The protective effect of tetrahydrobiopterin on the nitric oxide-mediated inhibition of purified nitric oxide synthase

Factors influencing the soluble guanylate cyclase heme redox state in blood vessels

Soluble guanylate cyclase (sGC) plays an important role in maintaining vascular homeostasis, as an acceptor for the biological messenger nitric oxide (NO). However, only reduced sGC (with a ferrous heme) can be activated by NO; oxidized (ferric heme) and apo (absent heme) sGC cannot. In addition, the proportions of reduced, oxidized, and apo sGC change under pathological conditions. Although diseased blood vessels often show decreased NO bioavailability in the vascular wall, a shift of sGC heme redox balance in favor of the oxidized/apo forms can also occur. Therefore, sGC is of growing interest as a drug target for various cardiovascular diseases. Notably, the balance between NO-sensitive reduced sGC and NO-insensitive oxidized/apo sGC in the body is regulated in a reversible manner by various biological molecules and proteins. Many studies have attempted to identify endogenous factors and determinants that influence this redox state. For example, various reactive nitrogen and oxygen species are capable of inducing the oxidation of sGC heme. Conversely, a heme reductase and some antioxidants reduce the ferric heme in sGC to the ferrous state. This review summarizes the factors and mechanisms identified by these studies that operate to regulate the sGC heme redox state.

Tetrahydrobiopterin Administration Augments Exercise-Induced Hyperemia and Endothelial Function in Patients With Systemic Sclerosis

Systemic sclerosis (SSc) is a rare, auto-immune disease with variably progressive fibrosis of the skin and internal organs, as well as vascular dysfunction. Recently, we demonstrated a decrement in exercising skeletal muscle blood flow and endothelium-dependent vasodilation in SSc, but the mechanisms responsible for these impairments have not been investigated. Thus, we sought to determine if acute administration of tetrahydrobiopterin (BH₄), an essential cofactor for endothelial nitric oxide synthase (eNOS), would improve hyperemia and brachial artery vasodilation during progressive handgrip exercise in SSc. Thirteen patients with SSc (63 ± 11 years) participated in this placebo-controlled, randomized, double-blind, crossover study. Tetrahydrobiopterin (10 mg/kg) administration resulted in a ~4-fold increase in circulating BH₄ concentrations (P < 0.05). Cardiovascular variables at rest were unaffected by BH₄ (P > 0.05). During handgrip exercise, BH₄ administration increased brachial artery blood flow (placebo: 200 ± 87; BH₄: 261 ± 115 ml/min; P < 0.05) and vascular conductance (placebo: 2.0 ± 0.8; BH₄: 2.5 ± 1.0 ml/min/mmHg; P < 0.05), indicating augmented resistance artery vasodilation. Tetrahydrobiopterin administration also increased brachial artery vasodilation in response to exercise (placebo: 12 ± 6; BH₄: 17 ± 7%; P < 0.05), resulting in a significant upward shift in the slope relationship between Δ brachial artery vasodilation and Δ shear rate (placebo: 0.030 ± 0.007; BH₄: 0.047 ± 0.007; P < 0.05) that indicates augmented sensitivity of the brachial artery to vasodilate to the sustained elevations in shear rate during handgrip exercise. These results demonstrate the efficacy of acute BH₄ administration to improve both resistance and conduit vessel endothelial function in SSc, suggesting that eNOS recoupling may be an effective strategy for improving vasodilatory capacity in this patient group.

Tetrahydrobiopterin augments endothelium-dependent dilatation in sedentary but not in habitually exercising older adults

Endothelium-dependent dilatation (EDD) is impaired with ageing in sedentary, but not in regularly exercising adults. We tested the hypotheses that differences in tetrahydrobiopterin (BH(4)) bioactivity are key mechanisms explaining the impairment in EDD with sedentary ageing, and the maintenance of EDD with ageing in regularly exercising adults. Brachial artery flow-mediated dilatation (FMD), normalized for local shear stress, was measured after acute oral placebo or BH(4) in young sedentary (YS) (n = 10; 22 +/- 1 years, mean +/- s.e.m.), older sedentary (OS) (n = 9; 62 +/- 2), and older habitually aerobically trained (OT) (n = 12; 66 +/- 1) healthy men. At baseline, FMD was approximately 50% lower in OS versus YS (1.12 +/- 0.09 versus 0.57 +/- 0.09 (Deltamm (dyn cm(-2))) x 10(-2), P < 0.001; 1 dyn = 10(-5) N), but was preserved in OT (0.93 +/- 0.08 (Deltamm (dyn cm(-2))) x 10(-2)). BH4 administration improved FMD by approximately 45% in OS (1.00 +/- 0.10 (Deltamm (dyn cm(-2))) x 10(-2), P < 0.01 versus baseline), but did not affect FMD in YS or OT. Endothelium-independent dilatation neither differed between groups at baseline nor changed with BH4 administration. These results suggest that BH4 bioactivity may be a key mechanism involved in the impairment of conduit artery EDD with sedentary ageing, and the EDD-preserving effect of habitual exercise.

