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

A colorimetric assay adapted to fragment screening revealing aurones and chalcones as new arginase inhibitors

Arginase, a difficult-to-target metalloenzyme, is implicated in a wide range of diseases, including cancer, infectious, and cardiovascular diseases. Despite the medical need, existing inhibitors have limited structural diversity, consisting predominantly of amino acids and their derivatives. The search for innovative arginase inhibitors has now extended to screening approaches. Due to the small and narrow active site of arginase, screening must meet the criteria of fragment-based screening. However, the limited binding capacity of fragments requires working at high concentrations, which increases the risk of interference and false positives. In this study, we investigated three colorimetric assays and selected one based on interference for screening under these challenging conditions. The subsequent adaptation and application to the screening a library of metal chelator fragments resulted in the identification of four compounds with moderate activity. The synthesis and evaluation of a series of compounds from one of the hits led to compound 21a with an IC50 value of 91.1 μM close to the reference compound piceatannol. Finally, molecular modelling supports the potential binding of aurones and chalcones to the active site of arginase, suggesting them as new candidates for the development of novel arginase inhibitors.

Old Acquaintances and Novel Complex Structures for the Ni(II) and Cu(II) Complexes of bis-Chelate Oxime-Amide Ligands

In the process of systematically studying the methylhydroxyiminoethaneamide bis-chelate ligands with polymethylene spacers of different lengths, L1-L3, and their transition metal complexes, a number of new Ni(II) and Cu(II) species have been isolated, and their molecular and crystal structures were determined using single-crystal X-ray diffraction. In all of these compounds, the divalent metal is coordinated by the ligand donor atoms in a square-planar arrangement. In addition, a serendipitously discovered new type of neutral Ni(II) complex, where the propane spacer of ligand L2 underwent oxidation to the propene spacer, and one of the amide groups was oxidised to the ketoimine, is also reported. The resulting ligand L2' affords the formation of neutral planar Ni(II) complexes, which are assembled in the solid state on top of each other, and yield two polymorphic structures. In both structures, the resulting infinite, exclusively parallel metal ion columns in ligand insulation may serve as precursor materials for sub-nano-conducting connectors. Overall, this paper reports the synthesis and characterisation of seven new anionic, cationic, and neutral Ni(II) and Cu(II) complexes, their crystal structures, as well as experimental and computed UV-Vis absorption spectra for two structurally similar Ni(II) complexes, yellow and red.

Opportunities for Nitric Oxide in Potentiating Cancer Immunotherapy

Despite nearly 30 years of development and recent highlights of nitric oxide (NO) donors and NO delivery systems in anticancer therapy, the limited understanding of exogenous NO's effects on the immune system has prevented their advancement into clinical use. In particular, the effects of exogenously delivered NO differing from that of endogenous NO has obscured how the potential and functions of NO in anticancer therapy may be estimated and exploited despite the accumulating evidence of NO's cancer therapy-potentiating effects on the immune system. After introducing their fundamentals and characteristics, this review discusses the current mechanistic understanding of NO donors and delivery systems in modulating the immunogenicity of cancer cells as well as the differentiation and functions of innate and adaptive immune cells. Lastly, the potential for the complex modulatory effects of NO with the immune system to be leveraged for therapeutic applications is discussed in the context of recent advancements in the implementation of NO delivery systems for anticancer immunotherapy applications. SIGNIFICANCE STATEMENT: Despite a 30-year history and recent highlights of nitric oxide (NO) donors and delivery systems as anticancer therapeutics, their clinical translation has been limited. Increasing evidence of the complex interactions between NO and the immune system has revealed both the potential and hurdles in their clinical translation. This review summarizes the effects of exogenous NO on cancer and immune cells in vitro and elaborates these effects in the context of recent reports exploiting NO delivery systems in vivo in cancer therapy applications.

Oxygen Binding by Co(II) Complexes with Oxime-Containing Schiff Bases in Solution

The present work describes the complexation properties of two oxime-containing Schiff bases (used as ligands), viz. 2-hydroxyimino-N′-[1-(2-pyridyl)ethylidene]propanohydrazone (Hpop) and 2-hydroxyimino-N′-[(pyridine-2-yl)methylidene]propanohydrazone (Hpoa), with Co(II) ions in DMSO/water solution. Volumetric (oxygenation) studies were carried out to determine the uptake of molecular oxygen O₂ in the formation of the complexes Co(II)-Hpop and Co(II)-Hpoa. The acquired data can be useful in the development of oxygen bioinorganic complexes of metal ions with Schiff base ligands in solution. Their properties allow them to be used as synthetic oxygen transporters. Moreover, the binding of dioxygen could play an important role in the research of catalytic activity by such systems.

