The catabolic fate of nitric oxide: the nitric oxide oxidase and peroxynitrite reductase activities of cytochrome oxidase

NO and Heme Proteins: Cross-Talk between Heme and Cysteine Residues

Heme proteins are a diverse group that includes several unrelated families. Their biological function is mainly associated with the reactivity of the heme group, which-among several other reactions-can bind to and react with nitric oxide (NO) and other nitrogen compounds for their production, scavenging, and transport. The S-nitrosylation of cysteine residues, which also results from the reaction with NO and other nitrogen compounds, is a post-translational modification regulating protein activity, with direct effects on a variety of signaling pathways. Heme proteins are unique in exhibiting this dual reactivity toward NO, with reported examples of cross-reactivity between the heme and cysteine residues within the same protein. In this work, we review the literature on this interplay, with particular emphasis on heme proteins in which heme-dependent nitrosylation has been reported and those for which both heme nitrosylation and S-nitrosylation have been associated with biological functions.

The light and dark sides of nitric oxide: multifaceted roles of nitric oxide in plant responses to light

Light drives photosynthesis and informs plants about their surroundings. Regarded as a multifunctional signaling molecule in plants, nitric oxide (NO) has been repeatedly demonstrated to interact with light signaling cascades to control plant growth, development and metabolism. During early plant development, light-triggered NO accumulation counteracts negative regulators of photomorphogenesis and modulates the abundance of, and sensitivity to, plant hormones to promote seed germination and de-etiolation. In photosynthetically active tissues, NO is generated at distinct rates under light or dark conditions and acts at multiple target sites within chloroplasts to regulate photosynthetic reactions. Moreover, changes in NO concentrations in response to light stress promote plant defenses against oxidative stress under high light or ultraviolet-B radiation. Here we review the literature on the interaction of NO with the complicated light and hormonal signaling cascades controlling plant photomorphogenesis and light stress responses, focusing on the recently identified molecular partners and action mechanisms of NO in these events. We also discuss the versatile role of NO in regulating both photosynthesis and light-dependent stomatal movements, two key determinants of plant carbon gain. The regulation of nitrate reductase (NR) by light is highlighted as vital to adjust NO production in plants living under natural light conditions.

Real-Time Energy Exposure Is Associated With Increased Oxidative Stress Among Feeding-Tolerant Critically Ill Patients: Results From the FEDOX Trial

Background

Prospective randomized controlled trials (PRCTs) that found harm in patients receiving higher levels of energy exposure have been largely ignored, in part because of the lack of a known mechanism of harm.

Objective

The current 7‐day pilot study is a PRCT and post hoc analysis designed to explore the relationship between energy exposure and oxidative stress (as plasma total F2‐isoprostanes) in mechanically ventilated intensive care unit patients with systemic inflammatory response syndrome.

Methods

Thirty‐five participants were randomized to receive either 100% or 40% of their estimated energy needs. Our intent‐to‐treat model found no differences in F2‐isoprostanes between groups. A post hoc analysis revealed that on days when participants were in the highest tertile of daily kcal/kg, the real‐time energy flow rate within 2 hours of the blood draw was predictive of increased oxidative stress. On these days, participants in the second or third vs the first tertile of real‐time energy flow rate experienced a 41.8% (P = .006) or 26.5% (P = .001) increase in F2‐isoprostane levels, respectively. This was confirmed through a within‐group subanalysis restricted to participants with measurements on both sides of the median of real‐time energy flow rate that found a 28.2% F2‐isoprostane increase on days in the upper vs lower median of flow rate (P = .002).

Conclusion

The benefits of feeding may be more nuanced than previously suspected. Our findings imply a potential mechanism of harm in meeting the current recommendations for nutrition support in the critically ill that warrants further investigation.

Potential Pathway of Nitrous Oxide Formation in Plants

Plants can produce and emit nitrous oxide (N₂O), a potent greenhouse gas, into the atmosphere, and several field-based studies have concluded that this gas is emitted at substantial amounts. However, the exact mechanisms of N₂O production in plant cells are unknown. Several studies have hypothesised that plants might act as a medium to transport N₂O produced by soil-inhabiting microorganisms. Contrarily, aseptically grown plants and axenic algal cells supplied with nitrate (NO₃) are reported to emit N₂O, indicating that it is produced inside plant cells by some unknown physiological phenomena. In this study, the possible sites, mechanisms, and enzymes involved in N₂O production in plant cells are discussed. Based on the experimental evidence from various studies, we determined that N₂O can be produced from nitric oxide (NO) in the mitochondria of plants. NO, a signaling molecule, is produced through oxidative and reductive pathways in eukaryotic cells. During hypoxia and anoxia, NO₃ in the cytosol is metabolised to produce nitrite (NO₂), which is reduced to form NO via the reductive pathway in the mitochondria. Under low oxygen condition, NO formed in the mitochondria is further reduced to N₂O by the reduced form of cytochrome c oxidase (CcO). This pathway is active only when cells experience hypoxia or anoxia, and it may be involved in N₂O formation in plants and soil-dwelling animals, as reported previously by several studies. NO can be toxic at a high concentration. Therefore, the reduction of NO to N₂O in the mitochondria might protect the integrity of the mitochondria, and thus, protect the cell from the toxicity of NO accumulation under hypoxia and anoxia. As NO₃ is a major source of nitrogen for plants and all plants may experience hypoxic and anoxic conditions owing to soil environmental factors, a significant global biogenic source of N₂O may be its formation in plants via the proposed pathway.

Gaseous Signaling Molecules in Cardiovascular Function: From Mechanisms to Clinical Translation

Carbon monoxide (CO), hydrogen sulfide (H₂S), and nitric oxide (NO) constitute endogenous gaseous molecules produced by specific enzymes. These gases are chemically simple, but exert multiple effects and act through shared molecular targets to control both physiology and pathophysiology in the cardiovascular system (CVS). The gases act via direct and/or indirect interactions with each other in proteins such as heme-containing enzymes, the mitochondrial respiratory complex, and ion channels, among others. Studies of the major impacts of CO, H₂S, and NO on the CVS have revealed their involvement in controlling blood pressure and in reducing cardiac reperfusion injuries, although their functional roles are not limited to these conditions. In this review, the basic aspects of CO, H₂S, and NO, including their production and effects on enzymes, mitochondrial respiration and biogenesis, and ion channels are briefly addressed to provide insight into their biology with respect to the CVS. Finally, potential therapeutic applications of CO, H₂S, and NO with the CVS are addressed, based on the use of exogenous donors and different types of delivery systems.

