Nitric oxide metabolism in mammalian cells: substrate and inhibitor profiles of a NADPH-cytochrome P450 oxidoreductase-coupled microsomal nitric oxide dioxygenase

Cardiac nitric oxide scavenging: role of myoglobin and mitochondria

Vascular production of nitric oxide (NO) regulates vascular tone. However, highly permeable NO entering the cardiomyocyte would profoundly impact metabolism and signalling without scavenging mechanisms. The purpose of this study was to establish mechanisms of cardiac NO scavenging. Quantitative optical studies of normoxic working hearts demonstrated that micromolar NO concentrations did not alter mitochondria redox state or respiration despite detecting NO oxidation of oxymyoglobin to metmyoglobin. These data are consistent with proposals that the myoglobin/myoglobin reductase (Mb/MbR) system is the major NO scavenging site. However, kinetic studies in intact hearts reveal a minor role (∼9%) for the Mb/MbR system in NO scavenging. In vitro, oxygenated mitochondria studies confirm that micromolar concentrations of NO bind cytochrome oxidase (COX) and inhibit respiration. Mitochondria had a very high capacity for NO scavenging, importantly, independent of NO binding to COX. NO is also known to quickly react with reactive oxygen species (ROS) in vitro. Stimulation of NO scavenging with antimycin and its inhibition by substrate depletion are consistent with NO interacting with ROS generated in Complex I or III under aerobic conditions. Extrapolating these in vitro data to the intact heart supports the hypothesis that mitochondria are a major site of cardiac NO scavenging. KEY POINTS: Cardiomyocyte scavenging of vascular nitric oxide (NO) is critical in maintaining normal cardiac function. Myoglobin redox cycling via myoglobin reductase has been proposed as a major NO scavenging site in the heart. Non-invasive optical spectroscopy was used to monitor the effect of NO on mitochondria and myoglobin redox state in intact beating heart and isolated mitochondria. These non-invasive studies reveal myoglobin/myoglobin reductase plays a minor role in cardiac NO scavenging. A high capacity for NO scavenging by heart mitochondria was demonstrated, independent of cytochrome oxidase binding but dependent on oxygen and high redox potentials consistent with generation of reactive oxygen species.

Dioxygen and glucose force motion of the electron-transfer switch in the iron(III) flavohemoglobin-type nitric oxide dioxygenase

Kinetic and structural investigations of the flavohemoglobin-type NO dioxygenase have suggested critical roles for transient Fe(III)O2 complex formation and O2-forced movements affecting hydride transfer to the FAD cofactor and electron-transfer to the Fe(III)O2 complex. Stark-effect theory together with structural models and dipole and internal electrostatic field determinations provided a semi-quantitative spectroscopic method for investigating the proposed Fe(III)O2 complex and O2-forced movements. Deoxygenation of the enzyme causes Stark effects on the ferric heme Soret and charge-transfer bands revealing the Fe(III)O2 complex. Deoxygenation also elicits Stark effects on the FAD that expose forces and motions that create a more restricted NADH access to FAD for hydride transfer and switch electron-transfer off. Glucose also forces the enzyme toward an off state. Amino acid substitutions at the B10, E7, E11, G8, D5, and F7 positions influence the Stark effects of O2 on resting heme spin states and FAD consistent with the proposed roles of the side chains in the enzyme mechanism. Deoxygenation of ferric myoglobin and hemoglobin A also induces Stark effects on the hemes suggesting a common 'oxy-met' state. The ferric myoglobin and hemoglobin heme spectra are also glucose-responsive. A conserved glucose or glucose-6-phosphate binding site is found bridging the BC-corner and G-helix in flavohemoglobin and myoglobin suggesting novel allosteric effector roles for glucose or glucose-6-phosphate in the NO dioxygenase and O2 storage functions. The results support the proposed roles of a ferric O2 intermediate and protein motions in regulating electron-transfer during NO dioxygenase turnover.

