Dihydrofolate reductase protects endothelial nitric oxide synthase from uncoupling in tetrahydrobiopterin deficiency

Tetrahydrobiopterin (BH4) is a required cofactor for the synthesis of NO by endothelial nitric oxide synthase (eNOS), and endothelial BH4 bioavailability is a critical factor in regulating the balance between NO and superoxide production (eNOS coupling). Biosynthesis of BH4 is determined by the activity of GTP-cyclohydrolase I (GTPCH). However, BH4 levels may also be influenced by oxidation, forming 7,8-dihydrobiopterin (BH2), which promotes eNOS uncoupling. Conversely, dihydrofolate reductase (DHFR) can regenerate BH4 from BH2, but whether DHFR is functionally important in maintaining eNOS coupling remains unclear. To investigate the mechanism by which DHFR might regulate eNOS coupling in vivo, we treated wild-type, BH4-deficient (hph-1), and GTPCH-overexpressing (GCH-Tg) mice with methotrexate (MTX), to inhibit BH4 recycling by DHFR. MTX treatment resulted in a striking elevation in BH2 and a decreased BH4:BH2 ratio in the aortas of wild-type mice. These effects were magnified in hph-1 but diminished in GCH-Tg mice. Attenuated eNOS activity was observed in MTX-treated hph-1 but not wild-type or GCH-Tg mouse lung, suggesting that inhibition of DHFR in BH4-deficient states leads to eNOS uncoupling. Taken together, these data reveal a key role for DHFR in regulating the BH4 vs BH2 ratio and eNOS coupling under conditions of low total biopterin availability in vivo.     

Thermodynamic analysis of L-arginine and N omega-hydroxy-L-arginine binding to nitric oxide synthase

Isothermal titration calorimetry has been used to determine thermodynamic parameters of substrate binding to the oxygenase domain of neuronal nitric oxide synthase (nNOS(oxy)) in the presence of the cofactor tetrahydrobiopterin. The intermediate N(omega)-hydroxy-L-arginine (NHA) has a larger affinity than L-Arginine (L-Arg) for nNOS(oxy), with K(d)=0.4+/-0.1 microM and 1.7+/-0.3 microM at 25 degrees C, respectively. nNOS(oxy) binds NHA and L-Arg with DeltaH -4.1+/-0.2 and -1.0+/-0.1 kcal/mol and DeltaS=15 and 23 cal/Kmol respectively. NHA binding is more exothermic probably due to formation of an extra hydrogen bond in the active site compared to L-Arg. The changes in heat capacity (DeltaC(p)) are relatively small for binding of both NHA and L-Arg (-53+/-18 and -95+/-23 cal/L mol, respectively), which indicates that hydrophobic interactions contribute little to binding.     

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

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

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

To check the stimulatory potency of the tetrahydro forms of the two major pteridines occurring in human tissues, neopterin and biopterin, NO synthase was purified 6000-fold from human cerebellum. Tetrahydrobiopterin stimulated the activity up to 4.5-fold in a concentration dependent manner with a maximum above 1 microM, whereas tetrahydroneopterin was completely inactive in concentrations up to 100 microM. Tetrahydrobiopterin, but not neopterin derivatives, were copurified with the NO synthase activity. Our results demonstrate that human cerebellum contains a tetrahydrobiopterin dependent NO synthase activity.     

Coordinate regulation of L-arginine uptake and nitric oxide synthase activity in cultured endothelial cells.

