Electron spin resonance spin-trapping detection of superoxide generated by neuronal nitric oxide synthase

Vascular redox signaling, eNOS uncoupling and endothelial dysfunction in the setting of transportation noise exposure or chronic treatment with organic nitrates

SIGNIFICANCE

Cardiovascular disease and drug-induced health side effects are frequently associated with - or even caused by - an imbalance between the concentrations of reactive oxygen and nitrogen species (RONS) and antioxidants respectively determining the metabolism of these harmful oxidants.

RECENT ADVANCES

According to the "kindling radical" hypothesis, initial formation of RONS may further trigger the additional activation of RONS formation under certain pathological conditions. The present review will specifically focus on a dysfunctional, uncoupled endothelial nitric oxide synthase (eNOS) caused by RONS in the setting of transportation noise exposure or chronic treatment with organic nitrates, especially nitroglycerin. We will further describe the various "redox switches" that are proposed to be involved in the uncoupling process of eNOS.

CRITICAL ISSUES

In particular, the oxidative depletion of tetrahydrobiopterin (BH4), and S-glutathionylation of the eNOS reductase domain will be highlighted as major pathways for eNOS uncoupling upon noise exposure or nitroglycerin treatment. In addition, oxidative disruption of the eNOS dimer, inhibitory phosphorylation of eNOS at threonine or tyrosine residues, redox-triggered accumulation of asymmetric dimethylarginine (ADMA) and L-arginine deficiency will be discussed as alternative mechanisms of eNOS uncoupling.

FUTURE DIRECTIONS

The clinical consequences of eNOS dysfunction due to uncoupling on cardiovascular disease will be summarized also providing a template for future clinical studies on endothelial dysfunction caused by pharmacological or environmental risk factors.

Recent Developments in the Probes and Assays for Measurement of the Activity of NADPH Oxidases

NADPH oxidases are a family of enzymes capable of transferring electrons from NADPH to molecular oxygen. A major function of NADPH oxidases is the activation of molecular oxygen into reactive oxygen species. Increased activity of NADPH oxidases has been implicated in various pathologies, including cardiovascular disease, neurological dysfunction, and cancer. Thus, NADPH oxidases have been identified as a viable target for the development of novel therapeutics exhibiting inhibitory effects on NADPH oxidases. Here, we describe the development of new assays for measuring the activity of NADPH oxidases enabling the high-throughput screening for NADPH oxidase inhibitors.

Spin Trapping: A Review for the Study of Obesity Related Oxidative Stress and Na+/K+-ATPase

Reactive oxygen species (ROS) have gained attention with mounting evidence of their importance in cell signaling and various disease states. ROS is produced continuously as a natural by-product of normal oxygen metabolism. However, high levels ROS causes oxidative stress and damage to biomolecules. This results in loss of protein function, DNA cleavage, lipid peroxidation, or ultimately cell injury or death. Obesity has become a worldwide epidemic; studies show fat accumulation is associated with increased ROS and oxidative stress. Evidence exists supporting oxidative stress as a factor driving forward insulin resistance (IR), potentially resulting in diabetes. Na⁺/K⁺-ATPase signaling is also a potential source of ROS promoting oxidative stress. The best way to observe radical species in biological systems is electron paramagnetic resonance spectroscopy with spin trapping. EPR spin trapping is an important technique to study the mechanisms driving disease states attributed to ROS.

