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

Neobavaisoflavone Protects H9c2 Cells Against H2O2-Induced Mitochondrial Dysfunction Through ALOX15/PGC1-α Axis

Neobavaisoflavone (NBIF) is a natural antioxidant that has a variety of pharmacological activities. To investigate the effects of NBIF on oxidative stress-induced myocardial injury, H9c2 cells were treated with H2O2. Cell counting kit-8 was used to detect cell viability. Intracellular as well as lipid radicals were detected. To measure mitochondrial function, tetramethylrhodamine ethyl ester was used to detect mitochondrial membrane potential. 12- and 15-hydroxyeicosatetraenoic acids (HETE) were measured by LC-MS/MS. ALOX15, which is the upstream protein of 12-, 15-HETE, was also measured by using western blot analysis. The results showed that H2O2 induced lipid peroxidation in cardiomyocytes and caused mitochondrial dysfunction which was relieved by NBIF treatment. Besides, H2O2 significantly increased the production of 12-HETE and 15-HETE and upregulated the expression of ALOX15 while PGC-1α was downregulated and triggered the release of cytochrome c. The treatment of NBIF decreased the expression of ALOX15 and inhibited the activation of caspase-3. NBIF protected mitochondrial membrane integrity through increasing PGC-1α and Nrf1. Our results indicated that NBIF could protect cardiomyocytes against H2O2-induced mitochondrial dysfunction via ALOX15/PGC-1α axis.

Mitochondrial Redox Balance of Fibroblasts Exposed to Ti-6Al-4V Microplates Subjected to Different Types of Anodizing

Despite the high biocompatibility of titanium and its alloys, the need to remove titanium implants is increasingly being debated due to the potential for adverse effects associated with long-term retention. Therefore, new solutions are being sought to enhance the biocompatibility of titanium implants. One of them is to increase the thickness of the passive layer of the implant made of titanium dioxide. We were the first to evaluate the effect of hard-anodized (type II) Ti-6Al-4V alloy discs on the cytotoxicity, mitochondrial function, and redox balance of fibroblasts mitochondria compared to standard-anodized (type III) and non-anodized discs. The study used fibroblasts obtained from human gingival tissue. The test discs were applied to the bottom of 12-well plates. Cells were cultured for 24 h and 7, 14, and 21 days and mitochondria were isolated. We demonstrated the occurrence of oxidative stress in the mitochondria of fibroblasts of all tested groups, regardless of the presence and type of anodization. Type II anodization prevented changes in complex II activity (vs. control). The lowest degree of citrate synthase inhibition occurred in mitochondria exposed to titanium discs with type II anodization. In the last phase of culture, the presence of type II anodization reduced the degree of cytochrome c oxidase inhibition compared to the other tests groups and the control group, and prevented apoptosis. Throughout the experiment, the release of titanium, aluminium, and vanadium ions from titanium discs with a hard-anodized passive layer was higher than from the other titanium discs, but decreased with time. The obtained results proved the existence of dysfunction and redox imbalance in the mitochondria of fibroblasts exposed to hard-anodized titanium discs, suggesting the need to search for new materials perhaps biodegradable in tissues of the human body.

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.

Interplay between oxidant species and energy metabolism

It has long been recognized that energy metabolism is linked to the production of reactive oxygen species (ROS) and critical enzymes allied to metabolic pathways can be affected by redox reactions. This interplay between energy metabolism and ROS becomes most apparent during the aging process and in the onset and progression of many age-related diseases (i.e. diabetes, metabolic syndrome, atherosclerosis, neurodegenerative diseases). As such, the capacity to identify metabolic pathways involved in ROS formation, as well as specific targets and oxidative modifications is crucial to our understanding of the molecular basis of age-related diseases and for the design of novel therapeutic strategies.

Herein we review oxidant formation associated with the cell's energetic metabolism, key antioxidants involved in ROS detoxification, and the principal targets of oxidant species in metabolic routes and discuss their relevance in cell signaling and age-related diseases.

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Energy metabolism is both a source and target of oxidant species.

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Reactive oxygen species are formed in redox reactions in catabolic pathways.

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Sensitive targets of oxidant species regulate the flux of metabolic pathways.

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Metabolic pathways and antioxidant systems are regulated coordinately.