Nitric oxide involvement in reperfusion injury of denervated muscle

PURPOSE

To investigate whether inhibition of inducible nitric oxide synthase (iNOS) improves microcirculation in denervated and reperfused skeletal muscle.

METHODS

The cremaster muscles of 52 rats received iNOS inhibitor 1400W (3 mg/kg) or phosphate buffered saline (PBS) and underwent either 3 hours of ischemia and 1.5 hours of reperfusion or a sham operation. During reperfusion the vessel diameters were measured by using intravital videomicroscopy and overall muscle blood flow was measured with laser Doppler flowmetry. The expression of NOS messenger RNA (mRNA) and protein was determined by using real-time reverse-transcription polymerase chain reaction and Western blot, respectively.

RESULTS

1400W treatment significantly increased the mean blood flow of the reperfused muscle compared with controls, and this was associated with significantly less vasospasm in 10 to 20 microm, 21 to 40 microm, and 41 to 70 microm arterioles. The expression of iNOS mRNA and protein in controls increased 23-fold and 6-fold from normal, respectively, but was reduced to only a 2-fold increase in the 1400W-treated muscles. The ischemia/reperfusion (I/R)-induced decrease of endothelial NOS (eNOS) and neuronal NOS (nNOS) expression in controls was not significantly changed after 1400W treatment.

CONCLUSIONS

Our data support a nitric oxide-mediated mechanism in reperfusion injury and show the importance of inhibition of iNOS in reducing reperfusion injury in denervated skeletal muscle. Our results suggest potential benefits via inhibition of iNOS to improve clinical outcomes not only for hand surgeons who work in the microsurgery field, but also for other physicians whose work involves ischemia/reperfusion injury.

Tetrahydrobiopterin prevents nitration of tyrosine hydroxylase by peroxynitrite and nitrogen dioxide

Tyrosine hydroxylase (TH) is the initial and rate-limiting enzyme in the synthesis of the neurotransmitter dopamine. TH is inhibited and nitrated at tyrosine residues in vitro by the reactive nitrogen species peroxynitrite and nitrogen dioxide (NO2) and in vivo by drugs that damage dopamine neurons. Tetrahydrobiopterin, which is the essential cofactor for TH and is concentrated in dopamine neurons, completely blocks nitration of tyrosine residues in TH caused by peroxynitrite or NO2. Various tetrahydro- and dihydro-analogs of tetrahydrobiopterin, including 6,7-dimethyl-tetrahydropterin, 6-methyl-tetrahydropterin, 6-hydroxymethyl-tetrahydropterin, tetrahydropterin, 7,8-dihydrobiopterin, 7,8-dihydroxanthopterin, and sepiapterin, also prevent nitration of tyrosines caused by the reactive nitrogen species. Biopterin and pterin, the fully oxidized forms of the pterin molecule, fail to block peroxynitrite- or NO2-induced nitration of TH. Reduced pterins prevent neither the inhibition of TH activity nor cysteine modification caused by peroxynitrite or NO2, despite blocking tyrosine nitration. However, dithiothreitol prevents and reverses these effects on TH of tetrahydrobiopterin and reactive nitrogen species. Using an enhanced green fluorescent protein-TH fusion construct as a real-time reporter of intracellular tyrosine nitration, tetrahydrobiopterin was found to prevent NO2-induced tyrosine nitration in intact cells but to leave TH activity inhibited. These results indicate that tetrahydrobiopterin prevents the tyrosine-nitrating properties of peroxynitrite and NO2. Tetrahydrobiopterin-derived radical species formed by reaction with reactive nitrogen species may account for inhibition of TH via mechanisms that do not involve tyrosine nitration.