Nox4: a hydrogen peroxide-generating oxygen sensor

Nox4 is an oddity among members of the Nox family of NADPH oxidases [seven isoenzymes that generate reactive oxygen species (ROS) from molecular oxygen] in that it is constitutively active. All other Nox enzymes except for Nox4 require upstream activators, either calcium or organizer/activator subunits (p47(phox), NOXO1/p67(phox), and NOXA1). Nox4 may also be unusual as it reportedly releases hydrogen peroxide (H₂O₂) in contrast to Nox1-Nox3 and Nox5, which release superoxide, although this result is controversial in part because of possible membrane compartmentalization of superoxide, which may prevent detection. Our studies were undertaken (1) to identify the Nox4 ROS product using a membrane-free, partially purified preparation of Nox4 and (2) to test the hypothesis that Nox4 activity is acutely regulated not by activator proteins or calcium, but by cellular pO₂, allowing it to function as an O₂ sensor, the output of which is signaling H₂O₂. We find that approximately 90% of the electron flux through isolated Nox4 produces H₂O₂ and 10% forms superoxide. The kinetic mechanism of H₂O₂ formation is consistent with a mechanism involving binding of one oxygen molecule, which is then sequentially reduced by the heme in two one-electron reduction steps first to form a bound superoxide intermediate and then H₂O₂; kinetics are not consistent with a previously proposed internal superoxide dismutation mechanism involving two oxygen binding/reduction steps for each H₂O₂ formed. Critically, Nox4 has an unusually high Km for oxygen (∼18%), similar to the values of known oxygen-sensing enzymes, compared with a Km of 2-3% for Nox2, the phagocyte NADPH oxidase. This allows Nox4 to generate H₂O₂ as a function of oxygen concentration throughout a physiological range of pO2 values and to respond rapidly to changes in pO₂.

Developmental susceptibility of neurons to transient tetrahydrobiopterin insufficiency and antenatal hypoxia-ischemia in fetal rabbits

Tetrahydrobiopterin (BH4) is important for normal brain development as congenital BH4 deficiencies manifest movement disorders at various childhood ages. BH4 transitions from very low levels in fetal brains to higher "adult" levels postnatally, with the highest levels in the thalamus. Maternal supplementation with the BH4 precursor sepiapterin reduces postnatal motor deficits and perinatal deaths after 40-min fetal hypoxia-ischemia (HI) at 70% gestation, suggesting that brain BH4 is important in improving function after HI. We tested the hypothesis that the intrinsically low concentrations of BH4 made fetal neurons vulnerable to added insults. Brains were obtained from naïve fetal rabbits or after 40-min HI, at 70% (E22) and 92% gestation (E29). Neuronal cultures were prepared from basal ganglia, cortex, and thalamus, regions with different intrinsic levels of BH4. Cultures were grown with or without added BH4 for 48h. Cell survival and mitochondrial function were determined by flow cytometry. At E22, thalamic cells had the lowest survival rate in a BH4-free milieu, in both control and HI groups, whereas BH4 supplementation ex vivo increased neuronal survival only in HI cells. Neuronal survival was similar in all regions without BH4 at E29. BH4 supplementation increased cell survival and cells with intact mitochondrial membrane potential, from basal ganglia and cortex, but not thalamus. After E29 HI, however, the benefit of BH4 was limited to cortical neurons. We conclude that BH4 is important for fetal neuronal survival after HI especially in the premature thalamus. Supplementation of BH4 has a greater benefit at an earlier gestational age.

Rapid fluorescent visualization of multiple NAD(P)H oxidoreductases in homogenate, permeabilized cells, and tissue slices

Intracellular NAD(P)H oxidoreductases are a class of diverse enzymes that are the key players in a number of vital processes. The method we present and validate here is based on the ability of many NAD(P)H oxidoreductases to reduce the superoxide probe lucigenin, which is structurally similar to flavins, to its highly fluorescent water-insoluble derivative dimethylbiacridene. Two modifications of the method are proposed: (i) an express method for tissue homogenate and permeabilized cells in suspensions and (ii) a standard procedure for cells in culture and acute thin tissue slices. The method allows one to assess, visualize, and localize, using fluorescent markers of cellular compartments, multiple NADH and NADPH oxidoreductase activities. The application of selective inhibitors (e.g., VAS2870, a NOX2 inhibitor; plumbagin, a NOX4 inhibitor) allows one to distinguish and compare specific NAD(P)H oxidoreductase activities in cells and tissues and to attribute them to known enzymes. The method is simple, rapid, and flexible. It can be easily adapted to a variety of tasks. It will be useful for investigations of the role of various NAD(P)H oxidoreductases in a number of physiological and pathophysiological processes.

