Synthesis and bioactivity of N omega-hydroxyarginine: a possible intermediate in the biosynthesis of nitric oxide from arginine

Functional regulation of microglia by vitamin B12 alleviates ischemic stroke-induced neuroinflammation in mice

Ischemic stroke is the second leading cause of death and disability worldwide, and efforts to prevent stroke, mitigate secondary neurological damage, and promote neurological recovery remain paramount. Recent findings highlight the critical importance of microbiome-related metabolites, including vitamin B12 (VB12), in alleviating toxic stroke-associated neuroinflammation. Here, we showed that VB12 tonically programmed genes supporting microglial cell division and activation and critically controlled cellular fatty acid metabolism in homeostasis. Intriguingly, VB12 promoted mitochondrial transcriptional and metabolic activities and significantly restricted stroke-associated gene alterations in microglia. Furthermore, VB12 differentially altered the functions of microglial subsets during the acute phase of ischemic stroke, resulting in reduced brain damage and improved neurological function. Pharmacological depletion of microglia before ischemic stroke abolished VB12-mediated neurological improvement. Thus, our preclinical studies highlight the relevance of VB12 in the functional programming of microglia to alleviate neuroinflammation, minimize ischemic injury, and improve host neurological recovery after ischemic stroke.

Integrated multi-omics reveal important roles of gut contents in intestinal ischemia–reperfusion induced injuries in rats

Intestinal ischemia–reperfusion (IIR) is a life-threatening clinical event with damaging signals whose origin and contents are unclear. Here we observe that IIR significantly affect the metabolic profiles of most organs by unbiased organ-wide metabolic analysis of gut contents, blood, and fifteen organs in rats (n = 29). Remarkably, correlations between gut content metabolic profiles and those of other organs are the most significant. Gut contents are also the only ones to show dynamic correlations during IIR. Additionally, according to targeted metabolomics analysis, several neurotransmitters are considerably altered in the gut during IIR, and displayed noteworthy correlations with remote organs. Likewise, metagenomics analysis (n = 35) confirm the effects of IIR on gut microbiota, and identify key species fundamental to the changes in gut metabolites, particularly neurotransmitters. Our multi-omics results establish key roles of gut contents in IIR induced remote injury and provide clues for future exploration.

Die Dai et al. evaluate changes in the metabolomic and gut microbiome in response to experimental intestinal ischemia reperfusion (IIR) injury in rats. Their results provide further insight into how gut contents contribute to widespread injury in IIR patients.

A recent history of nitroxyl chemistry, pharmacology and therapeutic potential

Due to the excitement surrounding the discovery of NO as an endogenously generated signalling molecule, a number of other nitrogen oxides were also investigated as possible physiological mediators. Among these was nitroxyl (HNO). Over the past 25 years or so, a significant amount of work by this laboratory and many others has disclosed that HNO possesses unique chemical properties and important pharmacological utility. Indeed, the pharmacological potential for HNO as a treatment for heart failure, among other uses, has garnered this curious molecule a considerable amount of recent attention. This review summarizes the events that led to this recent attention as well as poses important questions that are still to be answered with regards to understanding the chemistry and biology of HNO. LINKED ARTICLES: This article is part of a themed section on Nitric Oxide 20 Years from the 1998 Nobel Prize. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.2/issuetoc.

Unusual Amino Acids in Medicinal Chemistry

Unusual amino acids are fundamental building blocks of modern medicinal chemistry. The combination of readily functionalized amine and carboxyl groups attached to a chiral central core along with one or two potentially diverse side chains provides a unique three-dimensional structure with a high degree of functionality. This makes them invaluable as starting materials for syntheses of complex molecules, highly diverse elements for SAR campaigns, integral components of peptidomimetic drugs, and potential drugs on their own. This Perspective highlights the diversity of unnatural amino acid structures found in hit-to-lead and lead optimization campaigns and clinical stage and approved drugs, reflecting their increasingly important role in medicinal chemistry.

Development and characterization of glutamyl-protected N-hydroxyguanidines as reno-active nitric oxide donor drugs with therapeutic potential in acute renal failure

Acute renal failure (ARF) has high mortality and no effective treatment. Nitric oxide (NO) delivery represents a credible means of preventing the damaging effects of vasoconstriction, central to ARF, but design of drugs with the necessary renoselectivity is challenging. Here, we developed N-hydroxyguanidine NO donor drugs that were protected against spontaneous NO release by linkage to glutamyl adducts that could be cleaved by γ-glutamyl transpeptidase (γ-GT), found predominantly in renal tissue. Parent NO donor drug activity was optimized in advance of glutamyl adduct prodrug design. A lead compound that was a suitable substrate for γ-GT-mediated deprotection was identified. Metabolism of this prodrug to the active parent compound was confirmed in rat kidney homogenates, and the prodrug was shown to be an active vasodilator in rat isolated perfused kidneys (EC50 ~50 μM). The data confirm that glutamate protection of N-hydroxyguanidines is an approach that might hold promise in ARF.

