Tetrahydrobiopterin, a cofactor for rat cerebellar nitric oxide synthase, does not function as a reactant in the oxygenation of arginine

The Role of the BH4 Cofactor in Nitric Oxide Synthase Activity and Cancer Progression: Two Sides of the Same Coin

Cancer development is associated with abnormal proliferation, genetic instability, cell death resistance, metabolic reprogramming, immunity evasion, and metastasis. These alterations are triggered by genetic and epigenetic alterations in genes that control cell homeostasis. Increased reactive oxygen and nitrogen species (ROS, RNS) induced by different enzymes and reactions with distinct molecules contribute to malignant transformation and tumor progression by modifying DNA, proteins, and lipids, altering their activities. Nitric oxide synthase plays a central role in oncogenic signaling modulation and redox landscape. Overexpression of the three NOS isoforms has been found in innumerous types of cancer contributing to tumor growth and development. Although the main function of NOS is the production of nitric oxide (NO), it can be a source of ROS in some pathological conditions. Decreased tetrahydrobiopterin (BH4) cofactor availability is involved in NOS dysfunction, leading to ROS production and reduced levels of NO. The regulation of NOSs by BH4 in cancer is controversial since BH4 has been reported as a pro-tumoral or an antitumoral molecule. Therefore, in this review, the role of BH4 in the control of NOS activity and its involvement in the capabilities acquired along tumor progression of different cancers was described.

Tetrahydrobioterin (BH4) Pathway: From Metabolism to Neuropsychiatry

Tetrahydrobipterin (BH4) is a pivotal enzymatic cofactor required for the synthesis of serotonin, dopamine and nitric oxide. BH4 is essential for numerous physiological processes at periphery and central levels, such as vascularization, inflammation, glucose homeostasis, regulation of oxidative stress and neurotransmission. BH4 de novo synthesis involves the sequential activation of three enzymes, the major controlling point being GTP cyclohydrolase I (GCH1). Complementary salvage and recycling pathways ensure that BH4 levels are tightly kept within a physiological range in the body. Even if the way of transport of BH4 and its ability to enter the brain after peripheral administration is still controversial, data showed increased levels in the brain after BH4 treatment. Available evidence shows that GCH1 expression and BH4 synthesis are stimulated by immunological factors, notably pro-inflammatory cytokines. Once produced, BH4 can act as an anti- inflammatory molecule and scavenger of free radicals protecting against oxidative stress. At the same time, BH4 is prone to autoxidation, leading to the release of superoxide radicals contributing to inflammatory processes, and to the production of BH2, an inactive form of BH4, reducing its bioavailability. Alterations in BH4 levels have been documented in many pathological situations, including Alzheimer's disease, Parkinson's disease and depression, in which increased oxidative stress, inflammation and alterations in monoaminergic function are described. This review aims at providing an update of the knowledge about metabolism and the role of BH4 in brain function, from preclinical to clinical studies, addressing some therapeutic implications.

Immunobiology of Nitric Oxide and Regulation of Inducible Nitric Oxide Synthase

Nitric oxide (NO) is a bioactive gas that has multiple roles in innate and adaptive immune responses. In macrophages, nitric oxide is produced by inducible nitric oxide synthase upon microbial and cytokine stimulation. It is needed for host defense against pathogens and for immune regulation. This review will summarize the role of NO and iNOS in inflammatory and immune responses and will discuss the regulatory mechanisms that control inducible nitric oxide synthase expression and activity.

Tetrahydrobiopterin redox cycling in nitric oxide synthase: evidence supports a through-heme electron delivery

The nitric oxide synthases (NOS) catalyze a two-step oxidation of l-arginine (Arg) to generate NO. In the first step, O2 activation involves one electron being provided to the heme by an enzyme-bound 6R-tetrahydro-l-biopterin cofactor (H4 B), and the H4 B radical must be reduced back to H4 B in order for NOS to continue catalysis. Although an NADPH-derived electron is used to reduce the H4 B radical, how this occurs is unknown. We hypothesized that the NOS flavoprotein domain might reduce the H4 B radical by utilizing the NOS heme porphyrin as a conduit to deliver the electron. This model predicts that factors influencing NOS heme reduction should also influence the extent and rate of H4 B radical reduction in kind. To test this, we utilized single catalytic turnover and stop-freeze methods, along with electron paramagnetic resonance spectroscopy, to measure the rate and extent of reduction of the 5-methyl-H4 B radical formed in neuronal NOS (nNOS) during Arg hydroxylation. We used several nNOS variants that supported either a slower or faster than normal rate of ferric heme reduction. We found that the rates and extents of nNOS heme reduction correlated well with the rates and extents of 5-methyl-H4 B radical reduction among the various nNOS enzymes. This supports a model where the heme porphyrin transfers an electron from the NOS flavoprotein to the H4 B radical formed during catalysis, revealing that the heme plays a dual role in catalyzing O2 activation or electron transfer at distinct points in the reaction cycle.

Tetrahydrobiopterin in cardiovascular health and disease

Tetrahydrobiopterin (BH4) functions as a cofactor for several important enzyme systems, and considerable evidence implicates BH4 as a key regulator of endothelial nitric oxide synthase (eNOS) in the setting of cardiovascular health and disease. BH4 bioavailability is determined by a balance of enzymatic de novo synthesis and recycling, versus degradation in the setting of oxidative stress. Augmenting vascular BH4 levels by pharmacological supplementation has been shown in experimental studies to enhance NO bioavailability. However, it has become more apparent that the role of BH4 in other enzymatic pathways, including other NOS isoforms and the aromatic amino acid hydroxylases, may have a bearing on important aspects of vascular homeostasis, inflammation, and cardiac function. This article reviews the role of BH4 in cardiovascular development and homeostasis, as well as in pathophysiological processes such as endothelial and vascular dysfunction, atherosclerosis, inflammation, and cardiac hypertrophy. We discuss the therapeutic potential of BH4 in cardiovascular disease states and attempt to address how this modulator of intracellular NO-redox balance may ultimately provide a powerful new treatment for many cardiovascular diseases.

Tetrahydrobiopterin in nitric oxide synthase

SUMMARY

Nitric oxide synthase (NOS) is a critical enzyme for the production of the messenger molecule nitric oxide (NO) from L-arginine. NOS enzymes require tetrahydrobiopterin as a cofactor for NO synthesis. Besides being one of the few enzymes to use this cofactor, the role of tetrahydrobiopterin in NOS catalytic mechanism is different from other enzymes: during the catalytic cycle of NOS, tetrahydrobiopterin forms a radical species that is again reduced, thus effectively regenerating after each NO synthesis cycle. In this review, we summarize our current knowledge about the role of tetrahydrobiopterin in the structure, function, and catalytic mechanism of NOS enzymes.

