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

Complex Interplay of Heme-Copper Oxidases with Nitrite and Nitric Oxide

Nitrite and nitric oxide (NO), two active and critical nitrogen oxides linking nitrate to dinitrogen gas in the broad nitrogen biogeochemical cycle, are capable of interacting with redox-sensitive proteins. The interactions of both with heme-copper oxidases (HCOs) serve as the foundation not only for the enzymatic interconversion of nitrogen oxides but also for the inhibitory activity. From extensive studies, we now know that NO interacts with HCOs in a rapid and reversible manner, either competing with oxygen or not. During interconversion, a partially reduced heme/copper center reduces the nitrite ion, producing NO with the heme serving as the reductant and the cupric ion providing a Lewis acid interaction with nitrite. The interaction may lead to the formation of either a relatively stable nitrosyl-derivative of the enzyme reduced or a more labile nitrite-derivative of the enzyme oxidized through two different pathways, resulting in enzyme inhibition. Although nitrite and NO show similar biochemical properties, a growing body of evidence suggests that they are largely treated as distinct molecules by bacterial cells. NO seemingly interacts with all hemoproteins indiscriminately, whereas nitrite shows high specificity to HCOs. Moreover, as biologically active molecules and signal molecules, nitrite and NO directly affect the activity of different enzymes and are perceived by completely different sensing systems, respectively, through which they are linked to different biological processes. Further attempts to reconcile this apparent contradiction could open up possible avenues for the application of these nitrogen oxides in a variety of fields, the pharmaceutical industry in particular.

Bio-inspired nitrogen oxide (NOx) interconversion reactivities of synthetic heme Compound-I and Compound-II intermediates

Dioxygen activating heme enzymes have long predicted to be powerhouses for nitrogen oxide interconversion, especially for nitric oxide (NO) oxidation which has far-reaching biological and/or environmental impacts. Lending credence, reactivity of NO with high-valent heme‑oxygen intermediates of globin proteins has recently been implicated in the regulation of a variety of pivotal physiological events such as modulating catalytic activities of various heme enzymes, enhancing antioxidant activity to inhibit oxidative damage, controlling inflammatory and infectious properties within the local heme environments, and NO scavenging. To reveal insights into such crucial biological processes, we have investigated low temperature NO reactivities of two classes of synthetic high-valent heme intermediates, Compound-II and Compound-I. In that, Compound-II rapidly reacts with NO yielding the six-coordinate (NO bound) heme ferric nitrite complex, which upon warming to room temperature converts into the five-coordinate heme ferric nitrite species. These ferric nitrite complexes mediate efficient substrate oxidation reactions liberating NO; i.e., shuttling NO₂^- back to NO. In contrast, Compound-I and NO proceed through an oxygen-atom transfer process generating the strong nitrating agent NO₂, along with the corresponding ferric nitrosyl species that converts to the naked heme ferric parent complex upon warmup. All reaction components have been fully characterized by UV-vis, ²H NMR and EPR spectroscopic methods, mass spectrometry, elemental analyses, and semi-quantitative determination of NO₂^- anions. The clean, efficient, potentially catalytic NO_x interconversions driven by high-valent heme species presented herein illustrate the strong prospects of a heme enzyme/O₂/NO_x dependent unexplored territory that is central to human physiology, pathology, and therapeutics.

Nitrosative stress-mediated inhibition of OsDHODH1 gene expression suggests roots growth reduction in rice (Oryza sativa L.)

This study monitored the transcriptional response of OsDHODH1 under nitrosative stress conditions relative to the transcripts accumulations for the core mitochondrial cytochrome c oxidase1 (CcOX1) subunit, nuclear CcOX subunits 5b and 5c, two rice nitrate reductases (OsNIA1 and OsNIA2), and nitric oxide excess 1 (OsNOE1) genes. Our findings reveal that short-term exposure of rice seedlings to 1 mM SNP (Nitric oxide donor) applied exogenously for 1 h resulted in significant down-regulation of OsDHODH1 expression in all rice cultivars. In addition, the transcriptional patterns for the CcOX subunits, which are known to have a high affinity for nitric oxide, showed that the core catalytic subunit (OsCcOX1) and the nuclear subunit (OsCcOX5b) were up-regulated, while the nuclear subunit (OsCcOX5c) gene expression was suppressed. OsGSNOR1 expression was enhanced or decreased concomitant with a decrease or increase in SNO accumulation, particularly at the basal level. Moreover, high OsNIA1 expression was consistent with impaired root development, whereas low transcript accumulation matched a balanced root-growth pattern. This suggests that OsNIA1 expression would prevail over OsNIA2 expression under nitrosative stress response in rice. The level of malondialdehyde (MDA) content increased with the increase in SNP concentration, translating enhanced oxidative damage to the cell. We also observed increased catalase activity in response to 5 mM SNP suggesting that potential cross-talk exist between nitrosative and oxidative stress. These results collectively suggest a possible role of OsDHODH1 and OsCcOX5b role in plant root growth during nitrosative stress responses.

Modulating effect of tiron on the capability of mitochondrial oxidative phosphorylation in the brain of rats exposed to radiation or manganese toxicity

The brain is an important organ rich in mitochondria and more susceptible to oxidative stress. Tiron (sodium 4,5-dihydroxybenzene-1,3-disulfonate) is a potent antioxidant. This study aims to evaluate the effect of tiron on the impairment of brain mitochondria induced by exposure to radiation or manganese (Mn) toxicity. We assessed the capability of oxidative phosphorylation (OXPHOS) through determination of mitochondrial redox state, the activity of electron transport chain (ETC), and Krebs cycle as well as the level of adenosine triphosphate (ATP) production. Rats were exposed to 7 Gy of γ-rays or injected i.p. with manganese chloride (100 mg/kg), then treated with tiron (471 mg/kg) for 7 days. The results showed that tiron treatment revealed positive modulation on the mitochondrial redox state manifested by a marked decrease of hydrogen peroxide (H₂O₂), malondialdehyde (MDA), and total nitrate/nitrite (NO_x) associated with a significant increase in total antioxidant capacity (TAC), glutathione (GSH) content, manganese superoxide dismutase (MnSOD), and glutathione peroxidase (GPx) activities. Moreover, tiron can increase the activity of ETC through preventing the depletion in the activity of mitochondrial complexes (I, II, III, and IV), an elevation of coenzyme Q10 (CoQ10) and cytochrome c (Cyt-c) levels. Additionally, tiron showed a noticeable increase in mitochondrial aconitase (mt-aconitase) activity as the major component of Krebs cycle to maintain a high level of ATP production. Tiron also can restore mitochondrial metal homeostasis through positive changes in the levels of calcium (Ca), iron (Fe), Mn, and copper (Cu). It can be concluded that tiron may be used as a good mitigating agent to attenuate the harmful effects on the brain through the inhibition of mitochondrial injury post-exposure to radiation or Mn toxicity.