An Update on Arginase Inhibitors and Inhibitory Assays

Arginase, which converts arginine into ornithine and urea, is a promising therapeutic target. Arginase is involved in cardiovascular diseases, parasitic infections and, through a critical role in immunity, in some cancers. There is a need to develop effective arginase inhibitors and therefore efforts to identify and optimize new inhibitors are increasing. Several methods of evaluating arginase activity are available, but few directly measure the product. Radiometric assays need to separate urea and dying reactions require acidic conditions and sometimes heating. Hence, there are a variety of different approaches available, and each approach has its own limits and benefits. In this review, we provide an update on arginase inhibitors, followed by a discussion on available arginase assays and alternative methods, with a focus on the intrinsic biases and parameters that are likely to impact results.

Amidoximes and Oximes: Synthesis, Structure, and Their Key Role as NO Donors

Nitric oxide (NO) is naturally synthesized in the human body and presents many beneficial biological effects; in particular on the cardiovascular system. Recently; many researchers tried to develop external sources to increase the NO level in the body; for example by using amidoximes and oximes which can be oxidized in vivo and release NO. In this review; the classical methods and most recent advances for the synthesis of both amidoximes and oximes are presented first. The isomers of amidoximes and oximes and their stabilities will also be described; (Z)-amidoximes and (Z)-oximes being usually the most energetically favorable isomers. This manuscript details also the biomimetic and biological pathways involved in the oxidation of amidoximes and oximes. The key role played by cytochrome P450 or other dihydronicotinamide-adenine dinucleotide phosphate (NADPH)-dependent reductase pathways is demonstrated. Finally, amidoximes and oximes exhibit important effects on the relaxation of both aortic and tracheal rings alongside with other effects as the decrease of the arterial pressure and of the thrombi formation

Targeting Metalloenzymes for Therapeutic Intervention

Metalloenzymes are central to a wide range of essential biological activities, including nucleic acid modification, protein degradation, and many others. The role of metalloenzymes in these processes also makes them central for the progression of many diseases and, as such, makes metalloenzymes attractive targets for therapeutic intervention. Increasing awareness of the role metalloenzymes play in disease and their importance as a class of targets has amplified interest in the development of new strategies to develop inhibitors and ultimately useful drugs. In this Review, we provide a broad overview of several drug discovery efforts focused on metalloenzymes and attempt to map out the current landscape of high-value metalloenzyme targets.

Phenyl 2-pyridyl ketoxime induces cellular senescence-like alterations via nitric oxide production in human diploid fibroblasts

Phenyl-2-pyridyl ketoxime (PPKO) was found to be one of the small molecules enriched in the extracellular matrix of near-senescent human diploid fibroblasts (HDFs). Treatment of young HDFs with PPKO reduced the viability of young HDFs in a dose- and time-dependent manner and resulted in senescence-associated β-galactosidase (SA-β-gal) staining and G2/M cell cycle arrest. In addition, the levels of some senescence-associated proteins, such as phosphorylated ERK1/2, caveolin-1, p53, p16(ink4a), and p21(waf1), were elevated in PPKO-treated cells. To monitor the effect of PPKO on cell stress responses, reactive oxygen species (ROS) production was examined by flow cytometry. After PPKO treatment, ROS levels transiently increased at 30 min but then returned to baseline at 60 min. The levels of some antioxidant enzymes, such as catalase, peroxiredoxin II and glutathione peroxidase I, were transiently induced by PPKO treatment. SOD II levels increased gradually, whereas the SOD I and III levels were biphasic during the experimental periods after PPKO treatment. Cellular senescence induced by PPKO was suppressed by chemical antioxidants, such as N-acetylcysteine, 2,2,6,6-tetramethylpiperidinyloxy, and L-buthionine-(S,R)-sulfoximine. Furthermore, PPKO increased nitric oxide (NO) production via inducible NO synthase (iNOS) in HDFs. In the presence of NOS inhibitors, such as L-NG-nitroarginine methyl ester and L-NG-monomethylarginine, PPKO-induced transient NO production and SA-β-gal staining were abrogated. Taken together, these results suggest that PPKO induces cellular senescence in association with transient ROS and NO production and the subsequent induction of senescence-associated proteins.

Plasma and tissue homoarginine concentrations in healthy and obese humans

Increased cardiovascular risk associated with obesity cannot be fully explained by traditional risk markers. We therefore assessed plasma and interstitial concentrations of the novel cardiovascular risk biomarker homoarginine (hArg) in 18 individuals without signs of cardiovascular disease, including 4 morbidly obese subjects before and after bariatric surgery and subsequent weight reduction of 36 ± 7 kg. hArg concentrations were greater in skeletal muscle compared with adipose tissue. Plasma and tissue hArg concentrations did not correlate with BMI. Adipose tissue interstitial hArg concentrations were not affected by obesity, an oral glucose load, or dramatic weight loss. In conclusion, obesity seems not to have a major effect on hArg homeostasis, and hArg may not explain the added cardiovascular risk associated with obesity. Yet, given the small sample size of the study, the significance of hArg in obesity should be investigated in a larger population.

