Agrobacterium tumefaciens, but not for P. denitrificans and Pseudomonas aureofaciens. In summary, the results indicate that the early accumulation of nanomolar concentrations of NO has a significant, but strain-dependent effect on the expression of denitrification.

Mitochondria and nitric oxide: chemistry and pathophysiology

Cell respiration is controlled by nitric oxide (NO) reacting with respiratory chain complexes, particularly with Complex I and IV. The functional implication of these reactions is different owing to involvement of different mechanisms. Inhibition of complex IV is rapid (milliseconds) and reversible, and occurs at nanomolar NO concentrations, whereas inhibition of complex I occurs after a prolonged exposure to higher NO concentrations. The inhibition of Complex I involves the reversible S-nitrosation of a key cysteine residue on the ND3 subunit. The reaction of NO with cytochrome c oxidase (CcOX) directly involves the active site of the enzyme: two mechanisms have been described leading to formation of either a relatively stable nitrosyl-derivative (CcOX-NO) or a more labile nitrite-derivative (CcOX-NO (2) (-) ). Both adducts are inhibited, though with different K(I); one mechanism prevails on the other depending on the turnover conditions and availability of substrates, cytochrome c and O(2). SH-SY5Y neuroblastoma cells or lymphoid cells, cultured under standard O(2) tension, proved to follow the mechanism leading to degradation of NO to nitrite. Formation of CcOX-NO occurred upon rising the electron flux level at this site, artificially or in the presence of higher amounts of endogenous reduced cytochrome c. Taken together, the observations suggest that the expression level of mitochondrial cytochrome c may be crucial to determine the respiratory chain NO inhibition pathway prevailing in vivo under nitrosative stress conditions. The putative patho-physiological relevance of the interaction between NO and the respiratory complexes is addressed.

Cytochrome c oxidase and nitric oxide in action: molecular mechanisms and pathophysiological implications

BACKGROUND

The reactions between Complex IV (cytochrome c oxidase, CcOX) and nitric oxide (NO) were described in the early 60's. The perception, however, that NO could be responsible for physiological or pathological effects, including those on mitochondria, lags behind the 80's, when the identity of the endothelial derived relaxing factor (EDRF) and NO synthesis by the NO synthases were discovered. NO controls mitochondrial respiration, and cytotoxic as well as cytoprotective effects have been described. The depression of OXPHOS ATP synthesis has been observed, attributed to the inhibition of mitochondrial Complex I and IV particularly, found responsible of major effects.

SCOPE OF REVIEW

The review is focused on CcOX and NO with some hints about pathophysiological implications. The reactions of interest are reviewed, with special attention to the molecular mechanisms underlying the effects of NO observed on cytochrome c oxidase, particularly during turnover with oxygen and reductants.

MAJOR CONCLUSIONS AND GENERAL SIGNIFICANCE

The NO inhibition of CcOX is rapid and reversible and may occur in competition with oxygen. Inhibition takes place following two pathways leading to formation of either a relatively stable nitrosyl-derivative (CcOX-NO) of the enzyme reduced, or a more labile nitrite-derivative (CcOX-NO(2)(-)) of the enzyme oxidized, and during turnover. The pathway that prevails depends on the turnover conditions and concentration of NO and physiological substrates, cytochrome c and O(2). All evidence suggests that these parameters are crucial in determining the CcOX vs NO reaction pathway prevailing in vivo, with interesting physiological and pathological consequences for cells.

Effect of nitric oxide on anammox bacteria

The effects of nitrogen oxides on anammox bacteria are not well known. Therefore, anammox bacteria were exposed to 3,500 ppm nitric oxide (NO) in the gas phase. The anammox bacteria were not inhibited by the high NO concentration but rather used it to oxidize additional ammonium to dinitrogen gas under conditions relevant to wastewater treatment.

Dynamic and interacting profiles of *NO and O2 in rat hippocampal slices

Nitric oxide (*NO) is a ubiquitous signaling molecule that participates in the neuromolecular phenomena associated with memory formation. In the hippocampus, neuronal *NO production is coupled to the activation of the NMDA-type of glutamate receptor. Although *NO-mediated signaling has been associated with soluble guanylate cyclase activation, cytochrome oxidase is also a target for this gaseous free radical, for which *NO competes with O(2). Here we show, for the first time in a model preserving tissue cytoarchitecture (rat hippocampal slices) and at a physiological O(2) concentration, that endogenous NMDA-evoked *NO production inhibits tissue O(2) consumption for submicromolar concentrations. The simultaneous real-time recordings reveal a direct correlation between the profiles of *NO and O(2) in the CA1 subregion of the hippocampal slice. These results, obtained in a system close to in vivo models, strongly support the current paradigm for O(2) and *NO interplay in the regulation of cellular respiration.

Nitrite reduction: a ubiquitous function from a pre-aerobic past

In eukaryotes, small amounts of nitrite confer cytoprotection against ischemia/reperfusion-related tissue damage in vivo, possibly via reduction to nitric oxide (NO) and inhibition of mitochondrial function. Several hemeproteins are involved in this protective mechanism, starting with deoxyhemoglobin, which is capable of reducing nitrite. In facultative aerobic bacteria, such as Pseudomonas aeruginosa, nitrite is reduced to NO by specialized heme-containing enzymes called cd(1) nitrite reductases. The details of their catalytic mechanism are summarized below, together with a hypothesis on the biological role of the unusual d(1)-heme, which, in the reduced state, shows unique properties (very high affinity for nitrite and exceptionally fast dissociation of NO). Our results support the idea that the nitrite-based reactions of contemporary eukaryotes are a vestige of earlier bacterial biochemical pathways. The evidence that nitrite reductase activities of enzymes with different cellular roles and biochemical features still exist today highlights the importance of nitrite in cellular homeostasis.