Endogenous nitric oxide formation in cardiac myocytes does not control respiration during β-adrenergic stimulation

KEY POINTS

In the heart, endothelial nitric oxide (NO) controls oxygen consumption in the working heart through paracrine mechanisms. While cardiac myocytes contain several isoforms of NO synthases, it is unclear whether these can control respiration in an intracrine fashion. A long-standing controversy is whether a NOS exists within mitochondria. By combining fluorescence technologies with electrical field stimulation or the patch-clamp technique in beating cardiac myocytes, we identified a neuronal NO synthase (nNOS) as the most relevant source of intracellular NO during β-adrenergic stimulation, while no evidence for a mitochondria-located NOS was obtained. The amounts of NO produced by non-mitochondrial nNOS were insufficient to regulate respiration during β-adrenergic stimulation, arguing against intracrine control of respiration by NO within cardiac myocytes.

ABSTRACT

Endothelial nitric oxide (NO) controls cardiac oxygen (O₂ ) consumption in a paracrine way by slowing respiration at the mitochondrial electron transport chain. While NO synthases (NOSs) are also expressed in cardiac myocytes, it is unclear whether they control respiration in an intracrine way. Furthermore, the existence of a mitochondrial NOS is controversial. Here, by combining fluorescence imaging with electrical field stimulation, the patch-clamp method and knock-out technology, we determined the sources and consequences of intracellular NO formation during workload transitions in isolated murine and guinea pig cardiac myocytes and mitochondria. Using 4-amino-5-methylamino-2',7'-difluorofluorescein diacetate (DAF) as a fluorescent NO-sensor that locates to the cytosol and mitochondria, we observed that NO increased by ∼12% within 3 min of β-adrenergic stimulation in beating cardiac myocytes. This NO stems from neuronal NOS (nNOS), but not endothelial (eNOS). After patch clamp-mediated dialysis of cytosolic DAF, the remaining NO signals (mostly mitochondrial) were blocked by nNOS deletion, but not by inhibiting the mitochondrial Ca²⁺ uniporter with Ru360. While in isolated mitochondria exogenous NO inhibited respiration and reduced the NAD(P)H redox state, pyridine nucleotide redox states were unaffected by pharmacological or genetic disruption of endogenous nNOS or eNOS during workload transitions in cardiac myoctyes. We conclude that under physiological conditions, nNOS is the most relevant source for NO in cardiac myocytes, but this nNOS is not located in mitochondria and does not control respiration. Therefore, cardiac O₂ consumption is controlled by endothelial NO in a paracrine, but not intracrine, fashion.

Reactive Nitrogen Species in Mitochondria and Their Implications in Plant Energy Status and Hypoxic Stress Tolerance

Hypoxic and anoxic conditions result in the energy crisis that leads to cell damage. Since mitochondria are the primary organelles for energy production, the support of these organelles in a functional state is an important task during oxygen deprivation. Plant mitochondria adapted the strategy to survive under hypoxia by keeping electron transport operative even without oxygen via the use of nitrite as a terminal electrons acceptor. The process of nitrite reduction to nitric oxide (NO) in the mitochondrial electron transport chain recycles NADH and leads to a limited rate of ATP production. The produced ATP alongside with the ATP generated by fermentation supports the processes of transcription and translation required for hypoxic survival and recovery of plants. Non-symbiotic hemoglobins (called phytoglobins in plants) scavenge NO and thus contribute to regeneration of NAD(+) and nitrate required for the operation of anaerobic energy metabolism. This overall operation represents an important strategy of biochemical adaptation that results in the improvement of energy status and thereby in protection of plants in the conditions of hypoxic stress.

Three toxic gases meet in the mitochondria

The rationale of the study was two-fold: (i) develop a functional synthetic model of the Cytochrome c oxidase (CcO) active site, (ii) use it as a convenient tool to understand or predict the outcome of the reaction of CcO with ligands (physiologically relevant gases and other ligands). At physiological pH and potential, the model catalyzes the 4-electron reduction of oxygen. This model was immobilized on self-assembled-monolayer (SAM) modified electrode. During catalytic oxygen reduction, electron delivery through SAMs is rate limiting, similar to the situation in CcO. This model contains all three redox-active components in CcO's active site, which are required to minimize the production of partially-reduced-oxygen-species (PROS): Fe-heme (“heme a3”) in a myoglobin-like model fitted with a proximal imidazole ligand, and a distal tris-imidazole Copper (“Cu_B”) complex, where one imidazole is cross-linked to a phenol (mimicking “Tyr244”). This functional CcO model demonstrates how CcO itself might tolerate the hormone NO (which diffuses through the mitochondria). It is proposed that Cu_B delivers superoxide to NO bound to Fe-heme forming peroxynitrite, then nitrate that diffuses away. Another toxic gas, H₂S, has exceptional biological effects: at ~80 ppm, H₂S induces a state similar to hibernation in mice, lowering the animal's temperature and slowing respiration. Using our functional CcO model, we have demonstrated that at the same concentration range H₂S can reversibly inhibit catalytic oxygen reduction. Such a reversible catalytic process on the model was also demonstrated with an organic compound, tetrazole (TZ). Following studies showed that TZ reversibly inhibits respiration in isolated mitochondria, and induces deactivation of platelets, a mitochondria-rich key component of blood coagulation. Hence, this program is a rare example illustrating the use of a functional model to understand and predict physiologically important reactions at the active site of CcO.

Plant mitochondria: source and target for nitric oxide

Plant mitochondria generate nitric oxide (NO) under anoxia through the action of cytochrome c oxidase and other electron transport chain components on nitrite. This reductive mechanism operates under aerobic conditions at high electron transport rates. Indirect evidence also indicates that the oxidative pathway of NO production may be associated with mitochondria. We review the consequences of mitochondrial NO production, including the inhibition of oxygen uptake by cytochrome c oxidase, the inhibition of aconitase and succinate dehydrogenase, the induction of alternative oxidase, and the nitrosylation of several proteins, including glycine decarboxylase. The importance of these events in adaptation to abiotic and biotic stresses is discussed.