Myalgic encephalomyelitis or chronic fatigue syndrome: how could the illness develop?

A model of the development and progression of chronic fatigue syndrome (myalgic encephalomyelitis), the aetiology of which is currently unknown, is put forward, starting with a consideration of the post-infection role of damage-associated molecular patterns and the development of chronic inflammatory, oxidative and nitrosative stress in genetically predisposed individuals. The consequences are detailed, including the role of increased intestinal permeability and the translocation of commensal antigens into the circulation, and the development of dysautonomia, neuroinflammation, and neurocognitive and neuroimaging abnormalities. Increasing levels of such stress and the switch to immune and metabolic downregulation are detailed next in relation to the advent of hypernitrosylation, impaired mitochondrial performance, immune suppression, cellular hibernation, endotoxin tolerance and sirtuin 1 activation. The role of chronic stress and the development of endotoxin tolerance via indoleamine 2,3-dioxygenase upregulation and the characteristics of neutrophils, monocytes, macrophages and T cells, including regulatory T cells, in endotoxin tolerance are detailed next. Finally, it is shown how the immune and metabolic abnormalities of chronic fatigue syndrome can be explained by endotoxin tolerance, thus completing the model.

Hydrogen Sulfide Biochemistry and Interplay with Other Gaseous Mediators in Mammalian Physiology

Hydrogen sulfide (H₂S) has emerged as a relevant signaling molecule in physiology, taking its seat as a bona fide gasotransmitter akin to nitric oxide (NO) and carbon monoxide (CO). After being merely regarded as a toxic poisonous molecule, it is now recognized that mammalian cells are equipped with sophisticated enzymatic systems for H₂S production and breakdown. The signaling role of H₂S is mainly related to its ability to modify different protein targets, particularly by promoting persulfidation of protein cysteine residues and by interacting with metal centers, mostly hemes. H₂S has been shown to regulate a myriad of cellular processes with multiple physiological consequences. As such, dysfunctional H₂S metabolism is increasingly implicated in different pathologies, from cardiovascular and neurodegenerative diseases to cancer. As a highly diffusible reactive species, the intra- and extracellular levels of H₂S have to be kept under tight control and, accordingly, regulation of H₂S metabolism occurs at different levels. Interestingly, even though H₂S, NO, and CO have similar modes of action and parallel regulatory targets or precisely because of that, there is increasing evidence of a crosstalk between the three gasotransmitters. Herein are reviewed the biochemistry, metabolism, and signaling function of hydrogen sulfide, as well as its interplay with the other gasotransmitters, NO and CO.

Alkaloids, Nitric Oxide, and Nitrite Reductases: Evolutionary Coupling as Key Regulators of Cellular Bioenergetics with Special Relevance to the Human Microbiome

Typical alkaloids expressed by prokaryotic and eukaryotic cells are small heterocyclic compounds containing weakly basic nitrogen groups that are critically important for mediating essential biological activities. The prototype opiate alkaloid morphine represents a low molecular mass heterocyclic compound that has been evolutionarily fashioned from a relatively restricted role as a secreted antimicrobial phytoalexin into a broad spectrum regulatory molecule. As an essential corollary, positive evolutionary pressure has driven the development of a cognate 6-transmembrane helical (TMH) domain μ3 opiate receptor that is exclusively responsive to morphine and related opiate alkaloids. A key aspect of “morphinergic” signaling mediated by μ3 opiate receptor activation is its functional coupling with regulatory pathways utilizing constitutive nitric oxide (NO) as a signaling molecule. Importantly, tonic and phasic intra-mitochondrial NO production exerts profound inhibitory effects on the rate of electron transport, H+ pumping, and O2 consumption. Given the pluripotent role of NO as a selective, temporally-defined chemical regulator of mitochondrial respiration and cellular bioenergetics, the expansion of prokaryotic denitrification systems into mitochondrial NO/nitrite cycling complexes represents a series of evolutionary modifications of existential proportions. Presently, our short review provides selective discussion of evolutionary development of morphine, opiate alkaloids, μ3 opiate receptors, and NO systems, within the perspectives of enhanced mitochondrial function, cellular bioenergetics, and the human microbiome.

The Endoplasmic Reticulum Stress Response in Neuroprogressive Diseases: Emerging Pathophysiological Role and Translational Implications

The endoplasmic reticulum (ER) is the main cellular organelle involved in protein synthesis, assembly and secretion. Accumulating evidence shows that across several neurodegenerative and neuroprogressive diseases, ER stress ensues, which is accompanied by over-activation of the unfolded protein response (UPR). Although the UPR could initially serve adaptive purposes in conditions associated with higher cellular demands and after exposure to a range of pathophysiological insults, over time the UPR may become detrimental, thus contributing to neuroprogression. Herein, we propose that immune-inflammatory, neuro-oxidative, neuro-nitrosative, as well as mitochondrial pathways may reciprocally interact with aberrations in UPR pathways. Furthermore, ER stress may contribute to a deregulation in calcium homoeostasis. The common denominator of these pathways is a decrease in neuronal resilience, synaptic dysfunction and even cell death. This review also discusses how mechanisms related to ER stress could be explored as a source for novel therapeutic targets for neurodegenerative and neuroprogressive diseases. The design of randomised controlled trials testing compounds that target aberrant UPR-related pathways within the emerging framework of precision psychiatry is warranted.

Oxidative responses and fungal infection biology

The balance between reactive oxygen species and reactive nitrogen species production by the host and stress response by fungi is a key axis of the host-pathogen interaction. This review will describe emerging themes in fungal pathogenesis underpinning this axis.

The role of hypernitrosylation in the pathogenesis and pathophysiology of neuroprogressive diseases

There is a wealth of data indicating that de novo protein S-nitrosylation in general and protein transnitrosylation in particular mediates the bulk of nitric oxide signalling. These processes enable redox sensing and facilitate homeostatic regulation of redox dependent protein signalling, function, stability and trafficking. Increased S-nitrosylation in an environment of increasing oxidative and nitrosative stress (O&NS) is initially a protective mechanism aimed at maintaining protein structure and function. When O&NS becomes severe, mechanisms governing denitrosylation and transnitrosylation break down leading to the pathological state referred to as hypernitrosylation (HN). Such a state has been implicated in the pathogenesis and pathophysiology of several neuropsychiatric and neurodegenerative diseases and we investigate its potential role in the development and maintenance of neuroprogressive disorders. In this paper, we propose a model whereby the hypernitrosylation of a range of functional proteins and enzymes lead to changes in activity which conspire to produce at least some of the core abnormalities contributing to the development and maintenance of pathology in these illnesses.