Tetrahydrobiopterin: biochemistry and pathophysiology

BH4 (6R-L-erythro-5,6,7,8-tetrahydrobiopterin) is an essential cofactor of a set of enzymes that are of central metabolic importance, including four aromatic amino acid hydroxylases, alkylglycerol mono-oxygenase and three NOS (NO synthase) isoenzymes. Consequently, BH4 is present in probably every cell or tissue of higher organisms and plays a key role in a number of biological processes and pathological states associated with monoamine neurotransmitter formation, cardiovascular and endothelial dysfunction, the immune response and pain sensitivity. BH4 is formed de novo from GTP via a sequence of three enzymatic steps carried out by GTP cyclohydrolase I, 6-pyruvoyltetrahydropterin synthase and sepiapterin reductase. An alternative or salvage pathway involves dihydrofolate reductase and may play an essential role in peripheral tissues. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase, except for NOSs, in which the BH4 cofactor undergoes a one-electron redox cycle without the need for additional regeneration enzymes. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I. BH4 biosynthesis is controlled in mammals by hormones and cytokines. BH4 deficiency due to autosomal recessive mutations in all enzymes, except for sepiapterin reductase, has been described as a cause of hyperphenylalaninaemia. A major contributor to vascular dysfunction associated with hypertension, ischaemic reperfusion injury, diabetes and others, appears to be an effect of oxidized BH4, which leads to an increased formation of oxygen-derived radicals instead of NO by decoupled NOS. Furthermore, several neurological diseases have been suggested to be a consequence of restricted cofactor availability, and oral cofactor replacement therapy to stabilize mutant phenylalanine hydroxylase in the BH4-responsive type of hyperphenylalaninaemia has an advantageous effect on pathological phenylalanine levels in patients.

Synthesis and recycling of tetrahydrobiopterin in endothelial function and vascular disease

Nitric oxide, generated by the nitric oxide synthase (NOS) enzymes, plays pivotal roles in cardiovascular homeostasis and in the pathogenesis of cardiovascular disease. The NOS cofactor, tetrahydrobiopterin (BH4), is an important regulator of NOS function, since BH4 is required to maintain enzymatic coupling of L-arginine oxidation, to produce NO. Loss or oxidation of BH4 to 7,8-dihydrobiopterin (BH2) is associated with NOS uncoupling, resulting in the production of superoxide rather than NO. In addition to key roles in folate metabolism, dihydrofolate reductase (DHFR) can 'recycle' BH2, and thus regenerate BH4. It is therefore likely that net BH4 cellular bioavailability reflects the balance between de novo BH4 synthesis, loss of BH4 by oxidation to BH2, and the regeneration of BH4 by DHFR. Recent studies have implicated BH4 recycling in the direct regulation of eNOS uncoupling, showing that inhibition of BH4 recycling using DHFR-specific siRNA and methotrexate treatment leads to eNOS uncoupling in endothelial cells and the hph-1 mouse model of BH4 deficiency, even in the absence of oxidative stress. These studies indicate that not only BH4 level, but the recycling pathways regulating BH4 bioavailability represent potential therapeutic targets and will be discussed in this review.

Tetrahydrobiopterin restores diastolic function and attenuates superoxide production in ovariectomized mRen2.Lewis rats

After oophorectomy, mRen2.Lewis rats exhibit diastolic dysfunction associated with elevated superoxide, increased cardiac neuronal nitric oxide synthase (nNOS) expression, and diminished myocardial tetrahydrobiopterin (BH₄) content, effects that are attenuated with selective nNOS inhibition. BH₄ is an essential cofactor of nNOS catalytic activity leading to nitric oxide production. Therefore, we assessed the effect of 4 wk BH₄ supplementation on diastolic function and left ventricular (LV) remodeling in oophorectomized mRen2.Lewis rats compared with sham-operated controls. Female mRen2.Lewis rats underwent either bilateral ovariectomy (OVX) (n = 19) or sham operation (n = 13) at 4 wk of age. Beginning at 11 wk of age, OVX rats were randomized to receive either BH₄ (10 mg/kg · d) or saline, whereas the sham rats received saline via sc mini-pumps. Loss of ovarian hormones reduced cardiac BH₄ when compared with control hearts; this was associated with impaired myocardial relaxation, augmented filling pressures, increased collagen deposition, and thickened LV walls. Additionally, superoxide production increased and nitric oxide decreased in hearts from OVX compared with sham rats. Chronic BH₄ supplementation after OVX improved diastolic function and attenuated LV remodeling while restoring myocardial nitric oxide release and preventing reactive oxygen species generation. These data indicate that BH₄ supplementation protects against the adverse effects of ovarian hormonal loss on diastolic function and cardiac structure in mRen2.Lewis rats by restoring myocardial NO release and mitigating myocardial O₂⁻ generation. Whether BH₄ supplementation is a therapeutic option for the management of diastolic dysfunction in postmenopausal women will require direct testing in humans.

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

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

Catalytic reduction of a tetrahydrobiopterin radical within nitric-oxide synthase

Nitric-oxide synthases (NOS) are catalytically self-sufficient flavo-heme enzymes that generate NO from arginine (Arg) and display a novel utilization of their tetrahydrobiopterin (H(4)B) cofactor. During Arg hydroxylation, H(4)B acts as a one-electron donor and is then presumed to redox cycle (i.e. be reduced back to H(4)B) within NOS before further catalysis can proceed. Whereas H(4)B radical formation is well characterized, the subsequent presumed radical reduction has not been demonstrated, and its potential mechanisms are unknown. We investigated radical reduction during a single turnover Arg hydroxylation reaction catalyzed by neuronal NOS to document the process, determine its kinetics, and test for involvement of the NOS flavoprotein domain. We utilized a freeze-quench instrument, the biopterin analog 5-methyl-H(4)B, and a method that could separately quantify the flavin and pterin radicals that formed in NOS during the reaction. Our results establish that the NOS flavoprotein domain catalyzes reduction of the biopterin radical following Arg hydroxylation. The reduction is calmodulin-dependent and occurs at a rate that is similar to heme reduction and fast enough to explain H(4)B redox cycling in NOS. These results, in light of existing NOS crystal structures, suggest a "through-heme" mechanism may operate for H(4)B radical reduction in NOS.