Mitochondrial reactive oxygen species (ROS) and ROS-induced ROS release

Byproducts of normal mitochondrial metabolism and homeostasis include the buildup of potentially damaging levels of reactive oxygen species (ROS), Ca(2+), etc., which must be normalized. Evidence suggests that brief mitochondrial permeability transition pore (mPTP) openings play an important physiological role maintaining healthy mitochondria homeostasis. Adaptive and maladaptive responses to redox stress may involve mitochondrial channels such as mPTP and inner membrane anion channel (IMAC). Their activation causes intra- and intermitochondrial redox-environment changes leading to ROS release. This regenerative cycle of mitochondrial ROS formation and release was named ROS-induced ROS release (RIRR). Brief, reversible mPTP opening-associated ROS release apparently constitutes an adaptive housekeeping function by the timely release from mitochondria of accumulated potentially toxic levels of ROS (and Ca(2+)). At higher ROS levels, longer mPTP openings may release a ROS burst leading to destruction of mitochondria, and if propagated from mitochondrion to mitochondrion, of the cell itself. The destructive function of RIRR may serve a physiological role by removal of unwanted cells or damaged mitochondria, or cause the pathological elimination of vital and essential mitochondria and cells. The adaptive release of sufficient ROS into the vicinity of mitochondria may also activate local pools of redox-sensitive enzymes involved in protective signaling pathways that limit ischemic damage to mitochondria and cells in that area. Maladaptive mPTP- or IMAC-related RIRR may also be playing a role in aging. Because the mechanism of mitochondrial RIRR highlights the central role of mitochondria-formed ROS, we discuss all of the known ROS-producing sites (shown in vitro) and their relevance to the mitochondrial ROS production in vivo.

Cytochrome c oxidase dysfunction in oxidative stress

Cytochrome c oxidase (CcO) is the terminal oxidase of the mitochondrial electron transport chain. This bigenomic enzyme in mammals contains 13 subunits of which the 3 catalytic subunits are encoded by the mitochondrial genes. The remaining 10 subunits with suspected roles in the regulation, and/or assembly, are coded by the nuclear genome. The enzyme contains two heme groups (heme a and a3) and two Cu(2+) centers (Cu(2+) A and Cu(2+) B) as catalytic centers and handles more than 90% of molecular O(2) respired by the mammalian cells and tissues. CcO is a highly regulated enzyme which is believed to be the pacesetter for mitochondrial oxidative metabolism and ATP synthesis. The structure and function of the enzyme are affected in a wide variety of diseases including cancer, neurodegenerative diseases, myocardial ischemia/reperfusion, bone and skeletal diseases, and diabetes. Despite handling a high O(2) load the role of CcO in the production of reactive oxygen species still remains a subject of debate. However, a volume of evidence suggests that CcO dysfunction is invariably associated with increased mitochondrial reactive oxygen species production and cellular toxicity. In this paper we review the literature on mechanisms of multimodal regulation of CcO activity by a wide spectrum of physiological and pathological factors. We also review an array of literature on the direct or indirect roles of CcO in reactive oxygen species production.

Cytochrome c oxidase and its role in neurodegeneration and neuroprotection

A hallmark of neurodegenerative diseases, such as Alzheimer's and Parkinson's diseases, and stroke is a malfunction of mitochondria including cytochrome c oxidase (COX), the terminal enzyme complex of the respiratory chain. COX is ascribed a key role based on mainly two regulatory mechanisms. These are the expression of isoforms and the binding of specific allosteric factors to nucleus--encoded subunits. These characteristics represent a unique feature of COX compared with the other respiratory chain complexes. Additional regulatory mechanisms, such as posttranslational modification, substrate availability, and allosteric feedback inhibition by products of the COX reaction, control the enzyme activity in a complex way. In many tissues and cell types, COX represents the rate-limiting enzyme of the respiratory chain which further emphasizes the impact of the regulation of COX as a central site for regulating energy metabolism and oxidative stress. Two of the best-analyzed regulatory mechanisms of COX to date are the allosteric feedback inhibition of the enzyme by its indirect product ATP and the expression of COX subunit IV isoforms. This ATP feedback inhibition of COX requires the expression of COX isoform IV-1. At high ATP/ADP ratios, ADP is exchanged for ATP at the matrix side of COX IV-1 leading to an inhibition of COX activity, thus enabling COX to sense the energy level and to adjust ATP synthesis to energy demand. However, under hypoxic, toxic, and degenerative conditions, COX isoform IV-2 expression is up-regulated and exchanged for COX IV-1 in the enzyme complex. This COX IV isoform switch causes an abolition of the allosteric ATP feedback inhibition of COX and consequently the loss of sensing the energy level. Thus, COX activity is increased leading to higher levels of ATP in neural cells independently of the cellular energy level. Concomitantly, ROS production is increased. Thus, under pathological conditions, neural cells are provided with ATP to meet the energy demand, but at the expense of elevated oxidative stress. This mechanism explains the functional relevance of COX subunit IV isoform expression for cellular energy sensing, ATP production, and oxidative stress levels. This, in turn, affects neural cell function, signaling, and -survival. Thus, COX is a crucial factor in etiology, progression, and prevalence of numerous human neurodegenerative diseases and represents an important target for developing diagnostic and therapeutic tools against those diseases.