NO3-/NO2- assimilation in halophilic archaea: physiological analysis, nasA and nasD expressions

The haloarchaeon Haloferax mediterranei is able to assimilate nitrate or nitrite using the assimilatory nitrate pathway. An assimilatory nitrate reductase (Nas) and an assimilatory nitrite reductase (NiR) catalyze the first and second reactions, respectively. The genes involved in this process are transcribed as two messengers, one polycistronic (nasABC; nasA encodes Nas) and one monocistronic (nasD; codes for NiR). Here we report the Hfx mediterranei growth as well as the Nas and NiR activities in presence of high nitrate, nitrite and salt concentrations, using different approaches such as physiological experiments and enzymatic activities assays. The nasA and nasD expression profiles are also analysed by real-time quantitative PCR. The results presented reveal that the assimilatory nitrate/nitrite pathway in Hfx mediterranei takes place even if the salt concentration is higher than those usually present in the environments where this microorganism inhabits. This haloarchaeon grows in presence of 2 M nitrate or 50 mM nitrite, which are the highest nitrate and nitrite concentrations described from a prokaryotic microorganism. Therefore, it could be attractive for bioremediation applications in sewage plants where high salt, nitrate and nitrite concentrations are detected in wastewaters and brines.

Mechanisms of transient nitric oxide and nitrous oxide production in a complex biofilm

Nitric oxide (NO) and nitrous oxide (N(2)O) are formed during N-cycling in complex microbial communities in response to fluctuating molecular oxygen (O(2)) and nitrite (NO(2)(-)) concentrations. Until now, the formation of NO and N(2)O in microbial communities has been measured with low spatial and temporal resolution, which hampered elucidation of the turnover pathways and their regulation. In this study, we combined microsensor measurements with metabolic modeling to investigate the functional response of a complex biofilm with nitrifying and denitrifying activity to variations in O(2) and NO(2)(-). In steady state, NO and N(2)O formation was detected if ammonium (NH(4)(+)) was present under oxic conditions and if NO(2)(-) was present under anoxic conditions. Thus, NO and N(2)O are produced by ammonia-oxidizing bacteria (AOB) under oxic conditions and by heterotrophic denitrifiers under anoxic conditions. NO and N(2)O formation by AOB occurred at fully oxic conditions if NO(2)(-) concentrations were high. Modeling showed that steady-state NO concentrations are controlled by the affinity of NO-consuming processes to NO. Transient accumulation of NO and N(2)O occurred upon O(2) removal from, or NO(2)(-) addition to, the medium only if NH(4)(+) was present under oxic conditions or if NO(2)(-) was already present under anoxic conditions. This showed that AOB and heterotrophic denitrifiers need to be metabolically active to respond with instantaneous NO and N(2)O production upon perturbations. Transiently accumulated NO and N(2)O decreased rapidly after their formation, indicating a direct effect of NO on the metabolism. By fitting model results to measurements, the kinetic relationships in the model were extended with dynamic parameters to predict transient NO release from perturbed ecosystems.

Occurrence and turnover of nitric oxide in a nitrogen-impacted sand and gravel aquifer

Little is known about nitric oxide (NO) production or consumption in the subsurface, an environment which may be conducive to NO accumulation. A study conducted in a nitrogen-contaminated aquifer on Cape Cod, Massachusetts assessed the occurrence and turnover of NO within a contaminant plume in which nitrification and denitrification were known to occur. NO (up to 8.6 nM) was detected in restricted vertical zones located within a nitrate (NO3-) gradient and characterized by low dissolved oxygen (< 10 microM). NO concentrations correlated best with nitrite (NO2-) (up to 35 microM), but nitrous oxide (N2O) (up to 1 microM) also was present. Single-well injection tests were used to determine NO production and consumption in situ within these zones. First-order rate constants for NO consumption were similar (0.05-0.08 h(-1)) at high and low (260 and 10 nM) NO concentrations, suggesting a turnover time at in situ concentrations of 10-20 h. Tracer tests with 15N[NO] demonstrated that oxidation to 15N[NO2-] occurred only during the initial stages, but after 4 h reduction to 15N[N2O] was the primary reaction product. Added NO2- (31 microM) or NO3- (53 microM) resulted in a linear NO accumulation at 2.4 and 1.0 nM h(-1) for the first 6 h of in situ tests. These results suggest that NO was primarily produced by denitrification within this aquifer.

A dynamic model of nitric oxide inhibition of mitochondrial cytochrome c oxidase

Nitric oxide can inhibit mitochondrial cytochrome oxidase in both oxygen competitive and uncompetitive modes. A previous model described these interactions assuming equilibrium binding to the reduced and oxidised enzyme respectively (Mason, et al. Proc. Natl. Acad. Sci. U S A 103 (2006) 708-713). Here we demonstrate that the equilibrium assumption is inappropriate as it requires unfeasibly high association constants for NO to the oxidised enzyme. Instead we develop a model which explicitly includes NO binding and its enzyme-bound conversion to nitrite. Removal of the nitrite complex requires electron transfer to the binuclear centre from haem a. This revised model fits the inhibition constants at any value of substrate concentration (ferrocytochrome c or oxygen). It predicts that the inhibited steady state should be a mixture of the reduced haem nitrosyl complex and the oxidized-nitrite complex. Unlike the previous model, binding to the oxidase is always proportional to the degree of inhibition of oxygen consumption. The model is consistent with data and models from a recent paper suggesting that the primary effect of NO binding to the oxidised enzyme is to convert NO to nitrite, rather than to inhibit enzyme activity (Antunes et al. Antioxid. Redox Signal. 9 (2007) 1569-1579).