Inducible nitric oxide synthase and inflammation

Nitric oxide (NO), derived from L-arginine (L-Arg) by the enzyme nitric oxide synthase (NOS), is involved in acute and chronic inflammatory events. In view of the complexity associated with the inflammatory response, the dissection of possible mechanisms by which NO modulates this response will be profitable in designing novel and more efficacious NOS inhibitors. In this review we describe the consequences associated with the induction of inducible nitric oxide synthase (iNOS) and its therapeutic implications.

Aromatic N-hydroxyguanidines as new reduction cosubstrates for dopamine β-hydroxylase

Conversion of neurotransmitter dopamine into norepinephrine is catalyzed by dopamine beta-hydroxylase (DbH). The reaction requires the presence of both molecular oxygen and a reducing cosubstrate, the assumed physiological cosubstrate being ascorbic acid. We have investigated the ability of a new family of molecules, N-aryl-N'-hydroxyguanidines, to serve as cosubstrates for DbH. N-(4-Methoxyphenyl)-N'-hydroxyguanidine proved to be an efficient reducing agent for DbH. The complete N-hydroxyguanidine moiety was required for activity, as any modification of this function resulted in non-cosubstrate compounds. Moreover, analysis of the products formed from N-(4-methoxyphenyl)-N'-hydroxyguanidine showed that the main oxidation product was a nitrosoimine. Modification of the aromatic para-substituent evidenced an influence of its electronic properties on the catalytic activity whereas steric factors seemed less important. In addition, changing the methoxy-substituent from the para- to the ortho-position led to an inactive compound. Our results demonstrate that N-aryl-N'-hydroxyguanidines are new efficient reducing cosubstrates for DbH and prove that specific interactions with the reducing cosubstrate do take place at the active site of the enzyme.

Triplex selective 2-(2-naphthyl)quinoline compounds: origins of affinity and new design principles

A novel competition dialysis assay was used to investigate the structural selectivity of a series of substituted 2-(2-naphthyl)quinoline compounds designed to target triplex DNA. The interaction of 14 compounds with 13 different nucleic acid sequences and structures was studied. A striking selectivity for the triplex structure poly dA:[poly dT](2) was found for the majority of compounds studied. Quantitative analysis of the competition dialysis binding data using newly developed metrics revealed that these compounds are among the most selective triplex-binding agents synthesized to date. A quantitative structure-affinity relationship (QSAR) was derived using triplex binding data for all 14 compounds used in these studies. The QSAR revealed that the primary favorable determinant of triplex binding free energy is the solvent accessible surface area. Triplex binding affinity is negatively correlated with compound electron affinity and the number of hydrogen bond donors. The QSAR provides guidelines for the design of improved triplex-binding agents.

Synthesis and activity of substituted 2-phenylquinolin-4-amines, antagonists of immunostimulatory CpG-oligodeoxynucleotides

Fifty-seven 2-phenylquinolines substituted at the phenyl group and C4 of the quinoline were synthesized and analyzed for inhibition of the immunostimulatory effect of oligodeoxynucleotides with a CpG-motif. The Fujita-Ban variant of the classical Free-Wilson analysis gave a highly significant correlation for a series of 48 relatively small molecules demonstrating that (i) the partial contributions of substituents to biological activity (EC(50)) are additive and (ii) assuming similar bioavailability for all quinolines studied, the larger molecules cannot be accommodated within a still unknown biological receptor. The results suggest interaction of a basic antagonist molecule with weakly acidic groups in the antagonist-receptor complex. N-[2-(Dimethylamino)ethyl]-2-[4-(4-methylpiperazino)phenyl]quinolin-4-amine (50) is the most effective antagonist found in this study (EC(50) = 0.76 nM).

N-Aryl N'-hydroxyguanidines, a new class of NO-donors after selective oxidation by nitric oxide synthases: structure-activity relationship