Despite intracellular L-arginine concentrations that should saturate endothelial nitric oxide synthase (eNOS), nitric oxide production depends on extracellular L-arginine. We addressed this 'arginine paradox' in bovine aortic endothelial cells by simultaneously comparing the substrate dependence of L-arginine uptake and intracellular eNOS activity, the latter measured as L-[3H]arginine conversion to L-[3H]citrulline. Whereas the Km of eNOS for L-arginine was 2 microM in cell extracts, the L-arginine concentration of half-maximal eNOS stimulation was increased to 29 microM in intact cells. This increase likely reflects limitation by L-arginine uptake, which had a Km of 108 microM. The effects of inhibitors of endothelial nitric oxide synthesis also suggested that extracellular L-arginine availability limits intracellular eNOS activity. Treatment of intact cells with the calcium ionophore A23187 reduced the L-arginine concentration of half-maximal eNOS activity, which is consistent with a measured increase in L-arginine uptake. Increases in eNOS activity induced by several agents were closely correlated with enhanced L-arginine uptake into cells (r = 0.89). The 'arginine paradox' may be explained in part by regulated L-arginine uptake into a compartment, probably represented by caveolae, that contains eNOS and that is distinct from the bulk cytosolic L-arginine.     

Structural basis for endothelial nitric oxide synthase binding to calmodulin

The enzyme nitric oxide synthase (NOS) is exquisitely regulated in vivo by the Ca(2+) sensor protein calmodulin (CaM) to control production of NO, a key signaling molecule and cytotoxin. The differential activation of NOS isozymes by CaM has remained enigmatic, despite extensive research. Here, the crystallographic structure of Ca(2+)-loaded CaM bound to a 20 residue peptide comprising the endothelial NOS (eNOS) CaM-binding region establishes their individual conformations and intermolecular interactions, and suggests the basis for isozyme-specific differences. The alpha-helical eNOS peptide binds in an antiparallel orientation to CaM through extensive hydrophobic interactions. Unique NOS interactions occur with: (i). the CaM flexible central linker, explaining its importance in NOS activation; and (ii). the CaM C-terminus, explaining the NOS-specific requirement for a bulky, hydrophobic residue at position 144. This binding mode expands mechanisms for CaM-mediated activation, explains eNOS deactivation by Thr495 phosphorylation, and implicates specific hydrophobic residues in the Ca(2+) independence of inducible NOS.     

CO binding studies of nitric oxide synthase: effects of the substrate, inhibitors and tetrahydrobiopterin

The dissociation constant (K_d) for CO from neuronal nitric oxide synthase heme in the absence of the substrate and cofactor was less than 10⁻³ μM. In the presence of

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-Arg, it dramatically increased up to 1 μM. In the presence of inhibitors such as N^G-nitro-

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-arginine methyl ester and 7-nitroindazole (NI), the K_d value further increased up to more than 100 μM. Addition of the cofactor, 5,6,7,8-tetrahydrobiopterin (H₄B), increased the K_d value by 10-fold in the presence of

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-Arg, whereas it decreased the value to less than one 250th in the presence of NI. Addition of H₄B increased the recombination rate constant (k_on) for CO by more than two-fold in the presence of

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-Arg or N⁶-(1-iminoethyl)-

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-lysine, whereas it decreased the k_on value by three-fold in the presence of

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-thiocitrulline. Thus, the binding fashion of some of inhibitors, such as NI, may be different from that of

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-Arg with respect to the H₄B effect.     

Electron paramagnetic resonance characterization of tetrahydrobiopterin radical formation in bacterial nitric oxide synthase compared to mammalian nitric oxide synthase

H(4)B is an essential catalytic cofactor of the mNOSs. It acts as an electron donor and activates the ferrous heme-oxygen complex intermediate during Arg oxidation (first step) and NOHA oxidation (second step) leading to nitric oxide and citrulline as final products. However, its role as a proton donor is still debated. Furthermore, its exact involvement has never been explored for other NOSs such as NOS-like proteins from bacteria. This article proposes a comparative study of the role of H(4)B between iNOS and bsNOS. In this work, we have used freeze-quench to stop the arginine and NOHA oxidation reactions and trap reaction intermediates. We have characterized these intermediates using multifrequency electron paramagnetic resonance. For the first time, to our knowledge, we report a radical formation for a nonmammalian NOS. The results indicate that bsNOS, like iNOS, has the capacity to generate a pterin radical during Arg oxidation. Our current electron paramagnetic resonance data suggest that this radical is protonated indicating that H(4)B may not transfer any proton. In the 2nd step, the radical trapped for iNOS is also suggested to be protonated as in the 1st step, whereas it was not possible to trap a radical for the bsNOS 2nd step. Our data highlight potential differences for the catalytic mechanism of NOHA oxidation between mammalian and bacterial NOSs.     

