S-Ethyl-Isothiocitrullin-Based Dipeptides and 1,2,4-Oxadiazole Pseudo-Dipeptides: Solid Phase Synthesis and Evaluation as NO Synthase Inhibitors

We previously reported dipeptidomimetic compounds as inhibitors of neuronal and/or inducible NO synthases (n/iNOS) with significant selectivity against endothelial NOS (eNOS). They were composed of an S-ethylisothiocitrullin-like moiety linked to an extension through a peptide bond or a 1,2,4-oxadiazole link. Here, we developed two further series where the extension size was increased to establish more favorable interactions in the NOS substrate access channel. The extension was introduced on the solid phase by the reductive alkylation of an amino-piperidine moiety or an aminoethyl segment in the case of dipeptide-like and 1,2,4-oxadiazole compounds, respectively, with various benzaldehydes. Compared to the previous series, more potent inhibitors were identified with IC₅₀ in the micromolar to the submicromolar range, with significant selectivity toward nNOS. As expected, most compounds did not inhibit eNOS, and molecular modeling was carried out to characterize the reasons for the selectivity toward nNOS over eNOS. Spectral studies showed that compounds were interacting at the heme active site. Finally, selected inhibitors were found to inhibit intra-cellular iNOS and nNOS expressed in RAW264.7 and INS-1 cells, respectively.

Human Orphan Cytochrome P450 2U1 Catalyzes the ω-Hydroxylation of Leukotriene B4

Cytochrome P450 2U1 (CYP2U1) identified from the human genome remains poorly known since few data are presently available on its physiological function(s) and substrate(s) specificity. CYP2U1 mutations are associated with complicated forms of hereditary spastic paraplegia, alterations of mitochondrial architecture and bioenergetics. In order to better know the biological roles of CYP2U1, we used a bioinformatics approach. The analysis of the data invited us to focus on leukotriene B₄ (LTB₄), an important inflammatory mediator. Here, we show that CYP2U1 efficiently catalyzes the hydroxylation of LTB₄ predominantly on its ω-position. We also report docking experiments of LTB₄ in a 3D model of truncated CYP2U1 that are in agreement with this hydroxylation regioselectivity. The involvement of CYP2U1 in the metabolism of LTB₄ could have strong physiological consequences in cerebral pathologies including ischemic stroke because CYP2U1 is predominantly expressed in the brain.

Reactions of Recombinant Neuronal Nitric Oxide Synthase with Redox Cycling Xenobiotics: A Mechanistic Study

Neuronal nitric oxide synthase (nNOS) catalyzes single-electron reduction of quinones (Q), nitroaromatic compounds (ArNO₂) and aromatic N-oxides (ArN → O), and is partly responsible for their oxidative stress-type cytotoxicity. In order to expand a limited knowledge on the enzymatic mechanisms of these processes, we aimed to disclose the specific features of nNOS in the reduction of such xenobiotics. In the absence or presence of calmodulin (CAM), the reactivity of Q and ArN → O increases with their single-electron reduction midpoint potential (E¹₇). ArNO₂ form a series with lower reactivity. The calculations according to an "outer-sphere" electron transfer model show that the binding of CAM decreases the electron transfer distance from FMNH₂ to quinone by 1-2 Å. The effects of ionic strength point to the interaction of oxidants with a negatively charged protein domain close to FMN, and to an increase in accessibility of the active center induced by high ionic strength. The multiple turnover experiments of nNOS show that, in parallel with reduced FAD-FMN, duroquinone reoxidizes the reduced heme, in particular its Fe²⁺-NO form. This finding may help to design the heme-targeted bioreductively activated agents and contribute to the understanding of the role of P-450-type heme proteins in the bioreduction of quinones and other prooxidant xenobiotics.

Implication of folate deficiency in CYP2U1 loss of function

Hereditary spastic paraplegias are heterogeneous neurodegenerative disorders. Understanding of their pathogenic mechanisms remains sparse, and therapeutic options are lacking. We characterized a mouse model lacking the Cyp2u1 gene, loss of which is known to be involved in a complex form of these diseases in humans. We showed that this model partially recapitulated the clinical and biochemical phenotypes of patients. Using electron microscopy, lipidomic, and proteomic studies, we identified vitamin B2 as a substrate of the CYP2U1 enzyme, as well as coenzyme Q, neopterin, and IFN-α levels as putative biomarkers in mice and fluids obtained from the largest series of CYP2U1-mutated patients reported so far. We also confirmed brain calcifications as a potential biomarker in patients. Our results suggest that CYP2U1 deficiency disrupts mitochondrial function and impacts proper neurodevelopment, which could be prevented by folate supplementation in our mouse model, followed by a neurodegenerative process altering multiple neuronal and extraneuronal tissues.

Distinction between 2'- and 3'-Phosphate Isomers of a Fluorescent NADPH Analogue Led to Strong Inhibition of Cancer Cells Migration

Specific inhibition of NADPH oxidases (NOX) and NO-synthases (NOS), two enzymes associated with redox stress in tumor cells, has aroused great pharmacological interest. Here, we show how these enzymes distinguish between isomeric 2′- and 3′-phosphate derivatives, a difference used to improve the specificity of inhibition by isolated 2′- and 3′-phosphate isomers of our NADPH analogue NS1. Both isomers become fluorescent upon binding to their target proteins as observed by in vitro assay and in vivo imaging. The 2′-phosphate isomer of NS1 exerted more pronounced effects on NOS and NOX-dependent physiological responses than the 3′-phosphate isomer did. Docking and molecular dynamics simulations explain this specificity at the level of the NADPH site of NOX and NOS, where conserved arginine residues distinguished between the 2′-phosphate over the 3′-phosphate group, in favor of the 2′-phosphate.

Pseudoxanthoma elasticum overlaps hereditary spastic paraplegia type 56

PURPOSE

Pseudoxanthoma elasticum (PXE) is a recessive disorder involving skin, eyes and arteries, mainly caused by ABCC6 pathogenic variants. However, almost one fifth of patients remain genetically unsolved despite extensive genetic screening of ABCC6, as illustrated in a large French PXE series of 220 cases. We searched for new PXE gene(s) to solve the ABCC6-negative patients.