Giardia duodenalis: Flavohemoglobin is involved in drug biotransformation and resistance to albendazole

Giardia duodenalis causes giardiasis, a major diarrheal disease in humans worldwide whose treatment relies mainly on metronidazole (MTZ) and albendazole (ABZ). The emergence of ABZ resistance in this parasite has prompted studies to elucidate the molecular mechanisms underlying this phenomenon. G. duodenalis trophozoites convert ABZ into its sulfoxide (ABZSO) and sulfone (ABZSOO) forms, despite lacking canonical enzymes involved in these processes, such as cytochrome P450s (CYP450s) and flavin-containing monooxygenases (FMOs). This study aims to identify the enzyme responsible for ABZ metabolism and its role in ABZ resistance in G. duodenalis. We first determined that the iron-containing cofactor heme induces higher mRNA expression levels of flavohemoglobin (gFlHb) in Giardia trophozoites. Molecular docking analyses predict favorable interactions of gFlHb with ABZ, ABZSO and ABZSOO. Spectral analyses of recombinant gFlHb in the presence of ABZ, ABZSO and ABZSOO showed high affinities for each of these compounds with Kd values of 22.7, 19.1 and 23.8 nM respectively. ABZ and ABZSO enhanced gFlHb NADH oxidase activity (turnover number 14.5 min-1), whereas LC-MS/MS analyses of the reaction products showed that gFlHb slowly oxygenates ABZ into ABZSO at a much lower rate (turnover number 0.01 min-1). Further spectroscopic analyses showed that ABZ is indirectly oxidized to ABZSO by superoxide generated from the NADH oxidase activity of gFlHb. In a similar manner, the superoxide-generating enzyme xanthine oxidase was able to produce ABZSO in the presence of xanthine and ABZ. Interestingly, we find that gFlHb mRNA expression is lower in albendazole-resistant clones compared to those that are sensitive to this drug. Furthermore, all albendazole-resistant clones transfected to overexpress gFlHb displayed higher susceptibility to the drug than the parent clones. Collectively these findings indicate a role for gFlHb in ABZ conversion to its sulfoxide and that gFlHb down-regulation acts as a passive pharmacokinetic mechanism of resistance in this parasite.

Development of a microtiter plate-based analysis method of nitric oxide dioxygenase activity

Nitric oxide (NO) functions in cell protection or cell death, depending on its concentration. Therefore, regulation of the intracellular concentrations of NO by its degradation systems is important for cellular functions. One of the NO degrading enzymes, flavohemoglobin (FHb), which has NO dioxygenase (NOD) activity, is a promising target for antibiotics, based on the finding that FHb-deficient pathogens exhibited reduced host toxicity. Here, we developed a high-throughput method to measure the NOD activity. Our newly developed method could contribute to the screening of potential antibiotics with NOD inhibitory activity.

Ordered Motions in the Nitric-Oxide Dioxygenase Mechanism of Flavohemoglobin and Assorted Globins with Tightly Coupled Reductases

Nitric-oxide dioxygenases (NODs) activate and combine O₂ with NO to form nitrate. A variety of oxygen-binding hemoglobins with associated partner reductases or electron donors function as enzymatic NODs. Kinetic and structural investigations of the archetypal two-domain microbial flavohemoglobin-NOD have illuminated an allosteric mechanism that employs selective tunnels for O₂ and NO, gates for NO and nitrate, transient O₂ association with ferric heme, and an O₂ and NO-triggered, ferric heme spin crossover-driven, motion-controlled, and dipole-regulated electron-transfer switch. The proposed mechanism facilitates radical-radical coupling of ferric-superoxide with NO to form nitrate while preventing suicidal ferrous-NO formation. Diverse globins display the structural and functional motifs necessary for a similar allosteric NOD mechanism. In silico docking simulations reveal monomeric erythrocyte hemoglobin alpha-chain and beta-chain intrinsically matched and tightly coupled with NADH-cytochrome b₅ oxidoreductase and NADPH-cytochrome P450 oxidoreductase, respectively, forming membrane-bound flavohemoglobin-like mammalian NODs. The neuroprotective neuroglobin manifests a potential NOD role in a close-fitting ternary complex with membrane-bound NADH-cytochrome b₅ oxidoreductase and cytochrome b₅. Cytoglobin interfaces weakly with cytochrome b₅ for O₂ and NO-regulated electron-transfer and coupled NOD activity. The mechanistic model also provides insight into the evolution of O₂ binding cooperativity in hemoglobin and a basis for the discovery of allosteric NOD inhibitors.