Transcription and activities of NOx reductases in Agrobacterium tumefaciens: the influence of nitrate, nitrite and oxygen availability

The ability of Agrobacetrium tumefaciens to perform balanced transitions from aerobic to anaerobic respiration was studied by monitoring oxygen depletion, transcription of nirK and norB, and the concentrations of nitrite, nitric oxide (NO) and nitrous oxide in stirred batch cultures with different initial oxygen, nitrate or nitrite concentrations. Nitrate concentrations (0.2-2 mM) did not affect oxygen depletion, nor the oxygen concentration at which denitrification was initiated (1-2 microM). Nitrite (0.2-2 mM), on the other hand, retarded the oxygen depletion as it reached approximately 20 microM, and caused initiation of active denitrification as oxygen concentrations reached 10-17 microM. Unbalanced transitions occurred in treatments with high cell densities (i.e. with rapid transition from oxic to anoxic conditions), seen as NO accumulation to muM concentrations and impeded nitrous oxide production. This phenomenon was most severe in nitrite treatments, and reduced the cells' ability to respire oxygen during subsequent oxic conditions. Transcripts of norB were only detectable during the period with active denitrification. In contrast, nirK transcripts were detected at low levels both before and after this period. The results demonstrate that the transition from aerobic to anaerobic metabolism is a regulatory challenge, with implications for survival and emission of trace gases from denitrifying bacteria.

The nitrate-nitrite-nitric oxide pathway in physiology and therapeutics

The inorganic anions nitrate (NO3-) and nitrite (NO2-) were previously thought to be inert end products of endogenous nitric oxide (NO) metabolism. However, recent studies show that these supposedly inert anions can be recycled in vivo to form NO, representing an important alternative source of NO to the classical L-arginine-NO-synthase pathway, in particular in hypoxic states. This Review discusses the emerging important biological functions of the nitrate-nitrite-NO pathway, and highlights studies that implicate the therapeutic potential of nitrate and nitrite in conditions such as myocardial infarction, stroke, systemic and pulmonary hypertension, and gastric ulceration.

Effects of dietary nitrate on oxygen cost during exercise

AIM

Nitric oxide (NO), synthesized from l-arginine by NO synthases, plays a role in adaptation to physical exercise by modulating blood flow, muscular contraction and glucose uptake and in the control of cellular respiration. Recent studies show that NO can be formed in vivo also from the reduction of inorganic nitrate (NO(3) (-)) and nitrite (NO(2) (-)). The diet constitutes a major source of nitrate, and vegetables are particularly rich in this anion. The aim of this study was to investigate if dietary nitrate had any effect on metabolic and circulatory parameters during exercise.

METHOD

In a randomized double-blind placebo-controlled crossover study, we tested the effect of dietary nitrate on physiological and metabolic parameters during exercise. Nine healthy young well-trained men performed submaximal and maximal work tests on a cycle ergometer after two separate 3-day periods of dietary supplementation with sodium nitrate (0.1 mmol kg(-1) day-1) or an equal amount of sodium chloride (placebo).

RESULTS

The oxygen cost at submaximal exercise was reduced after nitrate supplementation compared with placebo. On an average Vo(2) decreased from 2.98 +/- 0.57 during CON to 2.82 +/- 0.58 L min(-1) during NIT (P < 0.02) over the four lowest submaximal work rates. Gross efficiency increased from 19.7 +/- 1.6 during CON to 21.1 +/- 1.3% during NIT (P < 0.01) over the four lowest work rates. There was no difference in heart rate, lactate [Hla], ventilation (VE), VE/Vo(2) or respiratory exchange ratio between nitrate and placebo during any of the submaximal work rates.

CONCLUSION

We conclude that dietary nitrate supplementation, in an amount achievable through a diet rich in vegetables, results in a lower oxygen demand during submaximal work. This highly surprising effect occurred without an accompanying increase in lactate concentration, indicating that the energy production had become more efficient. The mechanism of action needs to be clarified but a likely first step is the in vivo reduction of dietary nitrate into bioactive nitrogen oxides including nitrite and NO.

Nitric oxide regulation of mitochondrial oxygen consumption II: Molecular mechanism and tissue physiology

Nitric oxide (NO) is an intercellular signaling molecule; among its many and varied roles are the control of blood flow and blood pressure via activation of the heme enzyme, soluble guanylate cyclase. A growing body of evidence suggests that an additional target for NO is the mitochondrial oxygen-consuming heme/copper enzyme, cytochrome c oxidase. This review describes the molecular mechanism of this interaction and the consequences for its likely physiological role. The oxygen reactive site in cytochrome oxidase contains both heme iron (a(3)) and copper (Cu(B)) centers. NO inhibits cytochrome oxidase in both an oxygen-competitive (at heme a(3)) and oxygen-independent (at Cu(B)) manner. Before inhibition of oxygen consumption, changes can be observed in enzyme and substrate (cytochrome c) redox state. Physiological consequences can be mediated either by direct "metabolic" effects on oxygen consumption or via indirect "signaling" effects via mitochondrial redox state changes and free radical production. The detailed kinetics suggest, but do not prove, that cytochrome oxidase can be a target for NO even under circumstances when guanylate cyclase, its primary high affinity target, is not fully activated. In vivo organ and whole body measures of NO synthase inhibition suggest a possible role for NO inhibition of cytochrome oxidase. However, a detailed mapping of NO and oxygen levels, combined with direct measures of cytochrome oxidase/NO binding, in physiology is still awaited.