METHODS

First, family-based exome sequencing was performed, in one ABCC6-negative PXE patient with additional neurological features, and her relatives. CYP2U1, involved in hereditary spastic paraplegia type 56 (SPG56), was selected based on this complex phenotype, and the presence of two candidate variants. Second, CYP2U1 sequencing was performed in a retrospective series of 46 additional ABCC6-negative PXE probands. Third, six additional SPG56 patients were evaluated for PXE skin and eye phenotype. Additionally, plasma pyrophosphate dosage and functional analyses were performed in some of these patients.

RESULTS

6.4% of ABCC6-negative PXE patients (n = 3) harboured biallelic pathogenic variants in CYP2U1. PXE skin lesions with histological confirmation, eye lesions including maculopathy or angioid streaks, and various neurological symptoms were present. CYP2U1 missense variants were confirmed to impair protein function. Plasma pyrophosphate levels were normal. Two SPG56 patients (33%) presented some phenotypic overlap with PXE.

CONCLUSION

CYP2U1 pathogenic variants are found in unsolved PXE patients with neurological findings, including spastic paraplegia, expanding the SPG56 phenotype and highlighting its overlap with PXE. The pathophysiology of ABCC6 and CYP2U1 should be explored to explain their respective role and potential interaction in ectopic mineralization.

Comparison of Various Aryl-Dithiolethiones and Aryl-Dithiolones As Hydrogen Sulfide Donors in the Presence of Rat Liver Microsomes

It has been reported that microsomal metabolism of ADT (5-(p-methoxyphenyl)-3H-1,2-dithiole-3-thione, anetholedithiolethione, Sulfarlem) and ADO (5-(p-methoxyphenyl)-3H-1,2-dithiole-3-one, anetholedithiolone) led to formation of H₂S mainly derived from oxidations catalyzed by cytochrome P450-dependent monooxygenases and that ADO was a better H₂S donor than ADT under these conditions. This article compares the H₂S donor abilities of 18 dithiolethione and dithiolone analogs of ADT and ADO upon incubation with rat liver microsomes. It shows that, for all the studied compounds, maximal H₂S formation was obtained after incubation with microsomes and NADPH and that this formation greatly decreased in the presence of N-benzylimidazole, a known inhibitor of cytochrome P450. This indicates that H₂S formation from all the studied compounds requires, as previously observed in the case of ADT and ADO, oxidations catalyzed by cytochrome P450-dependent monooxygenases. Under these conditions, the studied dithiolones were almost always better H₂S donors than the corresponding dithiolethiones. Interestingly, the best H₂S yields (up to 75%) were observed in microsomal oxidation of ADO and its close analogs, pCl-Ph-DO and Ph-DO, in the presence of glutathione (GSH), whereas only small amounts of H₂S were formed in microsomal incubations of those compounds with GSH but in the absence of NADPH. A possible mechanism for this effect of GSH is proposed on the basis of results obtained from reactions of GSH with 5-(p-methoxyphenyl)-3H-1,2-dithiole-3-one-1-sulfoxide, the ADO metabolite involved in H₂S formation in microsomal oxidation of ADO. SIGNIFICANCE STATEMENT: A series of 18 dithiolethiones and dithiolones were compared for their ability to form hydrogen sulfide (H₂S) in oxidations catalyzed by microsomal monooxygenases. The studied dithiolones were better H₂S donors than the corresponding dithiolethiones, and the addition of glutathione to the incubations strongly increased H₂S formation. A possible mechanism for this effect of GSH is proposed on the basis of results obtained from reactions of GSH with 5-(p-methoxyphenyl)-3H-1,2-dithiole-3-one-1-sulfoxide, a metabolite of the choleretic and sialologic drug Sulfarlem.

Solid-Phase Synthesis of Substrate-Based Dipeptides and Heterocyclic Pseudo-dipeptides as Potential NO Synthase Inhibitors

More than 160 arginine analogues modified on the C-terminus via either an amide bond or a heterocyclic moiety (1,2,4-oxadiazole, 1,3,4-oxadiazole and 1,2,4-triazole) were prepared as potential inhibitors of NO synthases (NOS). A methodology involving formation of a thiocitrulline intermediate linked through its side-chain on a solid support followed by modification of its carboxylate group was developed. Finally, the side-chain thiourea group was either let unchanged, S-alkylated (Me, Et) or guanidinylated (Me, Et) to yield respectively after TFA treatment the corresponding thiocitrulline, S-Me/Et-isothiocitrulline and N-Me/Et-arginine substrate analogues. They all were tested against three recombinant NOS isoforms. Several compounds containing a S-Et- or a S-Me-Itc moiety and mainly belonging to both the dipeptide-like and 1,2,4-oxadiazole series were shown to inhibit nNOS and iNOS with IC₅₀ in the 1-50 μM range. Spectral studies confirmed that these new compounds interacted at the heme active site. The more active compounds were found to inhibit intra-cellular iNOS expressed in RAW264.7 and INS-1 cells with similar efficiency than the reference compounds L-NIL and SEIT.