Regulation of β-adrenergic control of heart rate by GTP-cyclohydrolase 1 (GCH1) and tetrahydrobiopterin

AIMS

Clinical markers of cardiac autonomic function, such as heart rate and response to exercise, are important predictors of cardiovascular risk. Tetrahydrobiopterin (BH4) is a required cofactor for enzymes with roles in cardiac autonomic function, including tyrosine hydroxylase and nitric oxide synthase. Synthesis of BH4 is regulated by GTP cyclohydrolase I (GTPCH), encoded by GCH1. Recent clinical studies report associations between GCH1 variants and increased heart rate, but the mechanistic importance of GCH1 and BH4 in autonomic function remains unclear. We investigate the effect of BH4 deficiency on the autonomic regulation of heart rate in the hph-1 mouse model of BH4 deficiency.

METHODS AND RESULTS

In the hph-1 mouse, reduced cardiac GCH1 expression, GTPCH enzymatic activity, and BH4 were associated with increased resting heart rate; blood pressure was not different. Exercise training decreased resting heart rate, but hph-1 mice retained a relative tachycardia. Vagal nerve stimulation in vitro induced bradycardia equally in hph-1 and wild-type mice both before and after exercise training. Direct atrial responses to carbamylcholine were equal. In contrast, propranolol treatment normalized the resting tachycardia in vivo. Stellate ganglion stimulation and isoproterenol but not forskolin application in vitro induced a greater tachycardic response in hph-1 mice. β1-adrenoceptor protein was increased as was the cAMP response to isoproterenol stimulation.

CONCLUSION

Reduced GCH1 expression and BH4 deficiency cause tachycardia through enhanced β-adrenergic sensitivity, with no effect on vagal function. GCH1 expression and BH4 are novel determinants of cardiac autonomic regulation that may have important roles in cardiovascular pathophysiology.

Hsp90 regulates NADPH oxidase activity and is necessary for superoxide but not hydrogen peroxide production

The goal of this study was to identify whether heat-shock protein 90 (Hsp90) regulates the production of superoxide and other reactive oxygen species from the NADPH oxidases (Nox). We found that pharmacological and genetic inhibition of Hsp90 directly reduced Nox5-derived superoxide without secondarily modifying signaling events. Coimmunoprecipitation and bioluminescence resonance energy transfer studies suggest that the C-terminus of Nox5 binds to Hsp90. Long-term Hsp90 inhibition reduced Nox5 expression and provides further evidence that Nox5 is an Hsp90 client protein. Inhibitors of Hsp90 also reduced superoxide from Nox1, Nox2 (neutrophils), and Nox3. However, Nox4, which emits only hydrogen peroxide, was unaffected by Hsp90 inhibitors. Hydrogen peroxide production from the other Nox enzymes was not affected by short-term inhibition of Hsp90, but long-term inhibition reduced production of all reactive oxygen species coincident with loss of enzyme expression. Expression of chimeric Nox enzymes consisting of N-terminal Nox1 or Nox3 and C-terminal Nox4 resulted in only hydrogen peroxide formation that was insensitive to Hsp90 inhibitors. We conclude that Hsp90 binds to the C-terminus of Noxes1-3 and 5 and is necessary for enzyme stability and superoxide production. Hsp90 does not bind to the C-terminus of Nox4 and is not required for hydrogen peroxide formation.

The molecular mechanism of mammalian NO-synthases: a story of electrons and protons

Since its discovery, nitric oxide synthase (NOS), the enzyme responsible for NO biosynthesis in mammals, has been the subject of extensive investigations regarding its catalytic and molecular mechanisms. These studies reveal the high degree of sophistication of NOS functioning and regulation. However, the precise description of the NOS molecular mechanism and in particular of the oxygen activation chemistry is still lacking. The reaction intermediates implicated in NOS catalysis continue to elude identification and the current working paradigm is increasingly contested. Consequently, the last three years has seen the emergence of several competing models. All these models propose the same global reaction scheme consisting of two successive oxidation reactions but they diverge in the details of their reaction sequence. The major discrepancies concern the number, source and characteristics of proton and electron transfer processes. As a result each model proposes distinct reaction pathways with different implied oxidative species. This review aims to examine the different experimental evidence concerning NOS proton and electron transfer events and the role played by the substrates and cofactors in these processes. The resulting discussion should provide a comparative picture of all potential models for the NOS molecular mechanism.