Nitric oxide and the respiratory enzyme

Available information on the molecular mechanisms by which nitric oxide (NO) controls the activity of the respiratory enzyme (cytochrome-c-oxidase) is reviewed. We report that, depending on absolute electron flux, NO at physiological concentrations reversibly inhibits cytochrome-c-oxidase by two alternative reaction pathways, yielding either a nitrosyl- or a nitrite-heme a3 derivative. We address a number of hypotheses, envisaging physiological and/or pathological effects of the reactions between NO and cytochrome-c-oxidase.

Control of cytochrome c oxidase activity by nitric oxide

Over the past decade it was discovered that, over-and-above multiple regulatory functions, nitric oxide (NO) is responsible for the modulation of cell respiration by inhibiting cytochrome c oxidase (CcOX). As assessed at different integration levels (from the purified enzyme in detergent solution to intact cells), CcOX can react with NO following two alternative reaction pathways, both leading to an effective, fully reversible inhibition of respiration. A crucial finding is that the rate of electron flux through the respiratory chain controls the mechanism of inhibition by NO, leading to either a "nitrosyl" or a "nitrite" derivative. The two mechanisms can be discriminated on the basis of the differential photosensitivity of the inhibited state. Of relevance to cell pathophysiology, the pathway involving the nitrite derivative leads to oxidative degradation of NO, thereby protecting the cell from NO toxicity. The aim of this work is to review the information available on these two mechanisms of inhibition of respiration.

Nitric oxide and cytochrome oxidase: reaction mechanisms from the enzyme to the cell

The aim of this work is to review the information available on the molecular mechanisms by which the NO radical reversibly downregulates the function of cytochrome c oxidase (CcOX). The mechanisms of the reactions with NO elucidated over the past few years are described and discussed in the context of the inhibitory effects on the enzyme activity. Two alternative reaction pathways are presented whereby NO reacts with the catalytic intermediates of CcOX populated during turnover. The central idea is that at "cellular" concentrations of NO (</= microM), the redox state of the respiratory chain results in the formation of either the nitrosyl- or the nitrite-derivative of CcOX, with potentially different metabolic implications for the cell. In particular, the role played by CcOX in protecting the cell from excess NO, potentially toxic for mitochondria, is also reviewed highlighting the mechanistic differences between eukaryotes and some prokaryotes.

Flavorubredoxin, an inducible catalyst for nitric oxide reduction and detoxification in Escherichia coli

Nitric oxide (NO) is a poison, and organisms employ diverse systems to protect against its harmful effects. In Escherichia coli, ygaA encodes a transcription regulator (b2709) controlling anaerobic NO reduction and detoxification. Adjacent to ygaA and oppositely transcribed are ygaK (encoding a flavorubredoxin (flavoRb) (b2710) with a NO-binding non-heme diiron center) and ygbD (encoding a NADH:(flavo)Rb oxidoreductase (b2711)), which function in NO reduction and detoxification. Mutation of either ygaA or ygaK eliminated inducible anaerobic NO metabolism, whereas ygbD disruption partly impaired the activity. NO-sensitive [4Fe-4S] (de)hydratases, including the Krebs cycle aconitase and the Entner-Doudoroff pathway 6-phosphogluconate dehydratase, were more susceptible to inactivation in ygaK or ygaA mutants than in the parental strain, and these metabolic poisonings were associated with conditional growth inhibitions. flavoRb (NO reductase) and flavohemoglobin (NO dioxygenase) maximally metabolized and detoxified NO in anaerobic and aerobic E. coli, respectively, whereas both enzymes scavenged NO under microaerobic conditions. We suggest designation of the ygaA-ygaK-ygbD gene cluster as the norRVW modulon for NO reduction and detoxification.

Nitric oxide and cytochrome oxidase: substrate, inhibitor or effector?

Endogenously produced nitric oxide (NO) controls oxygen consumption by inhibiting cytochrome c oxidase, the terminal electron acceptor of the mitochondrial electron transport chain. The oxygen-binding site of the enzyme is an iron/copper (haem a3/CuB) binuclear centre. At high substrate (ferrocytochrome c) concentrations, NO binds reversibly to the reduced iron in competition with oxygen. At low substrate concentrations, NO binds to the oxidized copper. Inhibition at the haem iron site is relieved by dissociation of the NO from the reduced iron. Inhibition at the copper site is relieved by oxidation of the bound NO and subsequent dissociation of nitrite from the enzyme. Therefore, NO can be a substrate, inhibitor or effector of cytochrome oxidase, depending on cellular conditions.

