Direct measurement of nitric oxide generation from nitric oxide synthase

PIN inhibits nitric oxide and superoxide production from purified neuronal nitric oxide synthase

A protein inhibitor of neuronal nitric oxide synthase (nNOS) was identified and designated as PIN. PIN was reported to inhibit nNOS activity in cell lysates through disruption of enzyme dimerization. However, there has been lack of direct characterization of the effect of PIN on NO production from purified nNOS. Furthermore, nNOS also generates superoxide (.O(2)(-)) at low levels of L-arginine. It is unknown whether PIN affects .O(2)(-) generation from nNOS. Therefore, we performed direct measurements of the effects of PIN on NO and .O(2)(-) generation from purified nNOS using electron paramagnetic resonance spin trapping techniques. nNOS was isolated by affinity chromatography and a fusion protein CBP-PIN was used to probe the effect of PIN. While the tag CBP did not affect nNOS activity, CBP-PIN caused a dose-dependent inhibition on both NO and L-citrulline production. In the absence of L-arginine, strong .O(2)(-) generation was observed from nNOS, and this was blocked by CBP-PIN in a dose-dependent manner. With low-temperature polyacrylamide gel electrophoresis, neither CBP nor CBP-PIN was found to affect nNOS dimerization. Thus, these results suggested that PIN not only inhibits NO but also .O(2)(-) production from nNOS, and this is through a mechanism other than decomposition of nNOS dimers.

Detection of nitrous oxide in the neuronal nitric oxide synthase reaction by gas chromatography–mass spectrometry

Using headspace gas chromatography-mass spectrometry, we detected significant amounts of nitrous oxide in the reaction products of the monooxygenase reaction catalyzed by neuronal nitric oxide synthase. Nitrous oxide is a dimerization product of nitroxyl anion; its presence in the reaction products indicates that the nitroxyl anion is a product of the neuronal nitric oxide synthase-catalyzed reaction.

Application of electron spin resonance spin-trapping technique for evaluation of substrates and inhibitors of nitric oxide synthase

The electron spin resonance (ESR) spin-trapping technique coupled with iron-dithiocarbamate complexes is one of the most specific methods for nitric oxide (NO) detection. In this study, we applied this method for the evaluation of the substrate and the inhibitors of NO synthase (NOS). A three-line ESR signal was detected from the mixture of inducible NOS (iNOS), l-arginine (Arg), nicotinamide adenine dinucleotide phosphate (NADPH), tetrahydrobiopterin, dithiothreitol, and Fe(2+)-N-(dithiocarboxy) sarcosine (DTCS-Fe), and the signal intensity increased time-dependently. The signal was not observed by excluding either Arg or NADPH, and it was decreased by the addition of hemoglobin, which is an NO scavenger, and N(G)-monomethyl-l-arginine (l-NMMA), N(G)-nitro-l-arginine (l-NAME), and aminoguanidine (AG), which are NOS inhibitors, depending on the concentration. In comparison with l-NAME and AG, l-NMMA strongly inhibited iNOS activity. By using this method, the K(m) value of Arg and the K(i) value of l-NMMA for iNOS were determined to be 12.6 and 6.1muM, respectively. These values are consistent with the reported values measured by the oxyhemoglobin and citrulline assays. These results suggest that the ESR spin-trapping technique coupled with the iron-dithiocarbamate complex can be applied for the evaluation of substrates and inhibitors of NOS, and it would be a powerful tool due to its simplicity and high specificity to NO.

Vascular consequences of endothelial nitric oxide synthase uncoupling for the activity and expression of the soluble guanylyl cyclase and the cGMP-dependent protein kinase

Endothelial dysfunction in the setting of cardiovascular risk factors, such as hypercholesterolemia, hypertension, diabetes mellitus, chronic smoking, as well as in the setting of heart failure, has been shown to be at least partly dependent on the production of reactive oxygen species (ROS), such as the superoxide radical, and the subsequent decrease in vascular bioavailability of nitric oxide (NO). Superoxide-producing enzymes involved in increased oxidative stress within vascular tissue include the NAD(P)H oxidase, the xanthine oxidase, and mitochondrial superoxide-producing enzymes. Superoxide produced by the NADPH oxidase may react with NO released by endothelial nitric oxide synthase (eNOS), thereby generating peroxynitrite. Peroxynitrite in turn has been shown to uncouple eNOS, thereby switching an antiatherosclerotic NO-producing enzyme to an enzyme that may initiate or even accelerate the atherosclerotic process by producing superoxide. Increased oxidative stress in the vasculature, however, is not restricted to the endothelium and has also been demonstrated to occur within the smooth muscle cell layer in the setting of hypercholesterolemia, diabetes mellitus, hypertension, congestive heart failure, and nitrate tolerance. Increased superoxide production by the endothelial and/or smooth muscle cells has important consequences with respect to signaling by the soluble guanylyl cyclase (sGC) and the cGMP-dependent protein kinase I (cGK-I), the activity and expression of which has been shown to be regulated in a redox-sensitive fashion. The present review summarizes current concepts concerning eNOS uncoupling and also focuses on the consequences for downstream signaling with respect to activity and expression of the sGC and cGK-I in various diseases.

Roles of 3-Phosphoinositide-dependent Kinase 1 in the Regulation of Endothelial Nitric-oxide Synthase Phosphorylation and Function by Heat Shock Protein 90

The 90-kDa heat shock protein (Hsp90) plays an important role in endothelial nitric-oxide synthase (eNOS) regulation. Besides acting as an allosteric enhancer, Hsp90 was shown to serve as a module recruiting Akt to phosphorylate the serine 1179/1177 (bovine/human) residue of eNOS. Akt is activated by the phosphorylation of 3-phosphoinositide-dependent kinase 1 (PDK1). Whether PDK1 is involved in the actions of Hsp90 on eNOS phosphorylation and function remains unknown. To address this issue, we treated bovine eNOS stably transfected human embryonic kidney 293 cells with Hsp90 inhibitors and determined the alterations of phospho-eNOS, Akt, and PDK1. Both geldanamycin and radicicol, two structurally different Hsp90 inhibitors, selectively reduced serine 1179-phosphorylated eNOS, leading to decreased enzyme activity. In Hsp90-inhibited cells, eNOS-associated phospho-Akt was decreased, but the total amount of Akt associated with eNOS remained the same. Further studies showed that Hsp90 inhibition dramatically depleted intracellular PDK1. Proteasome but not caspase blockade prevented the loss of PDK1 caused by Hsp90 inhibition. Silencing the PDK1 gene by small interfering RNA was sufficient to induce reduction of phospho-Akt and consequent loss of serine 1179-phosphorylated eNOS. Moreover, overexpression of PDK1, but not Akt, reversed Hsp90 inhibition-induced loss of eNOS serine 1179 phosphorylation and salvaged enzymatic activity. Thus, in addition to functioning as a module to recruit Akt to eNOS, Hsp90 also critically stabilized PDK1 by preventing it from proteasomal degradation. Inhibition of Hsp90 function resulted in PDK1 depletion and thus triggered a cascade of Akt deactivation, loss of eNOS serine 1179 phosphorylation, and decrease of enzyme function.