Cytochrome bd and Gaseous Ligands in Bacterial Physiology

Cytochrome bd is a unique prokaryotic respiratory terminal oxidase that does not belong to the extensively investigated family of haem-copper oxidases (HCOs). The enzyme catalyses the four-electron reduction of O₂ to 2H₂O, using quinols as physiological reducing substrates. The reaction is electrogenic and cytochrome bd therefore sustains bacterial energy metabolism by contributing to maintain the transmembrane proton motive force required for ATP synthesis. As compared to HCOs, cytochrome bd displays several distinctive features in terms of (i) metal composition (it lacks Cu and harbours a d-type haem in addition to two haems b), (ii) overall three-dimensional structure, that only recently has been solved, and arrangement of the redox cofactors, (iii) lesser energetic efficiency (it is not a proton pump), (iv) higher O₂ affinity, (v) higher resistance to inhibitors such as cyanide, nitric oxide (NO) and hydrogen sulphide (H₂S) and (vi) ability to efficiently metabolize potentially toxic reactive oxygen and nitrogen species like hydrogen peroxide (H₂O₂) and peroxynitrite (ONOO^-). Compelling evidence suggests that, beyond its bioenergetic role, cytochrome bd plays multiple functions in bacterial physiology and affords protection against oxidative and nitrosative stress. Relevant to human pathophysiology, thanks to its peculiar properties, the enzyme has been shown to promote virulence in several bacterial pathogens, being currently recognized as a target for the development of new antibiotics. This review aims to give an update on our current understanding of bd-type oxidases with a focus on their reactivity with gaseous ligands and its potential impact on bacterial physiology and human pathophysiology.

Staphylococcus aureus nitric oxide synthase (saNOS) modulates aerobic respiratory metabolism and cell physiology

Nitric oxide (NO) is generated from arginine and oxygen via NO synthase (NOS). Staphylococcus aureus NOS (saNOS) has previously been shown to affect virulence and resistance to exogenous oxidative stress, yet the exact mechanism is unknown. Herein, a previously undescribed role of saNOS in S. aureus aerobic physiology was reported. Specifically, aerobic S. aureus nos mutant cultures presented with elevated endogenous reactive oxygen species (ROS) and superoxide levels, as well as increased membrane potential, increased respiratory dehydrogenase activity and slightly elevated oxygen consumption. Elevated ROS levels in the nos mutant likely resulted from altered respiratory function, as inhibition of NADH dehydrogenase brought ROS levels back to wild-type levels. These results indicate that, in addition to its recently reported role in regulating the switch to nitrate-based respiration during low-oxygen growth, saNOS also plays a modulatory role during aerobic respiration. Multiple transcriptional changes were also observed in the nos mutant, including elevated expression of genes associated with oxidative/nitrosative stress, anaerobic respiration and lactate metabolism. Targeted metabolomics revealed decreased cellular lactate levels, and altered levels of TCA cycle intermediates, the latter of which may be related to decreased aconitase activity. Collectively, these findings demonstrate a key contribution of saNOS to S. aureus aerobic respiratory metabolism.

Cardiovascular Mitochondrial Dysfunction Induced by Cocaine: Biomarkers and Possible Beneficial Effects of Modulators of Oxidative Stress

Cocaine abuse has long been known to cause morbidity and mortality due to its cardiovascular toxic effects. The pathogenesis of the cardiovascular toxicity of cocaine use has been largely reviewed, and the most recent data indicate a fundamental role of oxidative stress in cocaine-induced cardiovascular toxicity, indicating that mitochondrial dysfunction is involved in the mechanisms of oxidative stress. The comprehension of the mechanisms involving mitochondrial dysfunction could help in selecting the most appropriate mitochondria injury biological marker, such as superoxide dismutase-2 activity and glutathionylated hemoglobin. The potential use of modulators of oxidative stress (mitoubiquinone, the short-chain quinone idebenone, and allopurinol) in the treatment of cocaine cardiotoxic effects is also suggested to promote further investigations on these potential mitochondria-targeted antioxidant strategies.

Attempting to Compensate for Reduced Neuronal Nitric Oxide Synthase Protein with Nitrate Supplementation Cannot Overcome Metabolic Dysfunction but Rather Has Detrimental Effects in Dystrophin-Deficient mdx Muscle

Duchenne muscular dystrophy arises from the loss of dystrophin and is characterized by calcium dysregulation, muscular atrophy, and metabolic dysfunction. The secondary reduction of neuronal nitric oxide synthase (nNOS) from the sarcolemma reduces NO production and bioavailability. As NO modulates glucose uptake, metabolism, and mitochondrial bioenergetics, we investigated whether an 8-week nitrate supplementation regimen could overcome metabolic dysfunction in the mdx mouse. Dystrophin-positive control (C57BL/10) and dystrophin-deficient mdx mice were supplemented with sodium nitrate (85 mg/l) in drinking water. Following the supplementation period, extensor digitorum longus and soleus were excised and radioactive glucose uptake was measured at rest (basal) and during contraction. Gastrocnemius was excised and mitochondrial respiration was measured using the Oroboros Oxygraph. Tibialis anterior was analyzed immunohistochemically for the presence of dystrophin, nNOS, nitrotyrosine, IgG and CD45+ cells, and histologically to assess areas of damage and regeneration. Glucose uptake in the basal and contracting states was normal in unsupplemented mdx muscles but was reduced following nitrate supplementation in mdx muscles only. The mitochondrial utilization of substrates was also impaired in mdx gastrocnemius during phosphorylating and maximal uncoupled respiration, and nitrate could not improve respiration in mdx muscle. Although nitrate supplementation reduced mitochondrial hydrogen peroxide emission, it induced mitochondrial uncoupling in red gastrocnemius, increased muscle fiber peroxynitrite (nitrotyrosine), and promoted skeletal muscle damage. Our novel data suggest that despite lower nNOS protein expression and likely lower NO production in mdx muscle, enhancing NO production with nitrate supplementation in these mice has detrimental effects on skeletal muscle. This may have important relevance for those with DMD.

Oxidative and nitrosative stress defences of Helicobacter and Campylobacter species that counteract mammalian immunity

Helicobacter and Campylobacter species are Gram-negative microaerophilic host-associated heterotrophic bacteria that invade the digestive tract of humans and animals. Campylobacter jejuni is the major worldwide cause of foodborne gastroenteritis in humans, while Helicobacter pylori is ubiquitous in over half of the world's population causing gastric and duodenal ulcers. The colonisation of the gastrointestinal system by Helicobacter and Campylobacter relies on numerous cellular defences to sense the host environment and respond to adverse conditions, including those imposed by the host immunity. An important antimicrobial tool of the mammalian innate immune system is the generation of harmful oxidative and nitrosative stresses to which pathogens are exposed during phagocytosis. This review summarises the regulators, detoxifying enzymes and subversion mechanisms of Helicobacter and Campylobacter that ultimately promote the successful infection of humans.