Redox control of mitochondrial functions

Redox reactions and electron flow through the respiratory chain are the hallmarks of mitochondria. By supporting oxidative phosphorylation and metabolite transport, mitochondrial redox reactions are of central importance for cellular energy conversion. In the present review, we will discuss two other aspects of the mitochondrial redox state: (i) its control of mitochondrial Ca2+ homeostasis, and (ii) the intramitochondrial formation of reactive oxygen or nitrogen species that strongly influence electron flow of the respiratory chain.

Regulation of mitochondrial respiration by nitric oxide inhibition of cytochrome c oxidase

Nitric oxide (NO) and its derivatives inhibit mitochondrial respiration by a variety of means. Nanomolar concentrations of NO immediately, specifically and reversibly inhibit cytochrome oxidase in competition with oxygen, in isolated cytochrome oxidase, mitochondria, nerve terminals, cultured cells and tissues. Higher concentrations of NO and its derivatives (peroxynitrite, nitrogen dioxide or nitrosothiols) can cause irreversible inhibition of the respiratory chain, uncoupling, permeability transition, and/or cell death. Isolated mitochondria, cultured cells, isolated tissues and animals in vivo display respiratory inhibition by endogenously produced NO from constitutive isoforms of NO synthase (NOS), which may be largely mediated by NO inhibition of cytochrome oxidase. Cultured cells expressing the inducible isoform of NOS (iNOS) can acutely and reversibly inhibit their own cellular respiration and that of co-incubated cells due to NO inhibition of cytochrome oxidase, but after longer-term incubation result in irreversible inhibition of cellular respiration due to NO or its derivatives. Thus the NO inhibition of cytochrome oxidase may be involved in the physiological and/or pathological regulation of respiration rate, and its affinity for oxygen.

Production and consumption of nitric oxide by three methanotrophic bacteria

We studied nitrogen oxide production and consumption by methanotrophs Methylobacter luteus (group I), Methylosinus trichosporium OB3b (group II), and an isolate from a hardwood swamp soil, here identified by 16S ribosomal DNA sequencing as Methylobacter sp. strain T20 (group I). All could consume nitric oxide (nitrogen monoxide, NO), and produce small amounts of nitrous oxide (N(2)O). Only Methylobacter strain T20 produced large amounts of NO (>250 parts per million by volume [ppmv] in the headspace) at specific activities of up to 2.0 x 10(-17) mol of NO cell(-1) day(-1), mostly after a culture became O(2) limited. Production of NO by strain T20 occurred mostly in nitrate-containing medium under anaerobic or nearly anaerobic conditions, was inhibited by chlorate, tungstate, and O(2), and required CH(4). Denitrification (methanol-supported N(2)O production from nitrate in the presence of acetylene) could not be detected and thus did not appear to be involved in the production of NO. Furthermore, cd(1) and Cu nitrite reductases, NO reductase, and N(2)O reductase could not be detected by PCR amplification of the nirS, nirK, norB, and nosZ genes, respectively. M. luteus and M. trichosporium produced some NO in ammonium-containing medium under aerobic conditions, likely as a result of methanotrophic nitrification and chemical decomposition of nitrite. For Methylobacter strain T20, arginine did not stimulate NO production under aerobiosis, suggesting that NO synthase was not involved. We conclude that strain T20 causes assimilatory reduction of nitrate to nitrite, which then decomposes chemically to NO. The production of NO by methanotrophs such as Methylobacter strain T20 could be of ecological significance in habitats near aerobic-anaerobic interfaces where fluctuating O(2) and nitrate availability occur.

Nitric oxide reductases in bacteria

Nitric oxide reductases (NORs) that are found in bacteria belong to the large enzyme family which includes cytochrome oxidases. Two types of bacterial NORs have been characterised. One is a cytochrome bc-type complex (cNOR) that receives electrons from soluble redox protein donors, whereas the other type (qNOR) lacks the cytochrome c component and uses quinol as the electron donor. The latter enzyme is present in several pathogens that are not denitrifiers. We summarise the current knowledge on bacterial NORs, and discuss the evolutionary relationship between them and cytochrome oxidases in this review.

Effects of nitric oxide and peroxynitrite on the cytochrome oxidase K(m) for oxygen: implications for mitochondrial pathology

This review summarises current knowledge about the effect of oxygen on cytochrome oxidase activity in vitro and in vivo. Cytochrome oxidase normally operates above its K(m) for oxygen in vivo. However, decreases in the intracellular oxygen concentration (hypoxia) under physiological extremes, or during pathophysiology, can cause mitochondrial respiration to become oxygen limited. Inhibitors that raise the enzyme's K(m) will induce oxygen limitation under apparently normoxic conditions. It is known that the concentrations of nitric oxide and peroxynitrite are raised in a number of pathophysiological conditions. These compounds are capable of reversibly and irreversibly raising the cytochrome oxidase K(m) for oxygen. Therefore, measurements of cell and mitochondrial respiration in vitro that fail to systematically vary oxygen through the range of physiological concentrations are likely to underestimate the effects of nitric oxide and peroxynitrite in vivo.