Macrophage nitric oxide synthase: relationship between enzyme-bound tetrahydrobiopterin and synthase activity.

Nitric oxide synthase (NOS) (EC 1.14.23) catalyzes the oxidation of L-arginine to citrulline and nitric oxide. The complex reaction carried out by NOS, which involves NADPH, O2, and enzyme-bound FAD, FMN, and tetrahydrobiopterin (BH4), has only recently begun to be elucidated. Herein we report the characterization of the pterin requirement of murine macrophage NOS. Although purified NOS activity was not dependent on BH4, activity was significantly enhanced by BH4 in a concentration-dependent fashion. NOS purified in the absence of added BH4 was found to contain substoichiometric concentrations of enzyme-bound pterin, where increased concentrations of bound pterin correlated with an increase in activity when assayed in the absence of exogenous BH4. However, NOS purified in the presence of BH4 followed by gel filtration exhibited a 1 mol of pterin:1 mol of NOS 130-kDa subunit stoichiometry and activity that was essentially independent of exogenous BH4. Experiments to probe a redox role for the pterin were carried out using pterin analogues. 6(R,S)-Methyltetrahydropterin was found to increase NOS activity in enzyme purified in the absence of BH4. However, the deaza analogue, 6(R,S)-methyl-5-deazatetrahydropterin, was not only incapable of supporting enzymatic turnover but also inhibited citrulline formation in a concentration-dependent manner. Overall, these results support a role for BH4 in the NOS reaction that involves stabilization of the enzyme and redox chemistry wherein a 1:1 stoichiometry between bound pterin and NOS subunit results in maximum activity.     

Apelin activates L-arginine/nitric oxide synthase/nitric oxide pathway in rat aortas

Apelin was recently found to be an inotropic polypeptide in isolated rat hearts, and intravenous injection of apelin can induce a transient decrease in blood pressure. To illustrate the mechanism of apelin-induced vasodilation, we observed the in vitro effects of apelin on the L-arginine (L-Arg)/nitric oxide (NO) pathway in the incubated, isolated rat aorta. Apelin stimulated vascular NO(2)(-) product and NOS activation in a concentration- and time-dependent manner. Compared with no apelin treatment, incubation with apelin (10(-9), 10(-8), and 10(-7)mol/L) increased NO(2)(-) product by 33%, 46%, and 69% (all p<0.01), respectively, and Ca(2+)-dependent constitutive NOS (cNOS) activity by 200%, 460%, and 550% (all p<0.01), respectively. However, Ca(2+)-independent NOS (iNOS) activity was not significantly altered (p>0.05). Apelin incubation (10(-9), 10(-8), and 10(-7)mol/L) increased L-Arg uptake by 130%, 180%, and 240% (all p<0.01), respectively. The mRNA level of cationic amino acid transporters, CAT-1 and CAT-2B, in rat aortic tissues treated with 10(-7)mol/L apelin was increased by 110% and 128%, respectively (both p<0.01). Incubation with 10(-7)mol/L apelin elevated eNOS mRNA and protein levels, by 53% (p<0.05) and 319% (p<0.01), respectively. Collectively, these results demonstrate that apelin directly activated the vascular L-Arg/NOS/NO pathway, which could be one of the important mechanisms of apelin-regulated vascular function.     