The power of life--cytochrome c oxidase takes center stage in metabolic control, cell signalling and survival

Mitochondrial dysfunction is increasingly recognized as a major factor in the etiology and progression of numerous human diseases, such as (neuro-)degeneration, ischemia reperfusion injury, cancer, and diabetes. Cytochrome c oxidase (COX) represents the rate-limiting enzyme of the mitochondrial respiratory chain and is thus predestined for being a central site of regulation of oxidative phosphorylation, proton pumping efficiency, ATP and reactive oxygen species production, which in turn affect cell signaling and survival. A unique feature of COX is its regulation by various factors and mechanisms interacting with the nucleus-encoded subunits, whose actual functions we are only beginning to understand.

Lifespan and oxidative stress show a non-linear response to atmospheric oxygen in Drosophila

Oxygen provides the substrate for most ATP production, but also serves as a source of reactive oxygen species (ROS), which can induce cumulative macromolecular oxidative damage and cause aging. Pure oxygen atmospheres (100 kPa) are known to strongly reduce invertebrate lifespan and induce aging-related physiological changes. However, the nature of the relationship between atmospheric oxygen, oxidative stress, and lifespan across a range of oxygen levels is poorly known. Developmental responses are likely to play a strong role, as prior research has shown strong effects of rearing oxygen level on growth, size and respiratory system morphology. In this study, we examined (1) the effect of oxygen on adult longevity and (2) the effect of the oxygen concentration experienced by larvae on adult lifespan by rearing Drosophila melanogaster in three oxygen atmospheres throughout larval development (10, 21 and 40 kPa), then measuring the lifespan of adults in five oxygen tensions (2, 10, 21, 40, 100 kPa). We also assessed the rate of protein carbonyl production for flies kept at 2, 10, 21, 40 and 100 kPa as adults (all larvae reared in normoxia). The rearing of juveniles in varying oxygen treatments affected lifespan in a complex manner, and the effect of different oxygen tensions on adult lifespan was non-linear, with reduced longevity and heightened oxidative stress at extreme high and low atmospheric oxygen levels. Moderate hypoxia (10 kPa) extended maximum, but not mean lifespan.

Regulation of plant glycine decarboxylase by s-nitrosylation and glutathionylation

Mitochondria play an essential role in nitric oxide (NO) signal transduction in plants. Using the biotin-switch method in conjunction with nano-liquid chromatography and mass spectrometry, we identified 11 candidate proteins that were S-nitrosylated and/or glutathionylated in mitochondria of Arabidopsis (Arabidopsis thaliana) leaves. These included glycine decarboxylase complex (GDC), a key enzyme of the photorespiratory C(2) cycle in C3 plants. GDC activity was inhibited by S-nitrosoglutathione due to S-nitrosylation/S-glutathionylation of several cysteine residues. Gas-exchange measurements demonstrated that the bacterial elicitor harpin, a strong inducer of reactive oxygen species and NO, inhibits GDC activity. Furthermore, an inhibitor of GDC, aminoacetonitrile, was able to mimic mitochondrial depolarization, hydrogen peroxide production, and cell death in response to stress or harpin treatment of cultured Arabidopsis cells. These findings indicate that the mitochondrial photorespiratory system is involved in the regulation of NO signal transduction in Arabidopsis.

Mitochondrial redox metabolism: aging, longevity and dietary effects

Mitochondrial redox metabolism has long been considered to play important roles in mammalian aging and the development of age-related pathologies in the major oxidative organs. Both genetic and dietary manipulations of mitochondrial redox metabolism have been associated with the extension of lifespan. Here we provide a broad overview of the circumstantial evidence showing associations between mitochondrial reactive oxygen species (ROS) metabolism, aging and longevity. We address most aspects of mitochondrial ROS metabolism, from superoxide production, to ROS detoxification and the repair/removal of ROS-mediated macromolecular damage. Finally, we discuss the effects of dietary manipulations (e.g. caloric restriction, methionine restriction), dietary deficiencies (e.g. folate) and dietary supplementation (e.g. resveratrol) on mitochondrial ROS metabolism and lifespan.