Endogenous methylarginines modulate superoxide as well as nitric oxide generation from neuronal nitric-oxide synthase: differences in the effects of monomethyl- and dimethylarginines in the presence and absence of tetrahydrobiopterin

The endogenous methylarginines asymmetric dimethylarginine (ADMA) and N(G)-monomethyl-L-arginine (L-NMMA) regulate nitric oxide (NO) production from neuronal NO synthase (nNOS). Under conditions of L-arginine or tetrahydrobiopterin (BH(4)) depletion, nNOS also generates superoxide, O(2)(.); however, the effects of methylarginines on this O(2)(.) generation are poorly understood. Therefore, we measured the dose-dependent effects of ADMA and L-NMMA on the rate and amount of O(2)(.) production from nNOS under conditions of L-arginine and/or BH(4) depletion, using electron paramagnetic resonance spin trapping. In the absence of L-arginine, ADMA (1 microm) inhibited O(2)(.) generation by approximately 60% from a rate of 56 to 23 nmol/mg/min, whereas L-NMMA (0.1-100 microm) had no effect. L-Arginine markedly decreased the observed O(2)(.) adduct formation; however, O(2)(.) generation from the enzyme still occurs at a low rate (12.1 nmol/mg/min). This O(2)(.) leak is NOS-derived as it is not seen in the absence of calcium and calmodulin and demonstrates that O(2)(.) generation from NOS occurs even when normal substrate/ cofactor levels are present. Under conditions of BH(4) depletion, ADMA had no effect on O(2)(.), whereas L-NMMA increased O(2)(.) production almost 3-fold. This O(2)(.) generation was >90% inhibited by imidazole, indicating that it occurred at the heme center. Thus, methylarginines can profoundly shift the balance of NO and O(2)(.) generation from nNOS. These observations have important implications with regard to the therapeutic use of methylarginine-NOS inhibitors in the treatment of disease.

Molecular mechanisms underlying the role of nitric oxide in the cardiovascular system

In the cardiovascular system, nitric oxide (NO) is involved in the short and long-term regulation of haemodynamics, and in a number of their pathological alterations. Investigation into the biochemistry of NO-synthase isoforms has confirmed that they also all produce superoxide anion (O(*)). The free radical NO can interact with many targets on which novel information has been recently obtained. The major results of these interactions are not only the well known activation of guanylyl cyclase, but also the formation of potentially cytotoxic peroxynitrite (ONOO(-)), and the formation of S-nitrosothiols and non-haem iron-dinitrosyl dithiolate complexes. Tissue O(2), O(*), low molecular weight thiols and transition metals (especially FeII) play a pivotal role in directing NO towards targets responsible for biological effects, or storage or release from these stores. In addition, circulating forms of NO have been proposed with S-nitrosation of blood proteins. All these mechanisms provide potential pharmacological targets for future therapeutic strategies.

Superoxide dismutase and catalase are required to detect (.-)NO from both coupled and uncoupled neuronal no synthase

Despite numerous approaches to measuring nitric oxide ((.-)NO) formation from purified NO synthase (NOS), it is still not clear whether (.-)NO is a direct or indirect product of the NO synthase reaction. The direct detection of catalytically formed (.-)NO is complicated by side reactions with reactive oxide species like H(2)O(2) and superoxide. The aim of the present study was therefore to reinvestigate these reactions both electrochemically and by chemiluminescence detection with particular emphasis on the requirement for cofactors and their interference with (.-)NO detection. Flavins were found to generate large amounts of H(2)O(2) and were therefore excluded from subsequent incubations. Under conditions of both coupled and uncoupled catalysis, SOD was absolutely required to detect (.-)NO from NOS. H(2)O(2) formation took place also in the presence of SOD and gave a smaller yet significant interfering signal. Similar data were obtained when the proposed intermediate N(omega)-hydroxy-l-arginine was utilized as substrate. In conclusion, standard Clark-type ()NO electrodes are cross-sensitive to H(2)O(2) and therefore both SOD and catalase are absolutely required to specifically detect (.-)NO from NOS.

Cellular and subcellular localization of endogenous nitric oxide in young and senescent pea plants

The cellular and subcellular localization of endogenous nitric oxide (NO.) in leaves from young and senescent pea (Pisum sativum) plants was studied. Confocal laser scanning microscopy analysis of pea leaf sections with the fluorescent probe 4,5-diaminofluorescein diacetate revealed that endogenous NO. was mainly present in vascular tissues (xylem and phloem). Green fluorescence spots were also detected in the epidermal cells, palisade and spongy mesophyll cells, and guard cells. In senescent leaves, NO. generation was clearly reduced in the vascular tissues. At the subcellular level, by electron paramagnetic resonance spectroscopy with the spin trap Fe(MGD)(2) and fluorometric analysis with 4,5-diaminofluorescein diacetate, NO. was found to be an endogenous metabolite of peroxisomes. The characteristic three-line electron paramagnetic resonance spectrum of NO., with g = 2.05 and a(N) = 12.8 G, was detected in peroxisomes. By fluorometry, NO. was also found in these organelles, and the level measured of NO. was linearly dependent on the amount of peroxisomal protein. The enzymatic production of NO. from l-Arg (nitric oxide synthase [NOS]-like activity) was measured by ozone chemiluminiscence. The specific activity of peroxisomal NOS was 4.9 nmol NO. mg(-1) protein min(-1); was strictly dependent on NADPH, calmodulin, and BH(4); and required calcium. In senescent pea leaves, the NOS-like activity of peroxisomes was down-regulated by 72%. It is proposed that peroxisomal NO. could be involved in the process of senescence of pea leaves.

Spin trapping of nitric oxide with the iron-dithiocarbamate complex: chemistry and biology

This brief review describes chemical and biological aspects concerning spin trapping of nitric oxide (NO) with the iron-dithiocarbamate (Fe-DTC) complex as a spin trap. Knowledge on basic properties of the Fe-DTC complex would help in understanding the applicability and limitation of the Fe-DTC-based NO spin-trapping method when it is employed in viable biological systems.

Nitrosyl-heme complexes are formed in the ischemic heart: evidence of nitrite-derived nitric oxide formation, storage, and signaling in post-ischemic tissues

In addition to the generation from specific nitric-oxide (NO) synthases, NO formation from nitrite occurs in ischemic tissues, such as the heart. Although NO binding to heme-centers is the basis for NO-mediated signaling as occurs through guanylate cyclase, it is not known if this process is triggered with physiologically relevant periods of sublethal ischemia and if nitrite serves as a critical substrate. Therefore electron paramagnetic resonance studies were performed to measure nitrosylheme formation during the time course of myocardial ischemia and reperfusion and the role of nitrite in this process. Rat hearts were either partially nitrite-depleted by nitrite-free buffer perfusion or nitrite-enriched by preinfusion with 50 microm nitrite. Ischemic hearts loaded with nitrite showed prominent spectra of six-coordinate nitrosyl-heme complexes, primarily NO-myoglobin, that increased as a function of ischemic duration, whereas in nonischemic-controls these signals were not seen. Total nitrosyl-heme concentrations within the heart were 6.6 +/- 0.7 microm after 30 min of ischemia. Nitrite-depleted hearts also gave rise to NO-heme signals during ischemia, but levels were 8-fold lower. Nitrite-mediated NO-heme complex formation during ischemia was associated with activation of guanylate cyclase. Upon reperfusion, the levels of NO-heme complexes decreased 3-fold by the first 15 min but remained elevated for over 45 min. The decrease in NO-heme complex levels was paralleled by the formation of nitrate, suggesting the oxidation of heme-bound NO upon reperfusion. Thus, nitrite-mediated NO-heme formation occurs progressively during ischemia, with these complexes serving as a store of NO with concordant activation of NO signaling pathways.

