Characterization of bovine endothelial nitric oxide synthase as a homodimer with down-regulated uncoupled NADPH oxidase activity: tetrahydrobiopterin binding kinetics and role of haem in dimerization

(6R)-5,6,7,8-tetrahydro-L-biopterin and its stereoisomer prevent ischemia reperfusion injury in human forearm

OBJECTIVE

6R-5,6,7,8-tetrahydro-L-biopterin (6R-BH4) is a cofactor for endothelial nitric oxide synthase but also has antioxidant properties. Its stereo-isomer 6S-5,6,7,8-tetrahydro-L-biopterin (6S-BH4) and structurally similar pterin 6R,S-5,6,7,8-tetrahydro-D-neopterin (NH4) are also antioxidants but have no cofactor function. When endothelial nitric oxide synthase is 6R-BH4-deplete, it synthesizes superoxide rather than nitric oxide. Reduced nitric oxide bioavailability by interaction with reactive oxygen species is implicated in endothelial dysfunction (ED). 6R-BH4 corrects ED in animal models of ischemia reperfusion injury (IRI) and in patients with cardiovascular risks. It is uncertain whether the effect of exogenous 6R-BH4 on ED is through its cofactor or antioxidant action.

METHODS AND RESULTS

In healthy volunteers, forearm blood flow was measured by venous occlusion plethysmography during intra-arterial infusion of the endothelium-dependent vasodilator acetylcholine, or the endothelium-independent vasodilator glyceryl trinitrate, before and after IRI. IRI reduced plasma total antioxidant status (P=0.03) and impaired vasodilatation to acetylcholine (P=0.01), but not to glyceryl trinitrate (P=0.3). Intra-arterial infusion of 6R-BH4, 6S-BH4 and NH4 at approximately equimolar concentrations prevented IRI.

CONCLUSION

IRI causes ED associated with increased oxidative stress that is prevented by 6R-BH4, 6S-BH4, and NH4, an effect mediated perhaps by an antioxidant rather than cofactor function. Regardless of mechanism, 6R-BH4, 6S-BH4, or NH4 may reduce tissue injury during clinical IRI syndromes.

Nitric-oxide synthase: A cytochrome P450 family foster child

Nitric-oxide synthase (NOS), the enzyme responsible for mammalian NO generation, is no cytochrome P450, but there are striking similarities between both enzymes. First and foremost, both are heme-thiolate proteins, employing the same prosthetic group to perform similar chemistry. Moreover, they share the same redox partner, a diflavoprotein reductase, which in the case of NOS is incorporated with the oxygenase in one polypeptide chain. There are, however, also conspicuous differences, such as the presence in NOS of the additional cofactor tetrahydrobiopterin, which is applied as an auxiliary electron donor to prevent decay of the oxyferrous complex to ferric heme and superoxide. In this review similarities and differences between NOS and cytochrome P450 are analyzed in an attempt to explain why NOS requires BH4 and why NO synthesis is not catalyzed by a member of the cytochrome P450 family.

Janus-faced role of endothelial NO synthase in vascular disease: uncoupling of oxygen reduction from NO synthesis and its pharmacological reversal

Endothelial NO synthase (eNOS) is the predominant enzyme responsible for vascular NO synthesis. A functional eNOS transfers electrons from NADPH to its heme center, where L-arginine is oxidized to L-citrulline and NO. Common conditions predisposing to atherosclerosis, such as hypertension, hypercholesterolemia, diabetes mellitus and smoking, are associated with enhanced production of reactive oxygen species (ROS) and reduced amounts of bioactive NO in the vessel wall. NADPH oxidases represent major sources of ROS in cardiovascular pathophysiology. NADPH oxidase-derived superoxide avidly interacts with eNOS-derived NO to form peroxynitrite (ONOO(-)), which oxidizes the essential NOS cofactor (6R-)5,6,7,8-tetrahydrobiopterin (BH(4)). As a consequence, oxygen reduction uncouples from NO synthesis, thereby rendering NOS to a superoxide-producing pro-atherosclerotic enzyme. Supplementation with BH(4) corrects eNOS dysfunction in several animal models and in patients. Administration of high local doses of the antioxidant L-ascorbic acid (vitamin C) improves endothelial function, whereas large-scale clinical trials do not support a strong role for oral vitamin C and/or E in reducing cardiovascular disease. Statins, angiotensin-converting enzyme inhibitors and AT1 receptor blockers have the potential of reducing vascular oxidative stress. Finally, novel approaches are being tested to block pathways leading to oxidative stress (e.g. protein kinase C) or to upregulate antioxidant enzymes.

Exploring the redox reactions between heme and tetrahydrobiopterin in the nitric oxide synthases

The NO synthases (NOSs) catalyze a two-step oxidation of L-arginine (Arg) to generate nitric oxide (NO) plus L-citrulline. Because NOSs are the only hemeproteins known to contain tetrahydrobiopterin (H4B) as a bound cofactor, the function and role of H4B in their heme-based oxygen activation and catalysis is of current interest. Distinct oxidative and reductive transitions of bound H4B cofactor occur during catalysis and are associated with distinct redox transitions of the NOS heme and flavin prosthetic groups. In this perspective, we discuss the redox transitions of H4B and heme with regard to their kinetics, regulation, role in the catalytic mechanism, and how and why they may be linked.

Intracellular location regulates calcium-calmodulin-dependent activation of organelle-restricted eNOS

Mislocalization of endothelial nitric oxide (NO) synthase (eNOS) in response to oxidized low-density lipoprotein, cholesterol depletion, elevated blood pressure, and bound eNOS interacting protein/NOS traffic inducer is associated with reduced NO release via unknown mechanisms. The proper targeting of eNOS to the plasma membrane or intracellular organelles is an important regulatory step controlling enzyme activity. Previous studies have shown that plasma membrane eNOS is constitutively phosphorylated on serine 1179 and highly active. In contrast, the activity of eNOS targeted to intracellular organelles is more complex. The cis-Golgi eNOS is fully activated by Akt-dependent phosphorylation. However, eNOS targeted to the trans-Golgi is decidedly less active in response to all modes of activation, including mutation to the phosphomimetic aspartic acid. In this study, we establish that when expressed within other intracellular organelles, such as the mitochondria and nucleus, the activity of eNOS is also greatly reduced. To address the mechanisms underlying the impaired catalytic activity of eNOS within these locations, we generated subcellular-targeted constructs that express a calcium-independent NOS isoform, iNOS. With the use of organelle specific (plasma membrane, cis- vs. trans-Golgi, plasma membrane, and Golgi, nucleus, and mitochondria) targeting motifs fused to the wild-type iNOS, we measured NO release from intact cells. With the exception of the Golgi lumen, our results showed no impairment in the ability of targeted iNOS to synthesize NO. Confirmation of correct targeting was obtained through confocal microscopy using identical constructs fused to the green fluorescent protein. We conclude that the reduced activation of eNOS within discrete cytoplasmic regions of the Golgi, the mitochondria and the nucleus is primarily due to insufficient access to calcium-calmodulin.

The three nitric-oxide synthases differ in their kinetics of tetrahydrobiopterin radical formation, heme-dioxy reduction, and arginine hydroxylation

The nitric-oxide synthases (NOSs) make nitric oxide and citrulline from l-arginine. How the bound cofactor (6R)-tetrahydrobiopterin (H4B) participates in Arg hydroxylation is a topic of interest. We demonstrated previously that H4B radical formation in the inducible NOS oxygenase domain (iNOSoxy) is kinetically coupled to the disappearance of a heme-dioxy intermediate and to Arg hydroxylation. Here we report single turnover studies that determine and compare the kinetics of these transitions in Arg hydroxylation reactions catalyzed by the oxygenase domains of endothelial and neuronal NOSs (eNOSoxy and nNOSoxy). There was a buildup of a heme-dioxy intermediate in eNOSoxy and nNOSoxy followed by a monophasic transition to ferric enzyme during the reaction. The rate of heme-dioxy decay matched the rates of H4B radical formation and Arg hydroxylation in both enzymes. The rates of H4B radical formation differed such that nNOSoxy (18 s(-1)) > iNOSoxy (11 s(-1)) > eNOSoxy (6 s(-1)), whereas the lifetimes of the resulting H4B radical followed an opposite rank order. 5MeH4B supported a three-fold faster radical formation and greater radical stability relative to H4B in both eNOSoxy and nNOSoxy. Our results indicate the following: (i) the three NOSs share a common mechanism, whereby H4B transfers an electron to the heme-dioxy intermediate. This step enables Arg hydroxylation and is rate-limiting for all subsequent steps in the hydroxylation reaction. (ii) A direct correlation exists between pterin radical stability and the speed of its formation in the three NOSs. (iii) Uncoupled NO synthesis often seen for eNOS at low H4B concentrations may be caused by the slow formation and poor stability of its H4B radical.

