Two modes of binding of N-hydroxyguanidines to NO synthases: first evidence for the formation of iron-N-hydroxyguanidine complexes and key role of tetrahydrobiopterin in determining the binding mode

Oxygen activation in NO synthases: evidence for a direct role of the substrate

Nitric oxide (NO) and the other reactive nitrogen species (RNOS) play crucial patho‐physiological roles at the interface of oxidative stress and signalling processes. In mammals, the NO synthases (NOSs) are the source of these reactive nitrogen species, and so to understand the precise biological role of RNOS and NO requires elucidation of the molecular functioning of NOS. Oxygen activation, which is at the core of NOS catalysis, involves a sophisticated sequence of electron and proton transfers. While electron transfer in NOS has received much attention, the proton transfer processes has been scarcely investigated. Here, we report an original approach that combines fast‐kinetic techniques coupled to resonance Raman spectroscopy with the use of synthetic analogues of NOS substrate. We characterise Fe^II‐O₂ reaction intermediates in the presence of L‐arginine (Arg), alkyl‐ and aryl‐guanidines. The presence of new reaction intermediates, such as ferric haem‐peroxide, that was formerly postulated, was tracked by analysing the oxygen activation reaction at different times and with different excitation wavelengths. Our results suggest that Arg is not a proton donor, but indirectly intervenes in oxygen activation mechanism by modulating the distal H‐bond network and, in particular, by tuning the position and the role of the distal water molecule. This report supports a catalytic model with two proton transfers in step 1 (Arg hydroxylation) but only one proton transfer in step 2 (N^ω‐hydroxy‐L‐arginine oxidation).

Enzymatic and cryoreduction EPR studies of the hydroxylation of methylated N(ω)-hydroxy-L-arginine analogues by nitric oxide synthase from Geobacillus stearothermophilus

Nitric oxide synthase (NOS) catalyzes the conversion of l-arginine to l-citrulline and NO in a two-step process involving the intermediate N^ω-hydroxy-l-arginine (NHA). It was shown that Cpd I is the oxygenating species for l-arginine; the hydroperoxo ferric intermediate is the reactive intermediate with NHA. Methylation of the N^ω-OH and N^ω-H of NHA significantly inhibits the conversion of NHA into NO and l-citrulline by mammalian NOS. Kinetic studies now show that N^ω-methylation of NHA has a qualitatively similar effect on H₂O₂-dependent catalysis by bacterial gsNOS. To elucidate the effect of methylating N^ω-hydroxy l-arginine on the properties and reactivity of the one-electron-reduced oxy-heme center of NOS, we have applied cryoreduction/annealing/EPR/ENDOR techniques. Measurements of solvent kinetic isotope effects during 160 K cryoannealing cryoreduced oxy-gsNOS/NHA confirm the hydroperoxo ferric intermediate as the catalytically active species of step two. Product analysis for cryoreduced samples with methylated NHA’s, NHMA, NMOA, and NMMA, annealed to 273 K, show a correlation of yields of l-citrulline with the intensity of the g 2.26 EPR signal of the peroxo ferric species trapped at 77 K, which converts to the reactive hydroperoxo ferric state. There is also a correlation between the yield of l-citrulline in these experiments and k_obs for the H₂O₂-dependent conversion of the substrates by gsNOS. Correspondingly, no detectable amount of cyanoornithine, formed when Cpd I is the reactive species, was found in the samples. Methylation of the NHA guanidinium N^ω-OH and N^ω-H inhibits the second NO-producing reaction by favoring protonation of the ferric-peroxo to form unreactive conformers of the ferric-hydroperoxo state. It is suggested that this is caused by modification of the distal-pocket hydrogen-bonding network of oxy gsNOS and introduction of an ordered water molecule that facilitates delivery of the proton(s) to the one-electron-reduced oxy-heme moiety. These results illustrate how variations in the properties of the substrate can modulate the reactivity of a monooxygenase.

EPR and ENDOR characterization of the reactive intermediates in the generation of NO by cryoreduced oxy-nitric oxide synthase from Geobacillus stearothermophilus