Plasma homoarginine, arginine, asymmetric dimethylarginine and total homocysteine interrelationships in rheumatoid arthritis, coronary artery disease and peripheral artery occlusion disease

Elevated circulating concentrations of total L-homocysteine (thCys) and free asymmetric dimethylarginine (ADMA) are long-established cardiovascular risk factors. Low circulating L-homoarginine (hArg) concentrations were recently found to be associated with increased cardiovascular morbidity and mortality. The biochemical pathways of these amino acids overlap and share the same cofactor S-adenosylmethionine (SAM). In the present study, we investigated potential associations between hArg, L-arginine (Arg), ADMA and thCys in plasma of patients suffering from rheumatoid arthritis (RA), coronary artery disease (CAD) or peripheral artery occlusive disease (PAOD). In RA, we did not find any correlation between ADMA or hArg and thCys at baseline (n = 100) and after (n = 83) combined add-on supplementation of omega-3 fatty acids, vitamin E, vitamin A, copper, and selenium, or placebo (soy oil). ADMA correlated with Arg at baseline (r = 0.446, P < 0.001) and after treatment (r = 0.246, P = 0.03). hArg did not correlate with ADMA, but correlated with Arg before (r = 0.240, P = 0.02) and after treatment (r = 0.233, P = 0.03). These results suggest that hArg, ADMA and Arg are biochemically familiar with each other, but unrelated to hCys in RA. In PAOD and CAD, ADMA and thCys did not correlate.

Vasorelaxation induced by a new naphthoquinone-oxime is mediated by NO-sGC-cGMP pathway

It has been established that oximes cause endothelium-independent relaxation in blood vessels. In the present study, the cardiovascular effects of the new oxime 3-hydroxy-4–(hydroxyimino)-2-(3-methylbut-2-enylnaphtalen-1(4H)-one (OximeS1) derived from lapachol were evaluated. In normotensive rats, administration of Oxime S1 (10, 15, 20 and 30 mg/Kg, i.v.) produced dose-dependent reduction in blood pressure. In isolated aorta and superior mesenteric artery rings, Oxime S1 induced endothelium-independent and concentration-dependent relaxations (10⁻⁸ M to 10⁻⁴ M). In addition, Oxime S1-induced vasorelaxations were attenuated by hydroxocobalamin or methylene blue in aorta and by PTIO or ODQ in mesenteric artery rings, suggesting a role for the nitric oxide (NO) pathway. Additionally, Oxime S1 (30 and 100 µM) significantly increased NO concentrations (13.9 ± 1.6 nM and 17.9 ± 4.1 nM, respectively) measured by nitric oxide microsensors. Furthermore, pre-contraction with KCl (80 mM) prevented Oxime S1-derived vasorelaxation in endothelium-denuded aortic rings. Of note, combined treatment with potassium channel inhibitors also reduced Oxime S1-mediated vasorelaxation suggesting a role for potassium channels, more precisely K_ir, K_v and K_ATP channels. We observed the involvement of BK_Ca channels in Oxime S1-induced relaxation in mesenteric artery rings. In conclusion, these data suggest that the Oxime S1 induces hypotension and vasorelaxation via NO pathway by activating soluble guanylate cyclase (sGC) and K⁺ channels.

Binding of the unreactive substrate analog L-2-amino-3-guanidinopropionic acid (dinor-L-arginine) to human arginase I

Human arginase I (HAI) is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to form L-ornithine and urea through a metal-activated hydroxide mechanism. Since HAI regulates L-Arg bioavailability for NO biosynthesis, it is a potential drug target for the treatment of cardiovascular diseases such as atherosclerosis. X-ray crystal structures are now reported of the complexes of Mn(2)(2+)-HAI and Co(2)(2+)-HAI with L-2-amino-3-guanidinopropionic acid (AGPA; also known as dinor-L-arginine), an amino acid bearing a guanidinium side chain two methylene groups shorter than that of L-arginine. Hydrogen bonds to the α-carboxylate and α-amino groups of AGPA dominate enzyme-inhibitor recognition; the guanidinium group does not interact directly with the metal ions.

From inflammation to wound healing: using a simple model to understand the functional versatility of murine macrophages

Macrophages are fundamental cells of the innate immune system. Their activation is essential for such distinct immune functions as inflammation (pathogen-killing) and tissue repair (wound healing). An open question has been the functional stability of an individual macrophage cell: whether it can change its functional profile between different immune responses such as between the repair pathway and the inflammatory pathway. We studied this question theoretically by constructing a rate equation model for the key substrate, enzymes and products of the pathways; we then tested the model experimentally. Both our model and experiments show that individual macrophages can switch from the repair pathway to the inflammation pathway but that the reverse switch does not occur.