The effect of high glucose on NO and O2- through endothelial GTPCH1 and NADPH oxidase

Although endothelial dysfunction deteriorates diabetic angiopathy, the mechanisms are obscure. We revealed that high glucose augmented eNOS through stimulation of eNOS mRNA in cultured BAECs. NO was decreased and O2- was increased simultaneously. NOS inhibitor, inhibited O2- release, so did NADPH oxidase inhibitor. The effects were synergistic. Both intracellular BH4 level and GTPCH1 activity were decreased by high glucose, in line with decrease of GTPCH1 mRNA. HMG-CoA reductase inhibitor, atorvastatin increased GTPCH1 mRNA and activity, and BH4 level. Conclusively, high glucose leads to eNOS dysfunction by inhibiting BH4 synthesis and atorvastatin stimulate BH4 synthesis directly, and it may work as atherogenic process.

Reactive oxygen species in vascular biology: implications in hypertension

Reactive oxygen species (ROS), including superoxide (*O2-), hydrogen peroxide (H2O2), and hydroxyl anion (OH-), and reactive nitrogen species, such as nitric oxide (NO) and peroxynitrite (ONOO-), are biologically important O2 derivatives that are increasingly recognized to be important in vascular biology through their oxidation/reduction (redox) potential. All vascular cell types (endothelial cells, vascular smooth muscle cells, and adventitial fibroblasts) produce ROS, primarily via cell membrane-associated NAD(P)H oxidase. Reactive oxygen species regulate vascular function by modulating cell growth, apoptosis/anoikis, migration, inflammation, secretion, and extracellular matrix protein production. An imbalance in redox state where pro-oxidants overwhelm anti-oxidant capacity results in oxidative stress. Oxidative stress and associated oxidative damage are mediators of vascular injury and inflammation in many cardiovascular diseases, including hypertension, hyperlipidemia, and diabetes. Increased generation of ROS has been demonstrated in experimental and human hypertension. Anti-oxidants and agents that interrupt NAD(P)H oxidase-driven *O2- production regress vascular remodeling, improve endothelial function, reduce inflammation, and decrease blood pressure in hypertensive models. This experimental evidence has evoked considerable interest because of the possibilities that therapies targeted against reactive oxygen intermediates, by decreasing generation of ROS and/or by increasing availability of antioxidants, may be useful in minimizing vascular injury and hypertensive end organ damage. The present chapter focuses on the importance of ROS in vascular biology and discusses the role of oxidative stress in vascular damage in hypertension.

Changes in the dimeric state of neuronal nitric oxide synthase affect the kinetics of secretagogue-induced insulin response

We previously showed that pancreatic beta-cells express a neuronal isoform of nitric oxide synthase (nNOS) that controls insulin secretion by exerting two enzymatic activities: nitric oxide (NO) production and cytochrome c reductase activity. We now bring evidence that two inhibitors of nNOS, N-omega-nitro-l-arginine methyl ester (l-NAME) and 7-nitroindazole (7-NI), increase glucose-induced insulin secretion but affect beta-cell function differently. In the presence of l-NAME, insulin response is monophasic, whereas 7-NI preserves the normal biphasic secretory pattern. In addition, the alterations of beta-cell functional response induced by the inhibitors also differ by their sensitivity to a substitutive treatment with sodium nitroprusside, a chemical NO donor. These differences are probably related to the nature of the two inhibitors. Indeed, using low-temperature SDS-PAGE and real-time analysis of nNOS dimerization by surface plasmon resonance, we could show that 7-NI, which competes with arginine and tetrahydrobiopterin (BH(4)), an essential cofactor for nNOS dimer formation, inhibits dimerization of the enzyme, whereas the substrate-based inhibitor l-NAME stabilizes the homodimeric state of nNOS. The latter effect could be reproduced by the two endogenous inhibitors of NOS, N-omega-methyl-l-arginine and asymmetric dimethylarginine, and resulted interestingly in a reduced ability of the protein inhibitor of nNOS (PIN) to dissociate nNOS dimers. We conclude that intracellular factors able to induce abnormalities in the nNOS monomer/dimer equilibrium could lead to pancreatic beta-cell dysfunction.

Tetrahydrobiopterin and nitric oxide: mechanistic and pharmacological aspects

In previous minireviews in this journal, we discussed work on induction of tetrahydrobiopterin biosynthesis by cytokines and its significance for nitric oxide (NO) production of intact cells as well as functions of H4-biopterin identified at this time for NO synthases (Proc Soc Exp Biol Med 203: 1-12, 1993; Proc Soc Exp Biol Med 219: 171-182, 1998). Meanwhile, the recognition of the importance of tetrahydrobiopterin for NO formation has led to new insights into complex biological processes and revealed possible novel pharmacological strategies to intervene in certain pathological conditions. Recent work could also establish that tetrahydrobiopterin, in addition to its allosteric effects, is redox-active in the NO synthase reaction. In this review, we summarize the current view of how tetrahydrobiopterin functions in the generation of NO and focus on pharmacological aspects of tetrahydrobiopterin availability with emphasis on endothelial function.

Synergistic activation of endothelial nitric-oxide synthase (eNOS) by HSP90 and Akt: calcium-independent eNOS activation involves formation of an HSP90-Akt-CaM-bound eNOS complex

Endothelial nitric-oxide synthase (eNOS), which generates the endogenous vasodilator, nitric oxide (NO), is highly regulated by post-translational modifications and protein interactions. We recently used purified proteins to characterize the mechanisms by which heat shock protein 90 (HSP90) increases eNOS activity at low and high Ca2+ levels (Takahashi, S. and Mendelsohn, M. E. (2003) J. Biol. Chem. 278, 9339-9344). Here we extend these studies to explore interactions between HSP90, Akt, and eNOS. In studies with purified proteins, HSP90 increased the initial rate and maximal extent of Akt-mediated eNOS phosphorylation and activation at low Ca2+ levels. Akt was not observed in the eNOS complex in the absence of HSP90, but both active and inactive Akt associated with eNOS in the presence of HSP90. Direct binding of Akt to HSP90 was observed even in the absence of eNOS. HSP90 also facilitated CaM binding to eNOS irrespective of Akt presence. Geldanamycin (GA) disrupted HSP90-eNOS binding, reduced HSP90-stimulated CaM binding, and blocked both recruitment of Akt to the eNOS complex and phosphorylation of eNOS at Ser-1179. Akt phosphorylated only CaM-bound eNOS, in an HSP90-independent manner. HSP90 and active Akt together increased eNOS activity synergistically, which was reversed by GA. In bovine aortic endothelial cells (BAECs), the effects of vascular endothelial growth factor (VEGF) and insulin on eNOS-HSP90-Akt complex formation and eNOS activation were compared. BAPTA-AM inhibited VEGF- but not insulin-induced eNOS-HSP90-Akt complex formation and eNOS phosphorylation. Insulin caused rapid, transient increase in eNOS activity correlated temporally with the formation of eNOS-HSP90-Akt complex. GA prevented insulin-induced association of HSP90, Akt and CaM with eNOS and inhibited eNOS activation in BAECs. Both platelet-derived growth factor (PDGF) and insulin induced activation of Akt in BAECs, but only insulin caused HSP90-Akt-eNOS association and eNOS phosphorylation. These results demonstrate that HSP90 and Akt synergistically activate eNOS and suggest that this synergy contributes to Ca2+-independent eNOS activation in response to insulin.