Specific potentiation of endothelium-dependent contractions in SHR by tetrahydrobiopterin

This study was designed to determine the effect of pteridines, R- and S-tetrahydrobiopterin, sepiapterin, and dihydrobiopterin on endothelium-dependent contractions to acetylcholine in isolated aortas from spontaneously hypertensive rat and normotensive Wistar-Kyoto rat. The noncumulative addition of redox-active pteridines R- and S-tetrahydrobiopterin (but not the oxidized analogues sepiapterin and dihydrobiopterin) produced a concentration-dependent transient contraction in isolated aortic rings from both normotensive and hypertensive rats. R- and S-tetrahydrobiopterin (but not sepiapterin or dihydrobiopterin) potentiated the endothelium-dependent contractions to acetylcholine but only in aortas from hypertensive rats and in the presence of N(G)-nitro-L-arginine. In these aortas, the generation of oxygen-derived free radicals by the combination of xanthine plus xanthine oxidase also potentiated the endothelium-dependent contractions to acetylcholine. The presence of R-tetrahydrobiopterin did not alter the characteristics of the endothelium-dependent contractions because they were inhibited by valeryl salicylate, an inhibitor of cyclooxygenase-1, by S18886, a TP-receptor antagonist or by Tiron, a cell permeable superoxide anion scavenger. However, the contractions to acetylcholine, which are unaffected by the combination of superoxide dismutase and catalase, become significantly inhibited by these two scavengers in the presence of R-tetrahydrobiopterin. In the presence of N(G)-nitro-L-arginine, R-tetrahydrobiopterin did not affect the contractions to phenylephrine, U 46619, or to oxygen-derived free radicals generated by xanthine plus xanthine oxidase. These results indicate that the production of superoxide by the autoxidation of tetrahydrobiopterin selectively enhances endothelium-dependent contractions in the spontaneously hypertensive rat when nitric oxide synthase is inhibited.

Reaction of tetrahydrobiopterin with superoxide: EPR-kinetic analysis and characterization of the pteridine radical

It has been shown that BH(4) ameliorates endothelial dysfunction associated with conditions such as hypertension, cigarette smoking, and diabetes. This effect has been proposed to be due to a superoxide scavenging activity of BH(4). To examine this possibility we determined the rate constant for the reaction between BH(4) and superoxide using electron paramagnetic resonance (EPR) spin trapping competition experiments with 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide (DEPMPO). We calculated a rate constant for the reaction between BH(4) and superoxide of 3.9 +/- 0.2 x 10(5) M(-1)s(-1) at pH 7.4 and room temperature. This result suggests that superoxide scavenging by BH(4) is not a major reaction in vivo. HPLC product analysis showed that 7,8-BH(2) and pterin are the stable products generated from the reaction. The formation of BH(4) cation radical (BH(4)(*+)) was demonstrated by direct EPR only under acidic conditions. Isotopic substitution experiments demonstrated that the BH(4)(*+) is mainly delocalized on the pyrazine ring of BH(4). In parallel experiments, we investigated the effect of ascorbate on 7,8-BH(2) reduction and eNOS activity. We demonstrated that ascorbate does not reduce 7,8-BH(2) to BH(4), nor does it stimulate nitric oxide release from eNOS incubated with 7,8-BH(2). In conclusion, it is likely that BH(4)-dependent inhibition of superoxide formation from eNOS is the mechanism that better explains the antioxidant effects of BH(4) in the vasculature.

Survival and graft function in a large animal lung transplant model after 30 h preservation and substitution of the nitric oxide pathway

OBJECTIVE

Substitution of the nitric oxide- (NO-) pathway improves early graft function following lung transplantation. We previously demonstrated that 8-Br-cGMP (second messenger of NO) to the flush solution and tetrahydrobiopterin (BH4, coenzyme of NO synthase) given as additive during reperfusion improve post-transplant graft function. In the present study, the combined treatment with 8-Br-cGMP and BH4 was evaluated.

METHODS

Unilateral left lung transplantation was performed in weight matched outbred pigs (24-31 kg). In group I, grafts were preserved for 30 h (n=5). 8-Br-cGMP (1mg/kg) was added to the flush solution (Perfadex, 1.5l, 1 degrees C) and BH4 (10mg/kg/h) was given to the recipient for 5h after reperfusion. In group II, lungs were transplanted after a preservation time of 30 h (n=3) and prostaglandin E(1) (250 g) was given into the pulmonary artery (PA) prior to flush. In all recipients 1h after reperfusion the contralateral right PA and bronchus were ligated to assess graft function only. Survival time after reperfusion, extravascular lung water index (EVLWI), hemodynamic variables, and gas exchange (PaO(2)) were assessed during a 12h observation period.