Mechanism of H2S Formation from the Metabolism of Anetholedithiolethione and Anetholedithiolone by Rat Liver Microsomes

The drug anetholedithiolethione (ADT) and its analogs have been extensively used as H₂S donors. However, the mechanism of H₂S formation from ADT under biologic conditions remains almost completely unknown. This article shows that only small amounts of H₂S are formed during incubation of ADT and of its metabolite anetholedithiolone (ADO) with rat liver cytosol or with rat liver microsomes (RLM) in the absence of NADPH, indicating that H₂S formation under these conditions is of hydrolytic origin only to a minor extent. By contrast, much greater amounts of H₂S are formed upon incubation of ADT and ADO with RLM in the presence of NADPH and dioxygen, with a concomitant formation of H₂S and para-methoxy-acetophenone (pMA). Moreover, H₂S and pMA formation under those conditions are greatly inhibited in the presence of N-benzyl-imidazole indicating the involvement of cytochrome P450-dependent monooxygenases. Mechanistic studies show the intermediate formation of the ADT-derived 1,2-dithiolium cation and of the ADO sulfoxide during microsomal metabolism of ADT and ADO, respectively. This article proposes the first detailed mechanisms for the formation of H₂S from microsomal metabolism of ADT and ADO in agreement with those data and with previously published data on the metabolism of compounds involving a C=S bond. Finally, this article shows for the first time that ADO is a better H₂S donor than ADT under those conditions. SIGNIFICANCE STATEMENT: Incubation of anetholedithiolethione (ADT) or its metabolite anetholedithiolone (ADO) in the presence of rat liver microsomes, NADPH, and O₂ leads to H₂S. This article shows for the first time that this H₂S formation involves several steps catalyzed by microsomal monooxygenases and that ADO is a better H₂S donor than ADT. We propose the first detailed mechanisms for the formation of H₂S from the microsomal metabolism of ADT and ADO.

Light-induced formation of NO in endothelial cells by photoactivatable NADPH analogues targeting nitric-oxide synthase

BACKGROUND

Nitric-oxide synthases (NOS) catalyze the formation of NO using NADPH as electron donor. We have recently designed and synthesized a new series of two-photon absorbing and photoactivatable NADPH analogues (NT). These compounds bear one or two carboxymethyl group(s) on the 2'- or/and 3'-position(s) of the ribose in the adenosine moiety, instead of a 2'-phosphate group, and differ by the nature of the electron donor in their photoactivatable chromophore (replacing the nicotinamide moiety). Here, we addressed the ability of NTs to photoinduce eNOS-dependent NO production in endothelial cells.

METHODS

The cellular fate of NTs and their photoinduced effects were studied using multiphoton fluorescence imaging, cell viability assays and a BODIPY-derived NO probe for NO measurements. The eNOS dependence of photoinduced NO production was addressed using two NOS inhibitors (NS1 and L-NAME) targeting the reductase and the oxygenase domains, respectively.

RESULTS

We found that, two compounds, those bearing a single carboxymethyl group on the 3'-position of the ribose, colocalize with the Golgi apparatus (the main intracellular location of eNOS) and display high intracellular two-photon brightness. Furthermore, a eNOS-dependent photooxidation was observed for these two compounds only, which is accompanied by a substantial intracellular NO production accounting for specific photocytotoxic effects.

CONCLUSIONS

We show for the first time that NT photoactivation efficiently triggers electron flow at the eNOS level and increases the basal production of NO by endothelial cells.

GENERAL SIGNIFICANCE

Efficient photoactivatable NADPH analogues targeting NOS could have important implications for generating apoptosis in tumor cells or modulating NO-dependent physiological processes.

Importance of Val567 on heme environment and substrate recognition of neuronal nitric oxide synthase

Nitric oxide (NO) produced by mammalian nitric oxide synthases (mNOSs) is an important mediator in a variety of physiological functions. Crystal structures of mNOSs have shown strong conservation of the active‐site residue Val567 (numbering for rat neuronal NOS, nNOS). NOS‐like proteins have been identified in several bacterial pathogens, and these display striking sequence identity to the oxygenase domain of mNOS (NOSoxy), with the exception of a Val to Ile mutation at the active site. Preliminary studies have highlighted the importance of this Val residue in NO‐binding, substrate recognition, and oxidation in mNOSs. To further elucidate the role of this valine in substrate and substrate analogue recognition, we generated five Val567 mutants of the oxygenase domain of the neuronal NOS (nNOSoxy) and used UV‐visible and EPR spectroscopy to investigate the effects of these mutations on the heme distal environment, the stability of the heme‐Fe^II‐CO complexes, and the binding of a series of substrate analogues. Our results are consistent with Val567 playing an important role in preserving the integrity of the active site for substrate binding, stability of heme‐bound gaseous ligands, and potential NO production.

Mechanism of melanoma cells selective apoptosis induced by a photoactive NADPH analogue

Melanoma is one of the most lethal cancers when it reaches a metastatic stage. Despite the spectacular achievements of targeted therapies (BRAF inhibitors) or immuno-therapies (anti-CTLA4 or anti-PD1), most patients with melanoma will need additional treatments. Here we used a photoactive NADPH analogue called NS1 to induce cell death by inhibition of NADPH oxidases NOX in melanoma cells, including melanoma cells isolated from patients. In contrast, healthy melanocytes growth was unaffected by NS1 treatment.

NS1 established an early Endoplasmic Reticulum stress by the early release of calcium mediated by (a) calcium-dependent redox-sensitive ion channel(s). These events initiated autophagy and apoptosis in all tested melanoma cells independently of their mutational status. The autophagy promoted by NS1 was incomplete. The autophagic flux was blocked at late stage events, consistent with the accumulation of p62, and a close localization of LC3 with NS1 associated with NS1 inhibition of NOX1 in autophagosomes. This hypothesis of a specific incomplete autophagy and apoptosis driven by NS1 was comforted by the use of siRNAs and pharmacological inhibitors blocking different processes. This study highlights the potential therapeutic interest of NS1 inducing cell death by triggering a selective ER stress and incomplete autophagy in melanoma cells harbouring wt and BRAF mutation.

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).