Activation of Hsp90/NOS and increased NO generation does not impair mitochondrial respiratory chain by competitive binding at cytochrome c oxidase in low oxygen concentrations

Nitric oxide (NO) is known to regulate mitochondrial respiration, especially during metabolic stress and disease, by nitrosation of the mitochondrial electron transport chain (ETC) complexes (irreversible) and by a competitive binding at O2 binding site of cytochrome c oxidase (CcO) in complex IV (reversible). In this study, by using bovine aortic endothelial cells, we demonstrate that the inhibitory effect of endogenously generated NO by nitric oxide synthase (NOS) activation, by either NOS stimulators or association with heat shock protein 90 (Hsp90), is significant only at high prevailing pO2 through nitrosation of mitochondrial ETC complexes, but it does not inhibit the respiration by competitive binding at CcO at very low pO2. ETC complexes activity measurements confirmed that significant reduction in complex IV activity was noticed at higher pO2, but it was unaffected at low pO2 in these cells. This was further extended to heat-shocked cells, where NOS was activated by the induction/activation of (Hsp90) through heat shock at an elevated temperature of 42 degrees C. From these results, we conclude that the entire attenuation of respiration by endogenous NO is due to irreversible inhibition by nitrosation of ETC complexes but not through reversible inhibition by competing with O2 binding at CcO at complex IV.

Accumulation of 8-oxoguanine in liver DNA during hyperoxic resuscitation of newborn mice

Supplementary oxygen during resuscitation of the asphyxiated newborn is associated with long-term detrimental effects including increased risk of childhood cancer. It is suspected that the resuscitation procedure results in accumulated DNA damage and mutagenesis. Base excision repair (BER) is the major pathway for repair of premutagenic oxidative DNA lesions. This study addresses DNA base damage and BER in brain, lung, and liver in neonatal mice (P7) after hyperoxic resuscitation. Mice were randomized to 8% oxygen or room air for 60 min in a closed chamber and subsequent reoxygenation with 100% oxygen for 0 to 90 min. During this treatment, 8-oxoguanine accumulated in liver but not in lung or cerebellum. We observed a linear relation between 8-oxoguanine and reoxygenation time in liver DNA from hypoxic animals (n = 28; B = 0.011 [0.001, 0.020]; p = 0.037). BER activity was not significantly changed during resuscitation. Our data suggest that after hypoxia, the capacity for immediate repair in liver tissue is inadequate to meet increasing amounts of DNA damage. The duration of supplementary oxygen use during resuscitation should be kept as short as justifiable to minimize the risk of genetic instability.

Activation of Hsp90-eNOS and increased NO generation attenuate respiration of hypoxia-treated endothelial cells

Hypoxia induces various adoptive signaling in cells that can cause several physiological changes. In the present work, we have observed that exposure of bovine aortic endothelial cells (BAECs) to extreme hypoxia (1-5% O(2)) attenuates cellular respiration by a mechanism involving heat shock protein 90 (Hsp90) and endothelial nitric oxide (NO) synthase (eNOS), so that the cells are conditioned to consume less oxygen and survive in prolonged hypoxic conditions. BAECs, exposed to 1% O(2), showed a reduced respiration compared with 21% O(2)-maintained cells. Western blot analysis showed an increase in the association of Hsp90-eNOS and enhanced NO generation on hypoxia exposure, whereas there was no significant accumulation of hypoxia-inducible factor-1alpha (HIF-1alpha). The addition of inhibitors of Hsp90, phosphatidylinositol 3-kinase, and NOS significantly alleviated this hypoxia-induced attenuation of respiration. Thus we conclude that hypoxia-induced excess NO and its derivatives such as ONOO(-) cause inhibition of the electron transport chain and attenuate O(2) demand, leading to cell survival at extreme hypoxia. More importantly, such an attenuation is found to be independent of HIF-1alpha, which is otherwise thought to be the key regulator of respiration in hypoxia-exposed cells, through a nonphosphorylative glycolytic pathway. The present mechanistic insight will be helpful to understand the difference in the magnitude of endothelial dysfunction.