Interactions of peroxynitrite, tetrahydrobiopterin, ascorbic acid, and thiols: implications for uncoupling endothelial nitric-oxide synthase

Tetrahydrobiopterin (BH4) serves as a critical co-factor for the endothelial nitric-oxide synthase (eNOS). A deficiency of BH4 results in eNOS uncoupling, which is associated with increased superoxide and decreased NO* production. BH4 has been suggested to be a target for oxidation by peroxynitrite (ONOO-), and ascorbate has been shown to preserve BH4 levels and enhance endothelial NO* production; however, the mechanisms underlying these processes remain poorly defined. To gain further insight into these interactions, the reaction of ONOO- with BH4 was studied using electron spin resonance and the spin probe 1-hydroxy-3-carboxy-2,2,5-tetramethyl-pyrrolidine. ONOO- reacted with BH4 6-10 times faster than with ascorbate or thiols. The immediate product of the reaction between ONOO- and BH4 was the trihydrobiopterin radical (BH3.), which was reduced back to BH4 by ascorbate, whereas thiols were not efficient in recycling of BH4. Uncoupling of eNOS caused by peroxynitrite was investigated in cultured bovine aortic endothelial cells (BAECs) by measuring superoxide and NO* using spin probe 1-hydroxy-3-methoxycarbonyl-2,2,5,5-tetramethyl-pyrrolidine and the NO*-spin trap iron-diethyldithiocarbamate. Bolus ONOO-, the ONOO- donor 3-morpholinosydnonimine, and an inhibitor of BH4 synthesis (2,4-diamino-6-hydroxypyrimidine) uncoupled eNOS, increasing superoxide and decreasing NO* production. Exogenous BH4 supplementation restored endothelial NO* production. Treatment of BAECs with both BH4 and ascorbate prior to ONOO- prevented uncoupling of eNOS by ONOO-. This study demonstrates that endothelial BH4 is a crucial target for oxidation by ONOO- and that the BH4 reaction rate constant exceeds those of thiols or ascorbate. We confirmed that ONOO- uncouples eNOS by oxidation of tetrahydrobiopterin and that ascorbate does not fully protect BH4 from oxidation but recycles BH3. radical back to BH4.

Calmodulin-dependent and -independent activation of endothelial nitric-oxide synthase by heat shock protein 90

Endothelial nitric oxide synthase (eNOS), which generates the endogenous vasodilator, nitric oxide (NO), is highly regulated by post-translational modifications and protein interactions. Heat shock protein 90 (HSP90) binds directly to eNOS, augmenting NO production. We have used purified proteins to characterize further the mechanism by which HSP90 increases eNOS activity at low (100 nm) and high (10 microm) Ca(2+) levels. In the presence of calmodulin (CaM), HSP90 increased eNOS activity dose dependently at both low and high Ca(2+) concentrations. This effect was abolished by the specific HSP90 inhibitor geldanamycin (GA) at both calcium concentrations. The EC(50) values of eNOS for both Ca(2+) and CaM were decreased in the presence of HSP90. HSP90 also significantly increased the rate of NADPH-dependent cytochrome c reduction by eNOS at both low and high Ca(2+) concentrations. HSP90 bound to eNOS in a dose-dependent manner, and the amount of bound HSP90 also increased with increasing Ca(2+)/CaM. At 100 nm Ca(2+), HSP90 promoted dose-dependent CaM binding to eNOS that was fully inhibitable by GA. At high calcium, HSP90 did not affect CaM binding to eNOS, but GA inhibited HSP90 binding to eNOS. At high Ca(2+), HSP90 caused the V(max) of eNOS for l-arginine to increase by 2-fold, but the K(m) of eNOS was unchanged. HSP90 bound preferentially to CaM-prebound eNOS and significantly increased both its NO synthesis and reductase activities. These data support that HSP90 promotes eNOS activity by two mechanisms: (i) a CaM-dependent mechanism operative at low Ca(2+) concentrations, characterized by an increase in the affinity of eNOS for CaM and (ii) a CaM-independent mechanism apparent at high Ca(2+) concentrations, characterized by stimulation of eNOS reductase activity without further change in CaM binding. These studies contribute to our understanding of eNOS activation by HSP90 and provide a basis for in vitro studies of other eNOS-interacting proteins.

Genetic influence on cigarette-induced cardiovascular disease

Cigarette smoking as an addictive habit has accompanied human beings for more than 4 centuries. It is also one of the most potent and prevalent environmental health risks human beings are exposed to, and it is responsible for more than 1000 deaths each day in the United States. With recent research progress, it becomes clear that cigarette smoking can cause almost all major diseases prevalent today, such as cancer or heart disease. These detrimental effects are not only present in active smokers who choose the risk, but also to innocent bystanders, as passive smokers, who are exposed to cigarettes not-by-choice. While the cigarette-induced harm to human health is indiscriminate and severe, the degree of damage also varies from individual to individual. This intersubject variability in cigarette-induced pathologies is partly mediated by genetic variants of genes that may participate in detoxification process, eg, cytochrome P450 (CYP), cellular susceptibility to toxins, such as p53, or disease development. Through population studies, we have learned that certain CYP1A1 variants, such as Mspl polymorphism, may render the carriers more susceptible to cigarette-induced lung cancer or severe coronary atherosclerosis. The endothelial nitric oxide synthase intron 4 rare allele homozygotes are more likely to have myocardial infarction if they also smoke. In vitro experimental approach has further demonstrated that cigarettes may specifically regulate these genes in genotype-dependent fashion. While we still know little about genetic basis and molecular pathways for cigarette-induced pathological changes, understanding these mechanisms will be of great value in designing strategies to further reduce smoking in targeted populations, and to implement more effective measures in prevention and treatment of cigarette-induced diseases.

Protective role of endothelial nitric oxide synthase

Nitric oxide is a versatile molecule, with its actions ranging from haemodynamic regulation to anti-proliferative effects on vascular smooth muscle cells. Nitric oxide is produced by the nitric oxide synthases, endothelial NOS (eNOS), neural NOS (nNOS), and inducible NOS (iNOS). Constitutively expressed eNOS produces low concentrations of NO, which is necessary for a good endothelial function and integrity. Endothelial derived NO is often seen as a protective agent in a variety of diseases. This review will focus on the potential protective role of eNOS. We will discuss recent data derived from studies in eNOS knockout mice and other experimental models. Furthermore, the role of eNOS in human diseases is described and possible therapeutic intervention strategies will be discussed.

Distinct dimer interaction and regulation in nitric-oxide synthase types I, II, and III

Homodimer formation activates all nitric-oxide synthases (NOSs). It involves the interaction between two oxygenase domains (NOSoxy) that each bind heme and (6R)-tetrahydrobiopterin (H4B) and catalyze NO synthesis from L-Arg. Here we compared three NOSoxy isozymes regarding dimer strength, interface composition, and the ability of L-Arg and H4B to stabilize the dimer, promote its formation, and protect it from proteolysis. Urea dissociation studies indicated that the relative dimer strengths were NOSIIIoxy >> NOSIoxy > NOSIIoxy (endothelial NOSoxy (eNOSoxy) >> neuronal NOSOXY (nNOSoxy) > inducible NOSoxy (iNOSoxy)). Dimer strengths of the full-length NOSs had the same rank order as judged by their urea-induced loss of NO synthesis activity. NOSoxy dimers containing L-Arg plus H4B exhibited the greatest resistance to urea-induced dissociation followed by those containing either molecule and then by those containing neither. Analysis of crystallographic structures of eNOSoxy and iNOSoxy dimers showed more intersubunit contacts and buried surface area in the dimer interface of eNOSoxy than iNOSoxy, thus revealing a potential basis for their different stabilities. L-Arg plus H4B promoted dimerization of urea-generated iNOSoxy and nNOSoxy monomers, which otherwise was minimal in their absence, and also protected both dimers against trypsin proteolysis. In these respects, L-Arg alone was more effective than H4B alone for nNOSoxy, whereas for iNOSoxy the converse was true. The eNOSoxy dimer was insensitive to proteolysis under all conditions. Our results indicate that the three NOS isozymes, despite their general structural similarity, differ markedly in their strengths, interfaces, and in how L-Arg and H4B influence their formation and stability. These distinguishing features may provide a basis for selective control and likely help to regulate each NOS in its particular biologic milieu.