The formation of nitric oxide (NO) was followed during the oxidation of 37 N-hydroxyguanidines or related derivatives, including 18 new N-aryl N'-hydroxyguanidines, by recombinant inducible nitric oxide synthase (NOS II). Several N-aryl N'-hydroxyguanidines bearing a relatively small, electron-donating para subtituent, such as H, F, Cl, CH(3), OH, OCH(3), and NH(2), led to NO formation rates between 8 and 41% of that of NO formation from the natural NOS substrate, N(omega)-hydroxy-L-arginine (NOHA). The characteristics of these reactions were very similar to those previously reported for the oxidation of NOHA by NOS:(i) the strict requirement of NOS containing (6R)-5,6,7,8-tetrahydro-L-biopterin, reduced nicotinamide adenine dinucleotide phosphate, and O(2) for the oxidation to occur, (ii) the formation of NO and the corresponding urea in a 1:1 molar ratio, and (iii) a strong inhibitory effect of the classical NOS inhibitors such as N(omega)-nitro-L-arginine and S-ethyl-iso-thiourea. Structure-activity relationship studies showed that two structural factors are crucial for NO formation from compounds containing a C(triple bond)NOH function. The first one is the presence of a monosubstituted N-hydroxyguanidine function, since disubstituted N-hydroxyguanidines, amidoximes, ketoximes, and aldoximes failed to produce NO. The second one is the presence of a N-phenyl ring bearing a relatively small, not electron-withdrawing para substituent that could favorably interact with a hydrophobic cavity close to the NOS catalytic site. The k(cat) value for NOS II-catalyzed oxidation of N-para-fluorophenyl N'-hydroxyguanidine was 80% of that found for NOHA, and its k(cat)/K(m) value was only 9-fold lower than that of NOHA. Interestingly, the K(m) value found for NOS II-catalyzed oxidation of N-(3-thienyl) N'-hydroxyguanidine was 25 microM, almost identical to that of NOHA. Recombinant NOS I and NOS III also oxidize several N-aryl N'-hydroxyguanidines with the formation of NO, with a clearly different substrate specificity. The best substrates of the studied series for NOS I and NOS III were N-(para-hydroxyphenyl) and N-(meta-aminophenyl) N'-hydroxyguanidine, respectively. Among the studied compounds, the para-chlorophenyl and para-methylphenyl derivatives were selective substrates of NOS II. These results open the way toward a new class of selective NO donors after in situ oxidation by each NOS family.

N-hydroxyguanidines as new heme ligands: UV-visible, EPR, and resonance Raman studies of the interaction of various compounds bearing a C=NOH function with microperoxidase-8

Interaction between microperoxidase-8 (MP8), a water-soluble hemeprotein model, and a wide range of N-aryl and N-alkyl N'-hydroxyguanidines and related compounds has been investigated using UV-visible, EPR, and resonance Raman spectroscopies. All the N-hydroxyguanidines studied bind to the ferric form of MP8 with formation of stable low-spin iron(III) complexes characterized by absorption maxima at 405, 535, and 560 nm. The complex obtained with N-(4-methoxyphenyl) N'-hydroxyguanidine exhibits EPR g-values at 2.55, 2.26, and 1.86. The resonance Raman (RR) spectrum of this complex is also in agreement with an hexacoordinated low-spin iron(III) structure. The dissociation constants (K(s)) of the MP8 complexes with mono- and disubstituted N-hydroxyguanidines vary between 15 and 160 microM at pH 7.4. Amidoximes also form low-spin iron(III) complexes of MP8, although with much larger dissociation constants. Under the same conditions, ketoximes, aldoximes, methoxyguanidines, and guanidines completely fail to form such complexes with MP8. The K(s) values of the MP8-N-hydroxyguanidine complexes decrease as the pH of the solution is increased, and the affinity of the N-hydroxyguanidines toward MP8 increases with the pK(a) of these ligands. Altogether these results show that compounds involving a -C(NHR)=NOH moiety act as good ligands of MP8-Fe(III) with an affinity that depends on the electron-richness of this moiety. The analysis of the EPR spectrum of the MP8-N-hydroxyguanidine complexes according to Taylor's equations shows a strong axial distortion of the iron, typical of those observed for hexacoordinated heme-Fe(III) complexes with at least one pi donor axial ligand (HO(-), RO(-), or RS(-)). These data strongly suggest that N-hydroxyguanidines bind to MP8 iron via their oxygen atom after deprotonation or weakening of their O-H bond. It thus seems that N-hydroxyguanidines could constitute a new class of strong ligands for hemeproteins and iron(III)-porphyrins.

Oxidations of N(omega)-hydroxyarginine analogues and various N-hydroxyguanidines by NO synthase II: key role of tetrahydrobiopterin in the reaction mechanism and substrate selectivity

