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

Superoxide generation by endothelial nitric oxide synthase: the influence of cofactors

The mechanism of superoxide generation by endothelial nitric oxide synthase (eNOS) was investigated by the electron spin resonance spin-trapping technique using 5-diethoxyphosphoryl-5-methyl-1-pyrroline N-oxide. In the absence of calcium/calmodulin, eNOS produces low amounts of superoxide. Upon activating eNOS electron transfer reactions by calcium/calmodulin binding, superoxide formation is increased. Heme-iron ligands, cyanide, imidazole, and the phenyl(diazene)-derived radical inhibit superoxide generation. No inhibition is observed after addition of L-arginine, NG-hydroxy-L-arginine, L-thiocitrulline, and L-NG-monomethyl arginine to activated eNOS. These results demonstrate that superoxide is generated from the oxygenase domain by dissociation of the ferrous-dioxygen complex and that occupation of the L-arginine binding site does not inhibit this process. However, the concomitant addition of L-arginine and tetrahydrobiopterin (BH4) abolishes superoxide generation by eNOS. Under these conditions, L-citrulline production is close to maximal. Our data indicate that BH4 fully couples L-arginine oxidation to NADPH consumption and prevents dissociation of the ferrous-dioxygen complex. Under these conditions, eNOS does not generate superoxide. The presence of flavins, at concentrations commonly employed in NOS assay systems, enhances superoxide generation from the reductase domain. Our data indicate that modulation of BH4 concentration may regulate the ratio of superoxide to nitric oxide generated by eNOS.

The protein inhibitor of neuronal nitric oxide synthase (PIN): characterization of its action on pure nitric oxide synthases

Neuronal NO synthase (nNOS) was discovered recently to interact specifically with the protein PIN (protein inhibitor of nNOS) [Jaffrey, S.R. and Snyder, S.H. (1996) Science 274, 774-777]. We have studied the effects on pure NOS enzymes of the same GST-tagged PIN used in the original paper. Unexpectedly, all NOS isoenzymes were inhibited. The IC50 for nNOS was 18 +/- 6 microM GST-PIN with 63 nM nNOS after 30 min at 37 degrees C. Uncoupled NADPH oxidation was inhibited similarly, whereas cytochrome c reductase activity, the K(M) for L-arginine, and dimerization were unaffected. We reconsider the physiological role of PIN in the light of these results.

Effects of pH on the structure and function of neuronal nitric oxide synthase

We investigated how pH affects rat brain neuronal nitric oxide synthase (nNOS) with regard to spin-state equilibrium and the thiolate ligand bond of the haem group, catalytic activity, and monomerleft and right arrow dimer equilibrium. At neutral pH, nNOS containing 1 equiv. of (6R)-5,6,7,8-tetrahydro-l-biopterin (BH4) per dimer was mostly high-spin (lambdamax at 398 nm), whereas the BH4-free enzyme consisted of a mixture of the high-spin and two low-spin forms (lambdamax at 418 nm, and at 376 and 456 nm respectively). With BH4-free nNOS, an appreciable high-spin fraction was only observed between pH 7 and 8; at pH 6 and 9, the 418 and 376/456 nm low-spin forms predominated respectively. With nNOS containing 1 equiv. of BH4 per dimer, similar observations were made, but these involved only half of the enzyme; the other half, presumably the BH4-containing subunits, remained high-spin. Since the spin state in the BH4-free subunit appeared little affected by the state of the other subunit, we conclude that, in dimeric nNOS, the two haem groups function independently. Low pH destabilized thiolate binding and the interaction between NOS subunits, as indicated by CO-binding studies and gel electrophoresis respectively. Formation of l-citrulline was optimal between pH 7.0 and 7.5; the decrease in NOS activity at lower pH proved to be due to uncoupling of NADPH oxidation, resulting in increased formation of H2O2. At high pH strict coupling of l-arginine and NADPH oxidation was maintained, even in the absence of exogenous BH4. The possible pathophysiological implications of the uncoupling at low pH are discussed.