Mechanisms of dysfunction of the nitric oxide pathway in vascular diseases

Vascular nitric oxide (NO) is involved in many physiologic and pathophysiologic processes throughout the body. Many vascular diseases have a reduction in the activity of endothelium-derived NO as an important component involved in the initiation and/or progression of the disease. It is now known that there are multiple mechanisms for this reduction in NO activity with one or more mechanisms operating depending on the specific condition or stage of a disease. In other instances, the therapy for certain diseases is responsible for the reduction in NO activity and contributes to the acceleration of vascular disease. This review details the known mechanisms of dysfunction of the NO pathway leading to vascular diseases, which provides the rationale for why certain therapies can improve while other therapies adversely affect vascular health.

Glial-derived arginine, the nitric oxide precursor, protects neurons from NMDA-induced excitotoxicity

Excitotoxic neuronal cell death is characterized by an overactivation of glutamate receptors, in particular of the NMDA subtype, and the stimulation of the neuronal nitric oxide synthase (nNOS), which catalyses the formation of nitric oxide (NO) from l-arginine (L-Arg). At low L-Arg concentrations, nNOS generates NO and superoxide (O2(.)(-)), favouring the production of the toxin peroxynitrite (ONOO-). Here we report that NMDA application for five minutes in the absence of added L-Arg induces neuronal cell death, and that the presence of L-Arg during NMDA application prevents cell loss by blocking O2(.)(-) and ONOO- formation and by inhibiting mitochondrial depolarization. Because L-Arg is transferred from glial cells to neurons upon activation of glial glutamate receptors, we hypothesized that glial cells play an important modulator role in excitotoxicity by releasing L-Arg. Indeed, as we further show, glial-derived L-Arg inhibits NMDA-induced toxic radical formation, mitochondrial dysfunction and cell death. Glial cells thus may protect neurons from excitotoxicity by supplying L-Arg. This potential neuroprotective mechanism may lead to an alternative approach for the treatment of neurodegenerative diseases involving excitotoxic processes, such as ischemia.

Comparison of neuronal and endothelial isoforms of nitric oxide synthase in stably transfected HEK 293 cells

The neuronal and endothelial isoforms of nitric oxide (NO) synthase (nNOS and eNOS, respectively) both catalyze the production of NO but are regulated differently. Stably transfected HEK 293 cell lines containing nNOS, eNOS, and a soluble mutant of eNOS were therefore established to compare their activity in a common cellular environment. NOS activity was determined by measuring L-[3H]citrulline production in homogenates and intact cells, the conversion of oxyhemoglobin to methemoglobin, and the production of cGMP. The results indicate that nNOS is more active than eNOS, both in unstimulated as well as calcium-stimulated cells. Under basal conditions, the soluble mutant of eNOS appeared to be slightly more active than wild-type eNOS in terms of NO and cGMP formation, suggesting that membrane association may be crucial for inhibition of basal NO release but is not required for stimulation by Ca2+-mobilizing agents. The maximal activity of soluble guanylate cyclase was significantly reduced by transfection with wild-type eNOS due to downregulation of mRNA expression. These results demonstrate that nNOS and eNOS behave differently even in an identical cellular environment.

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.

Chimeras of nitric-oxide synthase types I and III establish fundamental correlates between heme reduction, heme-NO complex formation, and catalytic activity

Neuronal nitric-oxide synthase (nNOS or NOS I) and endothelial NOS (eNOS or NOS III) differ widely in their reductase and nitric oxide (NO) synthesis activities, electron transfer rates, and propensities to form a heme-NO complex during catalysis. We generated chimeras by swapping eNOS and nNOS oxygenase domains to understand the basis for these differences and to identify structural elements that determine their catalytic behaviors. Swapping oxygenase domains did not alter domain-specific catalytic functions (cytochrome c reduction or H(2)O(2)-supported N(omega)-hydroxy-l-arginine oxidation) but markedly affected steady-state NO synthesis and NADPH oxidation compared with native eNOS and nNOS. Stopped-flow analysis showed that reductase domains either maintained (nNOS) or slightly exceeded (eNOS) their native rates of heme reduction in each chimera. Heme reduction rates were found to correlate with the initial rates of NADPH oxidation and heme-NO complex formation, with the percentage of heme-NO complex attained during the steady state, and with NO synthesis activity. Oxygenase domain identity influenced these parameters to a lesser degree. We conclude: 1) Heme reduction rates in nNOS and eNOS are controlled primarily by their reductase domains and are almost independent of oxygenase domain identity. 2) Heme reduction rate is the dominant parameter controlling the kinetics and extent of heme-NO complex formation in both eNOS and nNOS, and thus it determines to what degree heme-NO complex formation influences their steady-state NO synthesis, whereas oxygenase domains provide minor but important influences. 3) General principles that relate heme reduction rate, heme-NO complex formation, and NO synthesis are not specific for nNOS but apply to eNOS as well.

Effect of vitamin C on the availability of tetrahydrobiopterin in human endothelial cells

Vitamin C has long been known for its beneficial vascular effects, but its mechanism of action remains unclear. Recent reports suggest that vitamin C may prevent endothelial dysfunction by scavenging free radicals and increasing the bioavailability of nitric oxide. To investigate this area further, we studied the effect of vitamin C (10(-4) M) and Mn(III) tetrakis (4-benzoic acid) porphyrin chloride (MnTBAP; 10(-5) M), a scavenger of superoxide, hydrogen peroxide, and peroxynitrite, on endothelial nitric oxide synthase (eNOS) enzymatic activity in cultured human umbilical vein endothelial cells. L-Citrulline formation (a measure of eNOS enzymatic activity) was significantly increased in cells treated for 24 h with vitamin C. No effect was observed after MnTBAP treatment. Chronic administration of vitamin C also had no effect on eNOS protein expression. Treatment with vitamin C for 24 h significantly increased levels of the eNOS co-factor tetrahydrobiopterin (BH4), whereas MnTBAP did not affect its levels. Sepiapterin (10(-4) M), a precursor of BH4, significantly increased eNOS activity, whereas addition of vitamin C to cells treated with sepiapterin did not cause any further increase in eNOS activity. Our results suggest that the beneficial effect of vitamin C on endothelial function is best explained by increased intracellular BH4 content and subsequent enhancement of eNOS activity. This effect appears to be independent of the ability of vitamin C to scavenge superoxide anions.

Role of bound zinc in dimer stabilization but not enzyme activity of neuronal nitric-oxide synthase

Nitric-oxide synthases (NOS) are homodimeric proteins and can form an intersubunit Zn(4S) cluster. We have measured zinc bound to NOS purified from pig brain (0.6 mol/mol of NOS) and baculovirus-expressed rat neuronal NOS (nNOS) (0.49 +/- 0.13 mol/mol of NOS), by on-line gel-filtration/inductively coupled plasma mass spectrometry. Cobalt, manganese, molybdenum, nickel, and vanadium were all undetectable. Baculovirus-expressed nNOS also bound up to 2. 00 +/- 0.58 mol of copper/mol of NOS. Diethylenetriaminepentaacetic acid (DTPA) reduced the bound zinc to 0.28 +/- 0.07 and the copper to 0.97 +/- 0.24 mol/mol of NOS. Desalting of samples into thiol-free buffer did not affect the zinc content but completely eliminated the bound copper (</=0.02 mol/mol of NOS). Most (> or =75%) of the bound zinc was released from baculovirus-expressed rat nNOS by p-chloromercuriphenylsulfonic acid (PMPS). PMPS-treated nNOS was strongly (90 +/- 5%) inactivated. To isolate functional effects of zinc release from other effects of PMPS, PMPS-substituted thiols were unblocked by excess reduced thiol in the presence of DTPA, which hindered reincorporation of zinc. The resulting enzyme contained 0.12 +/- 0.05 mol of zinc but had a specific activity of 426 +/- 46 nmol of citrulline.mg(-1).min(-1), corresponding to 93 +/- 10% of non-PMPS-treated controls. PMPS also caused dissociation of nNOS dimers under native conditions, an effect that was blocked by the pteridine cofactor tetrahydrobiopterin (H(4)biopterin). H(4)biopterin did not affect zinc release. Even in the presence of H(4)biopterin, PMPS prevented conversion of NOS dimers to an SDS-resistant form. We conclude that zinc binding is a prerequisite for formation of SDS-resistant NOS dimers but is not essential for catalysis.