Nitric oxide and mitochondrial respiration

Nitric oxide (NO) and its derivative peroxynitrite (ONOO-) inhibit mitochondrial respiration by distinct mechanisms. Low (nanomolar) concentrations of NO specifically inhibit cytochrome oxidase in competition with oxygen, and this inhibition is fully reversible when NO is removed. Higher concentrations of NO can inhibit the other respiratory chain complexes, probably by nitrosylating or oxidising protein thiols and removing iron from the iron-sulphur centres. Peroxynitrite causes irreversible inhibition of mitochondrial respiration and damage to a variety of mitochondrial components via oxidising reactions. Thus peroxynitrite inhibits or damages mitochondrial complexes I, II, IV and V, aconitase, creatine kinase, the mitochondrial membrane, mitochondrial DNA, superoxide dismutase, and induces mitochondrial swelling, depolarisation, calcium release and permeability transition. The NO inhibition of cytochrome oxidase may be involved in the physiological regulation of respiration rate, as indicated by the finding that isolated cells producing NO can regulate cellular respiration by this means, and the finding that inhibition of NO synthase in vivo causes a stimulation of tissue and whole body oxygen consumption. The recent finding that mitochondria may contain a NO synthase and can produce significant amounts of NO to regulate their own respiration also suggests this regulation may be important for physiological regulation of energy metabolism. However, definitive evidence that NO regulation of mitochondrial respiration occurs in vivo is still missing, and interpretation is complicated by the fact that NO appears to affect tissue respiration by cGMP-dependent mechanisms. The NO inhibition of cytochrome oxidase may also be involved in the cytotoxicity of NO, and may cause increased oxygen radical production by mitochondria, which may in turn lead to the generation of peroxynitrite. Mitochondrial damage by peroxynitrite may mediate the cytotoxicity of NO, and may be involved in a variety of pathologies.

Modulation of mitochondrial respiration by nitric oxide: investigation by single cell fluorescence microscopy

With the electro-driven import of rhodamine 123, we used single cell fluorescence microscopy to single out the contribution of nitric oxide (NO) in controlling mitochondrial membrane potential expressed by (stationary growing) rhabdomyosarcoma and neuroblastoma cells in culture. The experimental design and the computer-aided image analysis detected and quantitated variations of fluorescence signals specific to mitochondria. We observed that 1) the two cell lines display changes of fluorescence dependent on mitochondrial energization states; 2) mitochondrial fluorescence decreases after exposure of the cells to a NO releaser; 4) the different fluorescence intensity measured under stationary growing conditions, or after activation and inhibition of constitutive NO synthase, is consistent with a steady-state production of NO. Direct comparison of single cell fluorescence with bulk cytofluorimetry proved that the results obtained by the latter method may be misleading because of the intrinsic-to-measure lack of information about distribution of fluorescence within different cell compartments. The kinetic parameters describing the reactions between cytochrome oxidase, NO, and O2 may account for the puzzling (20-fold) increase of the KM for O2 reported for cells and tissues as compared to purified cytochrome c oxidase, allowing an estimate of in vivo NO flux.

Molecular genetics of the genus Paracoccus: metabolically versatile bacteria with bioenergetic flexibility

Paracoccus denitrificans and its near relative Paracoccus versutus (formerly known as Thiobacilllus versutus) have been attracting increasing attention because the aerobic respiratory system of P. denitrificans has long been regarded as a model for that of the mitochondrion, with which there are many components (e.g., cytochrome aa3 oxidase) in common. Members of the genus exhibit a great range of metabolic flexibility, particularly with respect to processes involving respiration. Prominent examples of flexibility are the use in denitrification of nitrate, nitrite, nitrous oxide, and nitric oxide as alternative electron acceptors to oxygen and the ability to use C1 compounds (e.g., methanol and methylamine) as electron donors to the respiratory chains. The proteins required for these respiratory processes are not constitutive, and the underlying complex regulatory systems that regulate their expression are beginning to be unraveled. There has been uncertainty about whether transcription in a member of the alpha-3 Proteobacteria such as P. denitrificans involves a conventional sigma70-type RNA polymerase, especially since canonical -35 and -10 DNA binding sites have not been readily identified. In this review, we argue that many genes, in particular those encoding constitutive proteins, may be under the control of a sigma70 RNA polymerase very closely related to that of Rhodobacter capsulatus. While the main focus is on the structure and regulation of genes coding for products involved in respiratory processes in Paracoccus, the current state of knowledge of the components of such respiratory pathways, and their biogenesis, is also reviewed.

Nitric oxide and its congeners in mitochondria: implications for apoptosis

Apoptosis is an evolutionarily conserved form of physiologic cell death important for tissue development and homeostasis. The causes and execution mechanisms of apoptosis are not completely understood. Nitric oxide (NO) and its congeners, oxidative stress, Ca2+, proteases, nucleases, and mitochondria are considered mediators of apoptosis. Recent findings strongly suggest that mitochondria contain a factor or factors that upon release from the destabilized organelles, induce apoptosis. We have found that oxidative stress-induced release of Ca2+ from mitochondria followed by Ca2+ reuptake (Ca2+ cycling) causes destabilization of mitochondria and apoptosis. The protein product of the protooncogene bcl-2 protects mitochondria and thereby prevents apoptosis. We have also found that NO and its congeners can induce Ca2+ release from mitochondria. Thus, nitrogen monoxide (.NO) binds to cytochrome oxidase, blocks respiration, and thereby causes mitochondrial deenergization and Ca2+ release. Peroxynitrite (ONOO-), on the other hand, causes Ca2+ release from mitochondria by stimulating a specific Ca2+ release pathway. This pathway requires oxidized nicotinamide adenine dinucleotide (NAD+) hydrolysis to adenosine diphosphate ribose and nicotinamide. NAD+ hydrolysis is only possible when some vicinal thiols are cross-linked. ONOO- is able to oxidize them. Our findings suggest that NO and its congeners can induce apoptosis by destabilizing mitochondria via deenergization and/or by inducing a specific Ca2+ release followed by Ca2+ cycling.