Biopterin in the acute phase of hypoxia-ischemia in a neonatal pig model

To clarify the participation of inducible NOS (iNOS) in the hypoxia-ischemia, we examined iNOS and its tetrahydrobiopterin co-factor in the cerebral cortex and plasma in a newborn-piglet model. We also investigated the role of hypothermia in iNOS expression and biopterin production. Male newborn piglets were ventilated 6% oxygen for 45 min. Their common carotid arteries were clamped during hypoxia. Then they were resuscitated with 30% oxygen (HI group). Piglets of the hypothermia group were treated as the HI group and their body was cooled to 35.5 degrees C after hypoxic-ischemic insults. Sham-treated piglets were also reserved. In the HI group, iNOS was present in neurons and macrophages of the cerebral cortex 12h after the insult. The concentrations of nitrite and nitrate were elevated in the cerebral cortex 12h after hypoxic-ischemic insults but the biopterin level was unchanged. The plasma biopterin concentration after the insult (377.9+/-78.7 nM) was five times higher than before the insult (80.1+/-4.3 nM); this level peaked 4h after the insult (604.8+/-200.9 nM) and only slightly decreased after 12h (445.9+/-57.8 nM). In the hypothermia group, no iNOS expression was observed 12h after the insult. The plasma biopterin concentration after the insult (464.2+/-92.3 nM) was similar to that in the HI group, but was suppressed by 4h of hypothermia (229.3+/-106.8 nM). In this study, neuronal iNOS expression and increase of NO production were found in the acute phase of hypoxia-ischemia. Brain biopterin did not increase in hypoxia-ischemia although plasma biopterin was five-fold elevated. The discrepancy may also affect hypoxic-ischemic organ damage.

Estrogen therapy replenishes vascular tetrahydrobiopterin and reduces oxidative stress in ovariectomized rats

OBJECTIVE

We investigated whether the effect of estrogen therapy on vascular endothelial function is mediated through increasing the bioavailability of tetrahydrobiopterin (BH4) and associated antioxidant capacity in ovariectomized (Ovx) rats.

DESIGN

Aortas of sham-operated, Ovx, and Ovx plus estrogen therapy (Ovx + ET) female Sprague-Dawley rats were used to measure vascular reactivity. Plasma levels of nitric oxide (NO) metabolites, total antioxidant capacity, aortic superoxide anion (O2.), and BH4 contents were determined.

RESULTS

Vascular reactivity, assessed on isolated aortic segments, indicated that phenylephrine-induced contraction in the Ovx group was significantly greater than that in the sham and Ovx + ET groups. The vasodilator responses to acetylcholine (10 to 10 M) and L-arginine (L-Arg; 10 M) in the sham and Ovx + ET groups were significantly greater than those in the Ovx group. Pretreatment with BH4 (10 M) enhanced the vasodilator responses to L-Arg in the Ovx group compared with the untreated Ovx group. An inhibitor of BH4 synthesis, 2,4-diamino-6-hydroxypyrimidine (2 mM), significantly attenuated the vasodilator response to L-Arg in the sham and Ovx + ET groups. In addition, Ovx significantly increased O2. production in aortic tissues and decreased plasma NO metabolites levels, whereas ET significantly prevented these effects. Pretreatment with BH4 also significantly decreased aortic O2. production in the Ovx group; both plasma total antioxidant capacity and aortic BH4 contents in the Ovx group decreased significantly compared with those in the sham group, which were also improved by ET. There were no significant differences in the protein expression of endothelial NO synthase in aortas in these groups.

CONCLUSIONS

ET increases the availability of vascular BH4 to attenuate O2. production and restores total antioxidant capacity, leading to improved NO-mediated vasodilation in Ovx rats.

The journey from NADPH-cytochrome P450 oxidoreductase to nitric oxide synthases

This mini-review will reflect the perspective of its author on two fields of research, which have merged as the result of the insights of investigators whose work has influenced both areas immeasurably. It cannot be overlooked, however, that the research activities of many during a period of over five decades have produced the chemical and biological bases for the exciting discoveries now encompassing the cytochromes P450 and their redox partners, and the three isoforms of nitric oxide synthase as they function in their respective biological milieux. Following the remarkable discovery that, indeed, molecular oxygen can be adducted to organic molecules by enzymatic systems and that such processes require a supply of reducing equivalents, it is the purpose of this review to provide a chart, with some of its detours, of the road that followed in the pursuit of interesting biological phenomena involving these two major oxygenation systems. It is not intended to be a balanced review and apologies must be offered in advance to those whose contributions may be overlooked or simply were not directly germane to the development of the author's journey.

Dexamethasone suppresses iNOS yet induces GTPCH and CAT-2 mRNA expression in rat lungs

The in vivo mechanisms by which glucocorticoids inhibit nitric oxide expression await detailed investigation. In cell culture experiments, glucocorticoids have been shown to inhibit inducible nitric oxide synthase (iNOS) formation and activity. Glucocorticoids can inhibit iNOS activity in cultured cells by blocking arginine transport and inhibiting tetrahydrobiopterin biosynthesis. We recently reported that changes in intrapulmonary formation of nitric oxide in endotoxemic rats correspond with changes in transcription of the predominant arginine transporter cationic amino acid transporter (CAT)-2. Realizing that hemorrhagic shock induces nitric oxide overproduction in intact animals, we sought to explore whether glucocorticoids attenuate hemorrhagic shock-induced increases in intrapulmonary nitric oxide formation and whether they might do so by inhibiting the formation of tetrahydrobiopterin, iNOS protein, and CAT-2. We randomly assigned 10 male Sprague-Dawley rats to receive dexamethasone or normal saline. Bleeding the animals to a mean systemic blood pressure of between 40 and 45 mmHg created the hemorrhagic shock. Dexamethasone abrogated the increase in exhaled nitric oxide concentrations caused by hemorrhagic shock. At the end of the experiment, plasma nitrate/nitrite values were lower in the dexamethasone group than in the control group. The iNOS protein concentrations were also lower in the dexamethasone group than in the control group. Dexamethasone decreased the intrapulmonary iNOS mRNA concentrations yet increased both guanosine triphosphate cyclohydrolase I mRNA and CAT-2 mRNA. Our results support the idea that dexamethasone inhibits nitric oxide formation in a manner that is independent of tetrahydrobiopterin and arginine transport yet dependent on downregulation of iNOS mRNA expression.

Tetrahydrobiopterin improves vascular endothelial function in ovariectomized rats

The goal of the present study is to investigate the role of tetrahydrobiopterin (BH4) in the vascular response in ovariectomized rats. Rats were randomly assigned to two groups: (1) sham group: sham-operated female rats, and (2) Ovx group: rats were ovariectomized. Our results have shown that the plasma 17 beta-estradiol levels in the Ovx group at the end of the experiment were significantly lower than in the sham group. Vasoreactivity assessed with intact aortic rings indicated that the phenylephrine-induced vasocontractile response to aortic rings from the Ovx group was greater than that of the sham group. In contrast, the vasodilator responses to acetylcholine and L-arginine (L-Arg) in the sham group were significantly greater than in the Ovx group. Differences in vasoreactivity in denuded aorta between the two groups were not noted. Moreover, exogenous BH4 significantly restored L-Arg-induced vasodilator responses in the Ovx group. However, this improvement effect was not found in the sham group. In addition, there were significant increases in superoxide anion production in aortic tissue and significant decreases in plasma nitric oxide levels in the Ovx group. Furthermore, BH4 contents in the aorta in the Ovx group were significantly decreased compared with the sham group. In conclusion, the present study demonstrates that the impairment of vascular reactivity was found in the ovariectomized rats. The possible mechanism of this defect may have resulted from the deficiency of available BH4. Thus, this study may provide a novel therapeutic strategy for the treatment of postmenopausal cardiovascular disorders.