Metabolic fate of extrahepatic arginine in liver after burn injury

Increased nitrogen loss in the form of urea is a hallmark of the metabolic aberrations that occur after burn injury. As the immediate precursor for urea production is arginine, we have conducted an investigation on the metabolic fate of arginine in the liver to shed light on the metabolic characteristics of this increased nitrogen loss. Livers from 25% total surface burn (n = 8) and sham burn rats (n = 8) were perfused in a recycling fashion with a medium containing amino acids and stable isotope labeled l-[(15) N(2)-guanidino, 5,5-(2)H(2)]arginine for 120 minutes. The rates of glucose and urea production and oxygen consumption were measured. The rate of unidirectional arginine transport and the intrahepatic metabolic fate of arginine in relation to urea cycle activity were quantified by tracing the disappearance rate of the arginine tracer from and the appearance rate of [(15)N(2)]urea in the perfusion medium. Perfused livers from burned rats showed higher rates of total urea production (mean +/- SE, 4.471 +/- 0.274 v 3.235 +/- 0.261 mumol. g dry liver(-1). min(-1); P <.01). This was accompanied by increased hepatic arginine transport (1.269 +/- 0.263 v 0.365 +/- 0.021 mumol. g dry liver(-1). min(-1)) and an increased portion of urea production from the transported extrahepatic arginine (12.9% +/- 2.9% v 3.5% +/- 0.4%, P <.05). The disposal of arginine via nonurea pathways was also increased (0.702 +/- 0.185 v 0.257 +/- 0.025 mumol/g dry weight(-1)/min(-1); P <.05). We propose that increased inward transport and utilization of extrahepatic arginine by the liver contributes to the accelerated urea production after burn injury and accounts, in part, for its conditional essentiality in the nutritional support of burn patients.

Differential biological effects of products of nitric oxide (NO) synthase: it is not enough to say NO

The radical gas nitric oxide (NO) is implicated in an enormous number of biological function both in physiological and pathological conditions. Often it is not clear if it plays a deleterious or beneficial role. Here briefly, are analyzed some of the reasons of this multitude of effects. Emphasis is given to factors influencing NO formation and to the type and quantity of radicals formed by nitric oxide synthase. In particular, a comparison between the biological effects of nitroxyl anion (HNO/NO(-)) and nitric oxide NO(.) is considered. These redox siblings often exhibit orthogonal behavior in physiological and pathological conditions. In the light of the multitude of effects of NO, the role of this gas, their siblings and their derivatives in cardiac ischemic preconditioning scenario is more extensively analyzed.

The autoxidation of tetrahydrobiopterin revisited. Proof of superoxide formation from reaction of tetrahydrobiopterin with molecular oxygen

It has been known for quite some time that tetrahydrobiopterin (H4B) is prone to autoxidation in the presence of molecular oxygen. Evidence has been presented that in this process superoxide radicals may be released, although their intermediacy never has been directly proven. In the present study, the autoxidation of H4B was reinvestigated with the aim to find direct evidence for superoxide formation. By means of two specific assays, namely elicitation of luminescence from lucigenin and ESR-spectrometric detection of the DEPMPO-OOH radical adduct, the release of free superoxide radicals was unequivocally demonstrated. The production of superoxide radicals was further corroborated by interaction with nitric oxide. The kinetics of the autoxidation process was established. Our data fully confirm earlier conclusions that the direct reaction between H4B and oxygen serves as an initiation reaction for the further, rapid reaction of the thus formed superoxide with H4B, thereby very likely establishing a chain reaction process involving reduction of molecular oxygen by the intermediary tetrahydrobiopterin radical. Conclusively, because H4B can per se induce oxidative stress, an in vivo overproduction of this pterin, as is evident in various diseases, may be responsible for the observed acceleration of pathophysiological pathways.

HMG-CoA reductase inhibitor increases GTP cyclohydrolase I mRNA and tetrahydrobiopterin in vascular endothelial cells

OBJECTIVE

Endothelial nitric oxide synthase (eNOS) activity is supported by tetrahydrobiopterin (BH4), which appears to be important for generating protective NO but decreases uncoupling formation of superoxide. We investigated the effects of 3-hydroxy-3-methylglutaryl coenzyme A reductase inhibitors, or statins, in terms of BH4 metabolism in human umbilical vein endothelial cells (HUVECs).

METHODS AND RESULTS

We measured the mRNA levels of GTP cyclohydrolase I (GTPCH), the rate-limiting enzyme in the first step of de novo BH4 synthesis, by real-time polymerase chain reaction. The mRNA of GTPCH, as well as of eNOS, was upregulated in HUVECs treated with cerivastatin. This increase was time and dose dependent. Fluvastatin was also observed to enhance GTPCH and eNOS mRNA levels. In parallel with this observation, cerivastatin increased intracellular BH4. Incubating HUVECs with tumor necrosis factor (TNF-alpha) was observed to increase GTPCH mRNA while decreasing eNOS mRNA. In the presence of cerivastatin, the TNF-alpha-mediated increase in GTPCH mRNA was enhanced, and the TNF-alpha-mediated decrease in eNOS mRNA was attenuated. Cerivastatin increased the stability of eNOS mRNA. However, it did not alter the stability of GTPCH mRNA but increased GTPCH gene transcription, as shown by nuclear run-on assays. Preteatment of HUVECs with the selective GTPCH inhibitor, 2,4-diamino-6-hydroxypyrimidine, caused a decrease in intracellular BH4 and decreased citrulline formation after stimulation with ionomycin. Furthermore, the potentiating effect of cerivastatin was decreased by limiting the cellular availability of BH4.

CONCLUSIONS

Our data demonstrate that statins elevate GTPCH mRNA, thereby increasing BH4 levels in vascular endothelial cells. In addition to augmenting eNOS expression, statins potentiate GTPCH gene expression and BH4 synthesis, thereby increasing NO production and preventing relative shortages of BH4.

Nitroxyl affords thiol-sensitive myocardial protective effects akin to early preconditioning

Nitric oxide (NO) donors mimic the early phase of ischemic preconditioning (IPC). The effects of nitroxyl (HNO/NO(-)), the one-electron reduction product of NO, on ischemia/reperfusion (I/R) injury are unknown. Here we investigated whether HNO/NO(-), produced by decomposition of Angeli's salt (AS; Na(2)N(2)O(3)), has a cardioprotective effect in isolated perfused rat hearts. Effects were examined after intracoronary perfusion (19 min) of either AS (1 microM), the NO donor diethylamine/NO (DEA/NO, 0.5 microM), vehicle (100 nM NaOH) or buffer, followed by global ischemia (30 min) and reperfusion (30 min or 120 min in a subset of hearts). IPC was induced by three cycles of 3 min ischemia followed by 10 min reperfusion prior to I/R. The extent of I/R injury under each intervention was assessed by changes in myocardial contractility as well as lactate dehydrogenase (LDH) release and infarct size. Postischemic contractility, as indexed by developed pressure and dP/dt(max), was similarly improved with IPC and pre-exposure to AS, as opposed to control or DEA/NO-treated hearts. Infarct size and LDH release were also significantly reduced in IPC and AS groups, whereas DEA/NO was less effective in limiting necrosis. Co-infusion in the triggering phase of AS and the nitroxyl scavenger, N-acetyl-L-cysteine (4 mM) completely reversed the beneficial effects of AS, both at 30 and 120 min reperfusion. Our data show that HNO/NO(-) affords myocardial protection to a degree similar to IPC and greater than NO, suggesting that reactive nitrogen oxide species are not only necessary but also sufficient to trigger myocardial protection against reperfusion through species-dependent, pro-oxidative, and/or nitrosative stress-related mechanisms.

Implications of nitric oxide synthase isoforms in the pathophysiology of Peyronie's disease

Peyronie's disease is an idiopathic, localized connective tissue disorder of the penis which involves the tunica albuginea of the corpus cavernosum and the adjacent areolar space. Peyronie's disease is characterized by local changes in the collagen and elastic fiber composition of the tunica albuginea. The formation of fibrotic plaques alters penile anatomy and can cause different degrees of bending and narrowing, as well as penile pain and erectile dysfunction. Though long recognized as an important clinical entity of the male genitalia, the etiology of this disease has remained poorly understood. Until recently there have been no studies to examine the role nitric oxide (NO) and nitric oxide synthase (NOS) isoforms may play in the onset and progression of Peyronie's disease. NO is a potent biological mediator with diverse physiological and pathophysiological roles. The purpose of this review is to describe each of the NOS isoforms and their potential roles in the pathophysiology of Peyronie's disease, with particular emphasis on the regulation of endothelial and inducible NOS isoforms.