Layer-specific differences in reactive oxygen species levels after oxygen-glucose deprivation in acute hippocampal slices

The major role of reactive oxygen species (ROS) in the pathomechanism of ischemia have been widely recognized. Still, measurements of the precise time course and regional distribution of ischemia-induced ROS level changes in acute brain slices have been missing. By using acute hippocampal slices and the fluorescent dye CM-H2DCFDA, we showed that reoxygenation after in vitro ischemia (oxygen-glucose deprivation; OGD) increased ROS levels in the hippocampal CA1 layers vulnerable to ischemia but did not have significant effects in the resistant stratum granulosum in the dentate gyrus (DG). Production of ROS started during OGD, but, contrary to reoxygenation, it manifested as a ROS level increase exclusively in the presence of catalase and glutathione peroxidase inhibition. The mechanism of ROS production involves the activation of NMDA receptors and nitric oxide synthases. The inhibition of ROS response by either AP-5 or L-NAME together with the ROS sensitivity profile of the dye suggest that peroxynitrite, the reaction product of superoxide and nitric oxide, plays a role in the response. Direct visualization of layer-specific effects of ROS production and its scavenging, shown for the first time in acute hippocampal slices, suggests that distinct ROS homeostasis may underlie the different ischemic vulnerability of CA1 and DG.

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.

Mitochondrial dysfunction in atherosclerosis

Increased production of reactive oxygen species in mitochondria, accumulation of mitochondrial DNA damage, and progressive respiratory chain dysfunction are associated with atherosclerosis or cardiomyopathy in human investigations and animal models of oxidative stress. Moreover, major precursors of atherosclerosis-hypercholesterolemia, hyperglycemia, hypertriglyceridemia, and even the process of aging-all induce mitochondrial dysfunction. Chronic overproduction of mitochondrial reactive oxygen species leads to destruction of pancreatic beta-cells, increased oxidation of low-density lipoprotein and dysfunction of endothelial cells-factors that promote atherosclerosis. An additional mechanism by which impaired mitochondrial integrity predisposes to clinical manifestations of vascular diseases relates to vascular cell growth. Mitochondrial function is required for normal vascular cell growth and function. Mitochondrial dysfunction can result in apoptosis, favoring plaque rupture. Subclinical episodes of plaque rupture accelerate the progression of hemodynamically significant atherosclerotic lesions. Flow-limiting plaque rupture can result in myocardial infarction, stroke, and ischemic/reperfusion damage. Much of what is known on reactive oxygen species generation and modulation comes from studies in cultured cells and animal models. In this review, we have focused on linking this large body of literature to the clinical syndromes that predispose humans to atherosclerosis and its complications.

Sodium as the major mediator of NO-induced cell death in cultured hepatocytes

NO has been shown to induce cellular injury via inhibition of the mitochondrial respiratory chain and/or oxidative/nitrosative stress. Here, we studied which mechanism and downstream mediator is responsible for NO toxicity to hepatocytes. When cultured rat hepatocytes were incubated with spermineNONOate (0.01-2 mM) at 2, 5, 21 and 95% O(2) in Krebs-Henseleit buffer (37 degrees C), spermineNONOate caused concentration-dependent hepatocyte death (lactate dehydrogenase release, propidium iodide uptake) with morphological features of both apoptosis and necrosis. Increasing O(2) concentrations protected hepatocytes from NO-induced injury. Steady-state NO concentrations were lower at higher O(2) concentrations, suggesting formation of reactive nitrogen oxide species. Despite this, the scavenger ascorbic acid was hardly protective. In contrast, at equal NO concentrations loss of viability was higher at lower O(2) concentrations and inhibitors of hypoxic injury, fructose and glycine (10 mM), strongly decreased NO-induced injury. Upon addition of spermineNONOate, the cytosolic Na(+) concentration rapidly increased. The increase in sodium depended on the NO/O(2) ratio and was paralleled by hepatocyte death. Sodium-free Krebs-Henseleit buffer strongly protected from NO-induced injury. SpermineNONOate also increased cytosolic calcium levels but the Ca(2+) chelator quin-2-AM did not diminish cell injury. These results show that - in analogy to hypoxic injury - a sodium influx largely mediates the NO-induced death of cultured hepatocytes. Oxidative stress and disturbances in calcium homeostasis appear to be of minor importance for NO toxicity to hepatocytes.

Discrepancies between nitroglycerin and NO-releasing drugs on mitochondrial oxygen consumption, vasoactivity, and the release of NO