RESULTS

All recipients in group I survived the 12h assessment, whereas none of the group II animals survived more than 4h after reperfusion with a rapid increase of EVLWI up to 24.8+/-6.7 ml/kg. In contrast, in group I EVLWI reached up to 8.9+/-1.5 ml/kg and returned to nearly normal levels at 12h (6.1+/-0.8 ml/kg). In two animals of group I the gas exchange deteriorated slightly. The other three animals showed normal arterial oxygenation over the entire observation time.

CONCLUSION

Our data indicate that the combined substitution of the NO pathway during preservation and reperfusion reduces ischemia/reperfusion injury substantially and that this treatment even allows lung transplantation after 30 h preservation in this model.

Tetrahydrobiopterin attenuates modulation of platelet 12-lipoxygenase and cyclooxygenase activities by nitric oxide

Endothelial cells secrete large amounts of 5,6,7,8-tetrahydrobiopterin (BH(4)) in septic conditions. BH(4) is a cofactor for nitric oxide (NO) synthase and an essential regulator of its activity. We recently showed that NO can be a modulator of both platelet 12-lipoxygenase and cyclooxygenase activities. In the present study, we investigated the effect of BH(4) on the activities of 12-lipoxygenase and cyclooxygenase in rabbit platelets. The influence of BH(4) on NO-induced modulation of these enzyme activities was investigated. Exogenous BH(4) did not affect platelet 12-lipoxygenase and cyclooxygenase activities. The modulatory effects of NO on the two enzymatic pathways were reversed by addition of BH(4) but not by reduced glutathione. These results suggest that exogenous BH(4) is not essential for NO synthase activity of platelets, but that it is an important regulator of the action of NO released from other sources on platelet 12-lipoxygenase and cyclooxygenase activities.

Biology of nitric oxide signaling

The free radical nitric oxide (NO) has emerged in recent years as a fundamental signaling molecule for the maintenance of homeostasis, as well as a potent cytotoxic effector involved in the pathogenesis of a wide range of human diseases. Although this paradoxical fate has generated confusion, separating the biological actions of NO on the basis of its physiologic chemistry provides a conceptual framework which helps to distinguish between the beneficial and toxic consequences of NO, and to envision potential therapeutic strategies for the future. Under normal conditions, NO produced in low concentration acts as a messenger and cytoprotective (antioxidant) factor, via direct interactions with transition metals and other free radicals. Alternatively, when the circumstances allow the formation of substantial amounts of NO and modify the cellular microenvironment (formation of the superoxide radical), the chemistry of NO will turn into indirect effects consecutive to the formation of dinitrogen trioxide and peroxynitrite. These "reactive nitrogen species" will, in turn, mediate both oxidative and nitrosative stresses, which form the basis of the cytotoxicity generally attributed to NO, relevant to the pathophysiology of inflammation, circulatory shock, and ischemia-reperfusion injury.

The nitric oxide synthase cofactor tetrahydrobiopterin reduces allograft ischemia-reperfusion injury after lung transplantation

OBJECTIVE

Exogenous nitric oxide reduces ischemia-reperfusion injury after solid organ transplantation. Tetrahydrobiopterin, an essential cofactor for nitric oxide synthases, may restore impaired endothelium-dependent nitric oxide synthesis. We evaluated whether tetrahydrobiopterin administration to the recipient attenuates lung reperfusion injury after transplantation in swine.

METHODS

Unilateral left lung transplantation was performed in 15 weight-matched pigs (24-31 kg). Donor lungs were flushed with 1.5 L cold (1 degrees C) low-potassium-dextran solution and preserved for 20 hours. Group I animals served as controls. Group II and III animals were treated with a bolus of tetrahydrobiopterin (20 mg/kg). In addition, in group III a continuous infusion of tetrahydrobiopterin (10 mg/kg per hour over 5 hours) was given. One hour after reperfusion, the recipient right lung was occluded. Cyclic guanosine monophosphate levels were measured in the pulmonary venous and central venous blood. Extravascular lung water index, hemodynamic variables, lipid peroxidation, and neutrophil migration to the allograft were assessed.