Protein undernutrition during development and oxidative impairment in the central nervous system (CNS): potential factors in the occurrence of metabolic syndrome and CNS disease

Mitochondria play a regulatory role in several essential cell processes including cell metabolism, calcium balance and cell viability. In recent years, it has been postulated that mitochondria participate in the pathogenesis of a number of chronic diseases, including central nervous system disorders. Thus, the concept of mitochondrial function now extends far beyond the common view of this organelle as the ‘powerhouse’ of the cell to a new appreciation of the mitochondrion as a transducer of early metabolic insult into chronic disease in later life. In this review, we have attempted to describe some of the associations between nutritional status and mitochondrial function (and dysfunction) during embryonic development with the occurrence of neural oxidative imbalance and neurogenic disease in adulthood.

Bioenergetic relevance of hydrogen sulfide and the interplay between gasotransmitters at human cystathionine β-synthase

Merely considered as a toxic gas in the past, hydrogen sulfide (H2S) is currently viewed as the third 'gasotransmitter' in addition to nitric oxide (NO) and carbon monoxide (CO), playing a key signalling role in human (patho)physiology. H2S can either act as a substrate or, similarly to CO and NO, an inhibitor of mitochondrial respiration, in the latter case by targeting cytochrome c oxidase (CcOX). The impact of H(2)S on mitochondrial energy metabolism crucially depends on the bioavailability of this gaseous molecule and its interplay with the other two gasotransmitters. The H(2)S-producing human enzyme cystathionine β-synthase (CBS), sustaining cellular bioenergetics in colorectal cancer cells, plays a role in the interplay between gasotransmitters. The enzyme was indeed recently shown to be negatively modulated by physiological concentrations of CO and NO, particularly in the presence of its allosteric activator S-adenosyl-l-methionine (AdoMet). These newly discovered regulatory mechanisms are herein reviewed. This article is part of a Special Issue entitled 'EBEC 2016: 19th European Bioenergetics Conference, Riva del Garda, Italy, July 2-6, 2016', edited by Prof. Paolo Bernardi.

Changes in Methionine Metabolism and Histone H3 Trimethylation Are Linked to Mitochondrial Defects in Multiple Sclerosis

Mitochondrial changes, including decreased expression of electron transport chain subunit genes and impaired energetic, have been reported in multiple sclerosis (MS), but the mechanisms involved in these changes are not clear. To determine whether epigenetic mechanisms are involved, we measured the concentrations of methionine metabolites by liquid chromatography tandem mass spectrometry, histone H3 methylation patterns, and markers of mitochondrial respiration in gray matter from postmortem MS and control cortical samples. We found decreases in respiratory markers as well as decreased concentrations of the methionine metabolites S-adenosylmethionine, betaine, and cystathionine in MS gray matter. We also found expression of the enzyme betaine homocysteine methyltransferase in cortical neurons. This enzyme catalyzes the remethylation of homocysteine to methionine, with betaine as the methyl donor, and has previously been thought to be restricted to liver and kidney in the adult human. Decreases in the concentration of the methyl donor betaine were correlated with decreases in histone H3 trimethylation (H3K4me3) in NeuN+ neuronal nuclei in MS cortex compared with controls. Mechanistic studies demonstrated that H3K4me3 levels and mitochondrial respiration were reduced in SH-SY5Y cells after exposure to the nitric oxide donor sodium nitroprusside, and betaine was able to rescue H3K4me3 levels and respiratory capacity in these cells. Chromatin immunoprecipitation experiments showed that betaine regulates metabolic genes in human SH-SY5Y neuroblastoma cells. These data suggest that changes to methionine metabolism may be mechanistically linked to changes in neuronal energetics in MS cortex.

SIGNIFICANCE STATEMENT

For decades, it has been observed that vitamin B12 deficiency and multiple sclerosis (MS) share certain pathological changes, including conduction disturbances. In the present study, we have found that vitamin B12-dependent methionine metabolism is dysregulated in the MS brain. We found that concentrations of the methyl donor betaine are decreased in MS cortex and are correlated with reduced levels of the histone H3 methyl mark H3K4me3 in neurons. Cell culture and chromatin immunoprecipitation-seq data suggest that these changes may lead to defects in mitochondria and impact neuronal energetics. These data have uncovered a novel pathway linking methionine metabolism with mitochondrial respiration and have important implications for understanding mechanisms involved in neurodegeneration in MS.

The dual function of flavodiiron proteins: oxygen and/or nitric oxide reductases

Flavodiiron proteins have emerged in the last two decades as a newly discovered family of oxygen and/or nitric oxide reductases widespread in the three life domains, and present in both aerobic and anaerobic organisms. Herein we present the main features of these fascinating enzymes, with a particular emphasis on the metal sites, as more appropriate for this special issue in memory of the exceptional bioinorganic scientist R. J. P. Williams who pioneered the notion of (metal) element availability-driven evolution. We also compare the flavodiiron proteins with the other oxygen and nitric oxide reductases known until now, highlighting how throughout evolution Nature arrived at different solutions for similar functions, in some cases adding extra features, such as energy conservation. These enzymes are an example of the (bioinorganic) unpredictable diversity of the living world.

Morphine stimulates nitric oxide release in human mitochondria

The expression of morphine by plants, invertebrate, and vertebrate cells and organ systems, strongly indicates a high level of evolutionary conservation of morphine and related morphinan alkaloids as required for life. The prototype catecholamine, dopamine, serves as an essential chemical intermediate in morphine biosynthesis, both in plants and animals. We surmise that, before the emergence of specialized plant and animal cells/organ systems, primordial multi-potential cell types required selective mechanisms to limit their responsiveness to environmental cues. Accordingly, cellular systems that emerged with the potential for recruitment of the free radical gas nitric oxide (NO) as a multi-faceted autocrine/paracrine signaling molecule, were provided with extremely positive evolutionary advantages. Endogenous morphinergic signaling, in concert with NO-coupled signaling systems, has evolved as an autocrine/paracrine regulator of metabolic homeostasis, energy metabolism, mitochondrial respiration and energy production. Basic physiological processes involving morphinergic/NO-coupled regulation of mitochondrial function, with special emphasis on the cardiovascular system, are critical to all organismic survival. Key to this concept may be the phenomenon of mitochondrial enslavement in eukaryotic evolution via endogenous morphine.