L-arginine chlorination products inhibit endothelial nitric oxide production

The myeloperoxidase-derived oxidant hypochlorous acid (HOCl) is thought to contribute to endothelial dysfunction, but the mechanisms underlying this inhibitory effect are unknown. The present study tested the hypothesis that HOCl and L-arginine (L-Arg) react to form novel compounds that adversely affect endothelial function by inhibiting nitric oxide (NO) formation. Using spectrophotometric techniques, we found that HOCl and L-Arg react rapidly (k = 7.1 x 10(5) m(-1) s(-1)) to form two major products that were identified by mass spectrometry as monochlorinated and dichlorinated adducts of L-Arg. Pretreatment of bovine aortic endothelial cells with the chlorinated L-Arg metabolites (Cl-l-Arg) inhibited the -induced formation of the NO metabolites nitrate (NO(3)(-)) and nitrite (NO(2)(-)) in a concentration-dependent manner. Preincubation of rat aortic ring segments with Cl-L-Arg resulted in concentration-dependent inhibition of acetylcholine-induced relaxation. In contrast, blood vessels relaxed normally to the endothelium-independent vasodilator sodium nitroprusside. In vivo administration of Cl-L-Arg to anesthetized rats increased carotid artery vascular resistance. A greater than 10-fold excess of L-Arg was required to reverse the inhibitory effects of Cl-L-Arg in vivo and in vitro. Reaction of HOCl with D-arginine (D-Arg) did not result in the formation of inhibitory products. These results suggest that HOCl reacts with L-Arg to form chlorinated products that act as nitric-oxide synthase inhibitors.     

Evidence of two distinct oxygen complexes of reduced endothelial nitric oxide synthase

Oxygen binding to the oxygenase domain of reduced endothelial nitric oxide synthase (eNOS) results in two distinct species differing in their Soret and visible absorbance maxima and in their capacity to exchange oxygen by CO. At 7 degrees C, heme-oxy I (with maxima at 420 and 560 nm) is formed very rapidly (k(on) approximately 2.5.10(6) m(-1).s(-1)) in the absence of substrate but in the presence of pterin cofactor. It is capable of exchanging oxygen with CO at -30 degrees C. Heme-oxy II is formed more slowly (k(on) approximately equal to 3.10(5) m(-1).s(-1)) in the presence of substrate, regardless of the presence of pterin. It is also formed in the absence of both substrate and pterin. In contrast to heme-oxy I, it cannot exchange oxygen with CO at cryogenic temperature. In the presence of arginine, heme-oxy II is characterized by absorbance maxima near 432, 564, and 597 nm. When arginine is replaced by N-hydroxyarginine, and also in the absence of both substrate and pterin, its absorbance maxima are blue-shifted to 428, 560, and 593 nm. Heme-oxy I seems to resemble the ferrous dioxygen complex observed in many hemoproteins, including cytochrome P450. Heme-oxy II, which is the oxygen complex competent for product formation, appears to represent a distinct conformation in which the electronic configuration is essentially locked in the ferric superoxide complex.     

L-arginine analogs as alternate substrates for nitric oxide synthase

The L-arginine analogs, N(delta)-methyl-L-arginine (deltaMA) and L-canavanine, were used to probe the role of the N delta nitrogen of L-arginine in the reaction catalyzed by nitric oxide synthase (NOS). deltaMA was synthesized and found to be a partial alternate substrate and a weak, reversible inhibitor of NOS with a Ki equal to 1.4 mM. deltaMA undergoes hydroxylation; however, it is not converted further, hence it functions as a partial substrate. L-Canavanine was converted to an L-homoserine presumably via initial hydroxylation and decomposition. The mechanism of this reaction and products of this reaction were not probed further.     