Short-term hypoxic vasodilation in vivo is mediated by bioactive nitric oxide metabolites, rather than free nitric oxide derived from haemoglobin-mediated nitrite reduction

Local increases in blood flow--'hypoxic vasodilation'--confer cellular protection in the face of reduced oxygen delivery. The physiological relevance of this response is well established, yet ongoing controversy surrounds its underlying mechanisms. We sought to confirm that early hypoxic vasodilation is a nitric oxide (NO)-mediated phenomenon and to study putative pathways for increased levels of NO, namely production from NO synthases, intravascular nitrite reduction, release from preformed stores and reduced deactivation by cytochrome c oxidase. Experiments were performed on spontaneously breathing, anaesthetized, male Wistar rats undergoing short-term systemic hypoxaemia, who received pharmacological inhibitors and activators of the various NO pathways. Arterial blood pressure, cardiac output, tissue oxygen tension and the circulating pool of NO metabolites (oxidation, nitrosation and nitrosylation products) were measured in plasma and erythrocytes. Hypoxaemia caused a rapid and sustained vasodilation, which was only partially reversed by non-selective NO synthase inhibition. This was associated with significantly lower plasma nitrite, and marginally elevated nitrate levels, suggestive of nitrite bioinactivation. Administration of sodium nitrite had little effect in normoxia, but produced significant vasodilation and increased nitrosylation during hypoxaemia that could not be reversed by NO scavenging. Methodological issues prevented assessment of the contribution, if any, of reduced deactivation of NO by cytochrome c oxidase. In conclusion, acute hypoxic vasodilation is an adaptive NO-mediated response conferred through bioactive metabolites rather than free NO from haemoglobin-mediated reduction of nitrite.

The key role of nitric oxide in hypoxia: hypoxic vasodilation and energy supply-demand matching

SIGNIFICANCE

A mismatch between energy supply and demand induces tissue hypoxia with the potential to cause cell death and organ failure. Whenever arterial oxygen concentration is reduced, increases in blood flow--hypoxic vasodilation--occur in an attempt to restore oxygen supply. Nitric oxide (NO) is a major signaling and effector molecule mediating the body's response to hypoxia, given its unique characteristics of vasodilation (improving blood flow and oxygen supply) and modulation of energetic metabolism (reducing oxygen consumption and promoting utilization of alternative pathways).

RECENT ADVANCES

This review covers the role of oxygen in metabolism and responses to hypoxia, the hemodynamic and metabolic effects of NO, and mechanisms underlying the involvement of NO in hypoxic vasodilation. Recent insights into NO metabolism will be discussed, including the role for dietary intake of nitrate, endogenous nitrite (NO₂⁻) reductases, and release of NO from storage pools. The processes through which NO levels are elevated during hypoxia are presented, namely, (i) increased synthesis from NO synthases, increased reduction of NO₂⁻ to NO by heme- or pterin-based enzymes and increased release from NO stores, and (ii) reduced deactivation by mitochondrial cytochrome c oxidase.

CRITICAL ISSUES

Several reviews covered modulation of energetic metabolism by NO, while here we highlight the crucial role NO plays in achieving cardiocirculatory homeostasis during acute hypoxia through both vasodilation and metabolic suppression.

FUTURE DIRECTIONS

We identify a key position for NO in the body's adaptation to an acute energy supply-demand mismatch.

Interaction of NO with Cu and heme-bound Aβ peptides associated with Alzheimer's disease

Reduced Cu and heme has been invoked to be involved in Alzheimer's disease (AD). Recently the Aβ peptides have been demonstrated to bind heme and Cu simultaneously, and this complex produces significantly more toxic partially reduced oxygen species (PROS) than the Cu or heme-bound Aβ peptides. Here a combination of absorption, EPR, and resonance Raman spectroscopy along with kinetic assays are used to investigate the interaction of nitric oxide (NO) with the physiologically relevant form of Cu and heme-bound Aβ peptides, since a down-regulation of nitric oxide synthase activity is observed in patients suffering from AD. The data indicate that NO oxidizes the Cu(I) sites, making them less toxic toward PROS generation and releases heme from the Aβ peptides ameliorating the effects of heme binding to Aβ peptides associated with AD. This process involves a tyrosine-mediated electron transfer between the Cu and heme sites. These results provide a mechanistic pathway for the possible protective role of NO in AD.

Susceptibility of mitochondrial electron-transport complexes to oxidative damage. Focus on cytochrome c oxidase

Reactive oxygen species (ROS) are associated with a number of mitochondrial disorders. These include: ischemia/reperfusion injury, Parkinson's disease, Alzheimer's disease, neurodegenerative diseases, and other age-related degenerative changes. ROS can be generated at numerous sites within the cell, but the mitochondrial electron transport chain is recognized as the major source of intracellular ROS. Two mitochondrial electron-transfer complexes are major sources of ROS: complex I and complex III. Oxidative damage to either of these complexes, or to electron transport complexes that are in close proximity to these ROS sources, e.g., cytochrome c oxidase, would be expected to inhibit electron transport. Such inhibition would lead to increased electron leakage and more ROS production, much like the well-known effect of adding electron transport inhibitors. Recent studies reveal that ROS and lipid peroxidation products are effective inhibitors of the electron-transport complexes. In some cases, inactivation of enzymes correlates with chemical modification of only a small number of unusually reactive amino acids. In this article, we review current knowledge of ROS-induced alterations within three complexes: (1) complex IV; (2) complex III; and (3) complex I. Our goal is to identify "hot spots" within each complex that are easily chemically modified and could be responsible for ROS-induced inhibition of the individual complexes. Special attention has been placed on ROS-induced damage to cardiolipin that is tightly bound to each of the inner membrane protein complexes. Peroxidation of the bound cardiolipin is thought to be particularly important since its close proximity and long residence time on the protein make it an especially effective reagent for subsequent ROS-induced damage to these proteins.