Flow cytometry-based assay for the activity of NAD(P)H oxidoreductases of the outer mitochondrial membrane

NAD(P)H oxidoreductases of the outer mitochondrial membrane (OMM) are able to activate various xenobiotics and stimulate the production of reactive oxygen species and the opening of the mitochondrial permeability transition pore. However, the role of these systems in the cell damage by xenobiotics and chemotherapeutic drugs is poorly understood because the methods for the selective assessment of their activity have not been elaborated and specific inhibitors are unknown. Here we propose a method for the semiquantitative assessment of the activity of NAD(P)H oxidoreductases of the OMM in intact and permeabilized cells that is based on the flow cytometry detection of dimethylbiacridene, a fluorescent product of two-electron reduction of lucigenin. The method uses the structural feature of mitochondrial organization: the proximity of the sites of one-electron reduction of lucigenin to cation radical (NAD(P)H oxidoreductases of the OMM) to the sites of its subsequent oxidation (cytochrome c oxidase). The inhibition of cytochrome c oxidase by cyanide selectively activates the dimethylbiacridene formation by oxidoreductases of the OMM but not by other cellular oxidoreductases. The proposed protocol allows one to assess the lucigenin reductase (two-electron) activity of NAD(P)H oxidoreductases of the OMM and to compare it with the activity of other cellular systems that can be used for the analysis of the role of these systems in the cell damage by xenobiotics and antitumor drugs.

Distinct roles of Nox1 and Nox4 in basal and angiotensin II-stimulated superoxide and hydrogen peroxide production

NADPH oxidases are major sources of superoxide (O2*-) and hydrogen peroxide (H2O2) in vascular cells. Production of these reactive oxygen species (ROS) is essential for cell proliferation and differentiation, while ROS overproduction has been implicated in hypertension and atherosclerosis. It is known that the heme-containing catalytic subunits Nox1 and Nox4 are responsible for oxygen reduction in vascular smooth muscle cells from large arteries. However, the exact mechanism of ROS production by NADPH oxidases is not completely understood. We hypothesized that Nox1 and Nox4 play distinct roles in basal and angiotensin II (AngII)-stimulated production of O2*- and H2O2. Nox1 and Nox4 expression in rat aortic smooth muscle cells (RASMCs) was selectively reduced by treatment with siNox4 or antisense Nox1 adenovirus. Production of O2*- and H2O2 in intact RASMCs was analyzed by dihydroethidium and Amplex Red assay. Activity of NADPH oxidases was measured by NADPH-dependent O2*- and H2O2 production using electron spin resonance (ESR) and 1-hydroxy-3-carboxypyrrolidine (CPH) in the membrane fraction in the absence of cytosolic superoxide dismutase. It was found that production of O2*- by quiescent RASMC NADPH oxidases was five times less than H2O2 production. Stimulation of cells with AngII led to a 2-fold increase of O2*- production by NADPH oxidases, with a small 15 to 30% increase in H2O2 formation. Depletion of Nox4 in RASMCs led to diminished basal H2O2 production, but did not affect O2*- or H2O2 production stimulated by AngII. In contrast, depletion of Nox1 in RASMCs inhibited production of O2*- and AngII-stimulated H2O2 in the membrane fraction and intact cells. Our data suggest that Nox4 produces mainly H2O2, while Nox1 generates mostly O2*- that is later converted to H2O2. Therefore, Nox4 is responsible for basal H2O2 production, while O2*- production in nonstimulated and AngII-stimulated cells depends on Nox1. The difference in the products generated by Nox1 and Nox4 may help to explain the distinct roles of these NADPH oxidases in cell signaling. These findings also provide important insight into the origin of H2O2 in vascular cells, and may partially account for the limited pharmacological effect of antioxidant treatments with O2*- scavengers that do not affect H2O2.