Inhibition of human arginase I by substrate and product analogues

Human arginase I is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to generate L-ornithine and urea. We demonstrate that N-hydroxy-L-arginine (NOHA) binds to this enzyme with K(d)=3.6 microM, and nor-N-hydroxy-L-arginine (nor-NOHA) binds with K(d)=517 nM (surface plasmon resonance) or K(d) approximately 50 nM (isothermal titration calorimetry). Crystals of human arginase I complexed with NOHA and nor-NOHA afford 2.04 and 1.55 A resolution structures, respectively, which are significantly improved in comparison with previously-determined structures of the corresponding complexes with rat arginase I. Higher resolution structures clarify the binding interactions of the inhibitors. Finally, the crystal structure of the complex with L-lysine (K(d)=13 microM) is reported at 1.90 A resolution. This structure confirms the importance of hydrogen bond interactions with inhibitor alpha-carboxylate and alpha-amino groups as key specificity determinants of amino acid recognition in the arginase active site.

Schistosoma mansoni arginase shares functional similarities with human orthologs but depends upon disulphide bridges for enzymatic activity

Schistosome helminths constitute a major health risk for the human population in many tropical areas. We demonstrate for the first time that several developmental stages of the human parasite Schistosoma mansoni express arginase, which is responsible for the hydrolysis of l-arginine to l-ornithine and urea. Arginase activity by alternatively activated macrophages is an essential component of the mammalian host response in schistosomiasis. However, it has not been previously shown that the parasite also expresses arginase when it is in contact with the mammalian host. After cloning and sequencing the cDNA encoding the parasite enzyme, we found that many structural features of human arginase are well conserved in the parasite ortholog. Subsequently, we discovered that S. mansoni arginase shares many similar molecular, biochemical and functional properties with both human arginase isoforms. Nevertheless, our data also reveal striking differences between human and schistosome arginase. Particularly, we found evidence that schistosome arginase activity depends upon disulphide bonds by cysteines, in contrast to human arginase. In conclusion, we report that S. mansoni arginase is well adapted to the physiological conditions that exist in the human host.

Radiochemical HPLC detection of arginine metabolism: measurement of nitric oxide synthesis and arginase activity in vascular tissue

Nitric oxide (NO) plays a key role in vascular homeostasis. Accurate measurement of NO production by endothelial nitric oxide synthase (eNOS) is critical for the investigation of vascular disease mechanisms using genetically modified animal models. Previous assays of NO production measuring the conversion of arginine to citrulline have required homogenisation of tissue and reconstitution with cofactors including NADPH and tetrahydrobiopterin. However, the activity and regulation of NOS in vivo is critically dependant on tissue levels of these cofactors. Therefore, understanding eNOS regulation requires assays of NO production in intact vascular tissue that do not depend on the addition of exogenous cofactors and have sufficient sensitivity and specificity. We describe a novel technique, using radiochemical detection of arginine to citrulline conversion, to measure NO production within intact mouse aortas, without exogenous cofactors. We demonstrate the presence of arginase activity in mouse aortas which has the potential to confound this assay. Furthermore, we describe the use of N-hydroxy-nor-L-arginine (nor-NOHA) to inhibit arginase and permit specific detection of NO production in intact mouse tissue. Using this technique we demonstrate a 2.4-fold increase in NO production in aortas of transgenic mice overexpressing eNOS in the endothelium, and show that this technique has high specificity and high sensitivity for detection of in situ NO synthesis by eNOS in mouse vascular tissue. These results have important implications for the investigation of NOS regulation in cells and tissues.

Targeting the mechanisms of tumoral immune tolerance with small-molecule inhibitors

Cancer immunotherapy has been predominantly focused on biologically based intervention strategies. However, recent advances in the understanding of tumour-host interactions at the molecular level have revealed targets that might be amenable to intervention with small-molecule inhibitors. In particular, key effectors of tumoral immune escape have been identified that contribute to a dominant toleragenic state that is suspected of limiting the successful implementation of treatment strategies that rely on boosting immune function. Within the context of the pathophysiology of cancer-associated immune tolerance, this Review delineates potential molecular targets for therapeutic intervention and the progress that has been made in developing small-molecule inhibitors.