Heme insertion, assembly, and activation of apo-neuronal nitric-oxide synthase in vitro

It has been established that in the case of inducible NO synthase (NOS), a functionally active homodimer is assembled from the heme-deficient monomeric apo-NOS in vitro by the addition of heme, whereas the heme-deficient neuronal isoform (apo-nNOS) is at best only partially activated. In the current study we have discovered that reactive oxygen species, which can be removed by the addition of superoxide dismutase and catalase, destroy the heme and limit the activation of apo-nNOS in vitro. With the use of these improved conditions, we show for the first time that heme insertion is a rapid process that results in formation of a heme-bound monomeric nNOS that is able to form the ferrous-CO P450 complex but is unable to synthesize NO. A slow process requiring more than 90 min is required for dimerization and activation of this P450 intermediate to give an enzyme with a specific activity of approximately 1100 nmol of NO formed/min/mg of protein, similar to that of the native enzyme. Interestingly, the dimer is not SDS-resistant and is not the same dimer that forms in vivo. These studies indicate at least two intermediates in the assembly of nNOS and advance our understanding of the regulation of nNOS.

Contrasting effects of N5-substituted tetrahydrobiopterin derivatives on phenylalanine hydroxylase, dihydropteridine reductase and nitric oxide synthase

Tetrahydrobiopterin [(6R)-5,6,7,8-tetrahydro-L-biopterin, H(4)biopterin] is one of several cofactors of nitric oxide synthases (EC 1.14.13.39). Here we compared the action of N(5)-substituted derivatives on recombinant rat neuronal nitric oxide synthase with their effects on dihydropteridine reductase (EC 1.6.99.7) and phenylalanine hydroxylase (EC 1.14.16.1),the well-studied classical H(4)biopterin-dependent reactions. H(4)biopterin substituted at N(5) with methyl, hydroxymethyl, formyl and acetyl groups were used. Substitution at N(5) occurs at a position critical to the redox cycle of the cofactor in phenylalanine hydroxylase/dihydropteridine reductase. We also included N(2)'-methyl H(4)biopterin, a derivative substituted at a position not directly involved in redox cycling, as a control. As compared with N(5)-methyl H(4)biopterin, N(5)-formyl H(4)biopterin bound with twice the capacity but stimulated nitric oxide synthase to a lesser extent. Depending on the substituent used, N(5)-substituted derivatives were redox-active: N(5)-methyl- and N(5)-hydroxyl methyl H(4)biopterin, but not N(5)-formyl- and N(5)-acetyl H(4)biopterin, reduced 2,6-dichlorophenol indophenol. N(5)-Substituted H(4)biopterin derivatives were not oxidized to products serving as substrates for dihydropteridine reductase and,depending on the substituent, were competitive inhibitors of phenylalanine hydroxylase: N(5)-methyl- and N(5)-hydroxymethyl H(4)biopterin inhibited phenylalanine hydroxylase, whereas N(5)-formyl- and N(5)-acetyl H(4)biopterin had no effect. Our data demonstrate differences in the mechanism of stimulation of phenylalanine hydroxylase and nitric oxide synthase by H(4)biopterin. They are compatible with a novel, non-classical, redox-active contribution of H(4)biopterin to the catalysis of the nitric oxide synthase reaction.

Electron transfer, oxygen binding, and nitric oxide feedback inhibition in endothelial nitric-oxide synthase

We studied steps that make up the initial and steady-state phases of nitric oxide (NO) synthesis to understand how activity of bovine endothelial NO synthase (eNOS) is regulated. Stopped-flow analysis of NADPH-dependent flavin reduction showed the rate increased from 0. 13 to 86 s(-1) upon calmodulin binding, but this supported slow heme reduction in the presence of either Arg or N(omega)-hydroxy-l-arginine (0.005 and 0.014 s(-1), respectively, at 10 degrees C). O(2) binding to ferrous eNOS generated a transient ferrous dioxy species (Soret peak at 427 nm) whose formation and decay kinetics indicate it can participate in NO synthesis. The kinetics of heme-NO complex formation were characterized under anaerobic conditions and during the initial phase of NO synthesis. During catalysis heme-NO complex formation required buildup of relatively high solution NO concentrations (>50 nm), which were easily achieved with N(omega)-hydroxy-l-arginine but not with Arg as substrate. Heme-NO complex formation caused eNOS NADPH oxidation and citrulline synthesis to decrease 3-fold and the apparent K(m) for O(2) to increase 6-fold. Our main conclusions are: 1) The slow steady-state rate of NO synthesis by eNOS is primarily because of slow electron transfer from its reductase domain to the heme, rather than heme-NO complex formation or other aspects of catalysis. 2) eNOS forms relatively little heme-NO complex during NO synthesis from Arg, implying NO feedback inhibition has a minimal role. These properties distinguish eNOS from the other NOS isoforms and provide a foundation to better understand its role in physiology and pathology.

Superoxide production and reactive oxygen species signaling by endothelial nitric-oxide synthase

Reactive oxygen species can function as intracellular messengers, but linking these signaling events with specific enzymes has been difficult. Purified endothelial nitric-oxide synthase (eNOS) can generate superoxide (O(2)) under special conditions but is only known to participate in cell signaling through NO. Here we show that eNOS regulates tumor necrosis factor alpha (TNFalpha) through a mechanism dependent on the production of O(2) and completely independent of NO. Expression of eNOS in transfected U937 cells increased phorbol 12-myristate 13-acetate-induced TNFalpha promoter activity and TNFalpha production. N(omega)-Methyl-l-arginine, an inhibitor of eNOS that blocks NO production but not its NADPH oxidase activity, did not prevent TNFalpha up-regulation. Likewise, Gln(361)eNOS, a competent NADPH oxidase that lacks NOS activity, retained the ability to increase TNFalpha. Similar to the effect of eNOS, a O(2) donor dose-dependently increased TNFalpha production in differentiated U937 cells. In contrast, cotransfection of superoxide dismutase with eNOS prevented TNFalpha up-regulation, as did partial deletion of the eNOS NADPH binding site, a mutation associated with loss of O(2) production. Thus, eNOS may straddle a bifurcating pathway that can lead to the formation of either NO or O(2), interrelated but often opposing free radical messengers. This arrangement has possible implications for atherosclerosis and septic shock where endothelial dysfunction results from imbalances in NO and O(2) production.

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.

Genotype dependent and cigarette specific effects on endothelial nitric oxide synthase gene expression and enzyme activity

We explored the interactive effects of endothelial nitric oxide synthase (eNOS) genotypes and cigarette smoking on protein levels and enzyme activity in 33 postpartum placentas. Whilst the eNOS protein levels were lower in the rare allele (0.48+/-0.11, n=9 vs. 1. 05+/-0.10, n=24, P<0.01), the eNOS enzyme activity was about 7-fold higher in the rare allele (4556.2+/-255.4 vs. 621.8+/-180.5 cpm/mg/min, P<0.01). Smokers had lower eNOS protein levels (1.07+/-0. 09 vs. 0.50+/-0.19, P<0.05) in both alleles. It reduced the eNOS activities only in the rare allele (non-smokers: 6143.8+/-251.2, n=5, smokers: 2968.5+/-259.4, n=4, 52% reduction, P<0.01). We conclude that associations between eNOS polymorphism and protein levels and enzyme activities are modifiable by smoking, the effects of smoking are dependent on the eNOS genotypes.