NO•, CO and•OH Endogenous soluble guanylyl cyclase-activating factors

Several low molecular weight compounds are capable of activating soluble guanylyl cyclase. Recent evidence suggests that some of these are formed under physiological conditions: the nitric oxide radical, carbon monoxide and the hydroxyl radical. Thus, multiple signal transduction pathways appear to exist that form a family of guanylyl cyclase activating factors and thereby regulate the intracellular cyclic guanosine 3',5'-monophosphate level.

Mechanisms of synergistic cytokine-induced nitric oxide production in human alveolar epithelial cells

Nitric oxide (NO) derived from inducible NO synthase (iNOS) at sites of inflammation is closely related to host defense against infection and airway inflammation. Cytokines are known to stimulate NO production in human alveolar epithelial cells in a synergistic (nonlinear or nonadditive) manner. The mechanism of this synergy is not known. We measured the activation of the transcription factor NF-kappaB, the iNOS protein, and NO production in A549 monolayers (human alveolar epithelial cell line) in response to different combinations of IL-1beta, INF-gamma, and TNF-alpha (100 ng/ml), and the cofactors FMN, FAD, and BH4. We found that both IL-1beta and TNF-alpha could independently activate cytosolic NF-kappaB, direct its translocation into the nucleus, and induce iNOS monomer synthesis. In addition, different combinations of cytokines produced synergistic amounts of iNOS monomers. Exogenous BH4 (0.1 microM) had no impact on NO production induced by cytokine combinations that included IL-1beta, but significantly enhanced NO production in the presence of INF-gamma and TNF-alpha, and allowed TNF-alpha independently to produce NO. We conclude that there are at least three mechanisms of synergistic cytokine-induced NO production: (1) the biosynthesis of iNOS monomer due to nonlinear interactions by transcription factors, (2) synergistic cytosolic activation of NF-kappaB, and (3) parallel biosynthesis of BH4 in the presence of cytokine combinations that include IL-1beta.

[Perspectives on tetrahydrobiopterin research]

Tetrahydrobiopterin ((6R)-L-erythro-tetrahydrobiopterin, BH4) is de novo synthesized from GTP. Enzymes involved in its synthesis are the rate limiting enzyme GTP cyclohydrolase I, 6-pyruvoyl tetrahydropterin synthase (PTPS) and sepiapterin reductase. Abnormalities in the metabolism of BH4 have been demonstrated in some diseases affecting the central nervous systems such as atypical phenylketonuria, hereditary progressive dystonia (Segawa's disease). Furthermore, BH4 has been shown to be involved in vascular protection. It is suggested that the dysfunction of endothelial BH4 leads to atherosclerosis. Recently we established BH4-deficient mice by disrupting the PTPS gene to investigate the effects of BH4 depletion on the animals and the involvement of BH4 in regulating biological functions including neural systems. Investigation utilizing this model animal can contribute to the development of new therapeutic strategies toward various diseases involving neurological and vascular systems. Pterin derivatives other than biopterin may also be involved in the regulation of a variety of biological functions. We found that ciliated protozoan Tetrahymena pyriformis synthesizes tetrahydromonapterin, isomer of BH4, and its levels alter according to the progress of the cell cycle. How pterin derivatives are related to the human physiology and diseases is an interesting subject of investigation.

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.

Tetrahydrobiopterin augments arginine transport in rat cardiac myocytes through modulation of CAT-2 mRNA

Tetrahydrobiopterin (BH4) has been shown to be required for dimerization and acquisition of nitric oxide (NO) generating capacity by nitric oxide synthase (NOS). In the present study we have investigated the hypothesis that BH4 may affect NOS activity through a novel mechanism-namely, modulating arginine transport in rat cardiac myocytes. Cardiac myocytes have been previously shown to express cationic amino acid transport proteins (y+ system) CAT-1 and CAT-2. Increasing extracellular BH4 concentrations up to 0.5 mmol/L augments arginine transport in 1 mmol/L arginine media (no BH4, 558 +/- 42 fmol arginine/microg protein/min; 0.1 mmol/L BH4, 580 +/- 11 fmol arginine/microg protein/min; 0.5 mmol/L BH4, 944 +/- 71* fmol arginine/microg protein/min; 1.0 mmol/L BH4, 983+/-84* fmol arginine/microg protein/min, n = 4; *: P <.05 vs no BH4). Treating the cells with lipopolysaccharide (LPS) (10 microg/mL) significantly augmented arginine transport only in the presence of BH4 (no BH4, 600 +/- 33 fmol arginine/microg protein/min; 0.1 mmol/L BH4, 691 +/- 29*dagger fmol arginine/microg protein/min; 0.5 mmol/L BH4, 1123 +/- 32*dagger fmol arginine/microg protein/min; 1.0 mmol/L BH4, 1296 +/- 42*dagger fmol arginine/microg protein/min, n = 4; *: P <.01 vs no BH4, dagger: P <.05 vs no LPS). The administration of biopterin, sodium nitroprusside (NO donor), 2,4-diamino-6-hydroxy-pyrimidine (inhibitor of BH4 synthesis), and sepiapterin (the precursor of de novo synthesis of BH4) to unstimulated cells had no effect on arginine uptake values. Using reverse trancriptase-polymerase chain reaction, we next studied the steady state levels for CAT-1 and CAT-2 mRNA. Incubation with BH4 significantly increased CAT-2 mRNA expression in a concentration-dependent manner in 0.1, 0.5, and 1 mmol/L BH4, respectively. Northern blotting analysis further confirmed this observation. We also found that in the presence of BH4 in these concentrations, CAT-1 mRNA expression was abolished. We suggest that BH4 augments intracellular arginine availability by modulating CAT-2 mRNA expression and suggest that its presence is required for the LPS effect on trans-membrane arginine traffic.

Reactive oxygen species mediate endothelium-dependent relaxations in tetrahydrobiopterin-deficient mice

(6R)-5,6,7,8-Tetrahydro-biopterin (H(4)B) is essential for the catalytic activity of all NO synthases. The hyperphenylalaninemic mouse mutant (hph-1) displays 90% deficiency of the GTP cyclohydrolase I, the rate-limiting enzyme in H(4)B synthesis. A relative shortage of H(4)B may shift the balance between endothelial NO synthase (eNOS)-catalyzed generation of NO and reactive oxygen species. Therefore, the hph-1 mouse represents a unique model to assess the effect of chronic H(4)B deficiency on endothelial function. Aortas from 8-week-old hph-1 and wild-type mice (C57BLxCBA) were compared. H(4)B levels were determined by high-performance liquid chromatography and NO synthase activity by [(3)H]citrulline assay in homogenized tissue. Superoxide production by the chemiluminescence method was measured. Isometric tension was continuously recorded. The intracellular levels of H(4)B as well as constitutive NO synthase activity were significantly lower in hph-1 compared with wild-type mice. Systolic blood pressure was increased in hph-1 mice. However, endothelium-dependent relaxations to acetylcholine were present in both groups and abolished by inhibition of NO synthase with N(G)-nitro-L-arginine methyl ester as well. Only in hph-1 mice were the relaxations inhibited by catalase and enhanced by superoxide dismutase. After incubation with exogenous H(4)B, the differences between the 2 groups disappeared. Our findings demonstrate that H(4)B deficiency leads to eNOS dysfunction with the formation of reactive oxygen species, which become mediators of endothelium-dependent relaxations. A decreased availability of H(4)B may favor an impaired activity of eNOS and thus contribute to the development of vascular diseases.