Cryoreduction EPR/ENDOR/step-annealing measurements with substrate complexes of oxy-gsNOS (3; gsNOS is nitric oxide synthase from Geobacillus stearothermophilus) confirm that Compound I (6) is the reactive heme species that carries out the gsNOS-catalyzed (Stage I) oxidation of L-arginine to N-hydroxy-L-arginine (NOHA), whereas the active species in the (Stage II) oxidation of NOHA to citrulline and HNO/NO(-) is the hydroperoxy-ferric form (5). When 3 is reduced by tetrahydrobiopterin (BH4), instead of an externally supplied electron, the resulting BH4(+) radical oxidizes HNO/NO(-) to NO. In this report, radiolytic one-electron reduction of 3 and its complexes with Arg, Me-Arg, and NO(2)Arg was shown by EPR and (1)H and (14,15)N ENDOR spectroscopies to generate 5; in contrast, during cryoreduction of 3/NOHA, the peroxo-ferric-gsNOS intermediate (4/NOHA) was trapped. During annealing at 145 K, ENDOR shows that 5/Arg and 5/Me-Arg (but not 5/NO(2)Arg) generate a Stage I primary product species in which the OH group of the hydroxylated substrate is coordinated to Fe(III), characteristic of 6 as the active heme center. Analysis shows that hydroxylation of Arg and Me-Arg is quantitative. Annealing of 4/NOHA at 160 K converts it first to 5/NOHA and then to the Stage II primary enzymatic product. The latter contains Fe(III) coordinated by water, characteristic of 5 as the active heme center. It further contains quantitative amounts of citrulline and HNO/NO(-); the latter reacts with the ferriheme to form the NO-ferroheme upon further annealing. Stage I delivery of the first proton of catalysis to the (unobserved) 4 formed by cryoreduction of 3 involves a bound water that may convey a proton from L-Arg, while the second proton likely derives from the carboxyl side chain of Glu 248 or the heme carboxylates; the process also involves proton delivery by water(s). In the Stage II oxidation of NOHA, the proton that converts 4/NOHA to 5/NOHA likely is derived from NOHA itself, a conclusion supported by the pH invariance of the process. The present results illustrate how the substrate itself modulates the nature and reactivity of intermediates along the monooxygenase reaction pathway.

4,4-Difluorinated analogues of l-arginine and N(G)-hydroxy-l-arginine as mechanistic probes for nitric oxide synthase

4,4-Difluoro-l-arginine and 4,4-difluoro-N(G)-hydroxy-l-arginine were synthesized and shown to be substrates for the inducible isoform of nitric oxide synthase (iNOS). Binding of both fluorinated analogues to the NOS active site was also investigated using a spectral binding assay employing a heme domain construct of the inducible NOS isoform (iNOS(heme)). 4,4-Difluoro-N(G)-hydroxy-arginine was found to bind at the NOS active site in a unique manner consistent with a model involving ligation of the Fe(III) heme center by the oxygen atom of the N(G)-hydroxy moiety.

Oxygen-insensitive enzymatic reduction of oximes to imines

The reduction of oximes to imines under anaerobic and aerobic conditions was studied using (E)- and (Z)-2,4,6-trimethylacetophenone oxime, benzaldoxime and (E)-2,4,6-trimethylbenzaldoxime. Pig and human liver microsomes, pig liver mitochondria and cytosol to a minor extent catalyzed the conversion of both isomeric ketoximes to the corresponding stable imine, the (E)-isomer being the better substrate. All reactions were oxygen-insensitive and required active protein and NADH or NADPH; however, NADH was preferred as cofactor. The reconstituted liver microsomal system of a pig liver CYP2D enzyme (NADH-benzamidoxime reductase), which is known to reduce N-hydroxylated derivatives of strongly basic functional groups, such as amidoximes, is also capable of reducing oximes. As expected, the corresponding imine was detected in relevant amounts when incubating 2,4,6-trimethyl-acetophenone oxime using the reconstituted enzyme system, but reduction rates were significantly lower compared to rates obtained when incubating benzamidoxime. Steric hindrance due to the methyl groups in ortho position to the oxime functionality could be excluded as being responsible for the lower conversion rates according to results obtained in incubations of 2,4,6-trimethylbenzamidoxime. When incubating benzaldoxime, only benzoic acid could be detected as metabolite, since the aldehyde is easily oxidized during incubation procedures, whereas incubations of (E)-2,4,6-trimethylbenzaldoxime also showed the formation of the corresponding aldehyde. These results allow us to postulate that the metabolism of aldoximes like 2,4,6-trimethylbenzaldoxime most likely proceeds through enzymatic reduction of the oxime to yield the intermediate imine, which is subsequently hydrolyzed to the aldehyde and then oxidized to the corresponding benzoic acid.

Relationship between the structure of guanidines and N-hydroxyguanidines, their binding to inducible nitric oxide synthase (iNOS) and their iNOS-catalysed oxidation to NO