It has been generally acknowledged that the actions of glyceryl trinitrate (GTN) are a result of its bioconversion into NO. However, recent observations have thrown this idea into doubt, with many studies demonstrating that NO is present only when there are high concentrations of GTN. We have explored this discrepancy by developing a new approach that uses confocal microscopy to directly detect NO. Intracellular levels of NO in the rat aortic vascular wall have been compared with those present after incubation with 3 different NO donors (DETA-NO, 3-morpholinosydnonimine, and S-nitroso-N-acetylpenicillamine), endothelial activation with acetylcholine, or administration of GTN. We have also evaluated the relaxant effects of these treatments on isolated rings of aorta following activation of the enzyme soluble guanylyl cyclase and their inhibitory action on mitochondrial respiration, which is an index of the interaction of NO with the enzyme of the electron transport chain cytochrome C oxidase. In the case of the various NO donors and acetylcholine, we detected a concentration-dependent relationship in the intensity of vascular relaxation and degree of NO fluorescence and an increase in the Michaelis constant (Km) for O2. GTN did not produce similar effects, and although clinically relevant concentrations of this compound caused clear, concentration-related relaxations, there was neither any increase in NO-related fluorescence nor an augmented Km for O2. The nature of these differences suggests that these concentrations of GTN do not release free NO but probably a different species that, although it interacts with soluble guanylyl cyclase in vascular smooth muscle, does not inhibit O2 consumption by vascular mitochondria.

Calcium mobilization is required for peroxynitrite-mediated enhancement of spontaneous transient outward currents in arteriolar smooth muscle cells

Transiently local release of Ca(2+) from the sarcoplasmic reticulum (SR) activates nearby Ca(2+)-activated K(+) channels to produce spontaneous transient outward currents (STOCs) in smooth muscle cells. The purpose of the present study was to investigate the possible effect of peroxynitrite (ONOO(-)) on STOCs in mesenteric arteriolar smooth muscle cells (ASMCs) and decide whether Ca(2+) mobilization was involved in STOCs alteration by ONOO(-). STOCs were recorded and characterized using the perforated whole-cell patch-clamp configuration. The results demonstrated that STOCs activity was greatly suppressed by removal of extracellular Ca(2+); by addition of nifedipine, a specific inhibitor of L-type voltage-gated Ca(2+) channels (VGCCs); or by addition of ryanodine, a SR ryanodine receptors (RyRs) blocker. In contrast, both caffeine, a RyR activator, and 2-aminoethoxydiphenylborate (2-APB), a membrane-permeable inositol 1,4,5-trisphosphate receptors, (IP3R) antagonist, increased STOCs activity. 3-morpholinosydnonimine (SIN-1), an ONOO(-) donor, at concentrations of 20-200 microM, induced a dose-dependent enhancement of STOCs in ASMCs and led to conspicuous increases in STOCs frequency and amplitude, which were prevented by prior exposure to low external Ca(2+) (200 nM), ryanodine (10 microM), or nifedipine (10 microM). In contrast, caffeine (0.5 mM) did not further stimulate STOCs in ASMCs preincubated with SIN-1, and pretreatment with 2-APB (50 microM) had little effect on ONOO(-) -induced STOCs activation. These findings suggest that complex Ca(2+)-mobilizing pathways, including external Ca2+ influx through VGCCs activation and subsequent internal Ca(2+) release through RyRs but not IP3Rs, are involved in ONOO(-)mediated STOCs enhancement in ASMCs.

Critical O2 and NO concentrations in NO-induced cell death in a rat liver sinusoidal endothelial cell line

Nitric oxide (NO) plus oxygen (O2) are known to cause cell damage via formation of reactive nitrogen species. NO itself directly inhibits cytochrome oxidase of the mitochondrial respiratory chain in competition with O2, thus inducing a hypoxic-like injury. To assess the critical NO and O2 concentrations for both mechanisms of NO-induced cell injury, cells of a rat liver sinusoidal endothelial cell line were incubated in the presence of the NO donor spermineNONOate at different O2 concentrations, and their loss of viability was determined by the release of lactate dehydrogenase. Protection by ascorbic acid was used as indication for the involvement of reactive nitrogen species, whereas a hypoxic-like injury was indicated by the protective effects of glycine and glucose and the increase in NAD(P)H fluorescence. High concentrations of NO (approx. 10 microM NO) and O2 (21% O2) were required to induce endothelial cell death mediated by formation of reactive nitrogen species. On the other hand, pathophysiologically relevant NO concentrations at low but physiological O2 concentrations (ca. 2 microM NO at 5% O2 and about 1 microM NO at 2% O2) induced hypoxic-like cell death in the endothelial cells that was prevented by the presence of glucose.