Endogenous methylarginines regulate neuronal nitric-oxide synthase and prevent excitotoxic injury

Nitric oxide (NO) has a critical role in neuronal function; however, high levels lead to cellular injury. While guanidino-methylated arginines (MA) including asymmetric dimethylarginine (ADMA) and N(G)-methyl-l-arginine (NMA) are potent competitive inhibitors of nitric oxide synthase (NOS) and are released upon protein degradation, it is unknown whether their intracellular concentrations are sufficient to critically regulate neuronal NO production and secondary cellular function or injury. Therefore, we determine the intrinsic neuronal MA concentrations and their effects on neuronal NOS function and excitotoxic injury. Kinetic studies demonstrated that the K(m) for l-arginine is 2.38 microm with a V(max) of 0.229 micromol mg(-1) min(-1), while K(i) values of 0.67 microm and 0.50 microm were determined for ADMA and NMA, respectively. Normal neuronal concentrations of all NOS-inhibiting MA were determined to be approximately 15 microm, while l-arginine concentration is approximately 90 microm. These MA levels result in >50% inhibition of NO generation from neuronal NOS. Down-modulation or up-modulation of these neuronal MA levels, respectively, dramatically enhanced or suppressed NO-mediated excitotoxic injury. Thus, neuronal MA profoundly modulate NOS function and suppress NO mediated injury. Pharmacological modulation of the levels of these intrinsic NOS inhibitors offers a novel approach to modulate neuronal function and injury.

Statin enhances cytokine-mediated induction of nitric oxide synthesis in vascular smooth muscle cells

OBJECTIVE

We investigated the effects of the statins, cerivastatin and fluvastatin, on the induction of nitric oxide (NO) production in vascular smooth muscle cells (VSMC) stimulated by interleukin-1beta (IL-1) or in combination with interferon-gamma (IFN).

METHODS

We measured NO release, inducible NO synthase (iNOS) mRNA and protein levels, iNOS gene transcription rates, and iNOS mRNA stabilities in cytokine-activated VSMC. We also evaluated nuclear factor (NF)-kappaB activity and tetrahydrobiopterin (BH4) synthesis.

RESULTS

NO production induced by cytokines was dose-dependently enhanced by both statins. Incubating VSMC with IL-1/IFN stimulated iNOS mRNA and protein expression. Both statins significantly upregulated IL-1/IFN-stimulated iNOS mRNA and protein expression, and enhanced iNOS gene transcription as shown by nuclear run-on assays. However, they did not alter the stability of iNOS mRNA. Both statins slightly modulated IL-1/IFN-induced NF-kappaB activation, which was not associated with their effect on NO production. Cytokines induce the de novo synthesis of BH4 in VSMC. This event is essential for the induction of NO synthesis, which requires transcriptional induction of the genes that encode not only iNOS but also guanosine triphosphate cyclohydrolase I (GTPCH), the first and rate-limiting enzyme in de novo BH4 synthesis. The synthesis of BH4 and GTPCH mRNA induced by IL-1/IFN were enhanced by both statins. Exogenous mevalonate significantly prevented and geranylgeranylpyrophosphate reversed the stimulatory effect of both statins. Furthermore, the geranylgeranyltransferase I inhibitor GGTI-298 significantly increased IL-1/IFN-induced NO production.

CONCLUSION

Our data demonstrated that statins enhance immunostimulants-induced NO production by increasing iNOS gene expression at the transcriptional level via an NF-kappaB-independent pathway. The effect of statins on NO production is due at least partly through blocking the biosynthesis of mevalonate, which prevents isoprenoid biosynthesis. In addition to augmenting iNOS expression, statins potentiate GTPCH gene expression and BH4 synthesis, thereby preventing a relative shortage of BH4 which may shift the balance between NOS-catalyzed generation of protective NO and deleterious reactive oxygen species.

Formation of peroxynitrite from reaction of nitroxyl anion with molecular oxygen

Peroxynitrite (ONOO(-)/ONOOH) is generally expected to be formed in vivo from the diffusion-controlled reaction between superoxide (O(2)) and nitric oxide ((*)NO). In the present paper we show that under aerobic conditions the nitroxyl anion (NO(-)), released from Angeli's salt (disodium diazen-1-ium-1,2,2-triolate, (-)ON=NO(2)(-)), generated peroxynitrite with a yield of about 65%. Simultaneously, hydroxyl radicals are formed from the nitroxyl anion with a yield of about 3% via a minor, peroxynitrite-independent pathway. Further experiments clearly underline that the chemistry of NO(-) in the presence of oxygen is mainly characterized by peroxynitrite and not by HO( small middle dot) radicals. Quantum-chemical calculations predict that peroxynitrite formation should proceed via intermediary formation of (*)NO and O(2), probably by an electron-transfer mechanism. This prediction is supported by the fact that H(2)O(2) is formed during the decay of NO(-) in the presence of superoxide dismutase (Cu(II),Zn-SOD). Since the nitroxyl anion may be released endogenously by a variety of biomolecules, substantial amounts of peroxynitrite might be formed in vivo via NO(-) in addition to the "classical" ( small middle dot)NO + O(2)() pathway.

A brain nitric oxide synthase study in the rat: production of a nitroso-compound NA and absence of nitric oxide synthesis

The products of brain NO-synthase (NOS) were studied by different analytical techniques with the same incubation conditions. Voltammetric techniques used a micro cell containing NOS and its substrate (10 mM arginine). Using porphyrin microelectrodes with differential pulse amperometry nitric oxide (NO) was not detected when nafion membrane was present (less than 0.3 muM). Nitrite was detected with the same microelectrode without membrane (0.42 mM). Differential pulse voltammetry (DPV) with micro carbon electrode detected a nitroso-compound (NA) in reduction (1 mM) and not NO. In oxidation the observed DPV peak was due to nitrite (0.43 mM). Citrulline was detected by high performance liquid chromatography (0.51 mM). Using Diels Alder reaction in NOS preparation a NA-cycloadduct was observed by capillary electrophoresis (0.2 mM) and mass spectrometry (0.22 mM). Diels Alder reaction is the reaction of the identification of the nitroso group. NA-cycloadduct degradation by retro Diels Alder reaction gave equimolar concentrations of citrulline and nitrite without NO production. These observations lead us to affirm that NOS synthesizes NA.

Insulin-induced peroxynitrite production in human platelet-rich plasma

Recent data support the possible role of nitric oxide (NO*) in the development of insulin signalling. The aim of this study was to examine the effect of insulin on NO* production by platelets. The chemiluminescence of platelet-rich plasma prepared from the blood of healthy volunteers was measured in the presence of luminol. Indirect detection of NO* by luminol is possible in the form of peroxynitrite produced in the reaction of NO* with a superoxide free radical. Luminol oxidation induced by hydroxyl free radical and lipid peroxidation was prevented by 150 micromol/l of desferrioxamine mesylate. Insulin, in the range of 0.084-840 nmol/l, induced a concentration-dependent increase in chemiluminescence, which was inhibited both by the competitive antagonist of the NO* synthase enzyme. N(omega)-nitro-L-arginine methyl ester (at concentrations of 2.0-4.0 mmol/l, P<0.001), and by the elimination of superoxide free radicals using superoxide dismutase (72-144 IU/ml, P<0.001). In conclusion, we assume that the insulin-induced increase in chemiluminescence of platelet-rich plasma was due to increased production of NO* and superoxide free radicals forming peroxynitrite. The data are consistent with production of peroxynitrite from human platelets under insulin stimulation.