Presence of excess tetrahydrobiopterin during nitric oxide production from inducible nitric oxide synthase in LPS-treated rat aorta

Tetrahydrobiopterin (BH4) is one of the cofactors of nitric oxide synthase (NOS), and the synthesis of BH4 is induced as well as inducible NOS (iNOS) by lipopolysaccharide (LPS) and/or cytokines. BH4 has a protective effect against the cytotoxicity induced by nitric oxide (NO) and/or reactive oxygen species in various types of cells. The purpose of this study was to examine whether or not an excess of BH4 is present during the production of NO by iNOS in LPS-treated de-endothelialized rat aorta. Addition of LPS (10 microg/ml) to the aorta bath solution caused L-arginine (L-Arg)-induced relaxation from 1.5 hr after the addition of LPS in de-endothelialized rat aorta pre-contracted with 30 mM KCl. The L-Arg-induced relaxation was prevented by NOS inhibitors. BH4 content also increased from 3 hr after the addition of LPS. mRNAs of iNOS and GTP cyclohydrolase I (GTPCH), a rate-limiting enzyme of BH4 synthesis, were increased from 1.5 hr after addition of LPS. Although the expression of iNOS and GTPCH mRNAs was observed in the media, the expression levels in the media were much lower than those in the adventitia. Ten millimolar 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of GTPCH, strongly reduced L-Arg-induced relaxation, and decreased BH4 content to below the basal level in LPS-treated aorta, whereas 0.5 mM DAHP reduced the LPS-induced increase in BH4 content to the basal level but did not affect L-Arg-induced relaxation. The inhibition of L-Arg-induced relaxation by 10 mM DAHP was overcome by the addition of BH4 (10 microM). These results suggest that although BH4 is essential for NO production from iNOS, the increase in BH4 content above the basal level is not needed for eliciting L-Arg-induced relaxation by the treatment with LPS. Thus, an excess amount of BH4 may be synthesized during NO production by iNOS in LPS-treated rat aorta.     

High-density lipoprotein binding to scavenger receptor-BI activates endothelial nitric oxide synthase.

Atherosclerosis is the primary cause of cardiovascular disease, and the risk for atherosclerosis is inversely proportional to circulating levels of high-density lipoprotein (HDL) cholesterol. However, the mechanisms by which HDL is atheroprotective are complex and not well understood. Here we show that HDL stimulates endothelial nitric oxide synthase (eNOS) in cultured endothelial cells. In contrast, eNOS is not activated by purified forms of the major HDL apolipoproteins ApoA-I and ApoA-II or by low-density lipoprotein. Heterologous expression experiments in Chinese hamster ovary cells reveal that scavenger receptor-BI (SR-BI) mediates the effects of HDL on the enzyme. HDL activation of eNOS is demonstrable in isolated endothelial-cell caveolae where SR-BI and eNOS are colocalized, and the response in isolated plasma membranes is blocked by antibodies to ApoA-I and SR-BI, but not by antibody to ApoA-II. HDL also enhances endothelium- and nitric-oxide-dependent relaxation in aortae from wild-type mice, but not in aortae from homozygous null SR-BI knockout mice. Thus, HDL activates eNOS via SR-BI through a process that requires ApoA-I binding. The resulting increase in nitric-oxide production might be critical to the atheroprotective properties of HDL and ApoA-I.     

Alpha-tocopherol amplifies phosphorylation of endothelial nitric oxide synthase at serine 1177 and its short-chain derivative trolox stabilizes tetrahydrobiopterin

Alpha-tocopherol has been shown to increase nitric oxide (NO)-dependent relaxation but the underlying mechanisms have not been fully characterized. The present study investigates the effect of alpha-tocopherol and its derivative trolox on the synthesis of NO in human umbilical vein endothelial cells. NO was assayed as citrulline (co-product of NO) and cGMP (product of the NO-activated soluble guanylate cyclase) on ionomycin stimulation of cells. Ionomycin induced citrulline and cGMP formation partially through phosphorylation of endothelial NO synthase (eNOS) at its serine residue 1177, which was mediated mainly by calmodulin-dependent kinase II. Preincubation of cells with alpha-tocopherol or trolox increased eNOS activity in a concentration-dependent manner without changing eNOS expression. The effect of the water-soluble trolox was due to chemical stabilization of the eNOS cofactor tetrahydrobiopterin. On the contrary, alpha-tocopherol, located mainly in cellular membranes, did not affect tetrahydrobiopterin but increased ionomycin-induced eNOS phosphorylation at serine 1177. The effects of alpha-tocopherol on citrulline and cGMP formation and eNOS phosphorylation were amplified by co-incubation with ascorbate, which is suggested to regenerate oxidized alpha-tocopherol and to act synergistically with alpha-tocopherol. Our data describe a new vasoprotective function of alpha-tocopherol that may contribute to the prevention of endothelial dysfunction in vivo.     