The Chemical Interplay between Nitric Oxide and Mitochondrial Cytochrome c Oxidase: Reactions, Effectors and Pathophysiology

Nitric oxide (NO) reacts with Complex I and cytochrome c oxidase (CcOX, Complex IV), inducing detrimental or cytoprotective effects. Two alternative reaction pathways (PWs) have been described whereby NO reacts with CcOX, producing either a relatively labile nitrite-bound derivative (CcOX-NO₂  ⁻, PW1) or a more stable nitrosyl-derivative (CcOX-NO, PW2). The two derivatives are both inhibited, displaying different persistency and O₂ competitiveness. In the mitochondrion, during turnover with O₂, one pathway prevails over the other one depending on NO, cytochrome c²⁺ and O₂ concentration. High cytochrome c²⁺, and low O₂ proved to be crucial in favoring CcOX nitrosylation, whereas under-standard cell-culture conditions formation of the nitrite derivative prevails. All together, these findings suggest that NO can modulate physiologically the mitochondrial respiratory/OXPHOS efficiency, eventually being converted to nitrite by CcOX, without cell detrimental effects. It is worthy to point out that nitrite, far from being a simple oxidation byproduct, represents a source of NO particularly important in view of the NO cell homeostasis, the NO production depends on the NO synthases whose activity is controlled by different stimuli/effectors; relevant to its bioavailability, NO is also produced by recycling cell/body nitrite. Bioenergetic parameters, such as mitochondrial ΔΨ, lactate, and ATP production, have been assayed in several cell lines, in the presence of endogenous or exogenous NO and the evidence collected suggests a crucial interplay between CcOX and NO with important energetic implications.

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.

The anoxic plant mitochondrion as a nitrite: NO reductase

Under the conditions of oxygen deprivation, accumulating nitrite can be reduced in the mitochondrial electron transport chain forming free radical nitric oxide (NO). By reducing nitrite to NO, plant mitochondria preserve the capacity to oxidize external NADH and NADPH and retain a limited power for ATP synthesis complementing glycolytic ATP production. NO participates in O(2) balance in mitochondria by competitively inhibiting cytochrome c oxidase which can oxidize it to nitrite when oxygen concentration increases. Some of the NO escapes to the cytosol, where the efficient scavenging system involving non-symbiotic hemoglobin oxygenates NO to nitrate and supports continuous anaerobic turnover of nitrogen species.

Bioinspired heme, heme/nonheme diiron, heme/copper, and inorganic NOx chemistry: *NO((g)) oxidation, peroxynitrite-metal chemistry, and *NO((g)) reductive coupling

The focus of this Forum Article highlights work from our own laboratories and those of others in the area of biochemical and biologically inspired inorganic chemistry dealing with nitric oxide [nitrogen monoxide, *NO((g))] and its biological roles and reactions. The latter focus is on (i) oxidation of *NO((g)) to nitrate by nitric oxide dioxygenases (NODs) and (ii) reductive coupling of two molecules of *NO((g)) to give N(2)O(g). In the former case, NODs are described, and the highlighting of possible peroxynitrite/heme intermediates and the consequences of this are given by a discussion of recent works with myoglobin and a synthetic heme model system for NOD action. Summaries of recent copper complex chemistries with *NO((g)) and O(2)(g), leading to peroxynitrite species, are given. The coverage of biological reductive coupling of *NO((g)) deals with bacterial nitric oxide reductases (NORs) with heme/nonheme diiron active sites and on heme/copper oxidases such as cytochrome c oxidase, which can mediate the same chemistry. Recently designed protein and synthetic model compounds (heme/nonheme/diiron or heme/copper) as functional mimics are discussed in some detail. We also highlight examples from the chemical literature, not necessarily involving biologically relevant metal ions, that describe the oxidation of *NO((g)) to nitrate (or nitrite) and possible peroxynitrite intermediates or reductive coupling of *NO((g)) to give nitrous oxide.

In vitro and in vivo induction and activation of nNOS by LPS in oligodendrocytes

There are currently four known isoforms of nitric oxide synthase (NOS). Of these, neuronal NOS (nNOS) is known to be present exclusively in neurons, endothelial NOS (eNOS) in vascular endothelium, while the inducible form of NOS (iNOS) is known to be activated in oligodendrocytes, astrocytes and microglia. The fourth isoform, mitochondrial NOS (mtNOS), represents a post-translational modification of nNOS. Using western blotting and real time-PCR, we show induction and activation of nNOS following culture of oligodendrocyte progenitor cells (OPC) with lipopolysaccharide (LPS). Activation of nNOS results in accumulation of peroxynitrite and tyrosine nitration of proteins in oligodendrocytes resulting in reduced cell viability. Injection of LPS in vivo into the corpus callosum of rats leads to the development of extensive demyelination of the white matter tracts. Immunostaining of regions close to the injection site shows the presence of nNOS, but not iNOS, in oligodendrocytes. Neither iNOS nor nNOS was seen in astrocytes in areas of demyelination. These studies suggest that activation of nNOS in oligodendrocytes leads to oligodendrocyte injury resulting in demyelination.

Direct regulation of striated muscle myosins by nitric oxide and endogenous nitrosothiols

BACKGROUND

Nitric oxide (NO) has long been recognized to affect muscle contraction, both through activation of guanylyl cyclase and through modification of cysteines in proteins to yield S-nitrosothiols. While NO affects the contractile apparatus directly, the identities of the target myofibrillar proteins remain unknown. Here we report that nitrogen oxides directly regulate striated muscle myosins.

PRINCIPAL FINDINGS

Exposure of skeletal and cardiac myosins to physiological concentrations of nitrogen oxides, including the endogenous nitrosothiol S-nitroso-L-cysteine, reduced the velocity of actin filaments over myosin in a dose-dependent and oxygen-dependent manner, caused a doubling of force as measured in a laser trap transducer, and caused S-nitrosylation of cysteines in the myosin heavy chain. These biomechanical effects were not observed in response to S-nitroso-D-cysteine, demonstrating specificity for the naturally occurring isomer. Both myosin heavy chain isoforms in rats and cardiac myosin heavy chain from human were S-nitrosylated in vivo.

SIGNIFICANCE

These data show that nitrosylation signaling acts as a molecular "gear shift" for myosin--an altogether novel mechanism by which striated muscle and cellular biomechanics may be regulated.