Differential expression of iron-, carbon-, and oxygen-responsive mycobacterial genes in the lungs of chronically infected mice and tuberculosis patients

Pathogenetic processes that facilitate the entry, replication, and persistence of Mycobacterium tuberculosis (MTB) in the mammalian host likely include the regulated expression of specific sets of genes at different stages of infection. Identification of genes that are differentially expressed in vivo would provide insights into host-pathogen interactions in tuberculosis (TB); this approach might be particularly valuable for the study of human TB, where experimental opportunities are limited. In this study, the levels of selected MTB mRNAs were quantified in vitro in axenic culture, in vivo in the lungs of mice, and in lung specimens obtained from TB patients with active disease. We report the differential expression of MTB mRNAs associated with iron limitation, alternative carbon metabolism, and cellular hypoxia, conditions that are thought to exist within the granulomatous lesions of TB, in the lungs of wild-type C57BL/6 mice as compared with bacteria grown in vitro. Analysis of the same set of mRNAs in lung specimens obtained from TB patients revealed differences in MTB gene expression in humans as compared with mice.

Mitochondrial dysfunction in a long-term rodent model of sepsis and organ failure

Although sepsis is the major cause of mortality and morbidity in the critically ill, precise mechanism(s) causing multiorgan dysfunction remain unclear. Findings of impaired oxygen utilization in septic patients and animals implicate nitric oxide-mediated inhibition of the mitochondrial respiratory chain. We recently reported a relationship between skeletal muscle mitochondrial dysfunction, clinical severity, and poor outcome in patients with septic shock. We thus developed a long-term, fluid-resuscitated, fecal peritonitis model utilizing male Wistar rats that closely replicates human physiological, biochemical, and histological findings with a 40% mortality. As with humans, the severity of organ dysfunction and eventual poor outcome were associated with nitric oxide overproduction and increasing mitochondrial dysfunction (complex I inhibition and ATP depletion). This was seen in both vital (liver) and nonvital (skeletal muscle) organs. Likewise, histological evidence of cell death was lacking, suggesting the possibility of an adaptive programmed shutdown of cellular function. This study thus supports the hypothesis that multiorgan dysfunction induced by severe sepsis has a bioenergetic etiology. Despite the well-recognized limitations of laboratory models, we found clear parallels between this long-term model and human disease characteristics that will facilitate future translational research.

Mitochondrial formation of reactive oxygen species

The reduction of oxygen to water proceeds via one electron at a time. In the mitochondrial respiratory chain, Complex IV (cytochrome oxidase) retains all partially reduced intermediates until full reduction is achieved. Other redox centres in the electron transport chain, however, may leak electrons to oxygen, partially reducing this molecule to superoxide anion (O2-*). Even though O2-* is not a strong oxidant, it is a precursor of most other reactive oxygen species, and it also becomes involved in the propagation of oxidative chain reactions. Despite the presence of various antioxidant defences, the mitochondrion appears to be the main intracellular source of these oxidants. This review describes the main mitochondrial sources of reactive species and the antioxidant defences that evolved to prevent oxidative damage in all the mitochondrial compartments. We also discuss various physiological and pathological scenarios resulting from an increased steady state concentration of mitochondrial oxidants.

Nitric oxide and peroxynitrite cause irreversible increases in the Km for oxygen of mitochondrial cytochrome oxidase: in vitro and in vivo studies

Mitochondrial cytochrome oxidase is competitively and reversibly inhibited by inhibitors that bind to ferrous heme, such as carbon monoxide and nitric oxide. In the case of nitric oxide, nanomolar levels inhibit cytochrome oxidase by competing with oxygen at the enzyme's heme-copper active site. This raises the K(m) for cellular respiration into the physiological range. This effect is readily reversible and may be a physiological control mechanism. Here we show that a number of in vitro and in vivo conditions result in an irreversible increase in the oxygen K(m). These include: treatment of the purified enzyme with peroxynitrite or high (microM) levels of nitric oxide; treatment of the endothelial-derived cell line, b.End5, with NO; activation of astrocytes by cytokines; reperfusion injury in the gerbil brain. Studies of cell respiration that fail to vary the oxygen concentration systematically are therefore likely to significantly underestimate the degree of irreversible damage to cytochrome oxidase.

Inflammatory neurodegeneration mediated by nitric oxide, glutamate, and mitochondria