New 1,4-Dihydropyridines Down-regulate Nitric Oxide in Animals with Streptozotocin-induced Diabetes Mellitus and Protect Deoxyribonucleic Acid against Peroxynitrite Action

Diabetes mellitus (DM) and its complications cause numerous health and social problems throughout the world. Pathogenic actions of nitric oxide (NO) are responsible to a large extent for development of complications of DM. Search for compounds regulating NO production in patients with DM is thus important for the development of pharmacological drugs. Dihydropyridines (1,4-DHPs) are prospective compounds from this point of view. The goals of this study were to study the in vivo effects of new DHPs on NO and reactive nitrogen and oxygen species production in a streptozotocin (STZ)-induced model of DM in rats and to study their ability to protect DNA against nocive action of peroxynitrite. STZ-induced diabetes caused an increase in NO production in the liver, kidneys, blood and muscles, but a decrease in NO in adipose tissue of STZ-treated animals. Cerebrocrast treatment was followed by normalization of NO production in the liver, kidneys and blood. Two other DHPs, etaftorone and fenoftorone, were effective in decreasing NO production in kidneys, blood and muscles of diabetic animals. Furthermore, inhibitors of nitric oxide synthase (NOS) and an inhibitor of xanthine oxidoreductase (XOR) decreased NO production in kidneys of diabetic animals. Treatment with etaftorone decreased expression of inducible NOS and XOR in kidneys, whereas it increased the expression of endothelial NOS. In vitro, the studied DHPs did not significantly inhibit the activities of NOS and XOR but affected the reactivity of peroxynitrite with DNA. These new DHPs thus appear of strong interest for treatment of DM complications.

TLR4/IFNγ pathways induce tumor regression via NOS II-dependent NO and ROS production in murine breast cancer models

Toll-like receptor (TLR) 4 agonists have emerged as a new group of molecules used for cancer therapy. They have been exploited to enhance the immunogenicity of current chemotherapeutic regimens. However, their effects on cancer cells remain elusive. Here, we showed that a TLR4 agonist, namely a synthetic lipid A analog (ALA), OM-174, exhibits antitumor effects in several mammary tumor mouse models. We also showed that immune components are involved in such effects, as attested to by the failure of ALA to induce tumor regression or an increase of animal survival in mice knocked-out for interferon γ (IFNγ) or TLR4. TLR4 and IFNγ receptor (INFR2) expressed by cancer cells are involved in the antitumor efficacy of ALA since this last did not inhibit tumor growth in mice bearing a tumor but lacking TLR4 or IFNγ receptor 2 (IFNR2). Mechanistic investigations revealed that nitric oxide (NO), superoxide and peroxynitrite produced by uncoupling of inducible NO synthase (NOS II) in cancer cells are key mediators of ALA and IFNγ-mediated tumor growth inhibition. We present here a comprehensive picture of tumor cell death induction, in vivo and in vitro, by immunotherapy and for the first time the involvement of the TLR4/IFNγ/NOS II pathway in immunotherapy was investigated.

Regulation of NADPH-dependent Nitric Oxide and reactive oxygen species signalling in endothelial and melanoma cells by a photoactive NADPH analogue

Nitric Oxide (NO) and Reactive oxygen species (ROS) are endogenous regulators of angiogenesis-related events as endothelial cell proliferation and survival, but NO/ROS defect or unbalance contribute to cancers. We recently designed a novel photoactive inhibitor of NO-Synthases (NOS) called NS1, which binds their NADPH site in vitro. Here, we show that NS1 inhibited NO formed in aortic rings. NS1-induced NO decrease led to an inhibition of angiogenesis in a model of VEGF-induced endothelial tubes formation. Beside this effect, NS1 reduced ROS levels in endothelial and melanoma A375 cells and in aorta. In metastatic melanoma cells, NS1 first induced a strong decrease of VEGF and blocked melanoma cell cycle at G2/M. NS1 decreased NOX₄ and ROS levels that could lead to a specific proliferation arrest and cell death. In contrast, NS1 did not perturb melanocytes growth.

Altogether, NS1 revealed a possible cross-talk between eNOS- and NOX₄ –associated pathways in melanoma cells via VEGF, Erk and Akt modulation by NS1 that could be targeted to stop proliferation. NS1 thus constitutes a promising tool that modulates NO and redox stresses by targeting and directly inhibiting eNOS and, at least indirectly, NADPH oxidase(s), with great potential to control angiogenesis.

Coinfection. Virus-helminth coinfection reveals a microbiota-independent mechanism of immunomodulation

The mammalian intestine is colonized by beneficial commensal bacteria and is a site of infection by pathogens, including helminth parasites. Helminths induce potent immunomodulatory effects, but whether these effects are mediated by direct regulation of host immunity or indirectly through eliciting changes in the microbiota is unknown. We tested this in the context of virus-helminth coinfection. Helminth coinfection resulted in impaired antiviral immunity and was associated with changes in the microbiota and STAT6-dependent helminth-induced alternative activation of macrophages. Notably, helminth-induced impairment of antiviral immunity was evident in germ-free mice, but neutralization of Ym1, a chitinase-like molecule that is associated with alternatively activated macrophages, could partially restore antiviral immunity. These data indicate that helminth-induced immunomodulation occurs independently of changes in the microbiota but is dependent on Ym1.

Metabolic stability of superoxide adducts derived from newly developed cyclic nitrone spin traps

Reactive oxygen species are by-products of aerobic metabolism involved in the onset and evolution of various pathological conditions. Among them, the superoxide radical is of special interest as the origin of several damaging species such as H2O2, hydroxyl radical, or peroxynitrite (ONOO(-)). Spin trapping coupled with ESR is a method of choice to characterize these species in chemical and biological systems and the metabolic stability of the spin adducts derived from reaction of superoxide and hydroxyl radicals with nitrones is the main limit to the in vivo application of the method. Recently, new cyclic nitrones bearing a triphenylphosphonium or permethylated β-cyclodextrin moiety have been synthesized and their spin adducts demonstrated increased stability in buffer. In this article, we studied the stability of the superoxide adducts of four new cyclic nitrones in the presence of liver subcellular fractions and biologically relevant reductants using an original setup combining a stopped-flow device and an ESR spectrometer. The kinetics of disappearance of the spin adducts were analyzed using an appropriate simulation program. Our results highlight the interest of the new spin trapping agents CD-DEPMPO and CD-DIPPMPO for specific detection of superoxide with high stability of the superoxide adducts in the presence of liver microsomes.