Anoxic nitric oxide cycling in plants: participating reactions and possible mechanisms

At sufficiently low oxygen concentrations, hemeproteins are deoxygenated and become capable of reducing nitrite to nitric oxide (NO), in a reversal of the reaction in which NO is converted to nitrate or nitrite by oxygenated hemeproteins. The maximum rates of NO production depend on the oxygen avidity. The hemeproteins with the highest avidity, such as hexacoordinate hemoglobins, retain oxygen even under anoxic conditions resulting in their being extremely effective NO scavengers but essentially incapable of producing NO. Deoxyhemeprotein-related NO production can be observed in mitochondria (at the levels of cytochrome c oxidase, cytochrome c, complex III and possibly other sites), in plasma membrane, cytosol, endoplasmic reticulum and peroxisomes. In mitochondria, the use of nitrite as an alternative electron acceptor can contribute to a limited rate of ATP synthesis. Non-heme metal-containing proteins such as nitrate reductase and xanthine oxidase can also be involved in NO production. This will result in a strong anoxic redox flux of nitrogen through the hemoglobin-NO cycle involving nitrate reductase, nitrite: NO reductase, and NO dioxygenase. In normoxic conditions, NO is produced in very low quantities, mainly for signaling purposes and this nitrogen cycling is inoperative.

Kinetic model of the inhibition of respiration by endogenous nitric oxide in intact cells

Nitric oxide (NO) inhibits mitochondrial respiration by decreasing the apparent affinity of cytochrome c oxidase (CcO) for oxygen. Using iNOS-transfected HEK 293 cells to achieve regulated intracellular NO production, we determined NO and O(2) concentrations and mitochondrial O(2) consumption by high-resolution respirometry over a range of O(2) concentrations down to nanomolar. Inhibition of respiration by NO was reversible, and complete NO removal recovered cell respiration above its routine reference values. Respiration was observed even at high NO concentrations, and the dependence of IC(50) on [O(2)] exhibits a characteristic but puzzling parabolic shape; both these features imply that CcO is protected from complete inactivation by NO and are likely to be physiologically relevant. We present a kinetic model of CcO inhibition by NO that efficiently predicts experimentally determined respiration at physiological O(2) and NO concentrations and under hypoxia, and accurately predicts the respiratory responses under hyperoxia. The model invokes competitive and uncompetitive inhibition by binding of NO to the reduced and oxidized forms of CcO, respectively, and suggests that dissociation of NO from reduced CcO may involve its O(2)-dependent oxidation. It also explains the non-linear dependence of IC(50) on O(2) concentration, and the hyperbolic increase of c(50) as a function of NO concentration.

Myosin is reversibly inhibited by S-nitrosylation

Nitric oxide (NO*) is synthesized in skeletal muscle and its production increases during contractile activity. Although myosin is the most abundant protein in muscle, it is not known whether myosin is a target of NO* or NO* derivatives. In the present study, we have shown that exercise increases protein S-nitrosylation in muscle, and, among contractile proteins, myosin is the principal target of exogenous SNOs (S-nitrosothiols) in both skinned skeletal muscle fibres and differentiated myotubes. The reaction of isolated myosin with S-nitrosoglutathione results in S-nitrosylation at multiple cysteine thiols and produces two populations of protein-bound SNOs with different stabilities. The less-stable population inhibits the physiological ATPase activity, without affecting the affinity of myosin for actin. However, myosin is neither inhibited nor S-nitrosylated by the NO* donor diethylamine NONOate, indicating a requirement for transnitrosylation between low-mass SNO and myosin cysteine thiols rather than a direct reaction of myosin with NO* or its auto-oxidation products. Interestingly, alkylation of the most reactive thiols of myosin by N-ethylmaleimide does not inhibit formation of a stable population of protein-SNOs, suggesting that these sites are located in less accessible regions of the protein than those that affect activity. The present study reveals a new link between exercise and S-nitrosylation of skeletal muscle contractile proteins that may be important under (patho)physiological conditions.

A peroxynitrite complex of copper: formation from a copper-nitrosyl complex, transformation to nitrite and exogenous phenol oxidative coupling or nitration

Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)-(*NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO(-))-Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO(2) (-)) complex and 0.5 mol equiv O(2). In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper-nitrosyl and copper-peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

Mitochondrial generation of free radicals and hypoxic signaling

Most reactive oxygen species (ROS) are generated in cells by the mitochondrial respiratory chain. Mitochondrial ROS production is modulated largely by the rate of electron flow through respiratory chain complexes. Recently, it has become clear that under hypoxic conditions, the mitochondrial respiratory chain also produces nitric oxide (NO), which can generate other reactive nitrogen species (RNS). Although excess ROS and RNS can lead to oxidative and nitrosative stress, moderate to low levels of both function in cellular signaling pathways. Especially important are the roles of these mitochondrially generated free radicals in hypoxic signaling pathways, which have important implications for cancer, inflammation and a variety of other diseases.

Nitric oxide, cytochrome C oxidase, and the cellular response to hypoxia

Cytochrome c oxidase (CcO; complex IV of the mitochondrial electron transport chain) is the primary site of cellular oxygen consumption and, as such, is central to oxidative phosphorylation and the generation of adenosine-triphosphate. Nitric oxide (NO), an endogenously-generated gas, modulates the activity of CcO. Depending on the intracellular oxygen concentration and the resultant dominant redox state of CcO, the interaction between CcO and NO can have a range of signaling consequences for cells in the perception of changes in oxygen concentration and the initiation of adaptive responses. At higher oxygen concentrations, when CcO is predominantly in an oxidized state, it consumes NO. At lower oxygen concentrations, when CcO is predominantly reduced, NO is not consumed and accumulates in the microenvironment, with implications for both the respiratory rate of cells and the local vascular tone. Changes in the availability of intracellular oxygen and in the generation of reactive oxygen species that accompany these interactions result in cell signaling and in regulation of oxygen-sensitive pathways that ultimately determine the nature of the cellular response to hypoxia.