Oxidations of L-arginine 2, homo-L-arginine 1, their N(omega)-hydroxy derivatives 4 and 3 (NOHA and homo-NOHA, respectively), and four N-hydroxyguanidines, N(omega)-hydroxynor-L-arginine 5 (nor-NOHA), N(omega)-hydroxydinor-L-arginine 6 (dinor-NOHA), N-(4-chlorophenyl)-N'-hydroxyguanidine (8), and N-hydroxyguanidine (7) itself, by either NOS II or (6R)-5,6,7,8-tetrahydro-L-biopterin (BH4)-free NOS II, have been studied in a comparative manner. Recombinant BH4-free NOS II catalyzes the oxidation of all N-hydroxyguanidines by NADPH and O2, with formation of NO2(-) and NO3(-) at rates between 20 and 80 nmol min(-1) (mg of protein)(-1). In the case of compound 8, formation of the corresponding urea and cyanamide was also detected besides that of NO2(-) and NO3(-). These BH4-free NOS II-dependent reactions are inhibited by modulators of electron transfer in NOS such as thiocitrulline (TC) or imidazole (ImH), but not by Arg, and are completely suppressed by superoxide dismutase (SOD). They exhibit characteristics very similar to those previously reported for microsomal cytochrome P450-catalyzed oxidation of N-hydroxyguanidines. Both P450 and BH4-free NOS II reactions appear to be mainly performed by O2(.-) derived from the oxidase function of those heme proteins. In the presence of increasing concentrations of BH4, these nonselective oxidations progressively disappear while a much more selective monooxygenation takes place only with the N-hydroxyguanidines that are recognized well by NOS II, NOHA, homo-NOHA, and 8. These monooxygenations are much more chemoselective (8 being selectively transformed into the corresponding urea and NO) and are inhibited by Arg but not by SOD, as expected for reactions performed by the NOS Fe(II)-O2 species. Altogether, these results provide a further clear illustration of the key role of BH4 in regulating the monooxygenase/oxidase ratio in NOS. They also suggest a possible implication of NOSs in the oxidative metabolism of certain classes of xenobiotics such as N-hydroxyguanidines, not only via their monooxygenase function but also via their oxidase function.

Recognition of alpha-amino acids bearing various C=NOH functions by nitric oxide synthase and arginase involves very different structural determinants

Several alpha-amino acids bearing a C=NOH function separated from the Calpha carbon by two to five atoms have been synthesized and tested as substrates or inhibitors of recombinant nitric oxide synthases (NOS) I and II and as inhibitors of rat liver arginase (RLA). These include four N-hydroxyguanidines, N(omega)-hydroxy-L-arginine (NOHA) and its analogues homo-NOHA, nor-NOHA, and dinor-NOHA, two amidoximes bearing the -NH-C(CH(3))=NOH group, and two amidoximes bearing the -CH(2)-C(NH(2))=NOH group. Their behavior toward NOS and RLA was compared to that of the corresponding compounds bearing a C=NH function instead of the C=NOH function. The results obtained clearly show that efficient recognition of these alpha-amino acids by NOS and RLA involves very different structural determinants. NOS favors molecules bearing a -NH-C(R)=NH motif separated from Calpha by three or four CH(2) groups, such as arginine itself, with the necessary presence of delta-NH and omega-NH groups and a more variable R substituent. The corresponding molecules with a C=NOH function exhibit a much lower affinity for NOS. On the contrary, RLA best recognizes molecules bearing a C=NOH function separated from Calpha by three or four atoms, the highest affinity being observed in the case of three atoms. The presence of two omega-nitrogen atoms is important for efficient recognition, as in the two best RLA inhibitors, N(omega)-hydroxynorarginine and N(omega)-hydroxynorindospicine, which exhibit IC(50) values at the micromolar level. However, contrary to what was observed in the case of NOS, the presence of a delta-NH group is not important. These different structural requirements of NOS and RLA may be directly linked to the position of crucial residues that have been identified from crystallographic data in the active sites of both enzymes. Thus, binding of arginine analogues to NOS particularly relies on strong interactions of their delta-NH and omega-NH(2) groups with glutamate 371 (of NOS II), whereas binding of C=NOH molecules to RLA is mainly based on interactions of their terminal OH group with the binuclear Mn(II).Mn(II) cluster of the enzyme and on possible additional bonds between their omega-NH(2) group with histidine 141, glutamate 277, and one Mn(II) ion. The different modes of interaction displayed by both enzymes depend on their different catalytic functions and give interesting opportunities to design useful molecules to selectively regulate NOS and arginase.

Application of similarity matrices and genetic neural networks in quantitative structure-activity relationships of 2- or 4-(4-Methylpiperazino)pyrimidines: 5-HT(2A) receptor antagonists

Antagonists of the 5-HT(2A) receptor are being used to treat many psychiatric disorders. The present work focuses on a group of 27 antagonists possessing varying affinities toward the receptor. These are 26 title compounds and clozapine as a reference antagonist. The active conformers of the conformationally flexible ligands were proposed by using the active rigid analogue approach and performing similarity calculations. The calculations involved genetic neural network (GNN) computations deriving QSARs from similarity matrices (SM) with cross-validated correlation coefficients exceeding 0.92. The performance of neural networks with variety of architectures was studied. As the computations were performed for cations and neutral molecules separately, the relevance of the ligand charging is discussed.