Interaction of endothelial and neuronal nitric-oxide synthases with the bradykinin B2 receptor. Binding of an inhibitory peptide to the oxygenase domain blocks uncoupled NADPH oxidation

Endothelial nitric-oxide synthase (type III) (eNOS) was reported to form an inhibitory complex with the bradykinin receptor B2 (B2R) from which the enzyme is released in an active form upon receptor activation (Ju, H., Venema, V. J., Marrero, M. B., and Venema, R. C. (1998) J. Biol. Chem. 273, 24025-24029). Using a synthetic peptide derived from the known inhibitory sequence of the B2R (residues 310-329) we studied the interaction of the receptor with purified eNOS and neuronal nitric-oxide synthase (type I) (nNOS). The peptide inhibited formation of L-citrulline by eNOS and nNOS with IC(50) values of 10.6 +/- 0.4 microM and 7.1 +/- 0.6 microM, respectively. Inhibition was not due to an interference of the peptide with L-arginine or tetrahydrobiopterin binding. The NADPH oxidase activity of nNOS measured in the absence of L-arginine was inhibited by the peptide with an IC(50) of 3.7 +/- 0.6 microM, but the cytochrome c reductase activity of the enzyme was much less susceptible to inhibition (IC(50) >0.1 mM). Steady-state absorbance spectra of nNOS recorded during uncoupled NADPH oxidation showed that the heme remained oxidized in the presence of the synthetic peptide consisting of amino acids 310-329 of the B2R, whereas the reduced oxyferrous heme complex was accumulated in its absence. These data suggest that binding of the B2R 310-329 peptide blocks flavin to heme electron transfer. Co-immunoprecipitation of B2R and nNOS from human embryonic kidney cells stably transfected with human nNOS suggests that the B2R may functionally interact with nNOS in vivo. This interaction of nNOS with the B2R may recruit the enzyme to allow for the effective coupling of bradykinin signaling to the nitric oxide pathway.

Allosteric inhibitors of inducible nitric oxide synthase dimerization discovered via combinatorial chemistry

Potent and selective inhibitors of inducible nitric oxide synthase (iNOS) (EC ) were identified in an encoded combinatorial chemical library that blocked human iNOS dimerization, and thereby NO production. In a cell-based iNOS assay (A-172 astrocytoma cells) the inhibitors had low-nanomolar IC(50) values and thus were >1,000-fold more potent than the substrate-based direct iNOS inhibitors 1400W and N-methyl-l-arginine. Biochemical studies confirmed that inhibitors caused accumulation of iNOS monomers in mouse macrophage RAW 264.7 cells. High affinity (K(d) approximately 3 nM) of inhibitors for isolated iNOS monomers was confirmed by using a radioligand binding assay. Inhibitors were >1,000-fold selective for iNOS versus endothelial NOS dimerization in a cell-based assay. The crystal structure of inhibitor bound to the monomeric iNOS oxygenase domain revealed inhibitor-heme coordination and substantial perturbation of the substrate binding site and the dimerization interface, indicating that this small molecule acts by allosterically disrupting protein-protein interactions at the dimer interface. These results provide a mechanism-based approach to highly selective iNOS inhibition. Inhibitors were active in vivo, with ED(50) values of <2 mg/kg in a rat model of endotoxin-induced systemic iNOS induction. Thus, this class of dimerization inhibitors has broad therapeutic potential in iNOS-mediated pathologies.

The autoinhibitory control element and calmodulin conspire to provide physiological modulation of endothelial and neuronal nitric oxide synthase activity

NO production by the endothelial and neuronal isoforms of nitric oxide synthase (cNOS) is regulated on a moment-to-moment basis by calmodulin binding, triggered by transient elevations in intracellular-free calcium levels. Nonetheless, additional modes of cNOS regulation are implicit in the discoveries of stimuli that elicit a sustained increase in cNOS activity despite undetectable or transient increases in intracellular Ca2+ in endothelial cells; such stimuli include shear-stress, oestrogen, insulin or insulin-like growth factor treatment of endothelial cells. Recently, we identified a peptide insertion within the FMN-binding domain of mammalian NOSs that is unique to calcium-dependent isoforms, and not shared with inducible NOS or ancestral flavoproteins. Evidence suggests that this insertion serves as a fundamental control element, analogous to intrinsic autoinhibitory peptides that have been demonstrated to regulate activity of other calmodulin-dependent enzymes. Thus, the peptide insertion of cNOSs appears to function as structural element that is displaced upon calmodulin binding, resulting in dysinhibition of NO synthesis. Once displaced, the peptide may also be subject to transient chemical modifications and protein-protein interactions that modulate autoinhibitory function. Herein we summarize our present knowledge and speculate on mechanisms by which calmodulin and the autoinhibitory peptide conspire to regulate cNOS activity.

Characterization of recombinant human endothelial nitric-oxide synthase purified from the yeast Pichia pastoris

Human endothelial nitric-oxide synthase (eNOS) was expressed in the methylotrophic yeast Pichia pastoris, making use of the highly inducible alcohol oxidase promoter. The recombinant protein constituted approximately 3% of total protein and was largely soluble (>75%). About 1 mg of purified eNOS was obtained from 100-ml yeast cell cultures by affinity chromatography of crude cell supernatants. The purified enzyme had a V(max) of 192 +/- 18 nmol of L-citrulline x mg(-1) x min(-1), had a K(m) for L-arginine of 3.9 +/- 0.2 microM, and showed an absolute requirement for tetrahydrobiopterin (H(4)biopterin). NADPH oxidase activity was 136 +/- 9 and 342 +/- 24 nmol x mg(-1) x min(-1) in the absence and presence of 0.1 mM L-arginine, respectively, and not affected by H(4)biopterin. The protein contained 0.56 +/- 0.06 equivalents of FAD and 0.79 +/- 0.08 equivalents of FMN. On-line gel filtration/inductively coupled plasma mass spectrometry analysis confirmed that both iron (0.80 +/- 0.09 mol/subunit) and zinc (0.43 +/- 0.03 mol/subunit) were bound to the enzyme. Graphite furnace-atomic absorption spectroscopy yielded a value for bound iron of 0.84 +/- 0.04 mol/subunit. The absorbance of the enzyme at 398 nm implied a heme content of 0.85 +/- 0.09 mol/subunit, and the high pressure liquid chromatography heme assay gave an estimate of 0.71 +/- 0.02 mol heme/subunit. Gel permeation chromatography yielded one single peak with a Stokes radius of 6.62 +/- 0.7 nm, indicating that the native protein is dimeric. Upon low temperature gel electrophoresis the untreated protein appeared mainly as a monomer (88 +/- 3%), but pretreatment with H(4)biopterin and L-arginine led to a pronounced shift toward dimers (77 +/- 4%). Thus, in contrast to bovine eNOS (List, B. M., Klösch, B., Völker, C., Gorren, A. C. F., Sessa, W. C., Werner, E. R., Kukovetz, W. R., Schmidt, K., and Mayer, B. (1997) Biochem. J. 323, 159-165; Rodriguez-Crespo, I., Gerber, N. C., and Ortiz de Montellano, P. R. (1996) J. Biol. Chem. 271, 11462-11467), the human eNOS appears to be markedly stabilized by H(4)biopterin.

Pharmacological modulation of nitric oxide synthesis by mechanism-based inactivators and related inhibitors

Nitric oxide synthase (NOS) (EC 1.14.13.39) is a homodimeric cytochrome P450 monooxygenase analog that generates nitric oxide (NO) from the amino acid L-arginine. Enzymatically produced NO acts as an intracellular messenger in neuronal networks, blood pressure regulatory mechanisms, and immune responses. Isoform-selective pharmacological modulation of NO synthesis has emerged as a new therapeutic strategy for the treatment of diverse clinical conditions associated with NO overproduction. Mechanism-based inactivators (MBIs) represent a class of NOS mechanistic inhibitors that require catalytic turnover to produce irreversible inactivation of the ability of NOS to generate NO. Diverse isoform-selective NOS MBIs have been characterized with respect to their kinetic parameters and chemical mechanisms of inactivation. In studies with isolated and purified NOS isoforms, MBIs produce irreversible inactivation of NOS enzymatic activities. The inactivation process is associated with covalent modification of the NOS active site and proceeds either through heme destruction, its structural alteration, or covalent modification of the NOS protein chain. The behavior of NOS MBIs in intact cells is different from their behavior observed with the isolated NOS isoforms. In cytokine-induced RAW 264.7 macrophages, treatment with MBIs produces a complete loss of cellular NOS synthetic competence and inducible NOS activity. However, following drug removal, cells can recover at least partially in the absence of protein synthesis. In GH3 cells containing the neuronal NOS isoform, calcium transients are too low and abbreviated to allow significant NOS inactivation; hence, the cellular effects of MBIs on the neuronal isoform are almost completely and immediately reversible.