RESULTS

In group III a significant reduction of extravascular lung water was noted in comparison with the controls (P =.0047). Lipid peroxidation in lung allograft tissue was significantly reduced in group II (P =.0021) and group III ( P =. 0077) in comparison with group I. Pulmonary venous levels of cyclic guanosine monophosphate increased up to 23 +/- 1 pmol/mL at 5 hours in group II and up to 40 +/- 1 pmol/mL in group III (group I, 4.1 +/- 0.5 pmol/mL [I vs III]; P <.001), whereas central venous levels of cyclic guanosine monophosphate were unchanged in all groups.

CONCLUSION

Tetrahydrobiopterin administration during lung allograft reperfusion may reduce posttransplantation lung edema and oxygen-derived free radical injury in the graft. This effect is mediated by local enhancement of the nitric oxide/cyclic guanosine monophosphate pathway.

Evidence for a feedback inhibition of NO synthesis in enteric synaptosomes via a nitrosothiol intermediate

The exact mechanisms controlling nitric oxide synthase (NOS) activity within enteric neurons are largely unknown. In this study, the effect of exogenous nitric oxide (NO) on NOS activity was investigated in enteric synaptosomes of rat ileum. 3-Morpholinosydnonimine (SIN-1; 10(-4) M) and S-nitroso-N-acetylpenicillamine (SNAP; 10(-4) M) significantly inhibited NOS activity by 53% and 48%, respectively. However, superoxide dismutase (SOD; 160 U/ml) as well as the NO scavenger oxyhemoglobin (10(-3) M) did not influence NO donor-induced inhibition. In contrast, the inhibitory effect was antagonized by diethyldithiocarbamate (3 x 10(-4) M), an inhibitor of endogenous Cu/Zn SOD. Inhibition of NOS by exogenous NO was dependent on glutathione (GSH), since the inhibitory effect was augmented in the presence of GSH (5 x 10(-4) M) and antagonized by the GSH-depletor DL-buthionine-SR-sulfoximine (5 x 10(-4) M), suggesting that NO might be protected from extracellular breakdown by reaction with GSH. The reaction product of SIN-1/SNAP and GSH was identified as a nitrosothiol. In the presence of the Cu(+)-chelator neocuproine (10(-5) M), inhibition of NOS by SNAP/SIN-1 was reversed, suggesting that nitrosothiol formation is intermediary. These findings are indicative of a feedback inhibition of enteric NOS, presumably via formation of a nitrosothiol intermediate.

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.

Crystal structure of constitutive endothelial nitric oxide synthase: a paradigm for pterin function involving a novel metal center

Nitric oxide, a key signaling molecule, is produced by a family of enzymes collectively called nitric oxide synthases (NOS). Here, we report the crystal structure of the heme domain of endothelial NOS in tetrahydrobiopterin (H4B)-free and -bound forms at 1.95 A and 1.9 A resolution, respectively. In both structures a zinc ion is tetrahedrally coordinated to pairs of symmetry-related cysteine residues at the dimer interface. The phylogenetically conserved Cys-(X)4-Cys motif and its strategic location establish a structural role for the metal center in maintaining the integrity of the H4B-binding site. The unexpected recognition of the substrate, L-arginine, at the H4B site indicates that this site is poised to stabilize a positively charged pterin ring and suggests a model involving a cationic pterin radical in the catalytic cycle.

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.

Activation and dimerization of type III nitric oxide synthase by submicromolar concentrations of tetrahydrobiopterin in microsomal preparations from human primordial placenta

We have found in a previous study that 5-50 microM tetrahydrobiopterin (BH4) stimulated 1.2-2.5-fold Ca(2+)-dependent nitric-oxide synthase (NOS) activity in homogenates prepared from primordial human placentae. Now we report on the dramatic, about sixfold, activating effect of BH4 on this activity measured in microsomal preparations. Firstly, both in the absence and presence of BH4, arginine bound to kinetically homogeneous sites, with no significant change between the apparent KM values for arginine (3.12 +/- 1.99 microM and 2.06 +/- 1.13 microM in the absence and presence of 50 microM BH4 respectively, mean +/- s.d., n = 3). On the other hand, the Vmax values measured in different pools of placenta tissue varied between 2.5-7.55 (no BH4 added) and 13.3-58.5 (with BH4 added) pmol/min/mg protein. Secondly, the microsomal preparations responded sensitively to BH4 addition. A dose-response study indicated that as low as 79 nM final BH4 concentration stimulated NOS activity half-maximally, and 1 microM BH4 resulted in an almost maximal effect. Thirdly, immunoblot analysis combined with laser densitometric evaluation demonstrated that BH4 efficiently promoted the aggregation of microsomal NOS type III isozyme into a protein having the characteristics (electrophoretic mobility, resistance of SDS) of the dimeric form. Half-maximal dimerizing activity was reached at 148 +/- 33 nM BH4 (mean +/- s.d., n = 3), whereas 1 microM BH4 led to almost, maximal aggregation of monomers. This is the first time that BH4-induced dimerization of a NOS type III isoform has been demonstrated. Considering that human placenta predominantly expresses NOS type III isoform and BH4 concentration in this tissue is 207 +/- 87 nM, the present results strongly suggest that the dimerizing effect of BH4 is a crucial physiological mechanism for the assembly of active Ca(2+)-dependent NOS in the human primordial placenta.