Antagonism of nitric oxide toward the inhibition of cytochrome c oxidase by carbon monoxide and cyanide

The principle mitochondrial target where the respiratory inhibitors CO, CN(-), and NO act in the execution of their acute toxic effects is complex IV of the electron-transport chain, cytochrome c oxidase. However, there is a paucity of studies in the literature regarding the concerted effects of such poisons. Accordingly, the combined inhibitory effects of CO + CN(-), NO + CN(-), and NO + CO on the activity of cytochrome c oxidase preparations are reported. Only in the case of CO + CN(-) do the effects of the two inhibitors seem to be additive as expected. NO appears to be antagonistic toward the effects of the other two inhibitors; that is, the effects of both CO an CN(-) on enzyme activity are ameliorated by NO when present. To further clarify these observations, the ligand substitutions of heme-bound CN(-) by NO in cytochrome c oxidase and hemoglobin have also been briefly investigated. These results suggest that displacement of CN(-) from the ferric hemoproteins by NO is rate-limited by heme reduction-and in the case of the enzyme, the presence of nonligand-binding electron-transfer centers facilitates the reaction. The findings are discussed in relation to the idea that NO does not behave as a classic reversible (by dissociation) inhibitor.

Plant mitochondrial function during anaerobiosis

BACKGROUND

Under hypoxic conditions, plant mitochondria preserve the capacity to oxidize external NADH, NADPH and tricarboxylic acid cycle substrates. Nitrite serves as an alternative electron acceptor at the level of cytochrome oxidase, with possibly complex III and the alternative oxidase also being involved. Nitric oxide is a significant product of the reaction, which has a high affinity for cytochrome c oxidase, inhibiting it. The excess NO is scavenged by hypoxically induced class 1 haemoglobin in the reaction involving ascorbate.

SCOPE

By using nitrite, mitochondria retain a limited capacity for ATP synthesis. NADH, produced from glycolysis during anaerobiosis and oxidized in the mitochondrial electron transport chain, should shift the composition of metabolites formed during anaerobiosis with increased conversion of pyruvate to alanine and greater involvement of other transamination reactions, such as those involving gamma-aminobutyric acid formation.

CONCLUSIONS

Anaerobic mitochondrial metabolism may have a more significant role than previously thought in alleviating the effects of anoxia on plant cells. There is a need to re-examine mitochondrial carbon and nitrogen metabolism under anoxia to establish the extent of this involvement.

Interaction of nitric oxide with a functional model of cytochrome c oxidase

Cytochrome c oxidase (CcO) is a multimetallic enzyme that carries out the reduction of O2 to H2O and is essential to respiration, providing the energy that powers all aerobic organisms by generating heat and forming ATP. The oxygen-binding heme a(3) should be subject to fatal inhibition by chemicals that could compete with O2 binding. Near the CcO active site is another enzyme, NO synthase, which produces the gaseous hormone NO. NO can strongly bind to heme a(3), thus inhibiting respiration. However, this disaster does not occur. Using functional models for the CcO active site, we show how NO inhibition is avoided; in fact, it is found that NO can protect the respiratory enzyme from other inhibitors such as cyanide, a classic poison.

Reaction of a copper-dioxygen complex with nitrogen monoxide (*NO) leads to a copper(II)-peroxynitrite species

A discrete peroxynitrite-copper(II) complex, [(TMG3tren)CuII(-OONO)]+ (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting *NO(g) with superoxo complex [(TMG3tren)CuII(O2*-)]+ (2). Complex 3 undergoes a thermal transformation to give CuII-nitrite complex [(TMG3tren)CuII(-ONO)]+ (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k2-O,O'-OONO bound peroxynitrite moiety and dx2-y2 ground state is proposed. Experiments using 18O2 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 18O2 containing 3 undergoes O-O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological CuI/O2/(*NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus *NO reaction.

Cytochrome c oxidase regulates endogenous nitric oxide availability in respiring cells: a possible explanation for hypoxic vasodilation

One of the many routes proposed for the cellular inactivation of endogenous nitric oxide (NO) is by the cytochrome c oxidase of the mitochondrial respiratory chain. We have studied this possibility in human embryonic kidney cells engineered to generate controlled amounts of NO. We have used visible light spectroscopy to monitor continuously the redox state of cytochrome c oxidase in an oxygen-tight chamber, at the same time as which we measure cell respiration and the concentrations of oxygen and NO. Pharmacological manipulation of cytochrome c oxidase indicates that this enzyme, when it is in turnover and in its oxidized state, inactivates physiological amounts of NO, thus regulating its intra- and extracellular concentrations. This inactivation is prevented by blocking the enzyme with inhibitors, including NO. Furthermore, when cells generating low concentrations of NO respire toward hypoxia, the redox state of cytochrome c oxidase changes from oxidized to reduced, leading to a decrease in NO inactivation. The resultant increase in NO concentration could explain hypoxic vasodilation.

On the biologic role of the reaction of NO with oxidized cytochrome c oxidase

The inhibition of cytochrome c oxidase (CcOX) by nitric oxide (NO) is analyzed with a mathematical model that simulates the metabolism in vivo. The main results were the following: (a) We derived novel equations for the catalysis of CcOX that can be used to predict CcOX inhibition in any tissue for any [NO] or [O(2)]; (b) Competitive inhibition (resulting from the reversible binding of NO to reduced CcOX) emerges has the sole relevant component of CcOX inhibition under state 3 in vivo; (c) In state 4, contribution of uncompetitive inhibition (resulting from the reaction of oxidized CcOX with NO) represents a significant nonmajority fraction of inhibition, being favored by high [O(2)]; and (d) The main biologic role of the reaction between NO and oxidized CcOX is to consume NO. By reducing [NO], this reaction stimulates, rather than inhibits, respiration. Finally, we propose that the biologic role of NO as an inhibitor of CcOX is twofold: in state 4, it avoids an excessive buildup of mitochondrial membrane potential that triggers rapid production of oxidants, and in state 3, increases the efficiency of oxidative phosphorylation by increasing the ADP/O ratio, supporting the therapeutic use of NO in situations in which mitochondria are dysfunctional.