The binding of several alkyl- and aryl-guanidines and N-hydroxyguanidines to the oxygenase domain of inducible NO-synthase (iNOS(oxy)) was studied by UV/Vis difference spectroscopy. In a very general manner, monosubstituted guanidines exhibited affinities for iNOS(oxy) that were very close to those of the corresponding N-hydroxyguanidines. The highest affinities were observed for the natural substrates, L-arginine and N(omega)-hydroxy-L-arginine (K(d) at the microm level). The deletion of either the CO2H or the NH2 function of their amino acid moiety led to dramatic decreases in the affinity. However, alkylguanidines with a relatively small alkyl chain exhibited interesting affinities, the best being observed for a butyl chain (K(d) =20 microM). Arylguanidines also bound to iNOS(oxy), however, with lower affinities (K(d) > 250 microm). Many N-alkyl- and N-aryl-N'-hydroxyguanidines are oxidized by iNOS with formation of NO, whereas only few alkylguanidines led to significant production of NO under identical conditions, and all the arylguanidines tested to date were unable to lead to the production of NO. The k(cat) values of NO production from the oxidation by iNOS of the studied N-hydroxyguanidines were found to vary independently of their affinity for the protein. The k(cat) values determined for the two-step oxidation of alkylguanidines to NO were not clearly related to the K(d) of these substrates toward iNOS(oxy). However, there is a qualitative relationship between these k(cat) values and the apparent rate constants of dissociation of the complex between iNOS(oxy) and the corresponding N-alkyl-N'-hydroxyguanidine (k(off) (app)) that were determined by stopped-flow UV/Vis spectroscopy. These data indicate that a key factor for efficient oxidation of a guanidine by iNOS to NO is the ability of the corresponding N-hydroxyguanidine to bind to the active site without being too rapidly released before its further oxidation. This explains why 4,4,4-trifluorobutylguanidine is so far the best non-alpha-amino acid guanidine substrate of iNOS with formation of NO, because the k(off) (app) of the corresponding N-hydroxyguanidine is particularly low. This suggests that the rational design of guanidines as new NO donors upon in situ oxidation by NOSs should take into account both thermodynamic and kinetic characteristics of the interaction of the protein not only with the guanidine but also with the corresponding N-hydroxyguanidine.

Importance of valine 567 in substrate recognition and oxidation by neuronal nitric oxide synthase

Nitric oxide (NO) is synthesised by a two-step oxidation of -arginine (L-Arg) in the active site of nitric oxide synthase (NOS) with formation of an intermediate, N omega-hydroxy-L-Arg (NOHA). Crystal structures of NOSs have shown the importance of an active-site Val567 residue (numbered for rat neuronal NOS, nNOS) interacting with non-amino acid substrates. To investigate the role of this Val residue in substrate recognition and NO-formation activity by nNOS, we generated and purified four Val567 mutants of nNOS, Val567Leu, Val567Phe, Val567Arg and Val567Glu. We characterized these proteins and tested their ability to generate NO from the oxidation of natural substrates L-Arg and NOHA, and from N-hydroxyguanidines previously identified as alternative substrates for nNOS. The Val567Leu mutant displayed lower NO formation activities than the wild type (WT) in the presence of all tested compounds. Surprisingly, the Val567Phe mutant formed low amounts of NO only from NOHA. These two mutants displayed lower affinity for L-Arg and NOHA than the WT protein. Val576Glu and Val567Arg mutants were much less stable and did not lead to any formation of NO. These results suggest that Val567 is an important residue for preserving the integrity of the active site, for substrate binding, and subsequently for NO-formation in nNOS.

iNOS-mediated nitric oxide production and its regulation

This review focuses on the production of nitric oxide (NO) by inducible nitric oxide synthase (iNOS) and its regulation under physiological and pathophysiological conditions. NO is an important biological mediator in the living organism that is synthesized from L-arginine using NADPH and molecular oxygen. However, the overproduction of NO which is catalyzed by iNOS, a soluble enzyme and active in its dimeric form, is cytotoxic. Immunostimulating cytokines or bacterial pathogens activate iNOS and generate high concentrations of NO through the activation of inducible nuclear factors, including NFkB. iNOS activation is regulated mainly at the transcriptional level, but also at posttranscriptional, translational and postranslational levels through effects on protein stability, dimerization, phosphorylation, cofactor binding and availability of oxygen and L-arginine as substrates. The prevention of the overproduction of NO in the living organism through control of regulatory pathways may assist in the treatment of high NO-mediated disorders without changing physiological levels of NO.

Oltipraz inhibits inducible nitric oxide synthase in vitro and inhibits nitric oxide production in activated microglial cells

The clinically relevant drug oltipraz (OPZ) has previously been shown to inhibit cytochrome P450 enzymes [Chem. Res. Toxicol. 13 (2000) 245]. The current study reveals that OPZ is also able to inhibit *NO formation by purified inducible nitric oxide synthase (iNOS) but not by neuronal nitric oxide synthase in hemoglobin assays. The inhibition of iNOS by OPZ is reversible and competitive with an IC(50) of 5.9 microM and Ki of 0.6 microM. In murine BV-2 microglial cells, an immortalized cell line that produces *NO in response to lipopolysaccharide (LPS), OPZ is able to block the formation of nitrite in LPS treated cells. The inhibitory effect of OPZ on LPS treated cells is not due to cell toxicity. Finally, treatment of cells with OPZ does not induce or suppress expression of iNOS protein as shown by Western blot analysis.