Electrochemistry of pterin cofactors and inhibitors of nitric oxide synthase

Tetrahydrobiopterin (BH4) is an essential cofactor of nitric oxide synthase (NOS), but its function is not fully understood. Specifically, it is unclear whether BH4 participates directly in electron transfer. We investigated the redox properties of BH4 and several other pteridines with cyclic voltammetry and Osteryoung square wave voltammetry. BH4 was oxidized at a potential of +0.27 V vs normal hydrogen electrode (NHE); the corresponding reductive signal after the reversal of the scan direction was very small. Instead, reduction occurred at a potential of -0.16 V vs NHE; there was no corresponding oxidative signal. These two transitions were interdependent, indicating that the reductive wave at -0.16 V represented the regeneration of BH4 from its product of oxidation at +0.27 V. Similar voltammograms were obtained with tetrahydroneopterin and 6,7-dimethyltetrahydropterin, both of which can substitute for BH4 in NOS catalysis. Completely different voltammograms were obtained with 7,8-dihydrobiopterin, sepiapterin, 2'-deoxysepiapterin, and autoxidized BH4. These 7,8-dihydropterins, which do not sustain NOS catalysis, were oxidized at much higher potentials (+0.82-1.04 V vs NHE), and appreciable reduction did not occur between +1.2 and -0.8 V, in line with the concept of a redox role for BH4 in NOS catalysis. However, the electrochemical properties of the potent pterin-site NOS inhibitor 4-amino-BH4 resembled those of BH4, whereas the active pterin cofactor 5-methyl-BH4 was not re-reduced after oxidation. We conclude that the 2-electron redox cycling of the pterin cofactor between BH4 and quinonoid dihydrobiopterin is not essential for NO synthesis. The data are consistent with 1-electron redox cycling between BH4 and the trihydrobiopterin radical BH3(*).

Expression of GTP cyclohydrolase I in murine locus ceruleus is enhanced by peripheral administration of lipopolysaccharide

Among the enzymes involved in the system for catecholamine biosynthesis, GTP cyclohydrolase I (GCH) contributes to the system as the first and rate-limiting enzyme for the de novo biosynthesis of tetrahydrobiopterin (BH4), which is the cofactor for tyrosine hydroxylase (TH). Therefore, we investigated whether the endotoxemia caused by an intraperitoneal (i.p.) injection of lipopolysaccharide (LPS) can modulate BH4 production in the norepinephrine nuclei, i.e. the locus ceruleus (LC; A6) and central caudal pons (A5), in C3H/HeN mice and whether such a change in BH4, if any, can result in the modification of norepinephrine production in these nuclei. After a 5-microg i.p. injection of LPS, the protein expression of GCH and TH in both nuclei was examined by immunohistochemistry. The staining intensity of GCH-positive cells increased at 6 h, whereas no significant change in the staining intensity of TH-positive cells was detected. Next, we measured the contents of BH4, norepinephrine, and its metabolites 4-hydroxy-3-methoxyphenylglycol (MHPG) and DL-4-hydroxy-3-methoxymandelic acid (VMA) in these nuclei after LPS i.p. injection. The BH4 content increased to a statistically significant level at 2 and 4 h after the injection. The contents of MHPG and VMA also showed a time-course similar to that of BH4. These data can be rationalized to indicate that an increased supply of BH4 in the LC increased TH activity and resulted in an increase in norepinephrine production rate at the site. This is the first report that sheds light on BH4 as a molecule that intervenes during endotoxemia to increase norepinephrine production rate in the LC.

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.

Catalase activity of oxygenase domain of rat neuronal nitric oxide synthase. Evidence for product formation from L-arginine

Nitric oxide synthases (NOSs) catalyze the formation of nitric oxide from L-arginine. We purified the heme containing, tetrahydrobiopterin-free, oxygenase domain of rat neuronal nitric oxide synthase (nNOSox) overexpressed in Escherichia coli. We found catalase activity in nNOSox. This is significant because H(2)O(2) may also be a product of nitric oxide synthases. We found H(2)O(2) assisted product formation from N-hydroxy-L-arginine and even from L-arginine both in the presence and in absence of tetrahydrobiopterin. We propose how heme moiety of the oxygenase domain alone is sufficient to carry out both steps of the NOS catalysis.

Tetrahydrobiopterin biosynthesis, regeneration and functions

Tetrahydrobiopterin (BH(4)) cofactor is essential for various processes, and is present in probably every cell or tissue of higher organisms. BH(4) is required for various enzyme activities, and for less defined functions at the cellular level. The pathway for the de novo biosynthesis of BH(4) from GTP involves GTP cyclohydrolase I, 6-pyruvoyl-tetrahydropterin synthase and sepiapterin reductase. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase. Based on gene cloning, recombinant expression, mutagenesis studies, structural analysis of crystals and NMR studies, reaction mechanisms for the biosynthetic and recycling enzymes were proposed. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I, the expression of which may be under the control of cytokine induction. In the liver at least, activity is inhibited by BH(4), but stimulated by phenylalanine through the GTP cyclohydrolase I feedback regulatory protein. The enzymes that depend on BH(4) are the phenylalanine, tyrosine and tryptophan hydroxylases, the latter two being the rate-limiting enzymes for catecholamine and 5-hydroxytryptamine (serotonin) biosynthesis, all NO synthase isoforms and the glyceryl-ether mono-oxygenase. On a cellular level, BH(4) has been found to be a growth or proliferation factor for Crithidia fasciculata, haemopoietic cells and various mammalian cell lines. In the nervous system, BH(4) is a self-protecting factor for NO, or a general neuroprotecting factor via the NO synthase pathway, and has neurotransmitter-releasing function. With regard to human disease, BH(4) deficiency due to autosomal recessive mutations in all enzymes (except sepiapterin reductase) have been described as a cause of hyperphenylalaninaemia. Furthermore, several neurological diseases, including Dopa-responsive dystonia, but also Alzheimer's disease, Parkinson's disease, autism and depression, have been suggested to be a consequence of restricted cofactor availability.