Acute amelioration of diabetic endothelial dysfunction with a derivative of the nitric oxide synthase cofactor, tetrahydrobiopterin

Tetrahydrobiopterin is a cofactor for nitric oxide synthase. In low concentrations of this cofactor, nitric oxide synthase is known to produce less nitric oxide and, correspondingly, enhanced quantities of the oxidant species, hydrogen peroxide. In this study, we tested the hypothesis that an exogenous tetrahydrobiopterin derivative might improve endothelial nitric oxide synthase activity in diabetic endothelium. Diabetes was induced in Sprague-Dawley rats with intravenous injections of streptozotocin. After 8 weeks, endothelium-dependent relaxation was assessed in aortic rings by using acetylcholine, whereas endothelium-independent relaxation was assessed by using nitroglycerin. Acetylcholine-induced relaxation was impaired in diabetic rings, whereas nitroglycerin-induced relaxation was unimpaired. Exposure of rings for 30 min with 100 microM of the pteridine derivative, 6-methyl-5,6,7,8-tetrahydropterin (in the presence of diethylenetriaminepentaacetic acid to inhibit oxidation), followed by washing and equilibration in control media, augmented relaxation induced by acetylcholine in diabetic rings but had no effect on relaxation in control rings. Pteridine exposure did not alter relaxation or sensitivity to nitroglycerin in control rings either with or without endothelium. In diabetic rings, pteridine exposure augmented maximal relaxation to nitroglycerin in rings with or without endothelium while increasing the sensitivity only in rings with endothelium but not in rings without endothelium. In contrast, there was no effect of pteridine exposure on relaxation or sensitivity to nitroglycerin in diabetic rings (with or without endothelium) that are pretreated with L-nitroarginine. In summary, tetrahydrobiopterin availability can play a key role in the regulation of nitric oxide production by diabetic endothelium.

Role of tetrahydrobiopterin availability in the regulation of nitric-oxide synthase expression in human mesangial cells

Human mesangial cells express an inducible form of nitric-oxide synthase (iNOS) after treatment with cytokines. Tetrahydrobiopterin (BH4), an essential cofactor for NOS, is required for cytokine-induced NO generation. We report here that BH4 is necessary not only for the activity but also for the expression of iNOS in human mesangial cells. Inhibition of de novo BH4 synthesis with 2,4-diamino-6-hydroxypyrimidine (DAHP) significantly attenuated iNOS activity as well as mRNA and protein expression in response to interleukin 1beta plus tumor necrosis factor alpha (IL-1beta/TNF-alpha). In contrast, sepiapterin, which provides BH4 through the pterin salvage pathway, strongly potentiated IL-1beta/TNF-alpha-induced iNOS expression and abrogated the inhibitory effect of DAHP. Inhibition of the pterin salvage pathway with methotrexate abolished sepiapterin potentiation of iNOS induction but did not alter the effect of IL-1beta/TNF-alpha. Determination of intracellular pteridines confirmed that sepiapterin markedly raised BH4 content, an effect that was blocked by methotrexate. These results suggest that BH4 availability plays an important role in the regulation of iNOS expression. The effect of BH4 appears to be mediated, at least in part, by an increase in mRNA stability, as indicated by the observation that DAHP shortened, whereas sepiapterin prolonged the half-life of IL-1beta/TNF-alpha-induced iNOS mRNA. Taken together, our results suggest that the biosynthesis of BH4 contributes to cytokine induction of iNOS expression in human mesangial cells through the stabilization of iNOS mRNA.