Endotoxin-induced liver hypoxia: defective oxygen delivery versus oxygen consumption

In vivo EPR was used to investigate liver oxygenation in a hemodynamic model of septic shock in mice. Oxygen-sensitive material was introduced either (i) as a slurry of fine particles which localized at the liver sinusoids (pO2 = 44.39 +/- 5.13 mmHg) or (ii) as larger particles implanted directly into liver tissue to measure average pO2 across the lobule (pO2 = 4.56 +/- 1.28 mmHg). Endotoxin caused decreases in pO2 at both sites early (5-15 min) and at late time points (6 h after endotoxin; sinusoid = 11.22 +/- 2.48 mmHg; lobule = 1.16 +/- 0.42 mmHg). The overall pO2 changes observed were similar (74.56% versus 74.72%, respectively). Blood pressures decreased transiently between 5 and 15 min (12.88 +/- 8% decrease) and severely at 6 h (59 +/- 9% decrease) following endotoxin, despite volume replacement with saline. Liver and circulatory nitric oxide was elevated at these times. Liver oxygen extraction decreased from 44% in controls to only 15% following endotoxin, despite severe liver hypoxia. Arterial oxygen saturation, blood flow (hepatic artery), and cardiac output were unaffected. Pretreatment with l-NMMA failed to improve endotoxin-induced oxygen defects at either site, whereas interleukin-13 preserved oxygenation. These site-specific measurements of pO2 provide in vivo evidence that the principal cause of liver hypoxia during hypodynamic sepsis is reduced oxygen supply to the sinusoid and can be alleviated by maintaining sinusoidal perfusion.

Determination of the enhancing action of HSP90 on neuronal nitric oxide synthase by EPR spectroscopy

Recent studies showed that heat shock protein 90 (HSP90) enhances nitric oxide (NO) synthesis from endothelial and neuronal NO synthase (eNOS and nNOS, respectively). However, these findings were based on indirect NO measurements. Moreover, although our previous studies showed that the action of HSP90 involves increased Ca(2+)/calmodulin (Ca(2+)/CaM) binding, quantitative measurements of the effect of HSP90 on CaM binding to nNOS have been lacking. With electron paramagnetic resonance spectroscopy, we directly measured NO signals from purified nNOS. HSP90 augmented NO formation from nNOS in a dose-dependent manner. Tryptophan fluorescence-quenching measurements revealed that HSP90 markedly reduced the K(d) of CaM to nNOS (0.5 +/- 0.1 nM vs. 9.4 +/- 1.8 nM in the presence and absence of HSP90, P < 0.01). Ca(2+) ionophore triggered strong NO production from nNOS-transfected cells, and this was significantly reduced by the HSP90 inhibitor geldanamycin. Thus these studies provide direct evidence demonstrating that HSP90 enhances nNOS catalytic function in vitro and in intact cells. The effect of HSP90 is mediated by the enhancement of CaM binding to nNOS.

Nitric oxide synthases: structure, function and inhibition

This review concentrates on advances in nitric oxide synthase (NOS) structure, function and inhibition made in the last seven years, during which time substantial advances have been made in our understanding of this enzyme family. There is now information on the enzyme structure at all levels from primary (amino acid sequence) to quaternary (dimerization, association with other proteins) structure. The crystal structures of the oxygenase domains of inducible NOS (iNOS) and vascular endothelial NOS (eNOS) allow us to interpret other information in the context of this important part of the enzyme, with its binding sites for iron protoporphyrin IX (haem), biopterin, L-arginine, and the many inhibitors which interact with them. The exact nature of the NOS reaction, its mechanism and its products continue to be sources of controversy. The role of the biopterin cofactor is now becoming clearer, with emerging data implicating one-electron redox cycling as well as the multiple allosteric effects on enzyme activity. Regulation of the NOSs has been described at all levels from gene transcription to covalent modification and allosteric regulation of the enzyme itself. A wide range of NOS inhibitors have been discussed, interacting with the enzyme in diverse ways in terms of site and mechanism of inhibition, time-dependence and selectivity for individual isoforms, although there are many pitfalls and misunderstandings of these aspects. Highly selective inhibitors of iNOS versus eNOS and neuronal NOS have been identified and some of these have potential in the treatment of a range of inflammatory and other conditions in which iNOS has been implicated.

Nitric oxide synthases: structure, function and inhibition

This review concentrates on advances in nitric oxide synthase (NOS) structure, function and inhibition made in the last seven years, during which time substantial advances have been made in our understanding of this enzyme family. There is now information on the enzyme structure at all levels from primary (amino acid sequence) to quaternary (dimerization, association with other proteins) structure. The crystal structures of the oxygenase domains of inducible NOS (iNOS) and vascular endothelial NOS (eNOS) allow us to interpret other information in the context of this important part of the enzyme, with its binding sites for iron protoporphyrin IX (haem), biopterin, L-arginine, and the many inhibitors which interact with them. The exact nature of the NOS reaction, its mechanism and its products continue to be sources of controversy. The role of the biopterin cofactor is now becoming clearer, with emerging data implicating one-electron redox cycling as well as the multiple allosteric effects on enzyme activity. Regulation of the NOSs has been described at all levels from gene transcription to covalent modification and allosteric regulation of the enzyme itself. A wide range of NOS inhibitors have been discussed, interacting with the enzyme in diverse ways in terms of site and mechanism of inhibition, time-dependence and selectivity for individual isoforms, although there are many pitfalls and misunderstandings of these aspects. Highly selective inhibitors of iNOS versus eNOS and neuronal NOS have been identified and some of these have potential in the treatment of a range of inflammatory and other conditions in which iNOS has been implicated.

Nitric oxide synthases: structure, function and inhibition

This review concentrates on advances in nitric oxide synthase (NOS) structure, function and inhibition made in the last seven years, during which time substantial advances have been made in our understanding of this enzyme family. There is now information on the enzyme structure at all levels from primary (amino acid sequence) to quaternary (dimerization, association with other proteins) structure. The crystal structures of the oxygenase domains of inducible NOS (iNOS) and vascular endothelial NOS (eNOS) allow us to interpret other information in the context of this important part of the enzyme, with its binding sites for iron protoporphyrin IX (haem), biopterin, L-arginine, and the many inhibitors which interact with them. The exact nature of the NOS reaction, its mechanism and its products continue to be sources of controversy. The role of the biopterin cofactor is now becoming clearer, with emerging data implicating one-electron redox cycling as well as the multiple allosteric effects on enzyme activity. Regulation of the NOSs has been described at all levels from gene transcription to covalent modification and allosteric regulation of the enzyme itself. A wide range of NOS inhibitors have been discussed, interacting with the enzyme in diverse ways in terms of site and mechanism of inhibition, time-dependence and selectivity for individual isoforms, although there are many pitfalls and misunderstandings of these aspects. Highly selective inhibitors of iNOS versus eNOS and neuronal NOS have been identified and some of these have potential in the treatment of a range of inflammatory and other conditions in which iNOS has been implicated.

Heat-shock protein 90 augments neuronal nitric oxide synthase activity by enhancing Ca2+/calmodulin binding

Heat-shock protein 90 (hsp90) has been shown to facilitate neuronal NO synthase (nNOS, type 1) activity in vivo. But the direct effect of hsp90 on purified nNOS has not been determined yet. Moreover, the mechanism underlying the action of hsp90 is not known. nNOS activity is primarily initiated and regulated by the binding of Ca(2+)/calmodulin (CaM). Therefore, we explored whether hsp90 modulates nNOS activity by affecting CaM binding. Recombinant rat nNOS was purified from the stably transfected cells by affinity chromatography. hsp90 increased nNOS activity in a dose-dependent manner with an EC(50) of 24.1+/-6.4 nM. In the presence of hsp90, the CaM-nNOS dose-response curve was shifted markedly to the left and the maximal activity was also elevated. Further in vitro protein-binding experiments confirmed that hsp90 increased the binding of CaM to nNOS. Taken together, these data indicate that hsp90 directly augments nNOS catalytic function and that this effect is, at least partially, mediated by CaM-binding enhancement.