Mitochondrial arginase II modulates nitric-oxide synthesis through nonfreely exchangeable L-arginine pools in human endothelial cells

Reduced synthesis of nitric oxide (NO) contributes to the endothelial dysfunction and may be related to limited availability of L-arginine, the common substrate of constitutive nitric-oxide synthase (NOS) and cytosolic arginase I and mitochondrial arginase II. To determine whether arginases modulate the endothelial NO synthesis, we investigated the effects of the competitive arginase inhibitor N(omega)-hydroxy-nor-L-arginine (Nor-NOHA) on the activity of NOS, arginases, and L-arginine transporter and on NO release at surface of human umbilical vein endothelial cells (HUVECs). In unstimulated cells, Nor-NOHA dose-dependently reduced the arginase activity with maximal inhibition at 20 microM. When HUVECs were stimulated by thrombin without extracellular L-arginine, Nor-NOHA dose-dependently increased the NOS activity and the NO release with maximal effects at 20 microM. Extracellular L-arginine also dose-dependently increased NO release and arginase activity. When HUVECs were stimulated by thrombin in the presence of 100 microM L-arginine, NOS activity and NO release were similar in untreated and Nor-NOHA-treated cells. However, despite activation of L-arginine uptake, the inhibition of arginase activity by Nor-NOHA was still significant. The depletion of freely exchangeable L-arginine pools with extracellular L-lysine did not prevent Nor-NOHA from increasing the NO release. This indicates the presence of pools, which are accessible to NOS and arginase, but not exchangeable. Interestingly, the mitochondrial arginase II was constitutively expressed, whereas the cytosolic arginase I was barely detectable in HUVECs. These data suggest that endothelial NO synthesis depends on the activity of arginase II in mitochondria and l-arginine carriers in cell membrane.

Upregulation of arginase by H2O2 impairs endothelium-dependent nitric oxide-mediated dilation of coronary arterioles

OBJECTIVE

Overproduction of reactive oxygen species such as hydrogen peroxide (H2O2) has been implicated in various cardiovascular diseases. However, mechanism(s) underlying coronary vascular dysfunction induced by H2O2 is unclear. We studied the effect of H2O2 on dilation of coronary arterioles to endothelium-dependent and endothelium-independent agonists.

METHODS AND RESULTS

Porcine coronary arterioles were isolated and pressurized without flow for in vitro study. All vessels developed basal tone and dilated dose-dependently to activators of nitric oxide (NO) synthase (adenosine and ionomycin), cyclooxygenase (arachidonic acid), and cytochrome P450 monooxygenase (bradykinin). Intraluminal incubation of vessels with H2O2 (100 micromol/L, 60 minutes) did not alter basal tone but inhibited vasodilations to adenosine and ionomycin in a manner similar as that by NO synthase inhibitor L-NAME. H2O2 affected neither endothelium-dependent responses to arachidonic acid and bradykinin nor endothelium-independent dilation to sodium nitroprusside. The inhibited adenosine response was not reversed by removal of H2O2 but was restored by excess L-arginine. Inhibition of L-arginine consuming enzyme arginase by alpha-difluoromethylornithine or N(omega)-hydroxy-nor-L-arginine also restored vasodilation. Administering deferoxamine, an inhibitor of hydroxyl radical production, prevented the H2O2-induced impairment of vasodilation to adenosine. Western blot and reverse-transcription polymerase chain reaction results indicated that arginase I was upregulated after treating vessels with H2O2.

CONCLUSIONS

H2O2 specifically impairs endothelium-dependent NO-mediated dilation of coronary microvessels by reducing L-arginine availability through upregulation of arginase. The formation of hydroxyl radicals from H2O2 may contribute to this process.

Synthesis and evaluation of new omega-borono-alpha-amino acids as rat liver arginase inhibitors

Recent studies have demonstrated that arginase plays important roles in pathologies such as asthma or erectile dysfunctions. We have synthesized new omega-borono-alpha-amino acids that are analogues of the previously known arginase inhibitors S-(2-boronoethyl)-l-cysteine (BEC) and 2-amino-6-boronohexanoic acid (ABH) and evaluated them as inhibitors of purified rat liver arginase (RLA). In addition to the distance between the B(OH)(2) and the alpha-amino acid functions, the position of the sulfur atom in the side chain also appears as a key determinant for the interaction with the active site of RLA. Furthermore, substitution of the alkyl side chain of BEC by methyl groups and conformational restriction of ABH by incorporation of its side chain in a phenyl ring led to inactive compounds. These results suggest that subtle interactions govern the affinity of inhibitors for the active site of RLA.

Nomega-hydroxy-L-arginine homologues and hydroxylamine as nitric oxide-dependent vasorelaxant agents

Endothelium-independent relaxant activities of N(omega)-hydroxy-L-arginine (L-NOHA) homologues and hydroxylamine, a possible intermediate in nitric oxide (NO) formation, were examined in rat aortic rings. Addition of one -CH(2)- group to the -(CH(2))(x)- chain between the alpha-amino acid and the hydroxyguanidine group (x=4) almost abolished-while deletion of one or two -CH(2)- (x=1 or 2) enhanced-the relaxant activity of L-NOHA homologues. N(omega)-hydroxy-nor-L-arginine- (x=2) and hydroxylamine-induced relaxations were blunted by a NO scavenger and by inhibitors of the guanylyl cyclase pathway, but not by NO synthase or cytochrome P(450) inhibitors (except 7-ethoxyresorufin). However, aortic NO formation was detected (using electron paramagnetic resonance) in the presence of concentrations of these compounds higher than those producing relaxation. These findings support the view that endothelium-independent vasorelaxations induced by both L-NOHA homologues with a required chain length x</=3 and hydroxylamine are mediated by NO-dependent activation of guanylyl cyclase, through a 7-ethoxyresorufin-inhibited mechanism.