Nitrogen Oxide Atom-Transfer Redox Chemistry; Mechanism of NO(g) to Nitrite Conversion Utilizing μ-oxo Heme-Fe(III)-O-Cu(II)(L) Constructs

While nitric oxide (NO, nitrogen monoxide) is a critically important signaling agent, its cellular concentrations must be tightly controlled, generally through its oxidative conversion to nitrite (NO2(-)) where it is held in reserve to be reconverted as needed. In part, this reaction is mediated by the binuclear heme a3/CuB active site of cytochrome c oxidase. In this report, the oxidation of NO(g) to nitrite is shown to occur efficiently in new synthetic μ-oxo heme-Fe(III)-O-Cu(II)(L) constructs (L being a tridentate or tetradentate pyridyl/alkylamino ligand), and spectroscopic and kinetic investigations provide detailed mechanistic insights. Two new X-ray structures of μ-oxo complexes have been determined and compared to literature analogs. All μ-oxo complexes react with 2 mol equiv NO(g) to give 1:1 mixtures of discrete [(L)Cu(II)(NO2(-))](+) plus ferrous heme-nitrosyl compounds; when the first NO(g) equiv reduces the heme center and itself is oxidized to nitrite, the second equiv of NO(g) traps the ferrous heme thus formed. For one μ-oxo heme-Fe(III)-O-Cu(II)(L) compound, the reaction with NO(g) reveals an intermediate species ("intermediate"), formally a bis-NO adduct, [(NO)(porphyrinate)Fe(II)-(NO2(-))-Cu(II)(L)](+) (λmax = 433 nm), confirmed by cryo-spray ionization mass spectrometry and EPR spectroscopy, along with the observation that cooling a 1:1 mixture of [(L)Cu(II)(NO2(-))](+) and heme-Fe(II)(NO) to -125 °C leads to association and generation of the key 433 nm UV-vis feature. Kinetic-thermodynamic parameters obtained from low-temperature stopped-flow measurements are in excellent agreement with DFT calculations carried out which describe the sequential addition of NO(g) to the μ-oxo complex.

The many roads to mitochondrial dysfunction in neuroimmune and neuropsychiatric disorders

BACKGROUND

Mitochondrial dysfunction and defects in oxidative metabolism are a characteristic feature of many chronic illnesses not currently classified as mitochondrial diseases. Examples of such illnesses include bipolar disorder, multiple sclerosis, Parkinson's disease, schizophrenia, depression, autism, and chronic fatigue syndrome.

DISCUSSION

While the majority of patients with multiple sclerosis appear to have widespread mitochondrial dysfunction and impaired ATP production, the findings in patients diagnosed with Parkinson's disease, autism, depression, bipolar disorder schizophrenia and chronic fatigue syndrome are less consistent, likely reflecting the fact that these diagnoses do not represent a disease with a unitary pathogenesis and pathophysiology. However, investigations have revealed the presence of chronic oxidative stress to be an almost invariant finding in study cohorts of patients afforded each diagnosis. This state is characterized by elevated reactive oxygen and nitrogen species and/or reduced levels of glutathione, and goes hand in hand with chronic systemic inflammation with elevated levels of pro-inflammatory cytokines.

SUMMARY

This paper details mechanisms by which elevated levels of reactive oxygen and nitrogen species together with elevated pro-inflammatory cytokines could conspire to pave a major road to the development of mitochondrial dysfunction and impaired oxidative metabolism seen in many patients diagnosed with these disorders.

Glutathione S-transferase and MRP1 form an integrated system involved in the storage and transport of dinitrosyl-dithiolato iron complexes in cells

Nitrogen monoxide (NO) is vital for many essential biological processes as a messenger and effector molecule. The physiological importance of NO is the result of its high affinity for iron in the active sites of proteins such as guanylate cyclase. Indeed, NO possesses a rich coordination chemistry with iron and the formation of dinitrosyl-dithiolato iron complexes (DNICs) is well documented. In mammals, NO generated by cytotoxic activated macrophages has been reported to play a role as a cytotoxic effector against tumor cells by binding and releasing intracellular iron. Studies from our laboratory have shown that two proteins traditionally involved in drug resistance, namely multidrug-resistance protein 1 and glutathione S-transferase, play critical roles in intracellular NO transport and storage through their interaction with DNICs (R.N. Watts et al., Proc. Natl. Acad. Sci. USA 103:7670-7675, 2006; H. Lok et al., J. Biol. Chem. 287:607-618, 2012). Notably, DNICs are present at high concentrations in cells and are biologically available. These complexes have a markedly longer half-life than free NO, making them an ideal "common currency" for this messenger molecule. Considering the many critical roles NO plays in health and disease, a better understanding of its intracellular trafficking mechanisms will be vital for the development of new therapeutics.

Cytochrome bd oxidase and bacterial tolerance to oxidative and nitrosative stress

Cytochrome bd is a prokaryotic respiratory quinol:O2 oxidoreductase, phylogenetically unrelated to the extensively studied heme-copper oxidases (HCOs). The enzyme contributes to energy conservation by generating a proton motive force, though working with a lower energetic efficiency as compared to HCOs. Relevant to patho-physiology, members of the bd-family were shown to promote virulence in some pathogenic bacteria, which makes these enzymes of interest also as potential drug targets. Beyond its role in cell bioenergetics, cytochrome bd accomplishes several additional physiological functions, being apparently implicated in the response of the bacterial cell to a number of stress conditions. Compelling experimental evidence suggests that the enzyme enhances bacterial tolerance to oxidative and nitrosative stress conditions, owing to its unusually high nitric oxide (NO) dissociation rate and a notable catalase activity; the latter has been recently documented in one of the two bd-type oxidases of Escherichia coli. Current knowledge on cytochrome bd and its reactivity with O2, NO and H2O2 is summarized in this review in the light of the hypothesis that the preferential (over HCOs) expression of cytochrome bd in pathogenic bacteria may represent a strategy to evade the host immune attack based on production of NO and reactive oxygen species (ROS). This article is part of a Special Issue entitled: 18th European Bioenergetic Conference.

New evidence for cross talk between melatonin and mitochondria mediated by a circadian-compatible interaction with nitric oxide

Extending our previous observations, we have shown on HaCat cells that melatonin, at ~10⁻⁹ M concentration, transiently raises not only the expression of the neuronal nitric oxide synthase (nNOS) mRNA, but also the nNOS protein synthesis and the nitric oxide oxidation products, nitrite and nitrate. Interestingly, from the cell bioenergetic point of view, the activated NO-related chemistry induces a mild decrease of the oxidative phosphorylation (OXPHOS) efficiency, paralleled by a depression of the mitochondrial membrane potential. The OXPHOS depression is apparently balanced by glycolysis. The mitochondrial effects described have been detected only at nanomolar concentration of melatonin and within a time window of a few hours’ incubation; both findings compatible with the melatonin circadian cycle.