Tetrahydrobiopterin biosynthesis, regeneration and functions

Tetrahydrobiopterin (BH(4)) cofactor is essential for various processes, and is present in probably every cell or tissue of higher organisms. BH(4) is required for various enzyme activities, and for less defined functions at the cellular level. The pathway for the de novo biosynthesis of BH(4) from GTP involves GTP cyclohydrolase I, 6-pyruvoyl-tetrahydropterin synthase and sepiapterin reductase. Cofactor regeneration requires pterin-4a-carbinolamine dehydratase and dihydropteridine reductase. Based on gene cloning, recombinant expression, mutagenesis studies, structural analysis of crystals and NMR studies, reaction mechanisms for the biosynthetic and recycling enzymes were proposed. With regard to the regulation of cofactor biosynthesis, the major controlling point is GTP cyclohydrolase I, the expression of which may be under the control of cytokine induction. In the liver at least, activity is inhibited by BH(4), but stimulated by phenylalanine through the GTP cyclohydrolase I feedback regulatory protein. The enzymes that depend on BH(4) are the phenylalanine, tyrosine and tryptophan hydroxylases, the latter two being the rate-limiting enzymes for catecholamine and 5-hydroxytryptamine (serotonin) biosynthesis, all NO synthase isoforms and the glyceryl-ether mono-oxygenase. On a cellular level, BH(4) has been found to be a growth or proliferation factor for Crithidia fasciculata, haemopoietic cells and various mammalian cell lines. In the nervous system, BH(4) is a self-protecting factor for NO, or a general neuroprotecting factor via the NO synthase pathway, and has neurotransmitter-releasing function. With regard to human disease, BH(4) deficiency due to autosomal recessive mutations in all enzymes (except sepiapterin reductase) have been described as a cause of hyperphenylalaninaemia. Furthermore, several neurological diseases, including Dopa-responsive dystonia, but also Alzheimer's disease, Parkinson's disease, autism and depression, have been suggested to be a consequence of restricted cofactor availability.

Allosteric regulation of neuronal nitric oxide synthase by tetrahydrobiopterin and suppression of auto-damaging superoxide

The underlying mechanisms regulating the activity of the family of homodimeric nitric oxide synthases (NOSs) and, in particular, the requirement for (6R)-5,6,7,8-tetrahydro-L-biopterin (H(4)Bip) are not fully understood. Here we have investigated possible allosteric and stabilizing effects of H(4)Bip on neuronal NOS (NOS-I) during the conversion of substrate, L-arginine, into L-citrulline and nitric oxide. Indeed, in kinetic studies dual allosteric interactions between L-arginine and H(4)Bip activated recombinant human NOS-I to increase L-arginine turnover. Consistent with this was the observation that H(4)Bip, but not the pterin-based NOS inhibitor 2-amino-4,6-dioxo-3,4,5,6,8,8a,9,10-octahydrooxazolo[1, 2-f]-pteridine (PHS-32), caused an L-arginine-dependent increase in the haem Soret band, indicating an increase in substrate binding to recombinant human NOS-I. Conversely, L-arginine was observed to increase in a concentration-dependent manner H(4)Bip binding to pig brain NOS-I. Secondly, we investigated the stabilization of NOS quaternary structure by H(4)Bip in relation to uncoupled catalysis. Under catalytic assay conditions and in the absence of H(4)Bip, dimeric recombinant human NOS-I dissociated into inactive monomers. Monomerization was related to the uncoupling of reductive oxygen activation, because it was inhibited by both superoxide dismutase and the inhibitor N(omega)-nitro-L-arginine. Importantly, H(4)Bip was found to react chemically with superoxide (O(2)(-.)) and enzyme-bound H(4)Bip was consumed under O(2)(-.)-generating conditions in the absence of substrate. These results suggest that H(4)Bip allosterically activates NOS-I and stabilizes quaternary structure by a novel mechanism involving the direct interception of auto-damaging O(2)(-.).

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.

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 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.

Involvement of nitric oxide and biopterin in proinflammatory cytokine-induced apoptotic cell death in mouse osteoblastic cell line MC3T3-E1

We previously demonstrated that the addition of proinflammatory cytokines (tumor necrosis factor-alpha, interleukin-1beta, and interferon-gamma) caused induction of mRNAs for inducible nitric oxide (NO) synthase and GTP cyclohydrolase I, a rate-limiting enzyme for 5,6,7,8-tetrahydrobiopterin (BH4) biosynthesis, and produced their end-products, NO and BH4, in osteoblastic cells. In the present study, we examined whether NO and BH4, biologically active substances produced in response to proinflammatory cytokines, are involved in the effect of these cytokines on cell viability and apoptotic cell death involving DNA fragmentation. Cytokines as well as S-nitroso-N-acetyl-d,l-penicillamine, an NO generator, decreased cell viability, whereas sepiapterin, which was converted intracellularly to BH4, increased it. The examination of cytotoxicity measured in terms of lactate dehydrogenase release and apoptotic cell death assessed by flow cytometric analysis showed that cytokine-induced reduction of cell viability may be based upon cell death by apoptosis, but not lytic death as in necrosis. In the presence of sepiapterin, cytokine treatment resulted in a statistically pronounced reduction in the amount of DNA fragmentation. Furthermore, this fragmentation could be blocked by 2-(4-carboxy-phenyl)-4,4,5,5-tetramethylimidazole-1-oxyl 3-oxide, an NO scavenger. These results suggest that cytokine-induced apoptotic cell death is attributed to NO and is protected by BH4, and that osteoblastic cells in response to proinflammatory cytokines operate both a stimulatory process resulting in NO production and an inhibitory one resulting in BH4 production for apoptotic cell death. Cytokine-induced apoptotic cell death may be a consequence of the predominance of the stimulatory process over the inhibitory process.

Modulation of the remote heme site geometry of recombinant mouse neuronal nitric-oxide synthase by the N-terminal hook region

The role of two essential residues at the N-terminal hook region of neuronal nitric-oxide synthase (nNOS) in nitric-oxide synthase activity was investigated. Full-length mouse nNOS proteins containing single-point mutations of Thr-315 and Asp-314 to alanine were produced in the Escherichia coli and baculovirus-insect cell expression systems. The molecular properties of the mutant proteins were analyzed in detail by biochemical, optical, and electron paramagnetic resonance spectroscopic techniques and compared with those of the wild-type enzyme. Replacement of Asp-314 by Ala altered the geometry around the heme site and the substrate-binding pocket of the heme domain and abrogated the ability of nNOS to form catalytically active dimers. Replacement of Thr-315 by Ala reduced the protein stability and altered the geometry around the heme site, especially in the absence of bound (6R)-5,6,7, 8-tetrahydro-L-biopterin cofactor. These results suggest that Asp-314 and Thr-315 both play critical structural roles in stabilizing the heme domain and subunit interactions in mouse nNOS.