In inflammatory, infectious, ischemic, and neurodegenerative pathologies of the central nervous system (CNS) glia become "activated" by inflammatory mediators, and express new proteins such as the inducible isoform of nitric oxide synthase (iNOS). Although these activated glia have benefi- cial roles, in vitro they potently kill cocultured neurons, and there is increasing evidence that they contribute to pathology in vivo. Nitric oxide (NO) from iNOS appears to be a key mediator of such glial-induced neuronal death. The high sensitivity of neurons to NO is partly due to NO causing inhibition of respiration, rapid glutamate release from both astrocytes and neurons, and subsequent excitotoxic death of the neurons. NO is a potent inhibitor of mitochondrial respiration, due to reversible binding of NO to cytochrome oxidase in competition with oxygen, resulting in inhibition of energy production and sensitization to hypoxia. Activated astrocytes or microglia cause a potent inhibition of respiration in cocultured neurons due to glial NO inhibiting cytochrome oxidase within the neurons, resulting in ATP depletion and glutamate release. In some conditions, glutamate- induced neuronal death can itself be mediated by N-methyl-D-aspartate (NMDA)-receptor activation of the neuronal isoform of NO synthase (nNOS) causing mitochondrial damage. In addition NO can be converted to a number of reactive derivatives such as peroxynitrite, NO2, N2O3, and S-nitrosothiols that can kill cells in part by inhibiting mitochondrial respiration or activation of mitochondrial permeability transition, triggering neuronal apoptosis or necrosis.

Nitric oxide inhibition of mitochondrial respiration and its role in cell death

Nitric oxide (NO) or its derivatives (reactive nitrogen species, RNS) inhibit mitochondrial respiration in two different ways: (i) an acute, potent, and reversible inhibition of cytochrome oxidase by NO in competition with oxygen; and, (ii) irreversible inhibition of multiple sites by RNS. NO inhibition of respiration may impinge on cell death in several ways. Inhibition of respiration can cause necrosis and inhibit apoptosis due to ATP depletion, if glycolysis is also inhibited or is insufficient to compensate. Inhibition of neuronal respiration can result in excitotoxic death of neurons due to induced release of glutamate and activation of NMDA-type glutamate receptors. Inhibition of respiration may cause apoptosis in some cells, while inhibiting apoptosis in other cells, by mechanisms that are not clear. However, NO can induce (and inhibit) cell death by a variety of mechanisms unrelated to respiratory inhibition.

Carotid body chemosensory excitation induced by nitric oxide: involvement of oxidative metabolism

Nitric oxide (NO) produces a dual effect on carotid body (CB) oxygen chemoreception. At low concentration, NO inhibits chemosensory response to hypoxia, while in normoxia, medium and high [NO] increases the frequency of carotid chemosensory discharges (f(x)). Since NO and peroxynitrite inhibit mitochondrial respiration, it is plausible that the NO-induced excitation may depend on the mitochondrial oxidative metabolism. To test this hypothesis, we studied the effects of oligomycin, FCCP and antimycin A that produce selective blockade of hypoxic and NaCN-induced chemosensory responses, leaving nicotinic response less affected. CBs excised from pentobarbitone-anaesthetised cats were perfused in vitro with Tyrode (P(O(2)) approximately 125 Torr, pH 7.40 at 38 degrees C). Hypoxia (P(O(2)) approximately equal 30 Torr), NaCN and nicotine (1-100 microg) and S-nitroso-N-acetylpenicillamide (SNAP, 300-600 microg) increased f(x). Oligomycin (12.5-25 microg), antimycin A (10 microg) and FCCP (5 microM) transiently increased f(x). Subsequently, chemosensory responses to hypoxia, NaCN and SNAP were reduced or abolished, while the response to nicotine was less affected. The electron donor system tetramethyl-p-phenylene diamide and ascorbate that bypasses the electron chain blockade produced by antimycin A, restores the excitatory responses to NaCN and SNAP. Present results suggest that the chemoexcitatory effect of NO depends on the integrity of mitochondrial metabolism.

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.

Bioenergetics at low oxygen: dependence of respiration and phosphorylation on oxygen and adenosine diphosphate supply

Oxygen limitation is generally considered as impairment of mitochondrial respiration under hypoxia and ischemia. Low intracellular oxygen levels under normoxia, however, imply mild oxygen limitation, provide protection from oxidative stress, and result from economical strategies for oxygen transport through the respiratory cascade to cytochrome c oxidase. Both perspectives relate to the critical oxygen pressure, which inhibits mitochondrial respiration. Based on methodological considerations of oxygen kinetics and a presentation of high-resolution respirometry, mitochondrial oxygen affinities (1/P(50)) are reviewed with particular emphasis on the turnover effect under control of adenosine diphosphate ADP concentration, which increases the P(50) in active states. ADP/O(2) flux ratios are high even under severe oxygen limitation, as demonstrated by calorespirometry. Oxygen limitation reduces the uncoupled respiration observed under control by ADP, as shown by relationships derived between ADP/O(2) flux ratios, respiratory control ratios, and ADP kinetics. Bioenergetics at low oxygen versus oxidative stress must be considered in the context of limitation of maximum aerobic activity, ischemia-reperfusion injury, mitochondrial signalling to apoptosis, and mitochondrial theories of ageing.