Production and interaction of oxygen and nitric oxide free radicals in PMA stimulated macrophages during the respiratory burst

The activity of nitric oxide synthase (NOS) during the respiratory burst in phorbol-1,2-myristate-1,3-acetate (PMA) stimulated macrophages has been the topic of much debate in the literature. To help clarify the role of NOS, we have examined the chemiluminescence arising from peroxynitrite production, nitrite/nitrate and nitric oxide production, and oxygen consumption during the respiratory burst in PMA-stimulated macrophages. The Griess reaction was used to measure nitrite/nitrate, spin trapping with N-methyl D-glucamine dithiocarbamate (MGD)2-Fe2+ was used to quantify nitric oxide, and the spin probe 2,2,6,6-tetramethylpiperidine-N-oxyl-4-ol (TEMPOL) was used to measure oxygen consumption. Oxygen free radical production (hydroxyl and superoxide free radicals) was also investigated using the spin trap 5,5-dimethyl-1-pyroline-1-oxide (DMPO). The chemiluminescence emitted by the PMA-stimulated macrophages and nitrite/nitrate in the culture system were both found to increase. However, the rate of nitric oxide release remained constant, indicating that the activity of NOS is not enhanced during the respiratory burst in PMA stimulated macrophages.

Role of copper ions and cytochrome P450 in the vasodilator actions of the nitroxyl anion generator, Angeli's salt, on rat aorta

Since copper ions catalyse the oxidation of nitroxyl anion to nitric oxide, we investigated whether this might explain the vasodilator actions of the nitroxyl generator, Angeli's salt, in rat aorta. Parallel studies were conducted with S-nitroso-N-acetyl-D,L-penicillamine (SNAP), since Cu ions catalyse the liberation of nitric oxide from this compound. Copper sulphate enhanced relaxation to Angeli's salt and SNAP but this resulted from reduced destruction of nitric oxide by superoxide rather than from enhanced generation of nitric oxide, since it was mimicked by superoxide dismutase and by the superoxide dismutase mimetic, MnCl2. Results with the selective Cu2+ chelators, neocuproine and bathocuproine disulfonate, and the Cu2+ chelators, EDTA, cuprizone and diethyldithiocarbamate, confirmed an important role for endogenous copper in mediating relaxation to SNAP but suggested only a minor role for Angeli's salt. Relaxation to Angeli's salt was, however, powerfully blocked by proadifen, suggesting an important role for cytochrome P450.

A kinetic simulation model that describes catalysis and regulation in nitric-oxide synthase

After initiating NO synthesis a majority of neuronal NO synthase (nNOS) quickly partitions into a ferrous heme-NO complex. This down-regulates activity and increases enzyme K(m,O(2)). To understand this process, we developed a 10-step kinetic model in which the ferric heme-NO enzyme forms as the immediate product of catalysis, and then partitions between NO dissociation versus reduction to a ferrous heme-NO complex. Rate constants used for the model were derived from recent literature or were determined here. Computer simulations of the model precisely described both pre-steady and steady-state features of nNOS catalysis, including NADPH consumption and NO production, buildup of a heme-NO complex, changes between pre-steady and steady-state rates, and the change in enzyme K(m,O(2)) in the presence or absence of NO synthesis. The model also correctly simulated the catalytic features of nNOS mutants W409F and W409Y, which are hyperactive and display less heme-NO complex formation in the steady state. Model simulations showed how the rate of heme reduction influences several features of nNOS catalysis, including populations of NO-bound versus NO-free enzyme in the steady state and the rate of NO synthesis. The simulation predicts that there is an optimum rate of heme reduction that is close to the measured rate in nNOS. Ratio between NADPH consumption and NO synthesis is also predicted to increase with faster heme reduction. Our kinetic model is an accurate and versatile tool for understanding catalytic behavior and will provide new perspectives on NOS regulation.

Formation of nitric oxide from nitroxyl anion: role of quinones and ferricytochrome c

1. Our previous finding that copper ions oxidize nitroxyl anion released from Angeli's salt to nitric oxide prompted us to examine if copper-containing enzymes shared this property. 2. The copper-containing enzyme, tyrosinase, which catalyses the hydroxylation of monophenols to diphenols and the subsequent oxidation of these to the respective unstable quinone, failed to generate nitric oxide from Angeli's salt by itself, but did so in the presence of tyrosine. 3. L-DOPA, the initial product of the reaction of tyrosinase with tyrosine, was not the active species, since it failed to generate nitric oxide from Angeli's salt. Nevertheless, L-DOPA and two other substrates, namely, catechol and tyramine did produce nitric oxide from Angeli's salt in the presence of tyrosinase, suggesting involvement of the respective unstable quinones. In support, we found that 1,4-benzoquinone produced a powerful nitric oxide signal from Angeli's salt. 4. Coenzyme Q(o), an analogue of ubiquinone, failed to generate nitric oxide from Angeli's salt by itself, but produced a powerful signal in the presence of its mitochondrial complex III cofactor, ferricytochrome c. 5. Experiments conducted on rat aortic rings with the mitochondrial complex III inhibitor, myxothiazol, to determine if this pathway was responsible for the vascular conversion of nitroxyl to nitric oxide were equivocal: relaxation to Angeli's salt was inhibited but so too was that to unrelated relaxants. 6. Thus, certain quinones oxidize nitroxyl to nitric oxide. Further work is required to determine if endogenous quinones contribute to the relaxant actions of nitroxyl donors such as Angeli's salt.

Electrochemical detection of nitroso-arginine as an intermediate between N-hydroxy-arginine and citrulline. An in vitro versus in vivo study using microcarbon electrodes in neuronal nitric oxide synthase and mice brain

The aim of the study was to describe in vivo and in vitro the transformation of N-hydroxy-arginine (NHA) into nitrite and citrulline. The products of NHA oxidation were studied by electrochemical methods. Cyclic voltammetry of NHA on microcarbon electrode showed an oxidation in two steps with one electron and one proton exchanged at each step. The first step gave a radical species NHA(.) with a half-life shorter than 1 micros and the second step gave nitroso-arginine (NA) with a half-life of about 1 s (1.5 s). Coulometric oxidation of NHA gave citrulline and nitrite. Differential pulse voltammetry (DPV) in vivo and in vitro gave a peak in reduction at -1.66 V vs Ag/AgCl for NA. After reductive adsorption of NA on the microelectrode surface in mice brain it gave the two peaks of NHA in oxidation plus another peak identified as nitrite. DPV in native and recombinant rat brain nitric oxide (NO)-synthase gave NA signal permitting K(m) and V(max) determination. All these results showed that NA was synthetized by NO-synthases before the final products, citrulline and nitrite.