Regulation of pteridine-requiring enzymes by the cofactor tetrahydrobiopterin

Tetrahydrobiopterin (BH4) is synthesized from guanosine triphosphate (GTP) by GTP cyclohydrolase I (GCH), 6-pyruvoyltetrahydropterin synthase (PTS), and sepiapterin reductase (SPD). GCH is the rate-limiting enzyme. BH4 is a cofactor for three pteridine-requiring monooxygenases that hydroxylate aromatic L-amino acids, i.e., tyrosine hydroxylase (TH), tryptophan hydroxylase (TPH), and phenylalanine hydroxylase (PAH), as well as for nitric oxide synthase (NOS). The intracellular concentrations of BH4, which are mainly determined by GCH activity, may regulate the activity of TH (an enzyme-synthesizing catecholamines from tyrosine), TPH (an enzyme-synthesizing serotonin and melatonin from tryptophan), PAH (an enzyme required for complete degradation of phenylalanine to tyrosine, finally to CO2 + H2O), and also the activity of NOS (an enzyme forming NO from arginine), Dominantly inherited hereditary progressive dystonia (HPD), also termed DOPA-responsive dystonia (DRD) or Segawa's disease, is a dopamine deficiency in the nigrostriatal dopamine neurons, and is caused by mutations of one allele of the GCH gene. GCH activity and BH4 concentrations in HPD/DRD are estimated to be 2-20% of the normal value. By contrast, recessively inherited GCH deficiency is caused by mutations of both alleles of the GCH gene, and the GCH activity and BH4 concentrations are undetectable. The phenotypes of recessive GCH deficiency are severe and complex, such as hyperphenylalaninemia, muscle hypotonia, epilepsy, and fever episode, and may be caused by deficiencies of various neurotransmitters, including dopamine, norepinephrine, serotonin, and NO. The biosynthesis of dopamine, norepinephrine, epinephrine, serotonin, melatonin, and probably NO by individual pteridine-requiring enzymes may be differentially regulated by the intracellular concentration of BH4, which is mainly determined by GCH activity. Dopamine biosynthesis in different groups of dopamine neurons may be differentially regulated by TH activity, depending on intracellular BH4 concentrations and GCH activity. The nigrostriatal dopamine neurons may be most susceptible to a partial decrease in BH4, causing dopamine deficiency in the striatum and the HPD/DRD phenotype.

Anti-pterins as tools to characterize the function of tetrahydrobiopterin in NO synthase

Nitric oxide synthases (NOS) are homodimeric enzymes that NADPH-dependently convert L-arginine to nitric oxide and L-citrulline. Interestingly, all NOS also require (6R)-5,6,7, 8-tetrahydro-L-biopterin (H4Bip) for maximal activity although the mechanism is not fully understood. Basal NOS activity, i.e. that in the absence of exogenous H4Bip, has been attributed to enzyme-associated H4Bip. To elucidate further H4Bip function in purified NOS, we developed two types of pterin-based NOS inhibitors, termed anti-pterins. In contrast to type II anti-pterins, type I anti-pterins specifically displaced enzyme-associated H4Bip and inhibited H4Bip-stimulated NOS activity in a fully competitive manner but, surprisingly, had no effect on basal NOS activity. Moreover, for a number of different NOS preparations basal activity (percent of Vmax) was frequently higher than the percentage of pterin saturation and was not affected by preincubation of enzyme with H4Bip. Thus, basal NOS activity appeared to be independent of enzyme-associated H4Bip. The lack of intrinsic 4a-pterincarbinolamine dehydratase activity argued against classical H4Bip redox cycling in NOS. Rather, H4Bip was required for both maximal activity and stability of NOS by binding to the oxygenase/dimerization domain and preventing monomerization and inactivation during L-arginine turnover. Since anti-pterins were also effective in intact cells, they may become useful in modulating states of pathologically high nitric oxide formation.

Effects of transition metals on nitric oxide synthase catalysis

The biosynthesis of nitric oxide (NO) by the enzyme NO synthase (NOS) proceeds by the hydroxylation of L-arginine to form NG-hydroxy-L-arginine followed by the conversion of NG-hydroxy-L-arginine to L-citrulline and NO. The previously identified requirements of this relatively complicated reaction include several protein-bound cofactors: cytochrome P450-type heme, flavin mononucleotide (FMN), flavin adenine dinucleotide (FAD), and tetrahydrobiopterin (H4B). In addition to L-arginine, NOS also requires the substrates NADPH and molecular oxygen. The role of H4B in NOS catalysis has long been a subject of debate and uncertainty fueled, in part, by the failure to detect any dependence of the NOS reaction on nonheme iron, a cofactor integral to catalysis in every other H4B-dependent enzyme. Here we report the ability of NOS to bind transition metals stoichiometrically, and demonstrate that the rate of catalysis is enhanced by nonheme iron. We also show that other divalent transition metals, including Cu, Zn, Co, and Ni, inhibit NOS catalysis. Also, the addition of Cu2+ to NOS inhibits heme reduction, whereas the addition of Fe2+ does not. Overall, the results appear to connect NOS to the known H4B/nonheme iron-dependent hydroxylases, and suggest a similar, if not identical, step in the NOS reaction mechanism.

Increased inducible nitric oxide synthase protein but limited nitric oxide formation occurs in astrocytes of the hph-1 (tetrahydrobiopterin deficient) mouse

It has been suggested that decreased tetrahydrobiopterin (BH4) availability may be a useful tool for limiting excessive nitric oxide (NO) formation. In order to test this hypothesis we utilised cultured astrocytes derived from the brain of the hph-1 (BH4 deficient) mouse. In response to treatment with lipopolysaccharide and interferon-gamma (LPS/gammaIFN) levels of BH4 doubled in both wild type and hph-1 astrocytes. However, levels of BH4 in hph-1 astrocytes remained only 25% of the wild type astrocytes. Nitric oxide formation, measured with an NO-electrode, was 45% less from LPS/gammaIFN stimulated hph-1 astrocytes compared with wild type stimulated astrocytes. In contrast, iNOS specific activity and iNOS protein were enhanced in hph-1 stimulated astrocytes by 40 and 60%, respectively when compared with wild type. In conclusion it appears that whilst a decrease in BH4 may limit NO release per se, the possibility and consequences of long term 'over' induction of iNOS protein requires further consideration.