Free radical production from the interaction of 2-chloroethyl vesicants (mustard gas) with pyridine nucleotide-driven flavoprotein electron transport systems

The biochemical sequelae to chloroethyl mustard exposure correspond very well to toxic processes initiated by free radicals. Additionally, mustard solutions contain spontaneously formed cyclic onium ions which produce carbon free radicals when reduced electrochemically. Therefore, we hypothesized that the onium ions of sulfur or nitrogen mustards might produce carbon free radicals upon being reduced enzymatically, and that these radicals might constitute a metabolic activation. We set out to document radical production using an in vitro metabolic system and electron paramagnetic resonance (EPR). Our system consisted of NADPH, one of several pyridine nucleotide-driven flavoprotein reductases, cytochrome c as a terminal electron acceptor, various sulfur or nitrogen mustards and the spin trap alpha-[4-pyridyl-1-oxide]-N-tert-butylnitrone in buffer. Reactions were started by adding the reductase to the other materials, vortexing and immediately transferring the mixture to a 10 mm EPR flat cell. Repeated scans on a Bruker ESP 300E EPR spectrometer produced a triplet of doublets with hyperfine splitting constants of a(N)=15.483 G and a(H)=2.512 G. The outcome supported our hypothesis that carbon-centered free radicals are produced when mustard-related onium ions are enzymatically reduced. The EPR results varied little with the chloroethyl compound used or with porcine or human cytochrome P450 reductase, the reductase domain of rat brain neuronal nitric oxide synthase or rat liver thioredoxin reductase. Our results offer new insight into the basis for mustard-induced vesication and the outcome of exposure to different mustards. The free radical model provides an explanation for similarities in the lesions arising from mustard exposure and energy-based lesions such as those from heat, ultraviolet and nuclear radiation as well as damage across tissue types such as skin, eyes or airway epithelium.

EPR Spin Trapping of Oxygen Radicals in Plants: A Methodological Overview

We present a brief account of the difficulties involved in detection of oxygen free radicals in plants and give a rationale for using the EPR spin trapping technique in such studies. Comparative analysis of characteristics of different spin traps is given, having in mind their suitability in trapping oxygen-centered free radicals. Certain technical aspects of EPR experiments related to successful trapping of free radicals are discussed. Previous studies of trapping of oxygen radicals in plants are reviewed in terms of how efficient the experimental approach employed has been in their detection and how this influences conclusions about the mechanisms of their production. In addition, we analyze the potential of spin labels in the analysis of free radical production in plants and demonstrate that the combination of EPR spin traps and spin labels is extremely efficient for this purpose.

A comparative study of neuronal and inducible nitric oxide synthases: generation of nitric oxide, superoxide, and hydrogen peroxide

Nitric oxide synthases (NOS) independent of the isozyme, produce nitric oxide (.NO), superoxide (O2.-), and hydrogen peroxide (H2O2). Since .NO has been implicated in many physiological processes, the importance of O2.- and H2O2 in regulating cell signaling by .NO cannot be overlooked. Before addressing these questions, we investigated the production of .NO, O2.-, and H2O2 by purified NOS. NOS 1 and NOS 2 were chosen, as the flux of .NO from each isozyme supports differential biological activity. We found that the initial rate and sustained production of .NO was considerably greater for NOS 2 as compared to NOS 1. In the absence of L-arginine, however, NOS 1 generation of O2.- and H2O2 was found to be substantially greater than that measured for NOS 2. Differences between NOS 1 and NOS 2 production of .NO, O2.-, and H2O2 may define the specific physiologic function of each isozyme.