Recent advances in the use of phytochemicals to manage gastrointestinal oxidative stress in poultry and pigs

Plants are integral components of pig and poultry feed, and aside from their raw nutritive value, some phytochemicals contain bioactive compounds. The aim of the present paper is to review recent advances in the use of some phytochemicals in pig and poultry feed, focusing on the examples of isoquinoline alkaloids, polyphenol rich sugarcane extracts and superoxide dismutase-rich melon pulp extracts. As gut health is critical for efficient production, the review will focus on recent results modulating oxidative stress within the gastrointestinal tract and the potential mechanisms of action.

Autoamplificatory singlet oxygen generation sensitizes tumor cells for intercellular apoptosis-inducing signaling

Tumor cells express NADPH oxidase-1 (NOX1) in their membrane and control NOX1-based intercellular reactive oxygen and nitrogen species (ROS/RNS)-dependent apoptosis-inducing signaling through membrane-associated catalase and superoxide dismutase.

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of tumor cells with high concentrations of H₂O₂, peroxnitrite, HOCl, or increasing the concentration of cell-derived NO causes initial generation of singlet oxygen and local inactivation of membrane-associated catalase. As a result, free peroxynitrite and H₂O₂ interact and generate secondary singlet oxygen. Inactivation of further catalase molecules by secondary singlet oxygen leads to auto-amplification of singlet oxygen generation and catalase inactivation. This allows reactivation of intercellular ROS/RNS-signaling and selective apoptosis induction in tumor cells. The initial singlet oxygen generation seems to be the critical point in this complex biochemical multistep mechanism. Initial singlet oxygen generation requires the interaction between distinct tumor cell-derived ROS and RNS and may also depend on either the induction of NO synthase expression or NOX1 activation through the FAS receptor. FAS receptor activation can be achieved by singlet oxygen. Autoamplificatory generation of singlet oxygen through the interaction between peroxynitrite and hydrogen peroxide inherits a rich potential for the establishment of synergistic effects that may be instrumental for novel approaches of tumor therapy with high selectivity towards malignant cells.

Increasing the endogenous NO level causes catalase inactivation and reactivation of intercellular apoptosis signaling specifically in tumor cells