Reactivity of nitric oxide with cytochrome c oxidase: interactions with the binuclear centre and mechanism of inhibition

Nitric oxide (NO) has recently been recognized as an important biological mediator that inhibits respiration at cytochrome c oxidase (CcO). This inhibition is reversible and shows competition with oxygen, the Ki being lower at low oxygen concentrations. Although the species that binds NO in turnover has been suggested to contain a partially reduced binuclear center, the exact mechanism of the inhibition is not clear. Recently, rapid (ms) redox reactions of NO with the binuclear center have been reported, e.g., the ejection of an electron to cytochrome a and the depletion of the intermediates P and F. These observations have been rationalized within a scheme in which NO reacts with oxidized CuB leading to the reduction of this metal center and formation of nitrite in a very fast reaction. Electron migration from CuB to other redox sites within the enzyme is proposed to explain the optical transitions observed. The relevance of these reactions to the inhibition of CcO and metabolism of NO are discussed.

Cell biology and molecular basis of denitrification

Denitrification is a distinct means of energy conservation, making use of N oxides as terminal electron acceptors for cellular bioenergetics under anaerobic, microaerophilic, and occasionally aerobic conditions. The process is an essential branch of the global N cycle, reversing dinitrogen fixation, and is associated with chemolithotrophic, phototrophic, diazotrophic, or organotrophic metabolism but generally not with obligately anaerobic life. Discovered more than a century ago and believed to be exclusively a bacterial trait, denitrification has now been found in halophilic and hyperthermophilic archaea and in the mitochondria of fungi, raising evolutionarily intriguing vistas. Important advances in the biochemical characterization of denitrification and the underlying genetics have been achieved with Pseudomonas stutzeri, Pseudomonas aeruginosa, Paracoccus denitrificans, Ralstonia eutropha, and Rhodobacter sphaeroides. Pseudomonads represent one of the largest assemblies of the denitrifying bacteria within a single genus, favoring their use as model organisms. Around 50 genes are required within a single bacterium to encode the core structures of the denitrification apparatus. Much of the denitrification process of gram-negative bacteria has been found confined to the periplasm, whereas the topology and enzymology of the gram-positive bacteria are less well established. The activation and enzymatic transformation of N oxides is based on the redox chemistry of Fe, Cu, and Mo. Biochemical breakthroughs have included the X-ray structures of the two types of respiratory nitrite reductases and the isolation of the novel enzymes nitric oxide reductase and nitrous oxide reductase, as well as their structural characterization by indirect spectroscopic means. This revealed unexpected relationships among denitrification enzymes and respiratory oxygen reductases. Denitrification is intimately related to fundamental cellular processes that include primary and secondary transport, protein translocation, cytochrome c biogenesis, anaerobic gene regulation, metalloprotein assembly, and the biosynthesis of the cofactors molybdopterin and heme D1. An important class of regulators for the anaerobic expression of the denitrification apparatus are transcription factors of the greater FNR family. Nitrate and nitric oxide, in addition to being respiratory substrates, have been identified as signaling molecules for the induction of distinct N oxide-metabolizing enzymes.

Nitric oxide inhibition of cytochrome oxidase and mitochondrial respiration: implications for inflammatory, neurodegenerative and ischaemic pathologies

Nitric oxide (NO) at high levels is cytotoxic, and may be involved in a range of inflammatory, neurodegenerative, and cardiovascular/ischaemic pathologies. The mechanism of NO-induced cytotoxicity is unclear. Recently we and others have found that low (nanomolar) levels of NO reversibly inhibit mitochondrial respiration by binding to the oxygen binding site of cytochrome oxidase in competition with oxygen. This raises the apparent K(m) for oxygen of mitochondrial respiration into the physiological range, potentially making respiration sensitive to the oxygen level. The NO inhibition of oxygen consumption was seen in isolated cytochrome oxidase, mitochondria, brain nerve terminals, and cultured cells. Cultured astrocytes activated to express the inducible from of NO synthase produced up to 1 microM NO and strongly inhibited their own cellular respiration rate. This respiratory inhibition was rapidly reversed by removing the NO, and was due to the inhibition of cytochrome oxidase. These results suggest that any cell producing high levels of NO will inhibit its own respiration and that of surrounding cells, and make the respiration rate sensitive to the oxygen level. This inhibition of energy metabolism may contribute to cytotoxicity or cytostasis in some pathologies.

Nitric oxide ejects electrons from the binuclear centre of cytochrome c oxidase by reacting with oxidised copper: a general mechanism for the interaction of copper proteins with nitric oxide?

Small increases in NO concentration can inhibit mitochondrial oxygen consumption by reacting at the binuclear haem a3/CuB oxygen reduction site of cytochrome c oxidase. Here we demonstrate that under normal turnover conditions NO reacts initially with the oxidised CuB rather than the haem a3. We propose that hydration of an initial Cu+/NO+ complex forms nitrite, a proton and CuB+; the latter ejects an electron from the binuclear centre and results in the observed (100 s(-1)) reduction of other electron transfer centres in the enzyme (haem a and CuA). These reactions may have implications for the interactions of NO with other copper proteins.