Toward stable electron paramagnetic resonance oximetry probes: synthesis, characterization, and metabolic evaluation of new ester derivatives of a tris-(para-carboxyltetrathiaaryl)methyl (TAM) radical

Tris(p-carboxyltetrathiaaryl)methyl (TAM) radicals, such as 1a ("Finland" radical), are useful EPR probes for oximetry. However, they are rapidly metabolized by liver microsomes in the presence of NADPH, with the formation of diamagnetic quinone-methide metabolites resulting from an oxidative decarboxylation of one of their carboxylate substituents. In an effort to obtain TAM derivatives potentially more metabolically stable in vivo, we have synthesized four new TAM radicals in which the carboxylate substituents of 1a have been replaced with esters groups bearing various alkyl chains designed to render them water-soluble. The new compounds were completely characterized by UV-vis and EPR spectroscopies, high resolution mass spectrometry (HRMS), and electrochemistry. Two of them were water-soluble enough to undergo detailed microsomal metabolic studies in comparison with 1a. They were found to be stable in the presence of the esterases present in rat liver microsomes and cytosol, and, contrary to 1a, stable to oxidation in the presence of NADPH-supplemented microsomes. A careful study of their possible microsomal reduction under anaerobic or aerobic conditions showed that they were more easily reduced than 1a, in agreement with their higher reduction potentials. They were reduced into the corresponding anions not only under anaerobic conditions but also in the presence of dioxygen. These anions were much more stable than that of 1a and could be characterized by UV-vis spectroscopy, MS, and at the level of their protonated product. However, they were oxidized by O₂, giving back to the starting ester radicals and catalyzing a futile cycle of O₂ reduction. Such reactions should be considered in the design of future stable EPR probes for oximetry in vivo.

Single-electron reduction of quinone and nitroaromatic xenobiotics by recombinant rat neuronal nitric oxide synthase

We examined the kinetics of single-electron reduction of a large number of structurally diverse quinones and nitroaromatic compounds, including a number of antitumour and antiparasitic drugs, and nitroaromatic explosives by recombinant rat neuronal nitric oxide synthase (nNOS, EC 1.14.13.39), aiming to characterize the role of nNOS in the oxidative stress-type cytotoxicity of the above compounds. The steady-state second-order rate constants (kcat/Km) of reduction of the quinones and nitroaromatics varied from 10² M⁻¹s⁻¹ to 10⁶ M⁻¹s⁻¹, and increased with an increase in their single-electron reduction potentials (E¹₇). The presence of Ca²⁺/calmodulin enhanced the reactivity of nNOS. These reactions were consistent with an 'outer sphere' electron-transfer mechanism, considering the FMNH∙/FMNH₂ couple of nNOS as the most reactive reduced enzyme form. An analysis of the reactions of nNOS within the 'outer sphere' electron-transfer mechanism gave the approximate values of the distance of electron transfer, 0.39-0.47 nm, which are consistent with the crystal structure of the reductase domain of nNOS. On the other hand, at low oxygen concentrations ([O₂] = 40-50 μM), nNOS performs a net two-electron reduction of quinones and nitroaromatics. This implies that NOS may in part be responsible for the bioreductive alkylation by two-electron reduced forms of antitumour aziridinyl-substituted quinones under a modest hypoxia.

Single-electron reduction of quinone and nitroaromatic xenobiotics by recombinant rat neuronal nitric oxide synthase

We examined the kinetics of single-electron reduction of a large number of structurally diverse quinones and nitroaromatic compounds, including a number of antitumour and antiparasitic drugs, and nitroaromatic explosives by recombinant rat neuronal nitric oxide synthase (nNOS, EC 1.14.13.39), aiming to characterize the role of nNOS in the oxidative stress-type cytotoxicity of the above compounds. The steady-state second-order rate constants (kcat/Km) of reduction of the quinones and nitroaromatics varied from 10² M⁻¹s⁻¹ to 10⁶ M⁻¹s⁻¹, and increased with an increase in their single-electron reduction potentials (E¹₇). The presence of Ca²⁺/calmodulin enhanced the reactivity of nNOS. These reactions were consistent with an 'outer sphere' electron-transfer mechanism, considering the FMNH∙/FMNH₂ couple of nNOS as the most reactive reduced enzyme form. An analysis of the reactions of nNOS within the 'outer sphere' electron-transfer mechanism gave the approximate values of the distance of electron transfer, 0.39-0.47 nm, which are consistent with the crystal structure of the reductase domain of nNOS. On the other hand, at low oxygen concentrations ([O₂] = 40-50 μM), nNOS performs a net two-electron reduction of quinones and nitroaromatics. This implies that NOS may in part be responsible for the bioreductive alkylation by two-electron reduced forms of antitumour aziridinyl-substituted quinones under a modest hypoxia.

L-arginine uptake by cationic amino acid transporter 2 is essential for colonic epithelial cell restitution

Restoration of the colonic epithelial barrier is an important response during colitis. L-arginine (L-Arg) is a semiessential amino acid that reduces murine colitis induced by Citrobacter rodentium. Cationic amino acid transporter (CAT) proteins increase L-Arg uptake into cells. L-Arg is utilized to produce nitric oxide (NO), by inducible NO synthase (iNOS), or L-ornithine (L-Orn) by arginase (Arg) enzymes. The latter is followed by generation of polyamines by ornithine decarboxylase (ODC) and L-proline (L-Pro) by ornithine aminotransferase (OAT). We show that L-Arg enhanced epithelial restitution in conditionally immortalized young adult mouse colon (YAMC) cells in a wound repair model, and in isolated mouse colonic epithelial cells (CECs), using a cell migration assay. Restitution was impaired by C. rodentium. Wounding induced CAT2, and inhibition of L-Arg uptake by the competitive inhibitor L-lysine (L-Lys) or by CAT2 shRNA, but not CAT1 shRNA, decreased restitution. Migration was impaired in CECs treated with L-Lys or from CAT2(-/-) mice. Wounding increased Arg1 expression, and inhibition of arginase with S-(2-boronoethyl)-L-cysteine (BEC) or Arg1 shRNA inhibited restitution in YAMC cells; cell migration in CECs was also impaired by BEC. Inhibition of ODC or iNOS did not alter restitution. L-Orn or L-Pro restored restitution in cells treated with BEC or Arg1 shRNA, whereas the polyamine putrescine had no benefit. Wounding increased OAT levels, OAT shRNA inhibited restitution, and L-Pro restored restitution in cells with OAT knockdown. Uptake of L-Arg, and its metabolism by Arg1 to L-Orn and conversion to L-Pro by OAT is essential for colonic epithelial wound repair.