Tumor cells generate extracellular superoxide anions and are protected against intercellular apoptosis-inducing HOCl- and NO/peroxynitrite signaling through the expression of membrane-associated catalase. This enzyme decomposes H₂O₂ and thus prevents HOCl synthesis. It efficiently interferes with NO/peroxynitrite signaling through oxidation of NO and decomposition of peroxynitrite. The regulatory potential of catalase at the crosspoint of ROS and RNS chemical biology, as well as its high local concentration on the outside of the cell membrane of tumor cells, establish tight control of intercellular signaling and thus prevent tumor cell apoptosis. Therefore, inhibition of catalase or its inactivation by singlet oxygen reactivate intercellular apoptosis-inducing signaling. Nitric oxide and peroxynitrite are connected with catalase in multiple and meaningful ways, as (i) NO can be oxidated by compound I of catalase, (ii) NO can reversibly inhibit catalase, (iii) peroxynitrite can be decomposed by catalase and (iv) the interaction between peroxynitrite and H₂O₂ leads to the generation of singlet oxygen that inactivates catalase. Therefore, modulation of the concentration of free NO through addition of arginine, inhibition of arginase, induction of NOS expression or inhibition of NO dioxygenase triggers an autoamplificatory biochemical cascade that is based on initial formation of singlet oxygen, amplification of superoxide anion/H₂O₂ and NO generation through singlet oxygen dependent stimulation of the FAS receptor and caspase-8. Finally, singlet oxygen is generated at sufficiently high concentration to inactivate protective catalase and to reactivate intercellular apoptosis-inducing ROS signaling. This regulatory network allows to establish several pathways for synergistic interactions, like the combination of modulators of NO metabolism with enhancers of superoxide anion generation, modulators of NO metabolism that act at different targets and between modulators of NO metabolism and direct catalase inhibitors. The latter aspect is explicitely studied for the interaction between catalase inhibiting acetylsalicylic acid and an NO donor. It is also shown that hybrid molecules like NO-aspirin utilize this synergistic potential. Our data open novel approaches for rational tumor therapy based on specific ROS signaling and its control in tumor cells.

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Membrane-associated catalase protects tumor cells against ROS/RNS signaling.

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NO can be oxidated by catalase, but can also reversibly inhibit the enzyme.

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ONOO⁻ is decomposed by catalase but also drives its inactivation through singlet oxygen.

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Modulation of the NO level triggers singlet oxygen generation and catalase inactivation.

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This signaling network allows to establish synergistic antitumor effects.

Subpicomolar diphenyleneiodonium inhibits microglial NADPH oxidase with high specificity and shows great potential as a therapeutic agent for neurodegenerative diseases

Activation of microglial NADPH oxidase (NOX2) plays a critical role in mediating neuroinflammation, which is closely linked with the pathogenesis of a variety of neurodegenerative diseases, including Parkinson's disease (PD). The inhibition of NOX2-generated superoxide has become an effective strategy for developing disease-modifying therapies for PD. However, the lack of specific and potent NOX2 inhibitors has hampered the progress of this approach. Diphenyleneiodonium (DPI) is a widely used, long-acting NOX2 inhibitor. However, due to its non-specificity for NOX2 and high cytotoxicity at standard doses (µM), DPI has been precluded from human studies. In this study, using ultra-low doses of DPI, we aimed to: (1) investigate whether these problems could be circumvented and (2) determine whether ultra-low doses of DPI were able to preserve its utility as a potent NOX2 inhibitor. We found that DPI at subpicomolar concentrations (10(-14) and 10(-13) M) displays no toxicity in primary midbrain neuron-glia cultures. More importantly, we observed that subpicomolar DPI inhibited phorbol myristate acetate (PMA)-induced activation of NOX2. The same concentrations of DPI did not inhibit the activities of a series of flavoprotein-containing enzymes. Furthermore, potent neuroprotective efficacy was demonstrated in a post-treatment study. When subpicomolar DPI was added to neuron-glia cultures pretreated with lipopolysaccharide, 1-methyl-4-phenylpyridinium or rotenone, it potently protected the dopaminergic neurons. In summary, DPI's unique combination of high specificity toward NOX2, low cytotoxicity and potent neuroprotective efficacy in post-treatment regimens suggests that subpicomolar DPI may be an ideal candidate for further animal studies and potential clinical trials.

Reactive molecule species and antioxidative mechanisms in normal skin and skin aging