Effects of the new arginase inhibitor N(omega)-hydroxy-nor-L-arginine on NO synthase activity in murine macrophages

In stimulated murine macrophage, arginase and nitric oxide synthase (NOS) compete for their common substrate, l-arginine. The objectives of this study were (i) to test the new alpha-amino acid N(omega)-hydroxy-nor-l-arginine (nor-NOHA) as a new selective arginase inhibitor and (ii) to elucidate the effects of arginase inhibition on l-arginine utilization by an inducible NOS. Nor-NOHA is about 40-fold more potent than N(omega)-hydroxy-l-arginine (NOHA), an intermediate in the l-arginine/NO pathway, to inhibit the hydrolysis of l-arginine to l-ornithine catalyzed by unstimulated murine macrophages (IC(50) values 12 +/- 5 and 400 +/- 50 microM, respectively). Stimulation of murine macrophages with interferon-gamma and lipopolysaccharide (IFN-gamma + LPS) results in clear expression of an inducible NOS (iNOS) and to an increase in arginase activity. Nor-NOHA is also a potent inhibitor of arginase in IFN-gamma + LPS-stimulated macrophage (IC(50) value 10 +/- 3 microM). In contrast to NOHA, nor-NOHA is neither a substrate nor an inhibitor for iNOS and it appears as a useful tool to study the interplays between arginase and NOS. Inhibition of arginase by nor-NOHA increases nitrite and l-citrulline accumulation for incubation times higher than 12 h, under our conditions. Our results allow the determination of the kinetic parameters of the two competitive pathways and the proposal of a simple model which readily explains the differences observed between experiments. This model readily accounts for the observed effects and should be useful to predict the consequences of arginase inhibition in the presence of an active NOS on l-arginine availability.

alpha-Functionalized phosphonylphosphinates: synthesis and evaluation as transcarbamoylase inhibitors

Diverse alpha-methyl-substituted phosphonylphosphinates (P-C-P-C-X) are accessible from a protected, pentafluorophenylsulfonated phosphonylphosphinate via nucleophilic displacement. The utility of this route is demonstrated with several nitrogen nucleophiles. The resulting amine and amino acid phosphonylphosphinate derivatives were evaluated as inhibitors of Streptococcus faecalis ornithine transcarbamoylase (OTC). Compared with the structurally related phosphonoacetyl-L-ornithine (L-PALO), a known inhibitor of OTCs from various sources, the phosphonylphosphinates are surprisingly poor inhibitors, binding several orders of magnitude less tightly to the enzyme. These results suggest that the tetrahedral intermediate formed in the normal transcarbamoylase reaction is poorly mimicked by a tetrahedral and anionic phosphonate, either because of directly unfavorable interactions with a hydrogen-bond acceptor within the active site or because transition-state analogues are unable to induce the protein conformation changes that normally accompany reaction.

Inhibition of nitric oxide synthase as a potential therapeutic target

Nitric oxide (NO) regulates numerous physiological processes, including neurotransmission, smooth muscle contractility, platelet reactivity, and the cytotoxic activity of immune cells. Because of the ubiquitous nature of NO, inappropriate release of this mediator has been linked to the pathogenesis of a number of disease states. This provides the rationale for the design of therapies that modulate NO concentrations selectively. A well-characterized family of compounds are the inhibitors of NO synthase, the enzyme responsible for the generation of NO; such agents are potentially beneficial in the treatment of conditions associated with an overproduction of NO, including septic shock, neurodegenerative disorders, and inflammation. This article provides an overview of NO synthase inhibitors, focusing on agents that prevent binding of substrate L-arginine.

Efficient formation of nitric oxide from selective oxidation of N-aryl N'-hydroxyguanidines by inducible nitric oxide synthase

Inducible nitric oxide synthase (NOS II) efficiently catalyzes the oxidation of N-(4-chlorophenyl)N'-hydroxyguanidine 1 by NADPH and O2, with concomitant formation of the corresponding urea and NO. The characteristics of this reaction are very similar to those of the NOS-dependent oxidation of endogenous Nomega-hydroxy-L-arginine (NOHA), i.e., (i) the formation of products resulting from an oxidation of the substrate C=N(OH) bond, the corresponding urea and NO, in a 1:1 molar ratio, (ii) the absolute requirement of the tetrahydrobiopterin (BH4) cofactor for NO formation, and (iii) the strong inhibitory effects of L-arginine (L-arg) and classical inhibitors of NOSs. N-Hydroxyguanidine 1 is not as good a substrate for NOS II as is NOHA (Km = 500 microM versus 15 microM for NOHA). However, it leads to relatively high rates of NO formation which are only 4-fold lower than those obtained with NOHA (Vm = 390 +/- 50 nmol NO min-1 mg protein-1, corresponding roughly to 100 turnovers min-1). Preliminary results indicate that some other N-aryl N'-hydroxyguanidines exhibit a similar behavior. These results show for the first time that simple exogenous compounds may act as NO donors after oxidative activation by NOSs. They also suggest a possible implication of NOSs in the oxidative metabolism of certain classes of xenobiotics.