The regulation of dopamine release from striatum slices by tetrahydrobiopterin and L-arginine-derived nitric oxide

The regulation of dopamine release by 6(R)-tetrahydrobiopterin (BH4) and l-arginine-derived nitric oxide was examined by using a method of superfusion of rat striatum slices in vitro. l-Arginine, which can produce nitric oxide (NO) through the action of NO synthase, induces a concentration-dependent increase of [3H] dopamine release in the superfusate of striatum slices. Pretreatment with inhibitors of NO synthase or with inhibitors of BH4 synthesis diminishes the increase of [3H] dopamine release mediated by arginine. This increase is almost completely restored following repletion of intracellular BH4 levels by incubation of the slices with 7, 8-dihydrobiopterin. Adding exogenous BH4 directly to the superfusion fluid leads to a massive increase in [3H] dopamine release which can be inhibited 75% by superoxide dismutase and catalase, but is not inhibited by NG-nitro-arginine, a NO synthase inhibitor, or alpha-methyl-p-tyrosine, a tyrosine hydroxylase inhibitor. The increase of intracellular BH4 concentration by dihydrobiopterin administration causes a small increase of dopamine release which can be partially diminished by NG-nitro-arginine or alpha-methyl-p-tyrosine. It is suggested that the increase of dopamine release stimulated by an enhancement of intracellular BH4 is dependent on its cofactor activity with NO synthase and tyrosine hydroxylase. This study has also demonstrated that BH4 is a regulator of NO-mediated dopamine release in the striatum. Published by Elsevier Science B.V.

Coexpression of GTP cyclohydrolase I and inducible nitric oxide synthase mRNAs in mouse osteoblastic cells activated by proinflammatory cytokines

Proinflammatory cytokines, a combination of IL-1beta, TNF-alpha, and IFN-gamma, caused mRNA expression of GTP cyclohydrolase I (GTP-CH), the rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis, and of inducible nitric oxide synthase (iNOS) in a well-characterized osteoblastic clone MC3T3-E1 cell line. We found the expression of the GTP-CH gene in osteoblasts for the first time. The expression of GTP-CH and iNOS mRNAs was found to be maximal at 3 and 9 h, respectively. The expression of both genes elicited increases in BH4 and NO levels. Pharmacological studies using 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP-CH activity, showed that BH4 is involved in the activity of iNOS, but not in the induction of iNOS mRNA. The results using an inhibitor of nuclear factor (NF)-kappaB and activating protein-1 (AP-1) activation suggested that coinduction of the two genes in response to cytokines occurred via activation of NF-kappaB and AP-1. In MC3T3-E1 cells BH4 and sepiapterin, producing BH4, could protect against apoptosis, i.e. the degradation of nuclear DNA in the cells, induced by NO derived from S-nitroso-N-acetyl-D-L-penicillamine. These results suggest that the induction of BH4 together with NO by proinflammatory cytokines could protect against NO-induced apoptosis in MC3T3-E1 cells.

Structure of nitric oxide synthase oxygenase dimer with pterin and substrate

Crystal structures of the murine cytokine-inducible nitric oxide synthase oxygenase dimer with active-center water molecules, the substrate L-arginine (L-Arg), or product analog thiocitrulline reveal how dimerization, cofactor tetrahydrobiopterin, and L-Arg binding complete the catalytic center for synthesis of the essential biological signal and cytotoxin nitric oxide. Pterin binding refolds the central interface region, recruits new structural elements, creates a 30 angstrom deep active-center channel, and causes a 35 degrees helical tilt to expose a heme edge and the adjacent residue tryptophan-366 for likely reductase domain interactions and caveolin inhibition. Heme propionate interactions with pterin and L-Arg suggest that pterin has electronic influences on heme-bound oxygen. L-Arginine binds to glutamic acid-371 and stacks with heme in an otherwise hydrophobic pocket to aid activation of heme-bound oxygen by direct proton donation and thereby differentiate the two chemical steps of nitric oxide synthesis.

Localization of GTP cyclohydrolase in monoaminergic but not nitric oxide-producing cells

The first and rate-limiting enzyme in tetrahydrobiopterin (BH4) biosynthesis is GTP cyclohydrolase (GTPCH). BH4 serves as the essential cofactor for aromatic L-amino acid hydroxylases, such as tyrosine hydroxylase (TH) and tryptophan hydroxylase (TPH), as well as for nitric oxide synthase (NOS). We hypothesized that to provide access to the cofactor, a close association exists between BH4-synthesizing and BH4-dependent enzymes, and we determined the relationship among GTPCH, neuronal NOS (nNOS), and TH in rat brain and adrenal gland using immunohistochemistry and in situ hybridization. Analyses of adjacent sections revealed specific localization of GTPCH in TH-containing cells of the substantia nigra, ventral tegmental area, hypothalamus, locus ceruleus, and adrenal medulla, and also in TPH-containing cells of the dorsal raphe nucleus and pineal gland. Thus, BH4 can be synthesized in all monoaminergic cells and is readily available for the enzymes requiring it. In contrast, analysis of adjacent sections showed that nNOS was not colocalized with GTPCH. Scattered nNOS-positive cells were found in the cortex, striatum, cerebellum, and olfactory bulb, all areas that receive monoaminergic innervation. The absence of GTPCH in nNOS cells suggests that nitric oxide-producing cells may either obtain biopterin from monoamine-containing processes which terminate in close proximity, or take up biopterin released into the blood. Double labelling of the same section for TH and nNOS revealed the TH nerve terminals connecting with the nNOS-positive cell bodies, suggesting the possibility that the BH4-containing nerve terminals may directly donate this cofactor to the nNOS-containing cells.

Pulsatile stretch stimulates superoxide production in human aortic endothelial cells

BACKGROUND

Free radicals such as superoxide and nitric oxide (NO) play a key role in the pathophysiology of atherosclerosis. Mechanical forces such as pulsatile stretch may be involved in free radical production. We studied superoxide production by pulsatile stretch in human endothelial cells.

METHODS AND RESULTS

Human cultured aortic endothelial cells were exposed to pulsatile stretch up to 24 hours, and superoxide production was examined. Short-term stretch for 1 hour (10% average elongation, 50 cycles per minute) increased superoxide production 2.2-fold. This effect was reduced by diphenyleneiodonium chloride, an NADPH oxidase inhibitor, but not by the xanthine oxidase inhibitor oxypurinol or the cyclooxygenase inhibitor indomethacin. Prolonged stretch up to 6 hours increased superoxide production, but it returned to near the control level after 24 hours of stretch. However, after blockade of NO production, 24 hours of stretch did increase superoxide production 2.4-fold compared with 24 hours of stretch alone. Moreover, 24-hour stretch doubled NO synthase (NOS) (III) protein and mRNA expression. The tetrahydrobiopterin synthesis inhibitor 2,4-diamino-6-hydroxypyrimidine had no effect on unstretched cells but doubled superoxide production compared with 24-hour stretch alone; this increase was halved by cotreatment with 6-methyl-5,6,7,8-tetrahydropterine, a lipid-soluble form of tetrahydrobiopterine.

CONCLUSIONS

Short-term stretch increased superoxide production from human aortic endothelial cells via NADPH oxidase and NOS (III), whereas prolonged stretch increased both superoxide and NO production. The increase in NOS (III) protein with prolonged stretch acts as a scavenger mechanism whereby NO inactivates superoxide. Tetrahydrobiopterin determines the balance of superoxide and NO production from NOS (III) after prolonged stretch in which NOS (III) level is upregulated.