Reactive oxygen and nitrogen species (ROS/RNS) which may exist as radicals or nonradicals, as well as reactive sulfur species and reactive carbon species, play a major role in aging processes and in carcinogenesis. These reactive molecule species (RMS), often referred to as 'free radicals' or oxidants, are partly by-products of the physiological metabolism. When RMS concentrations exceed a certain threshold, cell compartments and cells are injured and destroyed. Endogenous physiological mechanisms are able to neutralize RMS to some extent, thereby limiting damage. In the skin, however, pollutants and particularly UV irradiation are able to produce additional oxidants which overload the endogenous protection system and cause early aging, debilitation of immune functions, and skin cancer. The application of antioxidants from various sources in skin care products and food supplements is therefore widespread, with increasingly effective formulations being introduced. The harmful effects of RMS (aside from impaired structure and function of DNA, proteins, and lipids) are: interference with specific regulatory mechanisms and signaling pathways in cell metabolism, resulting in chronic inflammation, weakening of immune functions, and degradation of tissue. Important control mechanisms are: MAP-kinases, the aryl-hydrocarbon receptor (AhR), the antagonistic transcription factors nuclear factor-κB and Nrf2 (nuclear factor erythroid 2-related factor 2), and, especially important, the induction of matrix metalloproteinases which degrade dermal connective tissue. Recent research, however, has revealed that RMS and in particular ROS/RNS are apparently also produced by specific enzyme reactions in an evolutionarily adapted manner. They may fulfill important physiologic functions such as the activation of specific signaling chains in the cell metabolism, defense against infectious pathogens, and regulation of the immune system. Normal physiological conditions are characterized by equilibrium of oxidative and antioxidative mechanisms. The application of antioxidants in the form of 'cosmeceuticals' or systemic 'nutraceuticals' should aim to support a physiologically balanced oxidation status in the skin.

Mechanistic insight into the enzymatic reduction of truncated hemoglobin N of Mycobacterium tuberculosis: role of the CD loop and pre-A motif in electron cycling

Many pathogenic microorganisms have evolved hemoglobin-mediated nitric oxide (NO) detoxification mechanisms, where a globin domain in conjunction with a partner reductase catalyzes the conversion of toxic NO to innocuous nitrate. The truncated hemoglobin HbN of Mycobacterium tuberculosis displays a potent NO dioxygenase activity despite lacking a reductase domain. The mechanism by which HbN recycles itself during NO dioxygenation and the reductase that participates in this process are currently unknown. This study demonstrates that the NADH-ferredoxin/flavodoxin system is a fairly efficient partner for electron transfer to HbN with an observed reduction rate of 6.2 μM/min(-1), which is nearly 3- and 5-fold faster than reported for Vitreoscilla hemoglobin and myoglobin, respectively. Structural docking of the HbN with Escherichia coli NADH-flavodoxin reductase (FdR) together with site-directed mutagenesis revealed that the CD loop of the HbN forms contacts with the reductase, and that Gly(48) may have a vital role. The donor to acceptor electron coupling parameters calculated using the semiempirical pathway method amounts to an average of about 6.4 10(-5) eV, which is lower than the value obtained for E. coli flavoHb (8.0 10(-4) eV), but still supports the feasibility of an efficient electron transfer. The deletion of Pre-A abrogated the heme iron reduction by FdR in the HbN, thus signifying its involvement during intermolecular interactions of the HbN and FdR. The present study, thus, unravels a novel role of the CD loop and Pre-A motif in assisting the interactions of the HbN with the reductase and the electron cycling, which may be vital for its NO-scavenging function.

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.

Nitric-oxide dioxygenase function of human cytoglobin with cellular reductants and in rat hepatocytes

Cytoglobin (Cygb) was investigated for its capacity to function as a NO dioxygenase (NOD) in vitro and in hepatocytes. Ascorbate and cytochrome b(5) were found to support a high NOD activity. Cygb-NOD activity shows respective K(m) values for ascorbate, cytochrome b(5), NO, and O(2) of 0.25 mm, 0.3 microm, 40 nm, and approximately 20 microm and achieves a k(cat) of 0.5 s(-1). Ascorbate and cytochrome b(5) reduce the oxidized Cygb-NOD intermediate with apparent second order rate constants of 1000 m(-1) s(-1) and 3 x 10(6) m(-1) s(-1), respectively. In rat hepatocytes engineered to express human Cygb, Cygb-NOD activity shows a similar k(cat) of 1.2 s(-1), a K(m)(NO) of 40 nm, and a k(cat)/K(m)(NO) (k'(NOD)) value of 3 x 10(7) m(-1) s(-1), demonstrating the efficiency of catalysis. NO inhibits the activity at [NO]/[O(2)] ratios >1:500 and limits catalytic turnover. The activity is competitively inhibited by CO, is slowly inactivated by cyanide, and is distinct from the microsomal NOD activity. Cygb-NOD provides protection to the NO-sensitive aconitase. The results define the NOD function of Cygb and demonstrate roles for ascorbate and cytochrome b(5) as reductants.