Na(+)/Ca(2+) exchange facilitates Ca(2+)-dependent activation of endothelial nitric-oxide synthase

Recent evidence suggests the expression of a Na(+)/Ca(2+) exchanger (NCX) in vascular endothelial cells. To elucidate the functional role of endothelial NCX, we studied Ca(2+) signaling and Ca(2+)-dependent activation of endothelial nitric-oxide synthase (eNOS) at normal, physiological Na(+) gradients and after loading of endothelial cells with Na(+) ions using the ionophore monensin. Monensin-induced Na(+) loading markedly reduced Ca(2+) entry and, thus, steady-state levels of intracellular free Ca(2+) ([Ca(2+)](i)) in thapsigargin-stimulated endothelial cells due to membrane depolarization. Despite this reduction of overall [Ca(2+)](i), Ca(2+)-dependent activation of eNOS was facilitated as indicated by a pronounced leftward shift of the Ca(2+) concentration response curve in monensin-treated cells. This facilitation of Ca(2+)-dependent activation of eNOS was strictly dependent on the presence of Na(+) ions during treatment of the cells with monensin. Na(+)-induced facilitation of eNOS activation was not due to a direct effect of Na(+) ions on the Ca(2+) sensitivity of the enzyme. Moreover, the effect of Na(+) was not related to Na(+) entry-induced membrane depolarization or suppression of Ca(2+) entry, since neither elevation of extracellular K(+) nor the Ca(2+) entry blocker 1-(beta-[3-(4-methoxyphenyl)-propoxy]-4-methoxyphenethyl)-1H-imidazol e hydrochloride (SK&F 96365) mimicked the effects of Na(+) loading. The effects of monensin were completely blocked by 3', 4'-dichlorobenzamil, a potent and selective inhibitor of NCX, whereas the structural analog amiloride, which barely affects Na(+)/Ca(2+) exchange, was ineffective. Consistent with a pivotal role of Na(+)/Ca(2+) exchange in Ca(2+)-dependent activation of eNOS, an NCX protein was detected in caveolin-rich membrane fractions containing both eNOS and caveolin-1. These results demonstrate for the first time a crucial role of cellular Na(+) gradients in regulation of eNOS activity and suggest that a tight functional interaction between endothelial NCX and eNOS may take place in caveolae.

Autoinhibition of endothelial nitric-oxide synthase. Identification of an electron transfer control element

The primary sequences of the three mammalian nitric- oxide synthase (NOS) isoforms differ by the insertion of a 52-55-amino acid loop into the reductase domains of the endothelial (eNOS) and neuronal (nNOS), but not inducible (iNOS). On the basis of studies of peptide derivatives as inhibitors of.NO formation and calmodulin (CaM) binding (Salerno, J. C., Harris, D. E., Irizarry, K., Patel, B., Morales, A. J., Smith, S. M., Martasek, P., Roman, L. J., Masters, B. S., Jones, C. L., Weissman, B. A., Lane, P., Liu, Q., and Gross, S. S. (1997) J. Biol. Chem. 272, 29769-29777), the insert has been proposed to be an autoinhibitory element. We have examined the role of the insert in its native protein context by deleting the insert from both wild-type eNOS and from chimeras obtained by swapping the reductase domains of the three NOS isoforms. The Ca2+ concentrations required to activate the enzymes decrease significantly when the insert is deleted, consistent with suppression of autoinhibition. Furthermore, removal of the insert greatly enhances the maximal activity of wild-type eNOS, the least active of the three isoforms. Despite the correlation between reductase and overall enzymatic activity for the wild-type and chimeric NOS proteins, the loop-free eNOS still requires CaM to synthesize.NO. However, the reductive activity of the CaM-free, loop-deleted eNOS is enhanced significantly over that of CaM-free wild-type eNOS and approaches the same level as that of CaM-bound wild-type eNOS. Thus, the inhibitory effect of the loop on both the eNOS reductase and. NO-synthesizing activities may have an origin distinct from the loop's inhibitory effects on the binding of CaM and the concomitant activation of the reductase and.NO-synthesizing activities. The eNOS insert not only inhibits activation of the enzyme by CaM but also contributes to the relatively low overall activity of this NOS isoform.

Mammalian nitric oxide synthases

The nitric oxide (NO) synthase family of enzymes generate NO from L-arginine, which acts as a biologic effector molecule in a broad number of settings. This report summarizes some of the current information regarding NO synthase structure-function, reaction mechanism, control of catalysis, and protein interactions.

Chimeric forms of neuronal nitric oxide synthase identify different regions of the reductase domain that are essential for dimerization and activity

Nitric oxide synthase (NOS) is the enzyme responsible for the conversion of L-arginine to L-citrulline and nitric oxide. Dimerization of the enzyme is an absolute requirement for catalytic activity. Each NOS monomer contains an N-terminal heme-binding domain and a C-terminal reductase domain. It is unclear how the reductase domain is involved in controlling dimerization and whether dimer formation alone controls enzyme activity. Our initial studies demonstrated that no dimerization or activity could be detected when the reductase domain of rat neuronal NOS (nNOS) was expressed either separately or in combination with the heme domain. To further evaluate the reductase domain, a set of expression plasmids was created by replacing the reductase domain of nNOS with other electron-transport proteins, thereby creating nNOS chimeric fusion proteins. The rat nNOS heme domain was linked with either cytochrome P450 reductase, adrenodoxin reductase, or the reductase domain from Bacillus megaterium cytochrome P450, BM-3. All the chimeric enzymes retained the ability to dimerize but were unable to metabolize L-arginine (<8% of wildtype activity levels), indicating that dimerization alone is insufficient to produce an active enzyme. Because the greatest regions of homology between electron-transport proteins are in the flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and nicotinamide adenine dinucleotide phosphate (NADPH) binding regions, we produced truncation mutants within the nNOS reductase domain to investigate the role of these sequences in the ability of nNOS to dimerize and to metabolize L-arginine. The results demonstrated that the deletion of the final 56 amino acids or the NADPH-binding region had no effect on dimerization but produced an inactive enzyme. However, when the FAD-binding site (located between amino acids 920 and 1161) was deleted, both activity and dimerization were abolished. These results implicate sequences within the FAD-binding site as essential for nNOS dimerization but sequences within amino acids 1373 to 1429 as essential for activity.

Temporal gradient in shear but not steady shear stress induces PDGF-A and MCP-1 expression in endothelial cells: role of NO, NF kappa B, and egr-1

Three well-defined laminar flow profiles were created to distinguish the influence of a gradient in shear and steady shear on platelet-derived growth factor A (PDGF-A) and monocyte chemoattractant protein-1 (MCP-1) expression in human endothelial cells. The flow profiles (16 dyne/cm2 maximum shear stress) were ramp flow (shear stress smoothly transited at flow onset), step flow (shear stress abruptly applied at flow onset), and impulse flow (shear stress abruptly applied for 3 s only). Ramp flow induced only minor expression of PDGF-A and did not increase MCP-1 expression. Step flow increased PDGF-A and MCP-1 mRNA levels 3- and 2-fold at 1.5 hours, respectively, relative to ramp flow. In contrast, impulse flow increased PDGF-A and MCP-1 expression 6- and 7-fold at 1.5 hours, and these high levels were sustained for at least 4 hours. These results indicate that a temporal gradient in shear (impulse flow and the onset of step flow) and steady shear (ramp flow and the steady component of step flow) stimulates and diminishes the expression of PDGF-A and MCP-1, respectively. NO synthase inhibitor NG-amino-L-arginine (L-NAA) was found to markedly enhance MCP-1 and PDGF-A expression induced by step flow, but decrease their expression induced by impulse flow, in a dose-dependent manner. NO donor spermine-NONOate (SPR/NO) dose-dependently reduced the MCP-1 and PDGF-A expression induced by impulse flow. Moreover, impulse flow was found to stimulate sustained (4 hours) I kappa B-alpha degradation and egr-1 mRNA induction. L-NAA prevented I kappa B-alpha degradation, whereas SPR/NO increased I kappa B-alpha resynthesis 2 hours after impulse flow. Both L-NAA and SPR/NO inhibited the impulse flow inducibility of egr-1 4 hours after the flow stimulation. The results show that both NO induced by steady shear and NO donor inhibit temporal gradient in shear-induced MCP-1 and PDGF-A expression by downregulation of their respective transcription factors NF kappa B and egr-1, whereas NO induced by impulse flow stimulates MCP-1 and PDGF-A expression by upregulation of the transcription factors. The above findings suggest distinct roles of temporal gradient in shear and steady shear in atherogenesis in vivo.