Alternative nitric oxide-producing substrates for NO synthases

Nitric oxide (NO) is a key inter- and intracellular molecule involved in the maintenance of vascular tone, neuronal signaling, and host response to infection. The biosynthesis of NO in mammals involves a two-step oxidation of L-arginine (L-Arg) to citrulline and NO catalyzed by a particular class of heme-thiolate proteins, called NO-synthases (NOSs). The NOSs successively catalyze the Nomega-hydroxylation of the guanidine group of L-Arg with formation of Nomega-hydroxy-L-arginine (NOHA) and the oxidative cleavage of the CN(OH) bond of NOHA with formation of citrulline and NO. During the last decade, a great number of compounds bearing a CNH or CNOH function have been synthesized and studied as possible NO-producing substrates of recombinant NOSs. This includes derivatives of L-Arg and NOHA, N-alkyl (or aryl) guanidines, N,N'- or N,N-disubstituted guanidines, N-alkyl (or aryl) N'-hydroxyguanidines, N- (or O-) disubstituted N'-hydroxyguanidines, as well as amidoximes, ketoximes, and aldoximes. However, only those involving the NHC(NH2)=NH (or NOH) moiety have led to a significant formation of NO. All the N-monosubstituted N'-hydroxyguanidines that are well recognized by the NOS active site lead to NO with catalytic efficiences (kcat/Km) up to 50% of that of NOHA. This is the case of many N-aryl and N-alkyl N'-hydroxyguanidines, provided that the aryl or alkyl substituent is small enough to be accommodated by a NOS hydrophobic site located in close proximity of the NOS "guanidine binding site." As far as N-substituted guanidines are concerned, few compounds bearing a small alkyl group have been found to act as NO-producing substrates. The kcat value found for the best compound may reach 55% of the kcat of L-Arg oxidation. However, the best catalytic efficiency (kcat/Km) that was obtained with N-(4,4,4-trifluorobutyl) guanidine is only 100-fold lower than that of L-Arg. In a general manner, NOS II is a better catalyst that NOS I and III for the oxidation of exogenous guanidines and N-hydroxyguanidines to NO. This is particularly true for guanidines as the ones acting as substrates for NOS II have been found to be almost inactive for NOS I and NOS III. Thus, a good NO-producing guanidine substrate for the two latter isozymes remains to be found.

Inhibitor Coordination Interactions in the Binuclear Manganese Cluster of Arginase,

Arginase is a manganese metalloenzyme that catalyzes the hydrolysis of L-arginine to form L-ornithine and urea. The structure and stability of the binuclear manganese cluster are critical for catalytic activity as it activates the catalytic nucleophile, metal-bridging hydroxide ion, and stabilizes the tetrahedral intermediate and its flanking states. Here, we report X-ray structures of a series of inhibitors bound to the active site of arginase, and each inhibitor exploits a different mode of coordination with the Mn(2+)(2) cluster. Specifically, we have studied the binding of fluoride ion (F(-); an uncompetitive inhibitor) and L-arginine, L-valine, dinor-N(omega)-hydroxy-L-arginine, descarboxy-nor-N(omega)-hydroxy-L-arginine, and dehydro-2(S)-amino-6-boronohexanoic acid. Some inhibitors, such as fluoride ion, dinor-N(omega)-hydroxy-L-arginine, and dehydro-2(S)-amino-6-boronohexanoic acid, cause the net addition of one ligand to the Mn(2+)(2) cluster. Other inhibitors, such as descarboxy-nor-N(omega)-hydroxy-L-arginine, simply displace the metal-bridging hydroxide ion of the native enzyme and do not cause any net change in the metal coordination polyhedra. The highest affinity inhibitors displace the metal-bridging hydroxide ion (and sometimes occupy a Mn(2+)(A) site found vacant in the native enzyme) and maintain a conserved array of hydrogen bonds with their alpha-amino and -carboxylate groups.