Reactions between iron porphyrins and tetrahydropterins

Data from the last few years have revealed a novel biological role of the tetrahydrobiopterin ( H 4 B ) cofactor, in one-electron transfers to the heme of the active site of NO-synthases (NOSs) with intermediate formation of a H 4 B -derived radical. These electron transfers play a key role in the catalytic cycles of the two steps catalyzed by NOS, the N ω-hydroxylation of L-arginine, and the three-electron oxidation of N ω-hydroxyarginine to L-citrulline and NO. Recent experiments performed between various tetrahydropterins and iron porphyrins have shown that the one-electron transfer from tetrahydropterins, such as the natural cofactors H 4 B and tetrahydrofolate or the synthetic 6,7-dimethyltetrahydropterin (diMeH4P), to Fe(III) porphyrins of sufficiently high redox potentials (> about -100 mV versus NHE for the Fe(III)/Fe(II) couple) is a very general reaction that occurs with formation of a tetrahydropterin-derived radical. Reaction of diMeH4P with a stable porphyrin Fe(II)-O 2 complex leads to a diMeH4P-derived radical and a transient Fe(III)-OOH complex, mimicking the reaction between H 4 B and heme Fe(II)-O 2 in the NOS catalytic cycle. Tetrahydropterins such as diMeH4P also reduce hemeproteins Fe(III) of sufficiently high redox potentials, such as cytochromes c and b5 or metmyoglobin, to the corresponding hemeproteins Fe(II) .

Spectroscopic, catalytic and binding properties of Bacillus subtilis NO synthase-like protein: comparison with other bacterial and mammalian NO synthases

Genome sequencing has shown the presence of genes coding for NO-synthase (NOS)-like proteins in bacteria. The roles and properties of these proteins remain unclear. UV-visible spectroscopy was used to characterize the recombinant NOS-like protein from Bacillus subtilis (bsNOS) in its ferric and ferrous states in the presence of various Fe(III)- and Fe(II)-heme-ligands and of a series of L-arginine (L-arg) analogs. BsNOS exhibited several spectroscopic and binding properties in common with Bacillus anthracis NOS (baNOS) that were clearly different from those of tetrahydrobiopterin (H4B)-free mammalian NOS oxygenase domains (mNOS(oxys)) and of Staphylococcus aureus NOS (saNOS). Interestingly, bsNOS and baNOS that do not contain H4B exhibited properties much closer to those of H4B-containing mNOS(oxys). Moreover, bsNOS was found to efficiently catalyze the oxidation of L-arginine into L-citrulline by H(2)O(2), whereas H4B-free mNOS(oxys) exhibited low activities for this reaction.

Helicobacter pylori arginase mutant colonizes arginase II knockout mice

AIM: To investigate the role of host and bacterial arginases in the colonization of mice by Helicobacter pylori (H. pylori).

METHODS: H. pylori produces a very powerful urease that hydrolyzes urea to carbon dioxide and ammonium, which neutralizes acid. Urease is absolutely essential to H. pylori pathogenesis; therefore, the urea substrate must be in ample supply for urease to work efficiently. The urea substrate is most likely provided by arginase activity, which hydrolyzes L-arginine to L-ornithine and urea. Previous work has demonstrated that H. pylori arginase is surprisingly not required for colonization of wild-type mice. Hence, another in vivo source of the critical urea substrate must exist. We hypothesized that the urea source was provided by host arginase II, since this enzyme is expressed in the stomach, and H. pylori has previously been shown to induce the expression of murine gastric arginase II. To test this hypothesis, wild-type and arginase (rocF) mutant H. pylori strain SS1 were inoculated into arginase II knockout mice.

RESULTS: Surprisingly, both the wild-type and rocF mutant bacteria still colonized arginase II knockout mice. Moreover, feeding arginase II knockout mice the host arginase inhibitor S-(2-boronoethyl)-L-cysteine (BEC), while inhibiting > 50% of the host arginase I activity in several tissues, did not block the ability of the rocF mutant H. pylori to colonize. In contrast, BEC poorly inhibited H. pylori arginase activity.

CONCLUSION: The in vivo source for the essential urea utilized by H. pylori urease is neither bacterial arginase nor host arginase II; instead, either residual host arginase I or agmatinase is probably responsible.

L-arginine binding to human inducible nitric oxide synthase: an antisymmetric funnel route toward isoform-specific inhibitors?

Nitric oxide (NO) is an important signaling molecule produced by a family of enzymes called nitric oxide synthases (NOS). Because NO is involved in various pathological conditions, the development of potent and isoform-selective NOS inhibitors is an important challenge. In the present study, the dimer of oxygenase domain of human iNOS (iNOSoxy) complexed to its natural substrate L-arginine (L-Arg) and both heme and tetrahydro-L-biopterin (BH4) cofactors was studied through multiple molecular dynamics simulations. Starting from the X-ray structure available for that complex (PDB: 1NSI ), a 16 ns equilibration trajectory was first obtained. Twelve dynamics of slow extraction of L-Arg out from the iNOSoxy active site were then performed. The steered molecular dynamics (SMD) approach was used starting from three different points of the reference trajectory for a total simulation time of 35 ns. A probable unbinding/binding pathway of L-Arg was characterized. It was suggested that a driving force directed the substrate toward the heme pocket. Key intermediate steps/residues along the access route to the active site were identified along this "funnel shape" pathway and compared to existing data. A quasi-normal mode analysis performed on the SMD data suggested that large collective motions of the protein may be involved in L-Arg binding and that opening the route to the active site in one monomer promoted an inverse, closing motion in the second monomer. Finally, our findings might help to rationalize the design of human iNOS isoform competitive inhibitors.