Nitrite-driven anaerobic ATP synthesis in barley and rice root mitochondria

Mitochondria isolated from the roots of barley (Hordeum vulgare L.) and rice (Oryza sativa L.) seedlings were capable of oxidizing external NADH and NADPH anaerobically in the presence of nitrite. The reaction was linked to ATP synthesis and nitric oxide (NO) was a measurable product. The rates of NADH and NADPH oxidation were in the range of 12-16 nmol min(-1) mg(-1) protein for both species. The anaerobic ATP synthesis rate was 7-9 nmol min(-1) mg(-1) protein for barley and 15-17 nmol min(-1) mg(-1) protein for rice. The rates are of the same order of magnitude as glycolytic ATP production during anoxia and about 3-5% of the aerobic mitochondrial ATP synthesis rate. NADH/NADPH oxidation and ATP synthesis were sensitive to the mitochondrial inhibitors myxothiazol, oligomycin, diphenyleneiodonium and insensitive to rotenone and antimycin A. The uncoupler FCCP completely eliminated ATP production. Succinate was also capable of driving ATP synthesis. We conclude that plant mitochondria, under anaerobic conditions, have a capacity to use nitrite as an electron acceptor to oxidize cytosolic NADH/NADPH and generate ATP.

Nitric oxide and peroxynitrite in health and disease

The discovery that mammalian cells have the ability to synthesize the free radical nitric oxide (NO) has stimulated an extraordinary impetus for scientific research in all the fields of biology and medicine. Since its early description as an endothelial-derived relaxing factor, NO has emerged as a fundamental signaling device regulating virtually every critical cellular function, as well as a potent mediator of cellular damage in a wide range of conditions. Recent evidence indicates that most of the cytotoxicity attributed to NO is rather due to peroxynitrite, produced from the diffusion-controlled reaction between NO and another free radical, the superoxide anion. Peroxynitrite interacts with lipids, DNA, and proteins via direct oxidative reactions or via indirect, radical-mediated mechanisms. These reactions trigger cellular responses ranging from subtle modulations of cell signaling to overwhelming oxidative injury, committing cells to necrosis or apoptosis. In vivo, peroxynitrite generation represents a crucial pathogenic mechanism in conditions such as stroke, myocardial infarction, chronic heart failure, diabetes, circulatory shock, chronic inflammatory diseases, cancer, and neurodegenerative disorders. Hence, novel pharmacological strategies aimed at removing peroxynitrite might represent powerful therapeutic tools in the future. Evidence supporting these novel roles of NO and peroxynitrite is presented in detail in this review.

Human heart mitochondria do not produce physiologically relevant quantities of nitric oxide

Previous studies raised the possibility that nitric oxide synthase is present in heart mitochondria (mtNOS) and the existence of such an enzyme became generally accepted. However, original experimental evidence is rather scarce and positive identification of the enzyme is lacking. We aimed to detect an NOS protein in human and mouse heart mitochondria and to measure the level of NO released from the organelles. Western blotting with 7 different anti-NOS antibodies failed to detect a NOS-like protein in mitochondria. Immunoprecipitation or substrate-affinity purification of the samples concentrated NOS in control preparations but not in mitochondria. Release of NO from live respiring human mitochondria was below 2 ppb after 45 min of incubation. In a bioassay system, mitochondrial suspension failed to cause vasodilation of human mammary artery segments. These results indicate that mitochondria do not produce physiologically relevant quantities of NO in the heart and are unlikely to have any physiological importance as NO donors, nor do they contain a recognizable mtNOS enzyme.

Mitochondria produce reactive nitrogen species via an arginine-independent pathway

We measured the contribution of mitochondrial nitric oxide synthase (mtNOS) and respiratory chain enzymes to reactive nitrogen species (RNS) production. Diaminofluorescein (DAF) was applied for the assessment of RNS production in isolated mouse brain, heart and liver mitochondria and also in a cultured neuroblastoma cell line by confocal microscopy and flow cytometry. Mitochondria produced RNS, which was inhibited by catalysts of peroxynitrite decomposition but not by nitric oxide (NO) synthase inhibitors. Disrupting the organelles or withdrawing respiratory substrates markedly reduced RNS production. Inhibition of complex I abolished the DAF signal, which was restored by complex II substrates. Inhibition of the respiratory complexes downstream from the ubiquinone/ubiquinol cycle or dissipating the proton gradient had no effect on DAF fluorescence. We conclude that mitochondria from brain, heart and liver are capable of significant RNS production via the respiratory chain rather than through an arginine-dependent mtNOS.

Mitochondrial NO and reactive nitrogen species production: Does mtNOS exist?

It is more than 10 years now that mitochondria are suspected to be sources of nitric oxide (NO). This hypothesis is intriguing since NO has multiple targets within the organelle and it is even suggested that mitochondria are the primary targets of NO in the cell. Most remarkably, nanomolar concentrations of NO can inhibit mitochondrial respiration, so even a small amount of NO in the mitochondrial matrix may regulate ATP synthesis. Therefore, the idea that mitochondria themselves are capable of NO production is an important concept in several physiological and pathological mechanisms. However, this field of research generates surprisingly few original papers and the published studies contain conflicting results. The reliability of the results is frequently questioned since they are seldom reproduced by independent investigators. Until 2003, all papers published in this field showed affirmative results but since then several studies directly challenged the existence of a mitochondrial nitric oxide synthase. The present review aims to summarize the most recent developments in mitochondrial NO production, highlights a few unsolved questions, and proposes new directions for future work in this research area.

Recent advances in the understanding of the role of nitric oxide in cardiovascular homeostasis

Nitric oxide synthases (NOS) are the enzymes responsible for nitric oxide (NO) generation. To date, 3 distinct NOS isoforms have been identified: neuronal NOS (NOS1), inducible NOS (NOS2), and endothelial NOS (NOS3). Biochemically, NOS consists of a flavin-containing reductase domain, a heme-containing oxygenase domain, and regulatory sites. NOS catalyse an overall 5-electron oxidation of one Nomega-atom of the guanidino group of L-arginine to form NO and L-citrulline. NO exerts a plethora of biological effects in the cardiovascular system. The basal formation of NO in mitochondria by a mitochondrial NOS seems to be one of the main regulators of cellular respiration, mitochondrial transmembrane potential, and transmembrane proton gradient. This review focuses on recent advances in the understanding of the role of enzyme and enzyme-independent NO formation, regulation of NO bioactivity, new aspects of NO on cardiac function and morphology, and the clinical impact and perspectives of these recent advances in our knowledge on NO-related pathways.