Kinetic studies on the successive reaction of neuronal nitric oxide synthase from L-arginine to nitric oxide and L-citrulline

Rat neuronal nitric oxide synthase (nNOS) was heterologously expressed in Escherichia coliand purified. The conversion of L-arginine to N(omega)-hydroxy-L-arginine and further to L-citrulline in one cycle of the reaction of the purified nNOS was measured with the reaction rapid quenching method using (3)H-L-arginine as the substrate. It was found that most of the produced (3)H-N(omega)-hydroxy-L-arginine was successively hydroxylated to (3)H-L-citrulline without leaving the enzyme. From the analysis of time courses, the rate constants for each reaction step, and also for the dissociation of the intermediate, were estimated at various temperature in which the rates for the first and the second reactions were not much different each other but the rate for the dissociation of (3)H-N(omega)-hydroxy-L-arginine from the enzyme was significantly slow. Under the steady-state reaction condition, almost all of the nNOS was estimated to be active from the amount of burst formation of L-citrulline in the pre-steady state. The rate constant for the dissociation of the product L-citrulline from nNOS was calculated from the combination of results of the rapid quenching experiments and the metabolism of L-arginine in the presence of an excess amount of substrate, which was the smallest among all the rate constants in one cycle of the nNOS reaction. The activation energies for all the reaction steps were determined from the temperature dependence of the rate constants, which revealed that the rate-determining step of the nNOS reaction in the steady state was the dissociation of the product L-citrulline from the enzyme.     

Crystallographic studies on endothelial nitric oxide synthase complexed with nitric oxide and mechanism-based inhibitors

The crystal structure of the endothelial nitric oxide synthase (NOS) heme domain complexed with NO reveals close hydrogen bonding interactions between NO and the terminal guanidino nitrogen of the substrate, L-arginine. Dioxygen is expected to bind in a similar mode which will facilitate proton abstraction from L-Arg to dioxygen, a required step for O-O bond cleavage. Structures of mechanism-based NOS inhibitors, N(5)-(1-iminoethyl)-L-ornithine and N-(3-(aminomethyl)benzyl)acetamidine, provide clues on how this class of compounds operate as suicide substrate inhibitors leading to heme oxidation.     

Expression of endothelial nitric oxide synthase in the porcine oocyte and its possible function

The present study was designed to investigate the localization of endothelial nitric oxide synthase (eNOS) in porcine oocytes and its possible function during in vitro development. RT-PCR and immunoblotting analyses revealed the presence of eNOS in the oocytes prepared from small follicles, with an amplified product of 456 bp and an apparent mol wt of 130 kDa, respectively. The synthesis of oocyte NO was suppressed during a 72-h culture of cumulus-oocyte complexes in the presence of follicle-stimulating hormone (FSH), but not luteinizing hormone (LH). However, the decrease in NO synthesis did not result from the levels of eNOS mRNA and its protein, as revealed by analyses of RT-PCR and Western blot analysis, suggesting that expression of oocyte eNOS is not dependent upon gonadotropin stimulation. In proliferated cumulus cells, LH receptor mRNA expression was detected after a 48-h culture with FSH, as revealed by RT-PCR analysis. mRNA expression was inhibited by an NO-releasing agent (S-nitroso-N-acetyl-DL-penicillamine) after an additional 24-h culture. These results suggest that oocytes may release eNOS-derived NO as a signal for somatic cells to steadily suppress the development of cumulus cells, if not FSH stimulation. Conversely, the synthesis of NO is suppressed during the action of FSH on the cumulus cells with no changes in eNOS expression.