Impacts of nitrate and nitrite on physiology of Shewanella oneidensis

Shewanella oneidensis exhibits a remarkable versatility in anaerobic respiration, which largely relies on its diverse respiratory pathways. Some of these are expressed in response to the existence of their corresponding electron acceptors (EAs) under aerobic conditions. However, little is known about respiration and the impact of non-oxygen EAs on the physiology of the microorganism when oxygen is present. Here we undertook a study to elucidate the basis for nitrate and nitrite inhibition of growth under aerobic conditions. We discovered that nitrate in the form of NaNO3 exerts its inhibitory effects as a precursor to nitrite at low concentrations and as an osmotic-stress provider (Na(+)) at high concentrations. In contrast, nitrite is extremely toxic, with 25 mM abolishing growth completely. We subsequently found that oxygen represses utilization of all EAs but nitrate. To order to utilize EAs with less positive redox potential, such as nitrite and fumarate, S. oneidensis must enter the stationary phase, when oxygen respiration becomes unfavorable. In addition, we demonstrated that during aerobic respiration the cytochrome bd oxidase confers S. oneidensis resistance to nitrite, which likely functions via nitric oxide (NO).

Effect of low-level laser therapy on odontoblast-like cells exposed to bleaching agent

The aim of the present study was to evaluate the effect of low-level laser therapy (LLLT) on odontoblast-like MDPC-23 cells exposed to carbamide peroxide (CP 0.01%-2.21 μg/mL of H2O2). The cells were seeded in sterile 24-well plates for 72 h. Eight groups were established according to the exposure or not to the bleaching agents and the laser energy doses tested (0, 4, 10, and 15 J/cm(2)). After exposing the cells to 0.01% CP for 1 h, this bleaching solution was replaced by fresh culture medium. The cells were then irradiated (three sections) with a near-infrared diode laser (InGaAsP-780 ± 3 nm, 40 mW), with intervals of 24 h. The 0.01% CP solution caused statistically significant reductions in cell metabolism and alkaline phosphate (ALP) activity when compared with those of the groups not exposed to the bleaching agent. The LLLT did not modulate cell metabolism; however, the dose of 4 J/cm(2) increased the ALP activity. It was concluded that 0.01% CP reduces the MDPC-23 cell metabolism and ALP activity. The LLLT in the parameters tested did not influence the cell metabolism of the cultured cells; nevertheless, the laser dose of 4 J/cm(2) increases the ALP activity in groups both with and without exposure to the bleaching agent.

Hemoglobin: a nitric-oxide dioxygenase

Members of the hemoglobin superfamily efficiently catalyze nitric-oxide dioxygenation, and when paired with native electron donors, function as NO dioxygenases (NODs). Indeed, the NOD function has emerged as a more common and ancient function than the well-known role in O2 transport-storage. Novel hemoglobins possessing a NOD function continue to be discovered in diverse life forms. Unique hemoglobin structures evolved, in part, for catalysis with different electron donors. The mechanism of NOD catalysis by representative single domain hemoglobins and multidomain flavohemoglobin occurs through a multistep mechanism involving O2 migration to the heme pocket, O2 binding-reduction, NO migration, radical-radical coupling, O-atom rearrangement, nitrate release, and heme iron re-reduction. Unraveling the physiological functions of multiple NODs with varying expression in organisms and the complexity of NO as both a poison and signaling molecule remain grand challenges for the NO field. NOD knockout organisms and cells expressing recombinant NODs are helping to advance our understanding of NO actions in microbial infection, plant senescence, cancer, mitochondrial function, iron metabolism, and tissue O2 homeostasis. NOD inhibitors are being pursued for therapeutic applications as antibiotics and antitumor agents. Transgenic NOD-expressing plants, fish, algae, and microbes are being developed for agriculture, aquaculture, and industry.

Integration of cellular bioenergetics with mitochondrial quality control and autophagy

Bioenergetic dysfunction is emerging as a cornerstone for establishing a framework for understanding the pathophysiology of cardiovascular disease, diabetes,cancer and neurodegeneration. Recent advances in cellular bioenergetics have shown that many cells maintain a substantial bioenergetic reserve capacity, which is a prospective index of ‘ healthy ’ mitochondrial populations.The bioenergetics of the cell are likely regulated by energy requirements and substrate availability. Additionally,the overall quality of the mitochondrial population and the relative abundance of mitochondria in cells and tissues also impinge on overall bioenergetic capacity and resistance to stress. Because mitochondria are susceptible to damage mediated by reactive oxygen/nitrogen and lipid species, maintaining a ‘ healthy ’ population of mitochondria through quality control mechanisms appears to be essential for cell survival under conditions of pathological stress. Accumulating evidence suggest that mitophagy is particularly important for preventing amplification of initial oxidative insults, which otherwise would further impair the respiratory chain or promote mutations in mitochondrial DNA (mtDNA). The processes underlying the regulation of mitophagy depend on several factors, including the integrity of mtDNA, electron transport chain activity, and the interaction and regulation of the autophagic machinery. The integration and interpretation of cellular bioenergetics in the context of mitochondrial quality control and genetics is the theme of this review.

Heme/copper assembly mediated nitrite and nitric oxide interconversion

The heme(a3)/Cu(B) active site of cytochrome c oxidase is responsible for cellular nitrite reduction to nitric oxide; the same center can return NO to the nitrite pool via oxidative chemistry. Here, we show that a partially reduced heme/Cu assembly reduces NO(2)(-) ion, producing nitric oxide. The heme serves as the reductant, but the Cu(II) ion is also required. In turn, a μ-oxo heme-Fe(III)-O-Cu(II) complex facilitates NO oxidation to nitrite; the final products are the reduced heme and Cu(II)-nitrito complexes.

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.

Reciprocal regulation of cellular nitric oxide formation by nitric oxide synthase and nitrite reductases

Our mini-review focuses on dual regulation of cellular nitric oxide (NO) signaling pathways by traditionally characterized enzymatic formation from L-arginine via the actions of NO synthases (NOS) and by enzymatic reduction of available cellular nitrite pools by a diverse class of cytosolic and mitochondrial nitrite reductases. Nitrite is a major metabolic product of NO and is found in all cell and tissue types that utilize NO signaling processes. Xanthine oxidoreductase (XOR) has been previously characterized as a housekeeping enzyme responsible for cellular uric acid formation via enzymatic conversion of hypoxanthine and xanthine. It has become apparent that XOR possesses multi-functional enzymatic activities outside the realm of xanthine metabolism and a small but significant literature also established a compelling functional association between administered sodium nitrite, XOR activation, and pharmacologically characterized NO transductive effects in positive cardiovascular function enhanced pulmonary perfusion, and protection against ischemia/reperfusion injury and hypoxic damage and oxidative stress. Similar positive vascular and cellular effects were observed to be functionally associated with mitochondrial aldehyde dehydrogenase and cytochrome c/cytochrome c oxidase. The profound implications of a reciprocal regulatory mechanism responsible for cytosolic and mitochondrial NO production are discussed below.