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

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

What is the real physiological NO concentration in vivo?

Clarity about the nitric oxide (NO) concentrations existing physiologically is essential for developing a quantitative understanding of NO signalling, for performing experiments with NO that emulate reality, and for knowing whether or not NO concentrations become abnormal in disease states. A decade ago, a value of about 1 μM seemed reasonable based on early electrode measurements and a provisional estimate of the potency of NO for its guanylyl cyclase-coupled receptors, which mediate physiological NO signal transduction. Since then, numerous efforts to measure NO concentrations directly using electrodes in cells and tissues have yielded an irreconcilably large spread of values. In compensation, data from several alternative approaches have now converged to provide a more coherent picture. These approaches include the quantitative analysis of NO-activated guanylyl cyclase, computer modelling based on the type, activity and amount of NO synthase enzyme contained in cells, the use of novel biosensors to monitor NO release from single endothelial cells and neurones, and the use of guanylyl cyclase as an endogenous NO biosensor in tissue subjected to a variety of challenges. All these independent lines of evidence suggest the physiological NO concentration range to be 100 pM (or below) up to ∼5 nM, orders of magnitude lower than was once thought.

Cytochrome P450 oxidoreductase participates in nitric oxide consumption by rat brain

In low nanomolar concentrations, NO (nitric oxide) functions as a transmitter in brain and other tissues, whereas near-micromolar NO concentrations are associated with toxicity and cell death. Control of the NO concentration, therefore, is critical for proper brain function, but, although its synthesis pathway is well-characterized, the major route of breakdown of NO in brain is unclear. Previous observations indicate that brain cells actively consume NO at a high rate. The mechanism of this consumption was pursued in the present study. NO consumption by a preparation of central glial cells was abolished by cell lysis and recovered by addition of NADPH. NADPH-dependent consumption of NO localized to cell membranes and was inhibited by proteinase K, indicating the involvement of a membrane-bound protein. Purification of this activity yielded CYPOR (cytochrome P450 oxidoreductase). Antibodies against CYPOR inhibited NO consumption by brain membranes and the amount of CYPOR in several cell types correlated with their rate of NO consumption. NO was also consumed by purified CYPOR but this activity was found to depend on the presence of the vitamin E analogue Trolox (6-hydroxy-2,5,7,8-tetramethylchromane-2-carboxylic acid), included in the buffer as a precaution against inadvertent NO consumption by lipid peroxidation. In contrast, NO consumption by brain membranes was independent of Trolox. Hence, it appears that, during the purification process, CYPOR becomes separated from a partner needed for NO consumption. Cytochrome P450 inhibitors inhibited NO consumption by brain membranes, making these proteins likely candidates.

Assay and characterization of the NO dioxygenase activity of flavohemoglobins

A variety of hemoglobins, including several microbial flavohemoglobins, enzymatically dioxygenate the free radical nitric oxide (*NO) to form nitrate. Many of these *NO dioxygenases have been shown to control *NO toxicity and signaling. Furthermore, *NO dioxygenation appears to be an ancient and intrinsic function for members of the hemoglobin superfamily found in Archaea, eukaryotes, and bacteria. Yet for many hemoglobins, a function remains to be elucidated. Methods for the assay and characterization of the *NO dioxygenase (EC 1.14.12.17) activity and function of flavohemoglobins are described. The methods may also be applied to the discovery and design of inhibitors for use as antibiotics or as modulators of *NO signaling.

