Stimulation of human nitric oxide synthase by tetrahydrobiopterin and selective binding of the cofactor

Tetrahydrobiopterin treatment in phenylketonuria: A repurposing approach

In phenylketonuria (PKU) patients, early diagnosis by neonatal screening and immediate institution of a phenylalanine-restricted diet can prevent severe intellectual impairment. Nevertheless, outcome remains suboptimal in some patients asking for additional treatment strategies. Tetrahydrobiopterin (BH4 ) could be one of those treatment options, as it may not only increase residual phenylalanine hydroxylase activity in BH4 -responsive PKU patients, but possibly also directly improves neurocognitive functioning in both BH4 -responsive and BH4 -unresponsive PKU patients. In the present review, we aim to further define the theoretical working mechanisms by which BH4 might directly influence neurocognitive functioning in PKU having passed the blood-brain barrier. Further research should investigate which of these mechanisms are actually involved, and should contribute to the development of an optimal BH4 treatment regimen to directly improve neurocognitive functioning in PKU. Such possible repurposing approach of BH4 treatment in PKU may improve neuropsychological outcome and mental health in both BH4 -responsive and BH4 -unresponsive PKU patients.

Nitric oxide synthase isoenzyme expression and activity in peripheral lung tissue of patients with chronic obstructive pulmonary disease

RATIONALE

Nitric oxide (NO) is increased in the lung periphery of patients with chronic obstructive pulmonary disease (COPD). However, expression of the NO synthase(s) responsible for elevated NO has not been identified in the peripheral lung tissue of patients with COPD of varying severity.

OBJECTIVES

METHODS

Protein and mRNA expression of nitric oxide synthase type I (neuronal NOS [nNOS]), type II (inducible NOS [iNOS]), and type III (endothelial NOS [eNOS]) were quantified by Western blotting and reverse transcription-polymerase chain reaction, respectively, in specimens of surgically resected lung tissue from nonsmoker control subjects, patients with COPD of varying severity, and smokers without COPD, and in a lung epithelial cell line (A549). The effects of nitrative/oxidative stress on NOS expression and activity were also evaluated in vitro in A549 cells. nNOS nitration was quantified by immunoprecipitation and dimerization of nNOS was detected by low-temperature SDS-PAGE/Western blot in the presence of the peroxynitrite generator, 3-morpholinosydnonimine-N-ethylcarbamide (SIN1), in vitro and in vivo.

MEASUREMENTS AND MAIN RESULTS

Lung tissue from patients with severe and very severe COPD had graded increases in nNOS (mRNA and protein) compared with nonsmokers and normal smokers. Hydrogen peroxide (H(2)O(2)) and SIN1 as well as the cytokine mixture (IFN-gamma, IL-1beta, and tumor necrosis factor-alpha) increased mRNA expression and activity of nNOS in A549 cells in a concentration-dependent manner compared with nontreated cells. Tyrosine nitration resulted in an increase in nNOS activity in vitro, but did not affect its dimerization.

CONCLUSIONS

Patients with COPD have a significant increase in nNOS expression and activity that reflects the severity of the disease and may be secondary to oxidative stress.

Cerebrospinal fluid nitric oxide levels in subacute sclerosing panencephalitis

Oxidative damage plays a role in neurodegenerative diseases. Levels of cerebrospinal fluid nitrite and nitrate levels (oxidation products that provide an indirect estimation of nitric oxide) were investigated in relation to clinical and laboratory features in subacute sclerosing panencephalitis (n = 47) and age-matched control (n = 43) groups. Significantly decreased levels of nitrite (median, 4.91 micromol/L) and nitrate (median, 6.14 micromol/L) were found in the patients. Nitrite and nitrate levels did not correlate with clinical or laboratory findings, except for presence of myoclonus. Cerebrospinal fluid nitrite levels of subacute sclerosing panencephalitis patients without myoclonic jerks were significantly higher than in those with myoclonus (median, 15.63 vs 4.34 micromol/L, respectively). The higher levels of nitrite in these patients can be explained by short disease duration and early stages of disease. Nitrate levels in subacute sclerosing panencephalitis patients with myoclonus (median, 9.26 micromol/L) were higher than in those without myoclonus (median, 4.25 micromol/L). Microbleeding resulting in conversion of nitrite to nitrate and increased production of superoxide can be suggested as possible mechanisms underlying these findings.