Nanomolar melatonin enhances nNOS expression and controls HaCaT-cells bioenergetics

A novel role of melatonin was unveiled, using immortalized human keratinocyte cells (HaCaT) as a model system. Within a time window compatible with its circadian rhythm, melatonin at nanomolar concentration raised both the expression level of the neuronal nitric oxide synthase mRNA and the nitric oxide oxidation products, nitrite and nitrate. On the same time scale, a depression of the mitochondrial membrane potential was detected together with a decrease of the oxidative phosphorylation efficiency, compensated by glycolysis as testified by an increased production of lactate. The melatonin concentration, ∼ nmolar, inducing the bioenergetic effects and their time dependence, both suggest that the observed nitric oxide-induced mitochondrial changes might play a role in the metabolic pathways characterizing the circadian melatonin chemistry.

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.

Analysis of the mitochondrial proteome in multiple sclerosis cortex

Mitochondrial dysfunction has been proposed to play a role in the neuropathology of multiple sclerosis (MS). Previously, we reported significant alterations in the transcription of nuclear-encoded electron transport chain genes in MS and confirmed translational alterations for components of Complexes I and III that resulted in reductions in their activity. To more thoroughly and efficiently elucidate potential alterations in the expression of mitochondrial and related proteins, we have characterized the mitochondrial proteome in postmortem MS and control cortex using Surface-Enhanced Laser Desorption Ionization Time of Flight Mass Spectrometry (SELDI-TOF-MS). Using principal component analysis (PCA) and hierarchical clustering techniques we were able to analyze the differential patterns of SELDI-TOF spectra to reveal clusters of peaks which distinguished MS from control samples. Four proteins in particular were responsible for distinguishing disease from control. Peptide fingerprint mapping unambiguously identified these differentially expressed proteins. Three proteins identified are involved in respiration including cytochrome c oxidase subunit 5b (COX5b), the brain specific isozyme of creatine kinase, and hemoglobin β-chain. The fourth protein identified was myelin basic protein (MBP). We then investigated whether these alterations were consistent in the experimental autoimmune encephalomyelitis (EAE) mouse model of MS. We found that MBP was similarly altered in EAE but the respiratory proteins were not. These data indicate that while the EAE mouse model may mimic aspects of MS neuropathology which result from inflammatory demyelinating events, there is another distinct mechanism involved in mitochondrial dysfunction in gray matter in MS which is not modeled in EAE.

Cyanide intoxication as part of smoke inhalation--a review on diagnosis and treatment from the emergency perspective

This paper reviews the current literature on smoke inhalation injuries with special attention to the effects of hydrogen cyanide. It is assumed that cyanide poisoning is still an overlooked diagnosis in fire victims. Treatment against cyanide poisoning in the emergency setting should be given based on the clinical diagnosis only. Oxygen in combination with a recommended antidote should be given immediately, the first to reduce cellular hypoxia and the second to eliminate cyanide. A specific antidote is hydroxycobalamin, which can be given iv. and has few side effects.

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.

Enhancement of nitric oxide release from nitrosyl hemoglobin and nitrosyl myoglobin by red/near infrared radiation: potential role in cardioprotection

Nitric oxide is an important messenger in numerous biological processes, such as angiogenesis, hypoxic vasodilation, and cardioprotection. Although nitric oxide synthases (NOS) produce the bulk of NO, there is increasing interest in NOS independent generation of NO in vivo, particularly during hypoxia or anoxia, where low oxygen tensions limit NOS activity. Interventions that can increase NO bioavailability have significant therapeutic potential. The use of far red and near infrared light (R/NIR) can reduce infarct size, protect neurons from methanol toxicity, and stimulate angiogenesis. How R/NIR modulates these processes in vivo and in vitro is unknown, but it has been suggested that increases in NO levels are involved. In this study we examined if R/NIR light could facilitate the release of NO from nitrosyl heme proteins. In addition, we examined if R/NIR light could enhance the protective effects of nitrite on ischemia and reperfusion injury in the rabbit heart. We show both in purified systems and in myocardium that R/NIR light can decay nitrosyl hemes and release NO, and that this released NO may enhance the cardioprotective effects of nitrite. Thus, the photodissociation to NO and its synergistic effect with sodium nitrite may represent a noninvasive and site specific means for increasing NO bioavailability.

Copper-Carbon Bonds in Mechanistic and Structural Probing of Proteins as well as in Situations where Copper is a Catalytic or Receptor Site

While copper-carbon bonds are well appreciated in organometallic synthetic chemistry, such occurrences are less known in biological settings. By far, the greatest incidence of copper-carbon moieties is in bioinorganic research aimed at probing copper protein active site structure and mechanism; for example, carbon monoxide (CO) binding as a surrogate for O2. Using infrared (IR) spectroscopy, CO coordination to cuprous sites has proven to be an extremely useful tool for determining active site copper ligation (e.g., donor atom number and type). The coupled (hemocyanin, tyrosinase, catechol oxidase) and non-coupled (peptidylglycine α-hydroxylating monooxygenase, dopamine β-monooxygenase) binuclear copper proteins as well as the heme-copper oxidases (HCOs) have been studied extensively via this method. In addition, environmental changes within the vicinity of the active site have been determined based on shifts in the CO stretching frequencies, such as for copper amine oxidases, nitrite reductases and again in the binuclear proteins and HCOs. In many situations, spectroscopic monitoring has provided kinetic and thermodynamic data on CuI-CO formation and CO dissociation from copper(I); recently, processes occurring on a femtosecond timescale have been reported. Copper-cyano moieties have also been useful for obtaining insights into the active site structure and mechanisms of copper-zinc superoxide dismutase, azurin, nitrous oxide reductase, and multi-copper oxidases. Cyanide is a good ligand for both copper(I) and copper(II), therefore multiple physical-spectroscopic techniques can be applied. A more obvious occurrence of a “Cu-C” moiety was recently described for a CO dehydrogenase which contains a novel molybdenum-copper catalytic site. A bacterial copper chaperone (CusF) was recently established to have a novel d-π interaction comprised of copper(I) with the arene containing side-chain of a tryptophan amino acid residue. Meanwhile, good evidence exists that a plant receptor site (ETR1) utilizes copper(I) to sense ethylene, a growth hormone. A copper olfactory receptor has also been suggested. All of the above mentioned occurrences or uses of carbon-containing substrates and/or probes are reviewed and discussed within the framework of copper proteins and other relevant systems.

Impaired regulation of electron transport chain subunit genes by nuclear respiratory factor 2 in multiple sclerosis

Multiple sclerosis (MS) is an inflammatory neurodegenerative disease. Recently, decreased expression of nuclear encoded electron transport chain genes was found in neurons in MS cortex. To understand the transcriptional mechanisms responsible for the coordinate down regulation of these genes, we performed electrophoretic mobility shifts with nuclear extracts isolated from gray matter from nonlesion areas of postmortem MS and control cortex. Nine tissue blocks from eight different MS brains and six matched control blocks from five control brains were analyzed. We identified a decrease in a transcription factor complex containing nuclear respiratory factor 2 (NRF-2) in nuclear extracts isolated from MS cortex. This decrease is correlated with decreased expression of electron transport chain subunit genes and increased oxidative damage measured by increased anti-nitrotyrosine immunoreactivity. We conclude that in MS cortex a chronic increase in oxidative stress leads to aberrant regulation of transcription of genes involved in energy metabolism.