Ischemia‐reperfusion selectively impairs nitric oxide‐ mediated dilation in coronary arterioles: counteracting role of arginase

A reduction in L-arginine availability has been implicated in the impairment of endothelium-dependent nitric oxide (NO)-mediated vasodilation by ischemia-reperfusion (I/R). However, the mechanisms contributing to dysregulation of the L-arginine pool remain unknown. Because endothelial cells can metabolize L-arginine via two major enzymes, that is, NO synthase (NOS) and arginase, we hypothesized that up-regulation of arginase during I/R reduces L-arginine availability to NOS and thus impairs NO-mediated vasodilation. To test this hypothesis, a local I/R was produced in the porcine heart by occlusion of a small branch of left anterior descending artery for 30 min, followed by reperfusion for 90 min. Arterioles (60-110 microm) isolated from non-ischemic and ischemic regions of subepicardium were cannulated and pressurized without flow for in vitro study. Vessels from both regions developed similar levels of basal tone. Although the dilation of I/R vessels to endothelium-independent agonist sodium nitroprusside was not altered, the endothelium-dependent NO-mediated dilations to adenosine and serotonin were attenuated. I/R not only inhibited arteriolar production of NO but also increased arteriolar arginase activity. Arginase inhibitor alpha-difluoromethylornithine enhanced NO production/dilation in normal vessels and also restored the NO-mediated function in I/R vessels. Treating I/R vessels with L-arginine also restored vasodilations. Immunohistochemical data revealed that I/R up-regulated arginase but down-regulated NOS expression in the arteriolar endothelium. Pretreating the animals with protein synthesis inhibitor cycloheximide prevented I/R-induced arginase up-regulation and also preserved NO-mediated vascular function. These results suggest that one mechanism by which I/R inhibits NO-mediated arteriolar dilation is through increased arginase activity, which limits the availability of L-arginine to NOS for NO production. In addition, the inability of arginase blockade or L-arginine supplementation to completely restore vasodilatory function may be attributable to the down-regulation of endothelial NOS expression.

Involvement of NO in the endothelium-independent relaxing effects of N(omega)-hydroxy-L-arginine and other compounds bearing a C=NOH function in the rat aorta

The mechanisms of vasorelaxation elicited by N(omega)-hydroxy-L-arginine (L-NOHA) and other compounds bearing a C=NOH function and the structural determinants governing this effect were investigated in rat aorta. L-NOHA, formamidoxime, five aromatic monosubstituted amidoximes, and one aromatic monosubstituted ketoxime elicited relaxation in endothelium-denuded rings. N-Hydroxyguanidine and substituted N-hydroxyguanidines were markedly less active. Relaxations induced by L-NOHA and by the most active studied compound, 4-chlorobenzamidoxime (ClBZA), were unmodified by the presence of endothelium. In endothelium-denuded rings, they were blunted by the NO scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide (300 microM) and by the inhibitor of guanylyl-cyclase activation 1H[1,2,4,]oxadiazolo[4,3-a]quinoxalin-1-one (1 microM). In addition, L-NOHA- and ClBZA both caused cGMP accumulation. L-Arginine, but not D-arginine (1 mM), antagonized the effect of L-NOHA but not ClBZA. Both L-NOHA- and ClBZA-induced relaxations were inhibited by the NAD(P)H-dependent enzymes inhibitor diphenyliodonium (30 microM) and the NAD(P)H-dependent reductases inhibitor 7-ethoxyresorufin (10 microM), but they were unmodified by the cytochrome P450 (P450) inhibitor proadifen (10 microM) and by the NO synthase inhibitor N(omega)-nitro-L-arginine methyl ester (L-NAME, 300 microM). These results show that L-NOHA and other compounds with a C=NOH function can cause endothelium-independent relaxation in the rat aorta. They suggest that activation of guanylyl cyclase and NO formation is implicated in relaxation and that a 7-ethoxyresorufin-sensitive NAD(P)H-dependent pathway is involved. On one hand, L-NOHA and amidoximes may be useful tools for characterizing this pathway in blood vessels and, on the other, may offer a novel approach for treating vascular diseases with impaired endothelial NO activity.

Novel substrates for nitric oxide synthases

Enzymatic generation of nitric oxide (NO) by nitric oxide synthase (NOS) consists of two oxidation steps. The first step converts L-arginine to N(G)-hydroxy-L-arginine (NOHA), a key intermediate, and the second step converts NOHA to NO and L-citrulline. To fully probe the substrate specificity of the second enzymatic step, an extensive structural screening was carried out using a series of N-alkyl (and N-aryl) substituted-N'-hydroxyguanidines (1-14). Among the eleven N-alkyl-N'-hydroxyguanidines evaluated, N-n-propyl (2), N-iso-propyl (3), N-n-butyl (4), N-s-butyl (5), N-iso-butyl (6), N-pentyl (8) and N-iso-pentyl (9) derivatives were efficiently oxidized by the three isoenzymes of NOS (nNOS, iNOS and eNOS) to generate NO. N-Butyl-N'-hydroxyguanidine (4) was the best substrate for iNOS (K(m)=33 microM) and N-iso-propyl-N'-hydroxyguanidine (3) was the best substrate for nNOS (K(m)=56 microM). When the alkyl substituents were too small (such as ethyl 1) or too large (such as hexyl 10 and cyclohexyl 11), the activity decreased significantly. This suggests that the van der Waals interaction between the alkyl group and the hydrophobic cavity in the NOS active site contributes significantly to the relative reactivity of compounds 3-11. Moreover, five N-aryl-N'-hydroxyguanidines were found to be good substrates for iNOS, but not substrates for eNOS and nNOS. N-phenyl-N'-hydroxyguanidine was the best substrate among them (K(m)=243 microM). This work demonstrates that N-alkyl substituted hydroxyguanidine compounds are novel NOS substrates which 'short-circuit' the first oxidation step of NOS, and N-aryl substituted hydroxyguanidine compounds are isoform selective NOS substrate.