Novel approaches to improving endothelium-dependent nitric oxide-mediated vasodilatation

Endothelial dysfunction, which is defined by decreased endothelium-dependent vasodilatation, is associated with an increased number of cardiovascular events. Nitric oxide (NO) bioavailability is reduced by altered endothelial signal transduction or increased formation of radical oxygen species reacting with NO. Endothelial dysfunction is therapeutically reversible and physical exercise, calcium channel blockers, angiotensin converting enzyme inhibitors, and angiotensin receptor antagonists improve flow-evoked endothelium-dependent vasodilation in patients with hypertension and diabetes. We have investigated three different approaches, with the aim of correcting endothelial dysfunction in cardiovascular disease. Thus, (1) we evaluated the effect of a cell permeable superoxide dismutase mimetic, tempol, on endothelial dysfunction in small arteries exposed to high pressure, (2) investigated the endothelial signal transduction pathways involved in vasorelaxation and NO release induced by an olive oil component, oleanolic acid, and (3) investigated the role of calcium-activated K channels in the release of NO induced by receptor activation. Tempol increases endothelium-dependent vasodilatation in arteries from hypertensive animals most likely through the lowering of radical oxygen species, but other mechanisms also appear to contribute to the effect. While oleanolic acid leads to the release of NO by calcium-independent phosphorylation of endothelial NO synthase, endothelial calcium-activated K channels and an influx of calcium play an important role in G-protein coupled receptor-evoked release of NO. Thus, all three approaches increase bioavailability of NO in the vascular wall, but it remains to be addressed whether these actions have any direct benefit at a clinical level.

(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.

GTP cyclohydrolase feedback regulatory protein controls cofactor 6-tetrahydrobiopterin synthesis in the cytosol and in the nucleus of epidermal keratinocytes and melanocytes

(6R)-L-erythro 5,6,7,8 tetrahydrobiopterin (6BH4) is crucial in the hydroxylation of L-phenylalanine-, L-tyrosine-, and L-tryptophan-regulating catecholamine and serotonin synthesis as well as tyrosinase in melanogenesis. The rate-limiting step of 6BH4 de novo synthesis is controlled by guanosine triphosphate (GTP) cyclohydrolase I (GTPCHI) and its feedback regulatory protein (GFRP), where binding of L-phenylalanine to GFRP increases enzyme activities, while 6BH4 exerts the opposite effect. Earlier it was demonstrated that the human epidermis holds the full capacity for autocrine 6BH4 de novo synthesis and recycling. However, besides the expression of epidermal mRNA for GFRP, the presence of a functioning GFRP feedback has never been shown. Therefore, it was tempting to investigate whether this important mechanism is present in epidermal cells. Our results identified indeed a functioning GFRP/GTPCHI axis in epidermal keratinocytes and melanocytes in the cytosol, adding the missing link for 6BH4 de novo synthesis which in turn controls cofactor supply for catecholamine and serotonin biosynthesis as well as melanogenesis in the human epidermis. Moreover, GFRP expression and GTPCHI activities have been found in the nucleus of both cell types. The significance of this result warrants further investigation.

Cellular basis of endothelial dysfunction in small mesenteric arteries from spontaneously diabetic (db/db -/-) mice: role of decreased tetrahydrobiopterin bioavailability

1. Endothelium-dependent and -independent regulation of vascular tone in small mesenteric arteries (SMA) from control (db/db +/?) and diabetic (db/db -/-) mice was compared. 2. Phenylephrine-induced maximum contraction, but not sensitivity, of SMA in db/db -/- compared to db/db +/? was enhanced. 3. Acetylcholine (ACh), but not sodium nitroprusside (SNP), -induced relaxation was reduced in SMA from db/db -/- compared to db/db +/?. 4. ACh-induced relaxation of SMA was inhibited by a combination of N(omega)-nitro-L-arginine and indomethacin in db/db +/?, but not in db/db -/-. 5. Acute incubation of SMA with tetrahydrobiopterin (BH(4), 10 microM) and sepiapterin (100 microM) enhanced ACh-induced relaxation in SMA from db/db -/-, but not from db/db +/? 2,4-diamino-6-hydroxypyrimidine, an inhibitor of GTP cyclohydrolase I, (10 mM), impaired the sensitivity of SMA from db/db +/? to ACh, which was restored by co-incubation with BH(4) (10 microM). 6. BH(4) and superoxide dismutase (SOD, 150 u ml(-1)), either alone or in combination, had no effect on either ACh or SNP-induced relaxation in SMA from eNOS -/- mice. 7. Incubation of SMA with SOD (150 iu ml(-1)), catalase (200 iu ml(-1)) and L-arginine (1 mM) had no effect on ACh-induced relaxation of SMA. However, the combination of polyethylene glycol-SOD (200 iu ml(-1)) and catalase (80 u ml(-1)) improved the sensitivity of ACh-induced relaxation in db/db -/-, but not in db/db +/?. 8. These data suggest that increased production of superoxide anions and decreased availability of BH(4) result in an 'uncoupling' of nitric oxide synthase and endothelial dysfunction in SMA from db/db -/- mice.