Glutathione depletion renders rat hepatocytes sensitive to nitric oxide donor-mediated toxicity

Nitric oxide (NO) can be either cytoprotective or cytotoxic in hepatocytes, depending on conditions within the cell. We hypothesized that redox status is a determinant of NO effects on cell viability. To cause the disturbance of redox homeostasis in the hepatocytes, cells were treated with the following glutathione (GSH) depleting agents: (1) chronic depletion by 18 hours pretreatment with buthionine sulfoximine (BSO), which depletes GSH by blocking its biosynthesis; and (2) acute depletion by 1 hour pretreatment with diethyl maleate (DEM), which conjugates GSH by the GSH-S-transferase catalyzed reaction. S-nitroso-N-acetyl-D,L-penicillamine (SNAP), a NO donor, was added after removal of GSH-depleting agents. Individual treatment with either SNAP or GSH depletion did not appreciably affect viability. A significant increase of cytotoxicity in hepatocytes was observed with the combination of a concentration and time course regimen of SNAP and GSH depletion. SNAP treatment of GSH-depleted hepatocytes led to an increase in LDH release and oxidative stress, disruption of mitochondrial membrane potential, the presence of nitrotyrosine (an indicator of peroxynitrite (ONOO-) generation), and a decrease in adenosine triphosphate (ATP) content. The interference of mitochondrial respiratory enzymes, especially with the combination treatments, indicated different levels of disturbance of electron transfer, superoxide generation, and ATP production. Other commonly used NO donors were found to exhibit lower and slower toxicity in the setting of GSH depletion than that evident with SNAP. In conclusion, the disruption of cellular redox homeostasis by GSH depletion leads hepatocytes to be more susceptible to NO (especially S-nitrosothiols) and subsequent necrotic cell death.

The haemoglobin/nitric oxide cycle: involvement in flooding stress and effects on hormone signalling

BACKGROUND

Class 1 haemoglobins (Hbs) are induced in plant cells under hypoxic conditions. They have a high affinity for oxygen, which is two orders of magnitude lower than that of cytochrome oxidase, permitting the utilization of oxygen by the molecule at extremely low oxygen concentrations. Their presence reduces the levels of nitric oxide (NO) that is produced from nitrate ion during hypoxia and improves the redox and energy status of the hypoxic cell.

SCOPE

The mechanism by which Hb interacts with NO under hypoxic conditions in plants is examined, and the effects of Hb expression on metabolism and signal transduction are discussed.

CONCLUSIONS

The accumulated evidence suggests that a metabolic pathway involving NO and Hb provides an alternative type of respiration to mitochondrial electron transport under limited oxygen. Hb in hypoxic plants acts as part of a soluble, terminal, NO dioxygenase system, yielding nitrate ion from the reaction of oxyHb with NO. NO is mainly formed due to anaerobic accumulation of nitrite. The overall reaction sequence, referred to as the Hb/NO cycle, consumes NADH and maintains ATP levels via an as yet unknown mechanism. Hb gene expression appears to influence signal transduction pathways, possibly through its effect on NO, as evidenced by phenotypic changes in normoxic Hb-varying transgenic plants. Ethylene levels are elevated when Hb gene expression is suppressed, which could be a factor leading to root aerenchyma formation during hypoxic stress.

Involvement of phospholipid, biomembrane integrity, and NO peroxidase activity in the NO catabolism by cytochrome c oxidase

The physiological regulation of mitochondrial respiration by NO has been reported to result from the reversible binding of NO to the two-electron reduced binuclear center (Fe(2+)(a3)-Cu(1+)(B)) of cytochrome c oxidase (CcO). Although the role of CcO and its derived catalytic intermediates in the catabolism of NO has been documented, little has been established for the enzyme in its fully oxidized state (Fe(3+)(a3)-Cu(2+)(B)). We report: (1) CcO, in its fully oxidized state, represents the major component of the mitochondrial electron transport chain for NO consumption as controlled by the binding of NO to its binuclear center. Phospholipid enhances NO consumption by fully oxidized CcO, whereas the consumption of NO is slowed down by membrane structure and membrane potential when CcO is embedded in the phospholipid bilayer. (2) In the presence of H(2)O(2), CcO was shown to serve as a mitochondria-derived NO peroxidase. A CcO-derived protein radical intermediate was induced and involved in the modulation of NO catabolism.

Melatonin and nitric oxide: two required antagonists for mitochondrial homeostasis

The presence of nitric oxide (NO* ) in the mitochondria led to analysis of its source and functions in mitochondrial homeostasis. Studies have revealed the existence of a mtNOS isoform with similar features to nNOS, with some post-traslational modifications, although without the typical signal peptide responsible for addressing proteins to mitochondrion. This isoform may account for the physiological production of NO* related to the respiratory control. During inflammatory conditions there is an excess of NO* in the mitochondria responsible for an increase in reactive oxygen and nitrogen species in sufficient amounts to compromise mitochondrial function. These conditions led to the discovery of the presence of an inducible mtNOS isoform with kinetic properties similar to iNOS. Experiments with knockout mice lacking either nNOS or iNOS further confirmed the existence of these two mtNOS isoforms in mitochondria. Although the increase in NO* in sepsis by inducible mtNOS may have important regulatory functions including the redistribution of oxygen into other pathways under hypoxia, it causes the production of excess NO* that is deleterious for the cell. Melatonin, an endogenous antioxidant, regulates mitochondrial respiration and bioenergetics and protects mitochondria from excess NO* by controlling the activity of mtNOS.

Nitric oxide, cytochromecoxidase and myoglobin: Competition and reaction pathways

It is relevant to cell physiology that nitric oxide (NO) reacts with both cytochrome oxidase (CcOX) and oxygenated myoglobin (MbO(2)). In this respect, it has been proposed [Pearce, L.L., et al. (2002) J. Biol. Chem. 277, 13556-13562] that (i) CcOX in turnover out-competes MbO(2) for NO, and (ii) NO bound to reduced CcOX is "metabolized" in the active site to nitrite by reacting with O(2). In contrast, rapid kinetics experiments reported in this study show that (i) upon mixing NO with MbO(2) and CcOX in turnover, MbO(2) out-competes the oxidase for NO and (ii) after mixing nitrosylated CcOX with O(2) in the presence of MbO(2), NO (and not nitrite) dissociates from the enzyme causing myoglobin oxidation.