Substrate specificity of NO synthases: detailed comparison of L-arginine, homo-L-arginine, their N omega-hydroxy derivatives, and N omega-hydroxynor-L-arginine

A detailed comparison of the oxidation of five compounds closely related to L-arginine (Arg) by purified recombinant neuronal and macrophage NO synthases (NOS I and NOS II) was performed. Homo-L-arginine (homo-Arg) is oxidized by both NOSs in the presence of NADPH with major formation of NO and homo-L-citrulline, with a molar ratio of close to 1, and minor formation of N omega-hydroxyhomo-L-arginine (homo-NOHA). Oxidation of homo-NOHA by the two NOSs also leads to NO and homocitrulline in a 1:1 molar ratio. On the contrary, N omega-hydroxynor-L-arginine (nor-NOHA) is a very poor substrate of NOS I and II, which fails to produce significant amounts of nitrite. The catalytic efficiency of both NOSs markedly decreases in the order Arg > NOHA > homo-Arg > homo-NOHA, as shown by the 20- and 10-fold decrease of kcat/Km observed for NOS I and NOS II, respectively, when comparing Arg to homo-NOHA. The greater loss of catalytic efficiency for homo-Arg, when compared to that for Arg, appears to occur at the first step (N-hydroxylation) of the reaction. In that regard, it is noteworthy that the Vm values for NOHA and homo-NOHA oxidation are very similar (about 1 and 2 micromol of NO min-1 mg of protein-1 for NOS I and II, respectively). In fact, lengthening of the Arg chain by one CH2 leads not only to markedly decreased kcat/Km but also to clear disturbances in NOS functioning. This is shown by a greater accumulation of the N omega-hydroxyguanidine intermediate (homo-NOHA:homocitrulline ratio between 0.2 and 0.4) and an increased consumption of NADPH for NO formation (between 2.0 and 2.6 mol of NADPH consumed for the formation of 1 mol of NO in the case of homo-Arg, instead of 1.5 mol in the case of Arg). Most of the above results could be interpreted by comparing the possible positionings of the various substrates relative to the two NOS active oxygen species which are believed to be responsible for the two steps of the reaction.

Structural requirements for human inducible nitric oxide synthase substrates and substrate analogue inhibitors

Inducible nitric oxide synthase (iNOS; EC 1.14.13.39) catalyzes the NADPH-dependent oxidation of one of the free guanidino nitrogens of L-Arg to form nitric oxide and L-citrulline. Analogues of L-Arg and the inhibitor, L-N6-(1-iminoethyl)lysine, were used to define structural elements required for the binding and catalysis of compounds. L-Arg analogues with sequentially shorter methylene spacing between the guanidino group and the amino acid portion of the molecule were not iNOS substrates but were reversible inhibitors. L-Arg analogues such as agmatine with a hydroxyl substitution at the 2-amino position were substrates. Desaminoarginine was not a substrate but a reversible inhibitor. Desaminoarginine, agmatine, and argininic acid bound to the enzyme to give type I difference spectra similar to that of L-Arg. The amidino compounds L-N6-(1-iminoethyl)lysine, L-N5-(1-iminoethyl)ornithine, and N5-(1-iminoethyl)cadaverdine, but not N6-(1-iminoethyl)-6-aminocaproic acid, were NADPH-dependent, irreversible inactivators of iNOS. For both the L-Arg and L-N6-(1-iminoethyl)lysine analogues, the 2-amino group appeared to play an important role in catalytic events leading to either substrate turnover or mechanism-based inactivation. Inactivation of iNOS by L-N6-(1-iminoethyl)lysine was NADPH- and dioxygen-dependent, but low incorporation of radiolabel with DL--4, 5-3H]-N6-(1-iminoethyl)lysine indicates that the mechanism of enzyme inactivation is not covalent modification of the protein.

Synthesis and structure-DNA binding relationship analysis of DNA triple-helix specific intercalators

4-[N-(Aminoalkyl)amino]-2-arylquinolines with conformational freedom around positions 2 and 4 of the quinoline stabilize strongly poly(dT.dA.dT) (triplex DNA) and bind weakly to poly-(dA.dT) (duplex DNA). Basicity of N1 of the quinoline parallels the interaction strength of these compounds with the triple-helical DNA structure suggesting that N1 of the quinoline is protonated in the complex with the DNA triplex. The experimental results support the interaction model suggested previously.