Pathogenic theories and intrathecal analysis of the sporadic form of Alzheimer's disease

Alzheimer's disease (AD) is an age-dependent dementia characterized by progressive loss of cognitive functions and by characteristic pathological changes in the brain: the formation of aggregates extracellularly by beta-amyloid (Abeta) peptide and intracellularly by tau proteins. The disease presents several major diagnostic difficulties: (1) AD develops slowly; (2) analysis of damaged brain tissues is difficult, requiring a biopsy which poses ethical problems; (3) no biochemical markers are available for the diagnosis and monitoring of the disease progression. Since the cerebrospinal fluid (CSF) is in contact with the extracellular space of the brain, many studies have tried to correlate the levels of the intrathecal peptides and amino acids and the development of dementia. The present review analyzes the main results of intrathecal content analyses in light of pathogenic theories proposed to explain the damage associated with AD and observed in the brain of patients by postmortem examination.

Effects of tetrahydrobiopterin on ataxia in Machado-Joseph disease may be based upon the theory of 'cerebellar long-term depression'

We previously verified the effectiveness of tetrahydrobiopterin (BH4) on the ataxia in Machado-Joseph disease (MJD; SCA3 [spinocerebellar ataxia type 3]) as one of the most common types of dominantly inherited spinocerebellar ataxias. We hypothesized as to the pharmacological mechanism of BH4 that on the basis of 'cerebellar long-term depression' theory, BH4 may exert its actions at the levels of soluble guanylate cyclase in the Purkinje cells and of nitric oxide synthase in the granule cells. If cerebellar long-term depression is the case as the theoretical basis of BH4, it will open a new page of therapeutic strategy for spinocerebellar ataxias.

Parallel induction of nitric oxide and tetrahydrobiopterin synthesis in alveolar macrophages

BACKGROUND

Nitric oxide (NO) and an essential cofactor for both constitutive and inducible NO synthase (NOS) activity, tetrahydrobiopterin (6R-L-erythro-1',2'-dihydroxypropyl-2-amino-4-hydroxy-5,6,7,8-tetrahydropteridine; BH4), are thought to be important modulators of function in normal and inflamed airways. However, the exact pathologic roles of NO and BH4 remain obscure. Even less is known about the effects of cytokines on alveolar macrophages.

OBJECTIVE

This study was designed to determine whether NO and BH4 are induced by cytokines in mouse alveolar macrophages and to investigate whether NO synthesis is affected by changes in intracellular BH4 levels in alveolar macrophages.

METHODS

We compared the induction by lipopolysaccharide (LPS), interferon-gamma (IFN-gamma), tumor necrosis factor-alpha (TNF-alpha), and interleukin-2 (IL-2) of NO production and BH4 synthesis in alveolar macrophages. To determine whether NO synthesis is affected by changes in intracellular BH4 levels in alveolar macrophages, we used inhibitors of BH4 biosynthesis.

RESULTS

Activation of alveolar macrophages induced parallel increases in NO and intracellular BH4 levels, although induction of the latter appears to be somewhat more sensitive than that of the latter to diverse cytokines. Inducible NO production in alveolar macrophages was blocked by inhibitors of BH4 biosynthesis. IL-2, an important component of the immunomodulatory system, was only a weak activator of alveolar macrophages by itself but potently synergized with IFN-gamma to stimulate the production of both NO and BH4.

CONCLUSION

Our results suggest that BH4 synthesis in alveolar macrophages is a potential target for therapeutic intervention in airway inflammatory diseases, such as asthma, cystic fibrosis, and acute bronchial infections whose pathology may be mediated by overproduction of NO.

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.