Potential nitrosamine formation and its prevention during biological denitrification of red beet juice

High nitrate intake has been shown to result in an increased risk of endogenous formation of N-nitroso compounds. Certain vegetables and vegetable juices contain high concentrations of nitrate. Biological denitrification using strains of Paracoccus denitrificans (P.d.) has been proposed as effective means to reduce nitrate contents in such vegetable juices. During this bacterial denitrification process, substantial nitrite concentrations are transiently formed. This study investigated whether N-nitrosation reactions might occur. The easily nitrosatable amine morpholine was added to red beet juice at high concentration (100 ppm) during denitrification 10 different batches of red beet juice served as raw material. Each batch was submitted to denitrification in the presence and absence of ascorbic acid. In the absence of ascorbic acid, formation of N-nitrosomorpholine (NMOR) was observed in the low ppb range (0.5-8 ppb). Addition of ascorbic acid (500 mg/litre) inhibited the formation of NMOR, except for those instances where the pH was less than 6 and/or nitrate turnover was low (< 200 mg NO3-/litre/hr). Under conditions leading to high rates of nitrate turnover (> 200 mg NO3-/litre/hr), nitrosamine formation can reliably be prevented by ascorbic acid. The results show that bacterial denitrification of red beet juice high in nitrate can be accomplished without the risk of nitrosamine formation.

Role of nitric oxide and superoxide anion in leukotoxin-, 9,10-epoxy-12-octadecenoate-induced mitochondrial dysfunction

The present study was carried out to explore the involvement of nitric oxide (NO) and superoxide anion (O2.-) in Leukotoxin (Lx)-induced suppression of mitochondrial respiration. Glutamate- and succinate-dependent oxygen consumption and cytochrome c oxidase activity were assayed. Lx-induced mitochondrial damage was significantly attenuated by the pretreatment of lung with 4 x 10(-4) M NG-monomethyl-L-arginine (L-NMMA) or 500 units/ml superoxide dismutase (SOD) in ex vivo. However, L-NMMA plus SOD pretreatment showed no additive effect on the recovery of mitochondrial functions. The same assay was performed after the exposure of intact mitochondria to NO containing solution (1.25 x 10(-5) M) or 0.1 mM KO2/18-Crown-6 solution, which generated O2.-(6.4 x 10(-5) M). NO, but not O2.-, significantly inhibited the respiration of isolated mitochondria in vitro. Thus, there were great discrepancies in the involvement of NO and O2.- between ex vivo and in vitro system. Together with the previous reports, these facts suggested that the mechanisms by which NO and O2.- probably from vascular constituent cells inhibit mitochondrial respiration function of isolated perfused rat lung may not be simply due to their direct reactions with mitochondrial electron transport chain components, but may rely on the formation of peroxynitrite, and/or peroxynitrite-derived oxidants.

Reversible binding and inhibition of catalase by nitric oxide

The interactions between nitric oxide (NO), H2O2, and catalase were investigated. H2O2 did not cause detectable breakdown of NO in the absence of catalase, but did cause NO breakdown in the presence of catalase. Catalase bound NO, and NO rapidly and reversibly inhibited catalase with a Ki of 0.18 microM. The significance of these results for NO cytotoxicity is discussed.

Nitric oxide regulates mitochondrial respiration and cell functions by inhibiting cytochrome oxidase

Nitric oxide (NO) reversibly inhibits mitochondrial respiration by competing with oxygen at cytochrome oxidase. Concentrations of NO measured in a range of biological systems are similar to those shown to inhibit cytochrome oxidase and mitochondrial respiration. Inhibition of NO synthesis results in a stimulation of respiration in a number of systems. It is proposed that NO exerts some of its main physiological and pathological effects on cell functions by inhibiting cytochrome oxidase. Further NO may be a physiological regulator of the affinity of mitochondrial respiration for oxygen, enabling mitochondria to act as sensors of oxygen over the physiological range.

Production and consumption of nitric oxide by denitrifyingFlexibacter canadensis

Production and consumption of nitric oxide (NO) by Flexibacter canadensis cells under anaerobic conditions was investigated using a chemiluminescence NO analyzer. Net NO production from nitrite in the presence of carbonyl cyanide m-chlorophenylhydrazone (CCCP) was pH dependent, increased in the pH range from 4.5 to 6.5, and sharply decreased at pH >6.5. CCCP inhibited NO consumption but only at pH values ≤6.5. This can explain why CCCP stimulation of NO production depends on the pH. Denitrification of nitrite at high concentrations (≥5 mM) also resulted in net NO accumulation. Diethyldithiocarbamate, a copper chelating agent, prevented not only net production of NO during the reduction of nitrite in the presence of CCCP, but also production of nitrous oxide (N2O) from nitrite in the presence of C2H2. This suggests that F. canadensis may possess a copper-type nitrite reductase. However, cytochrome cd1- and copper-containing nitrite reductase DNA probes from Pseudomonas species did not hybridize with the total DNA of F. canadensis, indicating that the nitrite reductase of F. canadensis may possess unique properties. In addition to diethyldithiocarbamate, sulfide, carbon monoxide, azide, cyanide, hydroxylamine and Triton X-100 prevented net NO production from nitrite in the presence of CCCP, and also inhibited NO consumption. C2H2, an inhibitor of N2O reductase, did not affect NO production or consumption.Key words: nitrite reductase, nitric oxide (NO), carbonyl cyanide m-chlorophenylhydrazone (CCCP), Flexibacter canadensis.