Electron transfer and catalytic activity of nitric oxide synthases. Chimeric constructs of the neuronal, inducible, and endothelial isoforms

The nitric oxide synthases (NOS) are single polypeptides that encode a heme domain, a calmodulin binding motif, and a flavoprotein domain with sequence similarity to P450 reductase. Despite this basic structural similarity, the three major NOS isoforms differ significantly in their rates of .NO synthesis, cytochrome c reduction, and NADPH utilization and in the Ca2+ dependence of these rates. To assign the origin of these differences to specific protein domains, we constructed chimeras in which the reductase domains of endothelial and inducible NOS, respectively, were replaced by the reductase domain of neuronal NOS. The results with the chimeric proteins confirm the modular organization of the NOS polypeptide chain and demonstrate that (a) similar residues establish the necessary contacts between the reductase and heme domains in the three NOS isoforms, (b) the maximal rate of .NO synthesis is determined by the maximum intrinsic ability of the reductase domain to deliver electrons to the heme domain, (c) the Ca2+ independence of inducible NOS requires interactions of calmodulin with both the calmodulin binding motif and the flavoprotein domain, and (d) the effects of tetrahydrobiopterin and L-arginine on electron transfer rates are mediated exclusively by heme domain interactions.

Biosynthesis and action of nitric oxide in mammalian cells

Nitric oxide (NO) can act as a vasorelaxant, a modulator of neurotransmission and a defence against pathogens. However, under certain conditions, NO can also have damaging effects to cells. Whether NO is useful or harmful depends on its chemical fate, and on the rate and location of its production. Here, we discuss progress in NO chemistry and the enzymology of NO synthases, and we will also attempt to explain its actions in the cardiovascular, nervous and immune systems.

Allosteric modulation of rat brain nitric oxide synthase by the pterin-site enzyme inhibitor 4-aminotetrahydrobiopterin

We investigated the functional and allosteric effects of the 4-amino analogue of tetrahydrobiopterin, (6R)-2,4-diamino- 5,6,7,8-tetrahydro-6-(L-erythro-1,2-dihydroxypropyl) pteridine (4-amino-H4biopterin) on pteridine-free rat neuronal nitric oxide synthase. In the presence of added (6R)-5,6,7,8-tetrahydro-L-erythrobiopterin (H4biopterin; 10 microM), 4-amino-H4biopterin completely inhibited the conversion of both L-arginine and NG-hydroxy-L-arginine with half-maximally effective concentrations of 1.1+/-0.09 and 1.3+/-0.09 microM, respectively. Inhibition was reversible, as shown by a time-dependent restoration of citrulline formation upon dilution of the inhibitor-treated enzyme (t1/2=3.0 min). Binding of 4-amino-H4biopterin led to a complete conversion of the haem from low-spin to high-spin state, and to the formation of stable homodimers which partially survived electrophoresis under denaturating conditions. These results show that oxidation of both L-arginine and NG-hydroxy-L-arginine is pteridine-dependent, and that the allosteric effects of H4biopterin do not fully explain the essential role of the pteridine cofactor in nitric oxide biosynthesis.

Tetrahydrobiopterin regulates superoxide and nitric oxide generation by recombinant endothelial nitric oxide synthase

Nitric oxide (NO) produced by the endothelial isoform of nitric oxide synthase (NOS III) is a key determinant of the anti-atherosclerotic properties of the endothelium. Recent in vivo studies suggest that NOS III may also be a source of superoxide production, which would limit its role as a NO-producing enzyme. In the current study we examined both the NO and the superoxide generating potential of recombinant NOS III obtained from a baculovirus/Sf9 expression system. Using lucigenin chemiluminesence we could indeed demonstrate (superoxide dismutase inhibitable) superoxide production by NOS III. This superoxide production was not affected by administration of L-arginine, but could be inhibited dose-dependently by the co-factor tetrahydrobiopterin (BH4). BH4 also dose dependently decreased superoxide generation by hypoxanthine/xantine oxidase suggesting a direct antioxidant effect. Superoxide generation by NOS III could be completely inhibited by diphenyleneiodonium (DPI), an inhibitor of the flavin moiety of the enzyme, indicating that this group is a main source of superoxide production by the enzyme. Using measurement of [3H-L-arginine] conversion to [3H-L-citrulline], it appeared that BH4 directly increased the production of NO by NOS III. In addition, we observed that BH4 stablized the NOS III in its dimeric form, suggesting that an effect on allosteric conformation could be involved in this effect on NO production. NOS III thus appears to be a superoxide generating enzyme probably through its flavin moiety, as well as a BH4-dependent NO producing enzyme.