Oxidation of the ketoxime acetoxime to nitric oxide by oxygen radical-generating systems

Ketoximes undergo a cytochrome P450-catalyzed oxidation to nitric oxide and ketones in liver microsomes. In addition, nitric oxide synthase (NOS) can catalyze the oxidative denitration of the >C=N-OH group of amidoximes. The objective of this work was to characterize the oxidation of a ketoxime (acetoxime) and to assess the ability of NOS to catalyze the generation of nitric oxide/nitrogen monoxide (*NO) from acetoxime. Acetoxime was oxidized to NO2- (and NO3-) by microsomes enriched with several P450 isoforms, including CYP2E1, CYP1A1, and CYP2B1. Nitric oxide was identified as an intermediate in the overall reaction. Superoxide dismutase and catalase significantly inhibited the reaction. Exogenous iron increased the microsomal generation of NO2- from acetoxime, while metal chelators (desferrioxamine, EDTA, DTPA) inhibited it. A Fenton-like system (Fe2+ plus H2O2, pH 7.4) consumed acetoxime with production of NO2- and NO3-, whereas oxidation by superoxide or by H2O2 was inefficient. The results presented suggest a role for hydroxyl radical-like oxidants in the oxidation of acetoxime to nitric oxide. O-Acetylacetoxime and O-tert-butylacetoxime were not oxidized by a Fenton system or by liver microsomes to any significant extent. Formation of the 5,5'-dimethyl-1-pyrroline-N-oxide/. OH adduct by a Fenton system was significantly inhibited by acetoxime, while O-acetylacetoxime and O-tert-butylacetoxime were inactive. These results suggest that the. OH-dependent oxidation of acetoxime initially proceeds via abstraction of a hydrogen atom from its hydroxyl group, as opposed to the oxidation of its >C=N- function. HepG2 cells with low levels of expression of P450 did not significantly produce NO2- from acetoxime, while HepG2 cells expressing CYP2E1 did, and this generation was blocked by a CYP2E1 inhibitor. Acetoxime was inactive either as a substrate or as an inhibitor of iNOS activity. These results indicate that reactive oxygen species play a key role in the oxidation of acetoxime to. NO by liver microsomes by a mechanism involving H abstraction from the OH moiety by hydroxyl radical-like oxidants and suggest the possibility that acetoxime may be an effective producer of. NO primarily in the liver by a pathway independent of NOS.

The inhibition of arginase by N(omega)-hydroxy-l-arginine controls the growth of Leishmania inside macrophages

Polyamine synthesis from l-ornithine is essential for Leishmania growth. We have investigated the dependence of Leishmania infection on arginase, which generates l-ornithine, in macrophages from BALB/c, C57BL/6, and nitric oxide synthase II (NOS II)-deficient mouse strains. We have found that N(omega)-hydroxy-l-arginine (LOHA), a physiological inhibitor of arginase, controls cellular infection and also specifically inhibits arginase activity from Leishmania major and Leishmania infantum parasites. The effect was proportional to the course of infection, concentration dependent up to 100 microM, and achieved without an increase in nitrite levels of culture supernatants. Moreover, when the l-arginine metabolism of macrophages is diverted towards ornithine generation by interleukin 4-induced arginase I, parasite growth is promoted. This effect can be reversed by LOHA. Inhibition of NOS II by N(G)-methyl-l-arginine (LNMMA) restores the killing obtained in the presence of interferon (IFN)-gamma plus lipolysaccharide (LPS), whereas the nitric oxide scavenger 2-(4-carboxyphenyl)-4,4,5,5,-tetramethylimidazoline-3-oxide-1-oxyl (carboxy-PTIO) was without effect. However, exogenous l-ornithine almost completely inhibits parasite killing when added in the presence of LOHA to macrophages from NOS II-deficient mice or to BALB/c-infected cells activated with IFN-gamma plus LPS. These results suggest that LOHA is an effector molecule involved in the control of Leishmania infection. In addition, macrophage arginase I induction by T helper cell type 2 cytokines could be a mechanism used by parasites to spread inside the host.