Synthesis of 15N omega-hydroxy-L-arginine and ESR and 15N-NMR studies for the elucidation of the molecular mechanism of enzymic nitric oxide formation from L-arginine

N omega-Hydroxy-L-arginine (2) was prepared by a multi-stage synthesis; the key step was the addition of hydroxylamine to the protected cyanamide 8. The presence of N-hydroxyguanidines was confirmed, above all, by 15N-NMR investigations. 15N omega-Hydroxy-L-arginine (2) was converted quantitatively to 15NO by NO synthases from macrophages. 15NO was identified by ESR-spectroscopy. These experiments confirm that 15N omega-hydroxy-L-arginine (2) is an intermediate in the biosynthesis of NO from arginine (1) and that the N-hydroxylated N-atom is present in the NO formed.

Exogenous NG-hydroxyl-L-arginine causes nitrite production in vascular smooth muscle cells in the absence of nitric oxide synthase activity

Nitric oxide (NO) production from exogenous NG-hydroxy-L-arginine (OH-L-Arg) was investigated in rat aortic smooth muscle cells in culture by measuring nitrite accumulation in the culture medium. As well, the interaction between OH-L-Arg and L-arginine uptake via the y+ cationic amino acid transporter was studied. In cells without NO-synthase activity, OH-L-Arg (1-1000 microM) induced a dose-dependent nitrite production with a half-maximal effective concentration (EC50) of 18.0 +/- 1.5 microM (n = 4-7). This nitrite accumulation was not inhibited by the NO-synthase inhibitor NG-nitro-L-arginine methyl ester, L-NAME (300 microM). In contrast, it was abolished by miconazole (100 microM), an inhibitor of cytochrome P450. Incubation of vascular smooth muscle cells with LPS (10 micrograms/ml) induced an L-NAME inhibited nitrite accumulation, but did not enhance the OH-L-Arg induced nitrite production. OH-L-Arg and other cationic amino acids, L-lysine and L-ornithine, competitively inhibited [3H]-L-arginine uptake in rat aortic smooth muscle cells, with inhibition constants of 195 +/- 23 microM (n = 12), 260 +/- 40 microM (n = 5) and 330 +/- 10 microM (n = 5), respectively. These results show that OH-L-Arg is recognized by the cationic L-amino acid carrier present in vascular smooth muscle cells can be oxidized to NO and nitrite in these cells in the absence of NO-synthase, probably by cytochrome P450 or by a reaction involving a cytochrome P450 by-product.

An investigation of some S-nitrosothiols, and of hydroxy-arginine, on the mouse anococcygeus

1. The effect of five S-nitrosothiols, and of the stereoisomers of NG-hydroxy-arginine (HOARG), were investigated on the mouse anococcygeus. 2. All five S-nitrosothiols produced concentration-related (0.1-100 microM) relaxations of carbachol (50 microM)-induced tone; the order of potency was S-nitroso-L-cysteine (CYSNO) > S-nitroso-N-acetyl-D,L-penicillamine (SNAP) > S-nitrosoglutathione (GSNO) > S-nitrosocoenzyme A (CoASNO) > S-nitroso-N-acetyl-L-cysteine (NACNO). The relaxations were unaffected by the nitric oxide synthase (NOS) inhibitor, L-NG-nitro-arginine (10 microM) (L-NOARG). 3. Cold-storage of the tissue for 72 h resulted in loss of sympathetic and non-adrenergic, non-cholinergic (NANC) nerve function. NOS activity in the tissue was reduced by 97%. Despite this, relaxations induced by the S-nitrosothiols were unaffected. 4. Haemoglobin (50 microM) attenuated relaxations induced by NO and the S-nitrosothiols, although responses to 3-isobutyl-1-methyl-xanthine were unaffected. N-methyl-hydroxylamine (2 mM) which has been shown previously to produce selective inhibition of NANC and nitrovasodilator responses in this tissue, also reduced responses to all S-nitrosothiols. 5. Hydroquinone (100 microM) greatly reduced relaxations to CYSNO (by 88%) but had no effect on those to SNAP, GSNO, CoASNO or NACNO. Since hydroquinone does not reduce responses to NANC stimulation, CYSNO is unlikely to be the NANC transmitter. 6. L-HOARG by itself (up to 100 microM) had no significant effect on carbachol-induced tone or on NANC (10 Hz; 10 strain every 100 s) relaxations. However, it produced reversal of the inhibitory effects of L-NOARG (10;pM), being only slightly less potent than L-arginine. D-HOARG was without effect.L-HOARG had no effect on relaxations induced by 1.51iM NO.7. The results show that S-nitrosothiols are potent relaxants of the mouse anococcygeus; they act directly on the smooth muscle with a mechanism similar to NO and other nitrovasodilators. In addition,the results are consistent with L-HOARG being an intermediate in the biosynthesis of NO from L-arginine, although there is no evidence for it acting to stabilize NO extracellularly.