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

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

Determination of nitrate by the IE-HPLC-UV method in the brain tissues of Wistar rats poisoned with paraquat

This work was a part of an initial study regarding the involvement of reactive nitrogen species (RNS) in paraquat (PQ) neurotoxicity. The nitrate concentration in the vulnerable regions of the brain (cortex, striatum and hippocampus) of Wistar rats was used as a measure of nitric oxide (NO) production or catabolism of the formed RNS. The tissue homogenates were deproteinized with acetonitrile and then centrifuged. Nitrate was measured in filtrated supernatants by simple and rapid isocratic ion-exchange high performance liquid chromatography with UV detection (IE-HPLC-UV) at 214 nm. The mobile phase (pH 8.5) consisted of borate buffer/gluconate concentrate, methanol, acetonitrile and deionized water (2:12:12:74, v/v/v/v), and the flow rate was 1.3 mL/min. Physiological nitrate levels (18.8 ? 6.1 nmol/mg of proteins), as well as a diverse range of nitrate concentrations could be determined with good precision (CV = 2.2 %) and accuracy (recovery of spiked samples was 99 ? 4%) in the brain tissue homogenates. Linearity was achieved in the range of nitrate from 0-80 ?M. The retention time of nitrate anion was 5.3 ? 0.3 min. .

Inactivation of nitric oxide by rat cerebellar slices

Nitric oxide (NO) functions as an intercellular messenger throughout the brain. For this role to be performed efficiently, there must be a mechanism for neutralizing NO, but whether an active biological process exists, or whether NO is lost mainly through diffusion is unclear. To investigate this issue, rat cerebellar slices were exposed to constant levels of NO and the cGMP generated within the slice used as an indicator of NO concentrations therein. NO was about 1000-fold less potent in slices (EC50, 1 microM) than in separated cells from the same tissue (EC50, 1.6 nM), consistent with access of NO to the slice interior being greatly hindered by inactivation. Supporting this interpretation, immunohistochemical analysis indicated a marked concentration gradient of cGMP across the thickness of slices exposed to subsaturating NO concentrations, signifying a marked NO gradient. Several known NO-degrading processes, including reaction with lipid peroxyl radicals, erythrocytes and superoxide ions, were eliminated as contributing factors, indicating a novel mechanism. A diffusion-inactivation model was used to estimate the kinetics of NO consumption by the slices. The best fits to experimental data indicated a Michaelis-Menten-type reaction having a Vmax of 1-2 microM s-1 and a Km of around 10 nM. The rates predict that inactivation would impose a very short half-life (<10 ms) on NO in physiological concentrations (up to 10 nM) and that it would play an important role in shaping the NO concentration profiles when it is synthesized by multiple nearby sites.

Consumption of nitric oxide by endothelial cells: evidence for the involvement of a NAD(P)H-, flavin- and heme-dependent dioxygenase reaction

In the present study, we investigated the mechanism of nitric oxide (NO) inactivation by endothelial cells. All experiments were performed in the presence of superoxide dismutase to minimize the peroxynitrite reaction. Incubation of the NO donor diethylamine/NO adduct with increasing amounts of intact cells led to a progressive decrease of the NO concentration, demonstrating a cell-dependent consumption of NO. In cell homogenates, consumption of NO critically depended on the presence of NADPH or NADH and resulted in the formation of nitrate. Both NO consumption and nitrate formation were largely inhibited by the heme poisons NaCN and phenylhydrazine as well as the flavoenzyme inhibitor diphenylene iodonium. Further characterization of this NO consumption pathway suggests that endothelial cells express a unique membrane-associated enzyme or enzyme system analogous to the bacterial NO dioxygenase that converts NO to nitrate in a NAD(P)H-, flavin- and heme-dependent manner.