The presence of thecagpathogenicity island is associated with increased superoxide anion radical scavenging activity byHelicobacter pylori

Reactive oxygen species (ROS) generated by Helicobacter pylori infection have been suggested to be important factors in induction of gastric malignancies. Utilizing electron spin resonance spectrometry, H. pylori-dependent radical formation and hydroxyl- and superoxide-anion radical scavenging activity was investigated. In contrast to previous reports, we found that H. pylori does not produce ROS, but displays superoxide scavenging activity. This scavenging activity was increased in cag-positive H. pylori strains when compared to strains lacking an intact cag pathogenicity island, and was dependent on enzyme activity. We hypothesize that the increased scavenging activity of cag-positive H. pylori strains is an adaptation to the increased inflammatory response associated with the cag-positive genotype of H. pylori.

Anesthetic preconditioning: triggering role of reactive oxygen and nitrogen species in isolated hearts

We postulated that anesthetic preconditioning (APC) is triggered by reactive oxygen/nitrogen species (ROS/RNS). We used the isolated guinea pig heart perfused with L-tyrosine, which reacts with ROS and RNS to form strong oxidants, principally peroxynitrite (ONOO(-)), and then forms fluorescent dityrosine. ROS scavengers superoxide dismutase, catalase, and glutathione (SCG) and NO. synthesis inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) were given 5 min before and after sevoflurane preconditioning stimuli. Drugs were washed out before 30 min of ischemia and 120 min of reperfusion. Groups were control (nontreated ischemia control), APC (two, 2-min periods of perfusion with 0.32 +/- 0.02 mM of sevoflurane; separated by a 6-min period of perfusion without sevoflurane), SCG, APC + SCG, L-NAME, and APC + L-NAME. Effluent dityrosine at 1 min reperfusion was 56 +/- 6 (SE), 15 +/- 5, 40 +/- 5(++), 39 +/- 4(++), 35 +/- 4(++) , and 33 +/- 5(++) units ((++)P< 0.05 vs. APC), respectively; left ventricular pressure (%baseline) at 60 min of reperfusion was 30 +/- 5(++), 60 +/- 4, 35 +/- 5(++), 37 +/- 5(++), 44 +/- 4, and 47 +/- 4; and infarct size (%total heart weight) was 50 +/- 5(++), 19 +/- 2, 48 +/- 3(++), 46 +/- 4(++), 42 +/- 4(++), and 45 +/- 2(++). Thus APC is initiated by ROS as shown by improved function, reduced infarct size, and reduced dityrosine on reperfusion; protective and ROS/RNS-reducing effect of APC were attenuated when bracketed by ROS scavengers or NO* inhibition.

Synthesis and biochemical applications of a solid cyclic nitrone spin trap: a relatively superior trap for detecting superoxide anions and glutathiyl radicals

A novel cyclic nitrone spin trap, 5-tert-butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) as a pure white solid has been synthesized for the first time. BMPO offers several advantages over the existing spin traps in the detection and characterization of thiyl radicals, hydroxyl radicals, and superoxide anions in biological systems. The corresponding BMPO adducts exhibit distinct and characteristic electron spin resonance (ESR) spectral patterns. Unlike the 5,5-dimethyl-1-pyrroline N-oxide (DMPO)-derived superoxide adduct, the BMPO superoxide adduct does not non-enzymatically decompose to the BMPO hydroxyl adduct. This feature is clearly perceived as a definite advantage of BMPO in its biological applications. In addition, the ESR spectrum of the BMPO glutathionyl adduct (BMPO/*SG) does not fully overlap with the spectrum of its hydroxyl adduct. This spectral feature is again distinctly different from that of DMPO because the ESR spectral lines of DMPO glutathionyl and hydroxyl radical adducts largely overlap. Finally, the ESR spectra of BMPO-derived adducts exhibit a much higher signal-to-noise ratio in biological systems. These favorable chemical and spectroscopic features make BMPO ideal for the detection of superoxide anions, hydroxyl and thiyl radicals in biochemical oxidation and reduction.