SHIP-deficient mice develop spontaneous intestinal inflammation and arginase-dependent fibrosis

Intestinal fibrosis is a serious complication of Crohn's disease (CD) that can lead to stricture formation, which requires surgery. Mechanisms underlying intestinal fibrosis remain elusive because of a lack of suitable mouse models. Herein, we describe a spontaneous mouse model of intestinal inflammation with fibrosis and the profibrotic role of arginase I. The Src homology 2 domain-containing inositol polyphosphate 5'-phosphatase-deficient (SHIP(-/-)) mice developed spontaneous discontinuous intestinal inflammation restricted to the distal ileum starting at the age of 4 weeks. Mice developed several key features resembling CD, including inflammation and fibrosis. Inflammation was characterized by abundant infiltrating Gr-1-positive immune cells, granuloma-like immune cell aggregates that contained multinucleated giant cells, and a mixed type 2 and type 17 helper T-cell cytokine profile. Fibrosis was characterized by a thickened ileal muscle layer, collagen deposition, and increased fibroblasts at the sites of collagen deposition. SHIP(-/-) ilea had increased arginase activity and arginase I expression that was inversely proportional to nitrotyrosine staining. SHIP(-/-) mice were treated with the arginase inhibitor S-(2-boronoethyl)-l-cysteine, and changes in the disease phenotype were measured. Arginase inhibition did not affect the number of immune cell infiltrates in the SHIP(-/-) mouse ilea; rather, it reduced collagen deposition and muscle hyperplasia. These findings suggest that arginase activity is a potential target to limit intestinal fibrosis in patients with CD.

Unprecedented ipso aromatic nucleophilic substitution upon oxidative decarboxylation of tris(p-carboxyltetrathiaaryl)methyl (TAM) radicals: a new access to diversely substituted TAM radicals

A new reaction of oxidative substitution of aromatic carboxyl groups on stable trityl radical derivatives (TAMs) by various nucleophiles is described; it leads to a wide variety of new persistent and diversely substituted TAM radicals.

TLR agonists that induce IFN-beta abrogate resident macrophage suppression of T cells

Resident tissue macrophages (Mφs) continually survey the microenvironment, ingesting Ags and presenting them on their surface for recognition by T cells. Because these Ags can be either host cell- or pathogen-derived, Mφs must be able to distinguish whether a particular Ag should provoke an immune response or be tolerated. However, the mechanisms that determine whether Mφs promote or inhibit T cell activation are not well understood. To investigate this, we first determined the mechanism by which murine resident peritoneal Mφs suppress in vitro T cell proliferation in the absence of pathogens and then explored the effects of different pathogen-derived molecules on Mφ immunosuppression. Our results suggest that, in response to IFN-γ, which is secreted by TCR-activated T cells, resident peritoneal Mφs acquire immunosuppressive properties that are mediated by NO. However, pretreatment of Mφs with LPS or dsRNA, but not CpG or peptidoglycan, eliminates their suppressive properties, in part via the induction of autocrine-acting IFN-β. These results suggest TLR agonists that activate TRIF, and consequently induce IFN-β, but not those that exclusively signal through MyD88, abrogate the immunosuppressive properties of Mφs, and thus promote T cell expansion and elimination of invading microorganisms.

Metabolic stability of superoxide and hydroxyl radical adducts of a cyclic nitrone toward rat liver microsomes and cytosol: A stopped-flow ESR spectroscopy study

The metabolic stability of the spin adducts derived from the reaction of superoxide and hydroxyl radicals with 5-tert-butoxycarbonyl-5-methyl-1-pyrroline N-oxide (BocMPO) in the presence of rat liver microsomes (RLM) and rat liver cytosol (RLC) was studied by using a stopped-flow device coupled to an electron spin resonance (ESR) spectrometer. The kinetics of the disappearance of the BocMPO-OH and BocMPO-OOH radicals could be followed by ESR spectroscopy with treatment of the ESR data by an appropriate computer program. The presence of cytosol led to a 60-fold decrease of the half-life of BocMPO-OOH with the intermediate formation of BocMPO-OH. This effect of cytosol was due to an ascorbate- and thiol-dependent reduction of BocMPO-OOH. RLC only led to a 5-fold decrease of the half-life of BocMPO-OH that was predominantly due to cytosolic ascorbate. RLM led to a 10-fold decrease of the BocMPO-OOH half-life that was mainly related to a direct reaction of the hydroperoxide function of BocMPO-OOH with cytochrome P450 Fe(III) (P450). Other ferric heme proteins, such as methemoglobin (metHb) and horseradish peroxidase (HRP), as well as hemin itself, exhibited a similar behavior. RLM and metHb showed a much weaker effect on BocMPO-OH half-life (2-fold decrease), whereas RLM in the presence of NADPH caused a greater decrease of the BocMPO-OH half-life ( approximately 5-fold). The effect of RLM without NADPH was mainly due to a direct reaction with microsomal P450, whereas the RLM- and NADPH-dependent effect was mainly due to flavin-containing reductases such as cytochrome P450 reductase. These data on the effects of liver subcellular fractions on the half-life of the BocMPO-OOH and the BocMPO-OH spin adducts highlight the role of heme as a biological cofactor involved in the disappearance of such spin adducts. They should be helpful for the design of new spin traps that would form more metabolically stable spin adducts in vitro and in vivo.