L-Ascorbic acid potentiates nitric oxide synthesis in endothelial cells

Ascorbic acid has been shown to enhance impaired endothelium-dependent vasodilation in patients with atherosclerosis by a mechanism that is thought to involve protection of nitric oxide (NO) from inactivation by free oxygen radicals. The present study in human endothelial cells from umbilical veins and coronary arteries investigates whether L-ascorbic acid additionally affects cellular NO synthesis. Endothelial cells were incubated for 24 h with 0.1-100 microM ascorbic acid and were subsequently stimulated for 15 min with ionomycin (2 microM) or thrombin (1 unit/ml) in the absence of extracellular ascorbate. Ascorbate pretreatment led to a 3-fold increase of the cellular production of NO measured as the formation of its co-product citrulline and as the accumulation of its effector molecule cGMP. The effect was saturated at 100 microM and followed a similar kinetics as seen for the uptake of ascorbate into the cells. The investigation of the precursor molecule L-gulonolactone and of different ascorbic acid derivatives suggests that the enediol structure of ascorbate is essential for its effect on NO synthesis. Ascorbic acid did not induce the expression of the NO synthase (NOS) protein nor enhance the uptake of the NOS substrate L-arginine into endothelial cells. The ascorbic acid effect was minimal when the citrulline formation was measured in cell lysates from ascorbate-pretreated cells in the presence of known cofactors for NOS activity. However, when the cofactor tetrahydrobiopterin was omitted from the assay, a similar potentiating effect of ascorbate pretreatment as seen in intact cells was demonstrated, suggesting that ascorbic acid may either enhance the availability of tetrahydrobiopterin or increase its affinity for the endothelial NOS. Our data suggest that intracellular ascorbic acid enhances NO synthesis in endothelial cells and that this may explain, in part, the beneficial vascular effects of ascorbic acid.

Preferential inhibition of inducible nitric oxide synthase in intact cells by the 4-amino analogue of tetrahydrobiopterin

In the present study we demonstrate that the 4-amino analogue of tetrahydrobiopterin, 2,4-diamino-5,6,7,8-tetrahydro-6-(l-erythro-1, 2-dihydroxypropyl)pteridine (4-amino-H4biopterin) binds with high affinity to recombinant endothelial NO synthase and concomitantly inhibits enzyme activity [IC50 = 14.8 +/- 7.5 microm in the presence of added 5,6,7,8-tetrahydro-l-erythrobiopterin (H4biopterin) 10 microm] as efficiently as previously shown for inducible NO synthase [Mayer, B., Wu, C.Q., Gorren, A.C.F., Pfeiffer, S., Schmidt, K., Clark, P., Stuehr, D.J. & Werner, E.R. (1997) Biochemistry 36, 8422-8427]. In cultured porcine endothelial cells, however, 4-amino-H4biopterin was less effective in inhibiting NO formation (IC50 = 420 +/- 36 microm) as compared with inhibition of the inducible isoform in murine fibroblasts (IC50 = 15 +/- 4.9 microm) and in human DLD-1 adenocarcinoma cells (IC50 = 55 +/- 10.3 microm). In all cells investigated, the inhibitory effect of 4-amino-H4biopterin was markedly enhanced by depletion of intracellular H4biopterin and could be overcome by increasing intracellular H4biopterin concentrations. Endothelial cells contained lower amounts of H4biopterin [5.2 +/- 0.3 pmol.(mg protein)-1] than fibroblasts [19.4 +/- 2.7 pmol.(mg protein)-1] and DLD-1 cells [8.3 +/- 1.1 pmol.(mg protein)-1], so that the selectivity of 4-amino-H4biopterin towards inducible NO synthase was not explained by differences in the H4biopterin levels. Because 4-amino-H4biopterin did not suppress expression of NO synthase in cytokine-treated cells, we suggest that high-affinity binding of the inhibitor during protein expression may be responsible for the preferential inhibition of the inducible isozyme in intact cells.

Superoxide generation from endothelial nitric-oxide synthase. A Ca2+/calmodulin-dependent and tetrahydrobiopterin regulatory process

It has been previously shown that besides synthesizing nitric oxide (NO), neuronal and inducible NO synthase (NOS) generates superoxide (O-2) under conditions of L-arginine depletion. However, there is controversy regarding whether endothelial NOS (eNOS) can also produce O-2. Moreover, the mechanism and control of this process are not fully understood. Therefore, we performed electron paramagnetic resonance spin-trapping experiments to directly measure and characterize the O-2 generation from purified eNOS. With the spin trap 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), prominent signals of O-2 adduct, DMPO-OOH, were detected from eNOS in the absence of added tetrahydrobiopterin (BH4), and these were quenched by superoxide dismutase. This O-2 formation required Ca2+/calmodulin and was blocked by the specific NOS inhibitor N-nitro-L-arginine methyl ester (L-NAME) but not its non-inhibitory enantiomer D-NAME. A parallel process of Ca2+/calmodulin-dependent NADPH oxidation was observed which was also inhibited by L-NAME but not D-NAME. Pretreatment of the enzyme with the heme blockers cyanide or imidazole also prevented O-2 generation. BH4 exerted dose-dependent inhibition of the O-2 signals generated by eNOS. Conversely, in the absence of BH4 L-arginine did not decrease this O-2 generation. Thus, eNOS can also catalyze O-2 formation, and this appears to occur primarily at the heme center of its oxygenase domain. O-2 synthesis from eNOS requires Ca2+/calmodulin and is primarily regulated by BH4 rather than L-arginine.

Interaction between caveolin-1 and the reductase domain of endothelial nitric-oxide synthase. Consequences for catalysis

Endothelial nitric-oxide synthase (eNOS) is targeted to caveoli through interaction with caveolin-1 (cav-1). cav-1 binding to a consensus site in the eNOS oxygenase domain is proposed to antagonize calmodulin (CaM) binding and thereby inhibit eNOS nitric oxide (NO) synthesis. To study the mechanism, we examined how cav-1 scaffolding domain peptide (amino acids 82-101; cav-1P) would affect NO synthesis, NADPH oxidation, cytochrome c reduction, and ferricyanide reduction by full-length eNOS or its isolated oxygenase and reductase domains. Cav-1P equivalently inhibited NO synthesis and NADPH oxidation by full-length eNOS in a manner reversible by CaM but did not affect NADPH-independent NO synthesis by full-length eNOS or its oxygenase domain, indicating inhibition required the reductase domain. Similar concentrations of cav-1P inhibited cytochrome c reduction by full-length eNOS or the reductase domain (amino acids 492-1205) in a CaM-reversible manner, indicating cav-1P interaction with reductase or full-length eNOS are equivalent. Ferricyanide reduction was unaffected by cav-1P in all cases. Immunoblotting showed that full-length eNOS, eNOS oxygenase, and eNOS reductase all bound to an immobilized glutathione S-transferase-cav-1 fusion protein. Thus, cav-1 interacts independently with both oxygenase and reductase domains of eNOS. The reductase interaction occurs independent of a cav-1 binding motif, is CaM-reversible, and is of sufficient affinity to match cav-1P inhibition of NO synthesis by full-length eNOS. We propose that cav-1 binding to eNOS reductase compromises its ability to bind CaM and to donate electrons to the eNOS heme, thereby inhibiting NO synthesis.