Cytochrome P450 catalyzes the oxidation of N omega-hydroxy-L-arginine by NADPH and O2 to nitric oxide and citrulline

Rat liver microsomes catalyze the oxidative denitration of N omega-hydroxy-L-arginine (NOHA) by NADPH and O2 with formation of citrulline and nitrogen oxides like NO and NO2-. Besides NO2- and citrulline, whose simultaneous formation is linear for at least 20 min, the formation of NO could be detected under the form of its P450 and P420-Fe(II) complexes by UV-visible and EPR spectroscopy. Classical inhibitors of NO-synthases, like N omega-methyl-and N omega-nitro-arginine, fail to inhibit the microsomal oxidation of NOHA to citrulline and NO2-. On the contrary classical inhibitors of hepatic cytochromes P450 like CO, miconazole, dihydroergotamine and troleandomycin, strongly inhibit this monooxygenase reaction. These results show that the oxygenation of NOHA by NADPH and O2 with formation of citrulline and NO can be efficiently catalyzed by cytochromes P450 (with rates up to 1.5 turnovers per min for the cytochromes of the 3A subfamily).

Mechanistic probes of N-hydroxylation of L-arginine by the inducible nitric oxide synthase from murine macrophages

NG-Hydroxy-L-arginine, [15N]-NG-hydroxy-L-arginine, and NG-hydroxy-NG- methyl-L-arginine were used as mechanistic probes of the initial step in the reaction catalyzed by nitric oxide synthase isolated from murine macrophages. NG-Hydroxy-L-arginine was found to be a substrate for nitric oxide synthase with a Km equal to 28.0 microM, yielding nitric oxide and L-citrulline. NADPH was required for the reaction and (6R)-tetrahydro-L-biopterin enhanced the initial rate of nitric oxide formation. The stoichiometry of NG-hydroxy-L-arginine loss to L-citrulline and nitric oxide (measured as nitrite and nitrate) formation was found to be 1:1:1. NG-Hydroxy-L-arginine was also observed in small amounts from L-arginine during the enzyme reaction. Studies with [15N]-NG-hydroxy-L-arginine indicated that the nitrogen in nitric oxide is derived from the oxime nitrogen of [15N]-NG-hydroxy-L- arginine. NG-Hydroxy-NG-methyl-L-arginine was found to be both a reversible and an irreversible inhibitor of nitric oxide synthase, displaying reversible competitive inhibition with K(i) equal to 33.5 microM. As an irreversible inhibitor, NG-hydroxy-NG-methyl-L-arginine gave kinact equal to 0.16 min-1 and KI equal to 26.5 microM. This inhibition was found to be both time- and concentration-dependent as well as showing substrate protection against inactivation. Gel filtration of an NG-hydroxy-NG-methyl-L-arginine-inactivated nitric oxide synthase failed to recover substantial amounts of enzyme activity.

Formation of nitrogen oxides and citrulline upon oxidation of N omega-hydroxy-L-arginine by hemeproteins

HRP catalyzes the oxidation of N omega-hydroxy-L-arginine (NOHA) by H2O2 with formation of citrulline and NO2- with initial rates of about 0.7 and 0.2 nmol per nmol HRP per min. In the same manner, cytochromes P450 from rat liver microsomes catalyze the oxidation of NOHA to citrulline and NO2- by cumylhydroperoxide. Inhibitors of these hemeproteins (N3- and CN- for HRP and miconazole for P450) strongly inhibit both citrulline and NO2- formation. Rates of NOHA oxidation by these hemeproteins markedly decrease with time presumably because of their denaturation by nitrogen oxides and of the formation of hemeprotein-iron-NO complexes. These results suggest that NO (and other nitrogen oxides) could be formed from oxidation of NOHA by other enzymes than NO-synthases.

Chemical oxidation of N-hydroxyguanidine compounds. Release of nitric oxide, nitroxyl and possible relationship to the mechanism of biological nitric oxide generation

N omega-Hydroxy-L-arginine was found to cause vasodilation in arginine-depleted rabbit aorta. It is, therefore, likely to be a biosynthetic intermediate in the conversion of arginine to nitric oxide in this tissue. N-Hydroxyalkylguanidine compounds, including N omega-hydroxy-L-arginine were oxidized with various oxidizing agents and examined for their ability to release nitric oxide. All oxidizing agents tested were capable of oxidizing the N-hydroxyguanidine function but only lead tetra-acetate (Pb(OAc)4) and potassium ferricyanide/hydrogen peroxide (K3FeCN6/H2O2) were capable of generating significant amounts of nitric oxide. Oxidation with K3FeCN6, lead oxide (PbO2) and silver carbonate (Ag2CO3) resulted instead in the release of nitrous oxide (N2O) presumably through the initial release of nitroxyl (HNO).