Detection of superoxide anion using an isotopically labeled nitrone spin trap: potential biological applications

We describe the synthesis and biological applications of a novel nitrogen-15-labeled nitrone spin trap, 5-ethoxycarbonyl-5-methyl-1-pyrroline N-oxide ([(15)N]EMPO) for detecting superoxide anion. Superoxide anion generated in xanthine/xanthine oxidase (100 nM min(-1)) and NADPH/calcium-calmodulin/nitric oxide synthase systems was readily detected using EMPO, a nitrone analog of 5,5'-dimethyl-1-pyrroline N-oxide (DMPO). Unlike DMPO-superoxide adduct (DMPO-OOH), the superoxide adduct of EMPO (EMPO-OOH) does not spontaneously decay to the corresponding hydroxyl adduct, making spectral interpretation less confounding. Although the superoxide adduct of 5-(diethoxyphosphoryl)-5-methyl-pyrroline N-oxide is more persistent than EMPO-OOH, the electron spin resonance spectra of [(14)N]EMPO-OOH and [(15)N]EMPO-OOH are less complex and easier to interpret. Potential uses of [(15)N]EMPO in elucidating the mechanism of superoxide formation from nitric oxide synthases, and in ischemia/reperfusion injury are discussed.

Effect of redox-active drugs on superoxide generation from nitric oxide synthases: biological and toxicological implications

In this article, we address the mechanism of superoxide formation from constitutive nitric oxide synthases (NOS). Merits and drawbacks of the various superoxide detection assays are reviewed. One of the most viable techniques for measuring superoxide from NOS is electron spin resonance (ESR) spin-trapping using a novel phosphorylated spin trap. Implications of superoxide and peroxynitrite formation from NOS enzymes in cardiovascular and cerebrovascular disorders are discussed.

Tetrahydrobiopterin-dependent inhibition of superoxide generation from neuronal nitric oxide synthase

The binding of calcium/calmodulin stimulates electron transfer between the reductase and oxygenase domains of neuronal nitric oxide synthase (nNOS). Here, we demonstrate using electron spin resonance spin-trapping with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide that pterin-free nNOS generates superoxide from the reductase and the oxygenase domain by a calcium/calmodulin-dependent mechanism. Tetrahydrobiopterin (BH(4)) diminishes the formation of superoxide by a mechanism that does not cause inhibition of NADPH consumption. In contrast, BH(4) analogs 7,8-dihydrobiopterin and sepiapterin do not affect superoxide yields. L-Arginine alone inhibits the generation of superoxide by nNOS but not by C331A-nNOS mutant that has a low affinity for L-arginine. A greater decrease in superoxide yields is observed when nNOS is preincubated with L-arginine. This effect is in accordance with the slow binding rates of L-arginine to NOS in the absence of BH(4). L-Arginine alone or in combination with BH(4) decreases the rates of NADPH consumption. The effect of L-arginine on superoxide yields, however, was less dramatic than that caused by BH(4) as much higher concentrations of L-arginine are necessary to attain the same inhibition. In combination, L-arginine and BH(4) inhibit the formation of superoxide generation and stimulate the formation of L-citrulline. We conclude that, in contrast to L-arginine, BH(4) does not inhibit the generation of superoxide by controlling electron transfer through the enzyme but by stimulating the formation of the heme-peroxo species.

Oxidant stress in the vasculature

Vascular disease and vasomotor responses are largely influenced by oxidant stress. Superoxide is generated via the cellular oxidase systems, xanthine oxidase, and NADH/NADPH oxidases. Once formed, superoxides participate in a number of reactions, yielding various free radicals such as hydrogen peroxide, peroxynitrite, oxidized low-density lipoprotein, or hypochlorous acid. Numerous cellular antioxidant systems exist to defend against oxidant stress; glutathione and the enzymes superoxide dismutase and glutathione peroxidase are critical for maintaining the redox balance of the cell. However, the redox state is disrupted by certain vascular diseases. It appears that oxidant stress both promotes and is induced by diseases such as hypertension, atherosclerosis, and restenosis as well as by certain risk factors for coronary artery disease including hyperlipidemia, diabetes, and cigarette smoking. Once oxidant stress is invoked, characteristic pathophysiologic features ensue, namely adverse vessel reactivity, vascular smooth muscle cell proliferation, macrophage adhesion, platelet activation, and lipid peroxidation.