Arginase II restricts host defense to Helicobacter pylori by attenuating inducible nitric oxide synthase translation in macrophages

Helicobacter pylori infection of the stomach causes peptic ulcer disease and gastric cancer. Despite eliciting a vigorous immune response, the bacterium persists for the life of the host. An important antimicrobial mechanism is the production of NO derived from inducible NO synthase (iNOS). We have reported that macrophages can kill H. pylori in vitro by an NO-dependent mechanism, but supraphysiologic levels of the iNOS substrate l-arginine are required. Because H. pylori induces arginase activity in macrophages, we determined if this restricts NO generation by reducing l-arginine availability. Inhibition of arginase with S-(2-boronoethyl)-l-cysteine (BEC) significantly enhanced NO generation in H. pylori-stimulated RAW 264.7 macrophages by enhancing iNOS protein translation but not iNOS mRNA levels. This effect resulted in increased killing of H. pylori that was attenuated with an NO scavenger. In contrast, inhibition of arginase in macrophages activated by the colitis-inducing bacterium Citrobacter rodentium increased NO without affecting iNOS levels. H. pylori upregulated levels of arginase II (Arg2) mRNA and protein, which localized to mitochondria, whereas arginase I was not induced. Increased iNOS protein and NO levels were also demonstrated by small interfering RNA knockdown of Arg2 and in peritoneal macrophages from C57BL/6 Arg2(-/-) mice. In H. pylori-infected mice, treatment with BEC or deletion of Arg2 increased iNOS protein levels and NO generation in gastric macrophages, but treatment of Arg2(-/-) mice with BEC had no additional effect. These studies implicate Arg2 in the immune evasion of H. pylori by causing intracellular depletion of l-arginine and thus reduction of NO-dependent bactericidal activity.

Role of arginine guanidinium moiety in nitric-oxide synthase mechanism of oxygen activation

Nitric-oxide synthases (NOS) are highly regulated heme-thiolate enzymes that catalyze two oxidation reactions that sequentially convert the substrate L-Arg first to N(omega)-hydroxyl-L-arginine and then to L-citrulline and nitric oxide. Despite numerous investigations, the detailed molecular mechanism of NOS remains elusive and debatable. Much of the dispute in the various proposed mechanisms resides in the uncertainty concerning the number and sources of proton transfers. Although specific protonation events are key features in determining the specificity and efficiency of the two catalytic steps, little is known about the role and properties of protons from the substrate, cofactors, and H-bond network in the vicinity of the heme active site. In this study, we have investigated the role of the acidic proton from the L-Arg guanidinium moiety on the stability and reactivity of the ferrous heme-oxy complex intermediate by exploiting a series of L-Arg analogues exhibiting a wide range of guanidinium pK(a) values. Using electrochemical and vibrational spectroscopic techniques, we have analyzed the effects of the analogues on the heme, including characteristics of its proximal ligand, heme conformation, redox potential, and electrostatic properties of its distal environment. Our results indicate that the substrate guanidinium pK(a) value significantly affects the H-bond network near the heme distal pocket. Our results lead us to propose a new structural model where the properties of the guanidinium moiety finely control the proton transfer events in NOS and tune its oxidative chemistry. This model may account for the discrepancies found in previously proposed mechanisms of NOS oxidation processes.

Effect of uncoupling endothelial nitric oxide synthase on calcium homeostasis in aged porcine endothelial cells

AIMS

The requirement of endothelial NO synthase (NOS3) calcium to produce NO is well described, although the effect of NO on intracellular calcium levels [Ca(2+)](i) is still confusing. Therefore, NO and [Ca(2+)](i) cross-talk were studied in parallel in endothelial cells possessing a functional or a dysfunctional NO pathway.

METHODS AND RESULTS

Dysfunctional porcine endothelial cells were obtained either in vitro by successive passages or in vivo from regenerated endothelium 1 month after coronary angioplasty. Activity of NOS3 was characterized by conversion of arginine to citrulline, BH(4) intracellular availability, cGMP, and superoxide anion production. Imaging of the Ca(2+) indicator FURA 2-AM was recorded and sarco/endoplasmic reticulum Ca(2+) ATPase (SERCA) pump activity was analysed by (45)Ca(2+) uptake into cells. In endothelial cells with a functional NO pathway, NOS3 inhibition increased [Ca(2+)](i) and, conversely, an NO donor decreased it. In aged cells with an uncoupled NOS3 as shown by the reduced BH(4) level, the increase in superoxide anion and the lower production of cGMP and the decrease in NO bioavailability were linearly correlated with the increase in basal [Ca(2+)](i). Moreover, when stimulated by bradykinin, the calcium response was reduced while its decay was slowed down. These effects on the calcium signalling were abolished in calcium-free buffer and were similarly induced by SERCA inhibitors. In aged cells, NO improved the reduced SERCA activity and tended to normalize the agonist calcium response.

CONCLUSION

In control endothelial cells, NO exerts a negative feedback on cytosolic Ca(2+) homeostasis. In aged cells, uncoupled NOS3 produced NO that was insufficient to control the [Ca(2+)](i). Consequently, under resting conditions, SERCA activity decreased and [Ca(2+)](i) increased. These alterations were reversible as exogenous NO, in a cGMP-independent way, refilled intracellular calcium stores, reduced calcium influx, and improved the agonist-evoked calcium response. Therefore, prevention of the decrease in NO in dysfunctional endothelium would normalize the calcium-dependent functions.

Synthesis and evaluation of pyrido[1,2-a]pyrimidines as inhibitors of nitric oxide synthases

A series of new 3-aroylpyrido[1,2-a]pyrimidines were synthesized from aryl methyl ketones in a simple two-step procedure and evaluated as nitric oxide synthases (NOS) inhibitors. In order to perform a structure-activity relationship study, different aroyl groups were introduced in 3-position and methyl groups were introduced at different positions of the pyrimidine heterocycle. Compounds with a biphenyloyl, benzyloxybenzoyl or naphthoyl group displayed the highest inhibitory effects which were further increased by introduction of a methyl group in position 8 of the pyrido[1,2-a]pyrimidine moiety. Some of the compounds exhibited promising inhibitory effects with selectivity toward the purified inducible NOS (iNOS) and were also active against iNOS expressed in stimulated RAW 264.7 cells.