Review article: the potential role of nitric oxide in chronic inflammatory bowel disorders

The aetiology of the chronic inflammatory bowel diseases-ulcerative colitis and Crohn's disease-as well as 'microscopic colitis'-both collagenous (COC) and lymphocytic colitis (LC)-remains unknown. Autoimmune mechanisms, cytokine polymorphism, commensal bacteria, infectious agents and vascular impairment have all been proposed as playing important roles in the pathogenesis of this spectrum of diseases. A variety of proinflammatory mediators, including tumour necrosis factor alpha, interleukin-1beta, interferon gamma, leukotriene B4 and platelet activating factor, promote the adherence of phagocytes to the venular endothelium and extravasation of these cells into the colonic mucosa. In addition to large amounts of nitric oxide (NO), injurious peroxynitrite may be formed in the epithelium by the inducible nitric oxide synthase (iNOS), which is considered to elicit cytotoxicity by the generation of superoxide with reduced L-arginine availability. In active ulcerative colitis, and to a lesser extent in Crohn's disease, a greatly increased production of NO has been demonstrated by indirect and direct measurements. Surprisingly, even higher rates of production have been observed in COC-a condition which is never associated with injurious inflammation. The latter observation favours the notion that NO promotes mucosal integrity. Further evidence for a protective role of NO in chronic inflammatory bowel disorders is provided by the observation of increased susceptibility to the induction of experi mental colitis in 'knock-out' mice deficient in iNOS. Selective inhibitors of iNOS activity, as well as topical L-arginine, may therefore prove beneficial in inflammatory bowel disease by reducing the production of superoxide by iNOS, while only the former option may be expected to reduce diarrhoea in chronic inflammatory bowel disorders. Clearly, further experimental work needs to be done before testing topical L-arginine in human inflammatory bowel disease.

Structural analysis of porcine brain nitric oxide synthase reveals a role for tetrahydrobiopterin and L-arginine in the formation of an SDS-resistant dimer

Nitric oxide synthases (NOSs), which catalyze the formation of the ubiquitous biological messenger molecule nitric oxide, represent unique cytochrome P-450s, containing reductase and mono-oxygenase domains within one polypeptide and requiring tetrahydrobiopterin as cofactor. To investigate whether tetrahydrobiopterin functions as an allosteric effector of NOS, we have analyzed the effect of the pteridine on the conformation of neuronal NOS purified from porcine brain by means of circular dichroism, velocity sedimentation, dynamic light scattering and SDS-polyacrylamide gel electrophoresis. We report for the first time the secondary structure of NOS, showing that the neuronal isozyme contains 30% alpha-helix, 14% antiparallel beta-sheet, 7% parallel beta-sheet, 19% turns and 31% other structures. The secondary structure of neuronal NOS was neither modulated nor stabilized by tetrahydrobiopterin, and the pteridine did not affect the quaternary structure of the protein, which appears to be an elongated homodimer with an axial ratio of approximately 20/1 under native conditions. Low temperature SDS-polyacrylamide gel electrophoresis revealed that tetrahydrobiopterin and L-arginine synergistically convert neuronal NOS into an exceptionally stable, non-covalently linked homodimer surviving in 2% SDS and 5% 2-mercaptoethanol. Ligand-induced formation of an SDS-resistant dimer is unprecedented and suggests a novel role for tetrahydrobiopterin and L-arginine in the allosteric regulation of protein subunit interactions.

Decreased cerebrospinal fluid nitrate levels in Parkinson's disease, Alzheimer's disease and multiple system atrophy patients

Nitric oxide (NO) is a recently discovered endogenous mediator of vasodilatation, neurotransmission, and macrophage cytotoxicity. NO is thought to have a function in memory and in long-term potentiation. At high concentrations NO is neurotoxic and may play a role in neurodegeneration. NO is formed from L-arginine by the enzyme NO synthase (NOS), for which tetrahydrobiopterin (BH4) is a necessary co-factor. Alzheimer's disease (AD) and, to a lesser degree, Parkinson's disease (PD) are thought to be associated with increased microglial activity, suggesting that NO production may be increased. Alternatively, in circumstances of reduced levels of intracellular L-arginine or BH4, NO production is diminished and neurotoxic oxygen radicals may be produced. Since BH4 is decreased in AD and PD brains, these diseases may be associated with decreased NO production. We investigated these two alternatives by measuring the NO degradation products nitrite and nitrate in cerebrospinal fluid (CSF) of PD (n = 103), AD (n = 13), and multiple system atrophy (MSA; n = 14) patients and controls (n = 20). We found for all patient groups, compared with controls, significantly decreased levels of nitrate, but not nitrite. This finding seems to indicate a decreased NO production of the central nervous system (CNS) in these neurodegenerative disorders.