Effects of continuous-wave (670-nm) red light on wound healing

BACKGROUND

Recent work suggests that injuries can heal faster if treated by lasers emitting 670-nm red light. LED lights emitting 670-nm light are now available. This suggests that inexpensive and easy-to-use 670-nm LED lights might help accelerate cutaneous wound healing.

OBJECTIVE

The objective was to evaluate the effect of 670-nm LED light on wound healing in SKH-1 hairless mice.

METHODS

To study 670-nm light effects on incisional injury, animals were left unexposed or exposed to equal doses of high-, medium-, or low-flux light. Burn injuries were treated with high-flux light or left unexposed. Healing was assessed by measurement of the burn area and the gap remaining to closure of incisional injury.

RESULTS

Mice exposed to 670-nm red light showed significantly faster healing than control mice. High, medium, and low fluxes of light were all effective after incisional injury. In burn injury, there was improvement in wound healing initially, but the time to repair was unchanged.

CONCLUSIONS

A 670-nm LED red light source accelerates healing in skin of SKH-1 hairless mice after incisional injuries, but is not as effective for burn injuries. These data that suggest red light exposure may be helpful in postoperative wound repair.

Control of cell respiration by nitric oxide in Ataxia Telangiectasia lymphoblastoid cells

Ataxia Telangiectasia (AT) patients are particularly sensitive to oxidative-nitrosative stress. Nitric oxide (NO) controls mitochondrial respiration via the reversible inhibition of complex IV. The mitochondrial response to NO of AT lymphoblastoid cells was investigated. Cells isolated from three patients and three intrafamilial healthy controls were selected showing within each group a normal diploid karyotype and homogeneous telomere length. Different complex IV NO-inhibition patterns were induced by varying the electron flux through the respiratory chain, using exogenous cell membrane permeable electron donors. Under conditions of high electron flux the mitochondrial NO inhibition of respiration was greater in AT than in control cells (P< or =0.05). This property appears peculiar to AT, and correlates well to the higher concentration of cytochrome c detected in the AT cells. This finding is discussed on the basis of the proposed mechanism of reaction of NO with complex IV. It is suggested that the peculiar response of AT mitochondria to NO stress may be relevant to the mitochondrial metabolism of AT patients.

Methylmercury Neurotoxicity: Exploring Potential Novel Targets

Mechanistic studies on the effects of MeHg in the central nervous system (CNS) have been limited to morphology, substrate uptake and macromolecular synthesis, differentiation, and changes in gene expression during development and adulthood, but its primary site of action has yet to be identified. Proper functioning of the nitric oxide synthase (NOS)-cyclic GMP and the cyclooxygenase (COX)-prostaglandin (PG) signaling pathways in the CNS depend on post-translational modifications of key enzymes by chaperone proteins. The ability of MeHg to alter or inhibit chaperone-client protein interactions is hitherto unexplored, and potentially offers an upstream unifying mechanism for the plethora of MeHg effects, ranging from reactive species generation (ROS) generation, mitochondrial dysfunction, changes in redox potential, macromolecule synthesis, and cell swelling. In view of the prominent function of astrocytes in the maintenance of the extracellular milieu and their critical role in mediating MeHg neurotoxicity, they afford a relevant and well-established experimental model. The present review is predicated on (a) the remarkable affinity of mercurials for the anionic form of sulfhydryl (-SH) groups, (b) the essential role of thiols in protein biochemistry, and (c) the role of molecular chaperone proteins, such as heat shock protein 90 (Hsp90) in the regulation of protein redox status by facilitating the formation and breakage of disulfide bridges. We offer potential sites where MeHg may interfere with cellular homeostasis and advance a novel mechanistic model for MeHg-induced neurotoxicity.

Interaction of cyanide and nitric oxide with cytochrome c oxidase: implications for acute cyanide toxicity

Acute cyanide toxicity is attributed to inhibition of cytochrome c oxidase (CcOX), the oxygen-reducing component of mitochondrial electron transport; however, the mitochondrial action of cyanide is complex and not completely understood. State-3 oxygen consumption and CcOX activity were studied in rat N27 mesencephalic cells to examine the functional interaction of cyanide and nitric oxide (NO). KCN produced a concentration-dependent inhibition of cellular respiration. Cyanide's median inhibitory concentration (IC50) of oxygen consumption (13.2 +/- 1.8microM) was higher than the CcOX IC50 (7.2 +/- 0.1microM). Based on respiratory threshold analysis, 60% inhibition of CcOX was necessary before oxygen consumption was decreased. Addition of high levels of exogenous NO (100microM S-nitroso-N-acetyl-DL-penicillamine) attenuated cyanide inhibition of both respiration and CcOX. On the other hand, when endogenous NO generation was blocked by an NOS inhibitor (N(omega)-monomethyl-L-arginine ester), the cyanide IC50 for both respiration and CcOX increased to 59.6 +/- 0.9microM and 102 +/- 10microM, respectively, thus showing constitutive, low-level NO production enhanced cyanide inhibition. Laser scanning cytometry showed that cyanide elevated mitochondrial NO, which then was available to interact with CcOX to enhance the inhibition. It is concluded that the rapid, potent action of cyanide is due in part to mitochondrial generation of NO, which enhances inhibition of CcOX. At low mitochondrial oxygen tensions, the cyanide-NO interaction would be increased. Also, the antidotal action of sodium nitrite is partly explained by generation of high mitochondrial levels of NO, which antagonizes the CcOX inhibition.

Direct evidence for S-nitrosation of mitochondrial complex I

NO* (nitric oxide) is a pleiotropic signalling molecule, with many of its effects on cell function being elicited at the level of the mitochondrion. In addition to the well-characterized binding of NO* to the Cu(B)/haem-a3 site in mitochondrial complex IV, it has been proposed by several laboratories that complex I can be inhibited by S-nitrosation of a cysteine. However, direct molecular evidence for this is lacking. In this investigation we have combined separation techniques for complex I (blue-native gel electrophoresis, Superose 6 column chromatography) with sensitive detection methods for S-nitrosothiols (chemiluminescence, biotin-switch assay), to show that the 75 kDa subunit of complex I is S-nitrosated in mitochondria treated with S-nitrosoglutathione (10 microM-1 mM). The stoichiometry of S-nitrosation was 7:1 (i.e. 7 mol of S-nitrosothiols per mol of complex I) and this resulted in significant inhibition of the complex. Furthermore, S-nitrosothiols were detected in mitochondria isolated from hearts subjected to ischaemic preconditioning. The implications of these results for the physiological regulation of respiration, for reactive oxygen species generation and for a potential role of S-nitrosation in cardioprotection are discussed.