Electron paramagnetic resonance spectroscopy with N-methyl-D-glucamine dithiocarbamate iron complexes distinguishes nitric oxide and nitroxyl anion in a redox-dependent manner: applications in identifying nitrogen monoxide products from nitric oxide synthase

Though a large number of studies indicate that nitric oxide synthase (NOS) is responsible for NO&z.rad; production in biological systems, controversy still remains concerning whether NOS directly produces NO&z.rad;. Schmidt et al. (PNAS 93:144492, 1996) proposed that NOS first synthesizes nitroxyl anion (NO(-)), which is then converted to NO&z.rad; by superoxide dismutase (SOD). With electron paramagnetic resonance spectroscopy using N-methyl-D-glucamine dithiocarbamate iron (Fe-MGD), we directly detected NO&z.rad; from purified NOS in the absence of SOD (Xia et al., PNAS 94:12705, 1997). We also showed that the requirement for SOD in the previous NO&z.rad; measurements appeared to be due to the high levels of exogenous superoxide production in their reaction system because of the presence of free FAD. However, it was recently questioned whether Fe-MGD can discriminate NO&z.rad; from NO(-) (Komarov et al., FRBM 28:739-742, 2000). In this study we examined the trapping specificity of different redox forms of Fe-MGD. With Fe(2+)-MGD, NO&z.rad; generated characteristic triplet NO&z.rad;-Fe(2+)-MGD signals (g = 2. 04, a(N) = 12.7 G), whereas NO(-) from Angeli's salt was EPR silent. Both NO&z.rad; and NO(-) gave rise to NO&z.rad;-Fe(2+)-MGD signals when Fe(3+)-MGD was used. Strong NO&z.rad; signals were measured from purified nNOS using the NO&z.rad; selective Fe(2+)-MGD and this was not affected by SOD. Thus, spin trapping with Fe-MGD can distinguish NO&z.rad; and NO(-) and this depends on the redox status of the iron. The detection of NO&z.rad; from purified NOS by Fe(2+)-MGD unambiguously reconfirms our previous report that NOS directly synthesizes NO&z.rad; but not NO(-).

Nitric oxide-induced autoinhibition of neuronal nitric oxide synthase in the presence of the autoxidation-resistant pteridine 5-methyltetrahydrobiopterin

Nitric oxide synthase (NOS) catalysis results in formation of NO or superoxide (O(2)(-.)) depending on the presence or absence of the cofactor tetrahydrobiopterin (BH4). In the absence of O(2)(-.) scavengers, net NO formation cannot be detected even at saturating BH4 concentrations, which is thought to be due to O(2)(-.) production by BH4 autoxidation. Because the N-5-methylated analogue of BH4 (5-Me-BH4) sustains NOS catalysis and is autoxidation-resistant, net NO formation by the neuronal isoform of NOS (nNOS) can be observed at saturating 5-Me-BH4 concentrations. Here we compare the effects of 5-Me-BH4 on L-citrulline formation, NADPH oxidation, H(2)O(2) production and soluble guanylate cyclase (sGC) stimulation. All activities were stimulated biphasically (EC(50) approx. 0.2 microM and more than 1 mM), with an intermediate inhibitory phase at the same pterin concentration as that required for net NO generation and sGC stimulation (4 microM). Concomitantly with inhibition, the NADP(+)/L-citrulline stoichiometry decreased from 2.0 to 1.6. Inhibition occurred only at high enzyme concentrations (IC(50) approx. 10 nM nNOS) and was antagonized by oxyhaemoglobin and by BH4. We ascribe the first stimulatory phase to high-affinity binding of 5-Me-BH4. The inhibitory phase is due to low-affinity binding, resulting in fully coupled catalysis, complete inhibition of O(2)(-.) production and net NO formation. At high enzyme concentrations and thus high NO levels, this causes autoinhibition. NO scavenging by 5-Me-BH4 at concentrations above 1 mM, resulting in the antagonization of inhibition of NOS, explains the second stimulatory phase. In agreement with these assignments 5-Me-BH4 was found to stimulate formation of a haem-NO complex during NOS catalysis. The observation of inhibition with 5-Me-BH4 but not with BH4 implies that, unless O(2)(-.) scavengers are present, a physiological role for NO-induced autoinhibition is unlikely.

Electron-paramagnetic resonance spectroscopy using N-methyl-D-glucamine dithiocarbamate iron cannot discriminate between nitric oxide and nitroxyl: implications for the detection of reaction products for nitric oxide synthase

Purified neuronal nitric oxide synthase (NOS) does not produce nitric oxide (NO) unless high concentrations of superoxide dismutase (SOD) are added, suggesting that nitroxyl (NO(-)) or a related molecule is the principal reaction product of NOS, which is SOD-dependently converted to NO. This hypothesis was questioned by experiments using electron paramagnetic resonance spectroscopy and iron N-methyl-D-glucamine dithiocarbamate (Fe-MGD) as a trap for NO. Although NOS and the NO donor S-nitroso-N-acetyl-penicillamine produced an electron paramagnetic resonance signal, the NO(-) donor, Angeli's salt (AS) did not. AS is a labile compound that rapidly hydrolyzes to nitrite, and important positive control experiments showing that AS was intact were lacking. On reinvestigating this crucial experiment, we find identical MGD(2)-Fe-NO complexes both from S-nitroso-N-acetyl-penicillamine and AS but not from nitrite. Moreover, the yield of MGD(2)-Fe-NO complex from AS was stoichiometric even in the absence of SOD. Thus, MGD(2)-Fe directly detects NO(-), and any conclusions drawn from MGD(2)-Fe-NO complexes with respect to the nature of the primary NOS product (NO, NO(-), or a related N-oxide) are invalid. Thus, NOS may form NO(-) or related N-oxides instead of NO.

Detection and imaging of endogenously produced nitric oxide with electron paramagnetic resonance spectroscopy

Nitric oxide (NO) represents a new paradigm for second messengers in regulation. Despite the numerous physiological and pathophysiological functions of NO, its importance as an endogenous second messenger and a cytostatic and/or cytotoxic agent was unknown until 1987. Recent developments in detection methods for endogenous NO produced directly or indirectly from NO synthases (NOSs) have enabled major advances in our understanding of the role of NO in biological systems. The spin-trapping technique combined with electron paramagnetic resonance (EPR) spectroscopy is a method for analyzing NO production directly both in vivo and in vitro. Iron complexes with dithiocarbamate derivatives are noteworthy among the spin-trapping reagents for NO because NO has a high affinity for iron complexes. The resultant stable nitrosyl iron complexes exhibit an intense three-line signal at room temperature and an axial signal at low temperature. Besides the facility and wide applicability of this method, its outstanding feature is that noninvasive in vivo measurements are available by using a low-frequency EPR spectrometer. In this article, we review on previous and recent developments of in vitro, in vivo, and ex vivo EPR detection and imaging of endogenously produced NO.

Opposite effects of nitric oxide and nitroxyl on postischemic myocardial injury

Recent experimental evidence suggests that reactive nitrogen oxide species can contribute significantly to postischemic myocardial injury. The aim of the present study was to evaluate the role of two reactive nitrogen oxide species, nitroxyl (NO(-)) and nitric oxide (NO(.)), in myocardial ischemia and reperfusion injury. Rabbits were subjected to 45 min of regional myocardial ischemia followed by 180 min of reperfusion. Vehicle (0.9% NaCl), 1 micromol/kg S-nitrosoglutathione (GSNO) (an NO(.) donor), or 3 micromol/kg Angeli's salt (AS) (a source of NO(-)) were given i.v. 5 min before reperfusion. Treatment with GSNO markedly attenuated reperfusion injury, as evidenced by improved cardiac function, decreased plasma creatine kinase activity, reduced necrotic size, and decreased myocardial myeloperoxidase activity. In contrast, the administration of AS at a hemodynamically equieffective dose not only failed to attenuate but, rather, aggravated reperfusion injury, indicated by an increased left ventricular end diastolic pressure, myocardial creatine kinase release and necrotic size. Decomposed AS was without effect. Co-administration of AS with ferricyanide, a one-electron oxidant that converts NO(-) to NO(.), completely blocked the injurious effects of AS and exerted significant cardioprotective effects similar to those of GSNO. These results demonstrate that, although NO(.) is protective, NO(-) increases the tissue damage that occurs during ischemia/reperfusion and suggest that formation of nitroxyl may contribute to postischemic myocardial injury.

Reaction of neuronal nitric oxide synthase with the nitric oxide spin-trapping agent, iron complexed with N-dithiocarboxysarcosine

A water-soluble iron complex with N-dithiocarboxysarcosine (Fe-DTCS) has been developed as an ESR spin-trapping agent for NO and successfully applied to ESR imaging of endogenous NO production in mice. We attempted to measure NO produced by purified neuronal NO synthase (nNOS) by this method, but could not detect NO. We speculated that Fe-DTCS inhibits NOS activity. In fact, it markedly inhibited NOS activity with an IC50 value of 9.7 +/- 0.7 microM in the citrulline-formation assay. DTCS alone did not inhibit the activity. An iron complex with N-methyl-D-glucamine dithiocarbamate, a similar spin-trapping agent for NO, also inhibited the activity, with an IC50 value of 25.1 +/- 2.9 microM. Fe-DTCS suppressed cytochrome c and ferricyanide reductase activities of nNOS, and markedly increased nNOS-mediated NADPH oxidation. Concomitantly, it accelerated oxygen consumption caused by activated nNOS. These results suggest that the ESR spin-trapping agent Fe-DTCS inhibits NO synthesis by interfering with the physiological electron flow from NADPH to nNOS heme iron.

Reactivity of tetrahydrobiopterin bound to nitric-oxide synthase

Levels of tetrahydrobiopterin (BH(4)) bound to nitric-oxide synthase (NOS) were examined during multiple turnovers of the enzyme in the presence of an NADPH-regenerating system. Our findings show that NOS-bound BH(4) does not remain in a static state but undergoes redox reactions. Under these experimental conditions, the redox state of BH(4) was determined by the balance between calcium/calmodulin (Ca(2+)/CaM)-dependent oxidation of BH(4) mediated by the uncoupled formation of superoxide/hydrogen peroxide on the one hand and by reductive regeneration of BH(4) on the other hand. BH(4) oxidation was appreciably increased in the presence of arginine. Levels of NOS-bound BH(4) were also examined under single turnover conditions in the absence of an NADPH-regenerating system and in the presence of added superoxide dismutase and catalase to suppress the accumulation of superoxide and hydrogen peroxide. BH(4) oxidation was again dependent on Ca(2+)/CaM. The insensitivity to superoxide dismutase and catalase suggested that the single turnover oxidation of BH(4) did not proceed through superoxide/peroxide, although the involvement of these oxidants could not be definitively excluded. The amount of BH(4) oxidized was highest in the presence of arginine, and this oxidation significantly exceeded that in the presence of N(G)-hydroxy-L-arginine. The findings that single turnover oxidation of BH(4) is stimulated by arginine in the presence of Ca(2+)/CaM and that BH(4) is regenerated are consistent with a role for the pterin as an electron donor in product formation; this role remains to be defined.

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.

Tetrahydrobiopterin inhibits monomerization and is consumed during catalysis in neuronal NO synthase

The biosynthesis of nitric oxide (NO) is catalyzed by homodimeric NO synthases (NOS). For unknown reasons, all NOS co-purify with substoichiometric amounts of (6R)-5,6,7,8-tetrahydrobiopterin (H(4)Bip) and require additional H(4)Bip for maximal activity. We examined the effects of H(4)Bip and pterin-derived inhibitors (anti-pterins) on purified neuronal NOS-I quaternary structure and H(4)Bip content. During L-arginine turnover, NOS-I dimers time dependently dissociated into inactive monomers, paralleled by a loss of enzyme-associated pterin. Dimer dissociation was inhibited when saturating levels of H(4)Bip were added during catalysis. Similar results were obtained with pterin-free NOS-I expressed in Escherichia coli. This stabilizing effect of H(4)Bip was mimicked by the anti-pterin 2-amino-4,6-dioxo-3,4,5,6,8,8a,9, 10-octahydro-oxazolo[1,2f]-pteridine (PHS-32), which also displaced NOS-associated H(4)Bip in a competitive manner. Surprisingly, H(4)Bip not only dissociated from NOS during catalysis, but was only partially recovered in the solute (50.0 +/- 16.5% of control at 20 min). NOS-associated H(4)Bip appeared to react with a NOS catalysis product to a derivative distinct from dihydrobiopterin or biopterin. Under identical conditions, reagent H(4)Bip was chemically stable and fully recovered (95.5 +/- 3.4% of control). A similar loss of both reagent and enzyme-bound H(4)Bip and dimer content was observed by NO generated from spermine NONOate. In conclusion, we propose a role for H(4)Bip as a dimer-stabilizing factor of neuronal NOS during catalysis, possibly by interfering with enzyme destabilizing products.

The inducible nitric oxide synthase in vascular and cardiac tissue

Expression of the inducible form of nitric oxide synthase (iNOS) has been reported in a variety of cardiovascular diseases. The resulting high output nitric oxide (NO) formation, besides the level of iNOS expression, depends also on the expression of the metabolic pathways providing the enzyme with substrate and cofactor. NO may trigger short and long term effects which are either beneficial or deleterious, depending on the molecular targets with which it interacts. These interactions are governed by local factors (like the redox state). In the cardiovascular system, the major targets involve not only guanylyl cyclase, but also other haem proteins, protein thiols, iron-non-haem complexes, and superoxide anion (forming peroxynitrite). The latter has several intracellular targets and may be cytotoxic, despite the existence of endogenous defence mechanisms. These interactions may either trigger NO effects or represent releasable NO stores, able to buffer NO and prolong its effects in blood vessels and in the heart. Besides selectively inhibiting iNOS, a number of other therapeutic strategies are conceivable to alleviate deleterious effects of excessive NO formation, including peroxynitrite (ONOO-) scavenging and inhibition of metabolic pathways triggered by ONOO-. When available, these approaches might have the advantage to preserve beneficial effects of iNOS induction. Counteracting vascular hyper-responsiveness to endogenous vasoconstrictor agonists in septic shock, or inducing cardiac protection against ischaemia-reperfusion injury are examples of such beneficial effects of iNOS induction.

Autoinhibition of neuronal nitric oxide synthase: distinct effects of reactive nitrogen and oxygen species on enzyme activity

Nitric oxide (NO) synthases (NOSs), which catalyse the oxidation of L-arginine to L-citrulline and an oxide of nitrogen, possibly NO or nitroxyl (NO-), are subject to autoinhibition by a mechanism that has yet to be fully elucidated. In the present study we investigated the actions of NO and other NOS-derived products as possible autoregulators of enzyme activity. With the use of purified NOS-I, L-arginine turnover was found to operate initially at Vmax (0-15 min, phase I) although, despite the presence of excess substrate and cofactors, prolonged catalysis (15-90 min, phase II) was associated with a rapid decline in L-arginine turnover. Taken together, these observations suggested that one or more NOS products inactivate NOS. Indeed, exogenously applied reactive nitrogen oxide species (RNSs) decreased Vmax during phase I, although with different potencies (NO->NO> ONOO-) and efficacies (NO>NO-=ONOO-). The NO scavengers oxyhaemoglobin (HbO2; 100 microM) and 1H-imidazol-1 - yloxy - 2 - (4-carboxyphenyl) - 4,5 - dihydro - 4,4,5,5 - tetramethyl - 3 -oxide (CPTIO; 10 microM) and the ONOO- scavenger GSH (7 mM) had no effect on NOS activity during phase I, except for an endogenous autoinhibitory influence of NO and ONOO-. However, superoxide dismutase (SOD; 300 units/ml), which is thought either to increase the half-life of NO or to convert NO- to NO, lowered Vmax in an NO-dependent manner because this effect was selectively antagonized by HbO2 (100 microM). This latter observation demonstrated the requirement of SOD to reveal endogenous NO-mediated autoinhibition. Importantly, during phase II of catalysis, NOS became uncoupled and began to form H2O2 because catalase, which metabolizes H2O2, increased enzyme activity. Consistent with this, exogenous H2O2 also inhibited NOS activity during phase I. Thus during catalysis NOS is subject to complex autoinhibition by both enzyme-derived RNS and H2O2, differentially affecting enzyme activity.