Substrate Arginine Research Articles

Systemic inflammation is linked to low arginine and high ADMA plasma levels resulting in an unfavourable NOS substrate-to-inhibitor ratio: the Hoorn Study

Inflammation is associated with a reduced availability of NO in the vasculature. We investigated the possible involvement of altered levels of the substrate (arginine) and the inhibitor [ADMA (asymmetric ω-NG,NG-dimethylarginine)] of NOS (NO synthase). Plasma concentrations of arginine and ADMA, the inflammatory markers CRP (C-reactive protein) and MPO (myeloperoxidase), and oxLDL [oxidized LDL (low-density lipoprotein)] were measured in 369 male and 377 female participants (aged 50-87 years) of a population-based cohort study. The arginine/ADMA ratio decreased significantly across increasing tertiles of CRP and MPO. These negative associations remained significant in a linear regression model with both MPO (P = 0.002) and CRP (P < 0.001) as independent variables and adjusted for age, sex and cardiovascular risk factors. In a fully adjusted regression model, MPO was positively associated with ADMA {5.4 [95% CI (confidence interval), 1.3-9.4] nmol/l change of ADMA per S.D. increase in MPO; P = 0.010}, whereas CRP was not (P = 0.36). Conversely, in a fully adjusted model, CRP was negatively associated with arginine [-2.8 (95% CI, -4.0 to -1.6) μmol/l arginine per S.D. of CRP; P < 0.001], without a significant contribution of MPO (P = 0.23). The relationship between MPO and ADMA became stronger with increasing levels of oxLDL (1.8, 5.2 and 8.7 nmol/l ADMA per S.D. of MPO for increasing tertiles of oxLDL), consistent with the ability of MPO to amplify oxidative stress. In contrast, the relationship between CRP and arginine was not modified by levels of oxLDL. In conclusion, an unfavourable NOS substrate/inhibitor ratio may contribute to the reduced NO bioavailability associated with inflammation.

Blastomyces dermatitidis Yeast Cells Inhibit Nitric Oxide Production by Alveolar Macrophage Inducible Nitric Oxide Synthase

The ability of pathogens to evade host antimicrobial mechanisms is crucial to their virulence. The dimorphic fungal pathogen Blastomyces dermatitidis can infect immunocompetent patients, producing a primary pulmonary infection that can later disseminate to other organs. B. dermatitidis possesses a remarkable ability to resist killing by alveolar macrophages. To date, no mechanism to explain this resistance has been described. Here, we focus on macrophage production of the toxic molecule nitric oxide as a potential target of subversion by B. dermatitidis yeast cells. We report that B. dermatitidis yeast cells reduce nitric oxide levels in the supernatants of activated alveolar macrophages. This reduction is not due to detoxification of nitric oxide, but rather to suppression of macrophage nitric oxide production. We show that B. dermatitidis yeast cells do not block upregulation of macrophage inducible nitric oxide synthase (iNOS) expression or limit iNOS access to its arginine substrate. Instead, B. dermatitidis yeast cells appear to inhibit iNOS enzymatic activity. Further investigation into the genetic basis of this potential virulence mechanism could lead to the identification of novel antifungal drug targets.

Arginase inhibition restores in vivo coronary microvascular function in type 2 diabetic rats

Nitric oxide (NO) is crucial for maintaining normal endothelial function and vascular integrity. Increased arginase activity in diabetes might compete with NO synthase (NOS) for their common substrate arginine, resulting in diminished production of NO. The aim of this study was to evaluate coronary microvascular function in type 2 diabetic Goto-Kakizaki (GK) rats using in vivo coronary flow velocity reserve (CFVR) and the effect of arginase inhibition to restore vascular function. Different groups of GK and Wistar rats were given vehicle, the arginase inhibitor N(ω)-hydroxy-nor-l-arginine (nor-NOHA), l-arginine, and the NOS inhibitor N(G)-monomethyl -l-arginine (l-NMMA). GK rats had impaired CFVR compared with Wistar rats (1.31 ± 0.09 vs. 1.87 ± 0.05, P < 0.001). CFVR was restored by nor-NOHA treatment compared with vehicle in GK rats (1.71 ± 0.13 vs. 1.23 ± 0.12, P < 0.05) but remained unchanged in Wistar rats (1.88 ± 0.10 vs. 1.79 ± 0.16). The beneficial effect of nor-NOHA in GK rats was abolished after NOS inhibition. CFVR was not affected by arginine compared with vehicle. Arginase II expression was increased in the aorta and myocardium from GK rats compared with Wistar rats. Citrulline-to-ornithine and citrulline-to-arginine ratios measured in plasma increased significantly more in GK rats than in Wistar rats after nor-NOHA treatment, suggesting a shift of arginine utilization from arginase to NOS. In conclusion, coronary artery microvascular function is impaired in the type 2 diabetic GK rat. Treatment with nor-NOHA restores the microvascular function by a mechanism related to increased utilization of arginine by NOS and increased NO availability.

Small molecule inhibitors that discriminate between protein arginine N-methyltransferases PRMT1 and CARM1

Protein arginine N-methyltransferases (PRMTs) selectively replace N-H for N-CH(3) at substrate protein guanidines, a post-translational modification important for a range of biological processes, such as epigenetic regulation, signal transduction and cancer progression. Selective chemical probes are required to establish the dynamic function of individual PRMTs. Herein, model inhibitors designed to occupy PRMT binding sites for an arginine substrate and S-adenosylmethionine (AdoMet) co-factor are described. Expedient access to such compounds by modular synthesis is detailed. Remarkably, biological evaluation revealed some compounds to be potent inhibitors of PRMT1, but inactive against CARM1. Docking studies show how prototype compounds may occupy the binding sites for a co-factor and arginine substrate. Overlay of PRMT1 and CARM1 binding sites suggest a difference in a single amino acid that may be responsible for the observed selectivity.

Nitric oxide and preeclampsia

Preeclampsia is one of the leading causes of obstetric morbidity and mortality. The placenta has a crucial role in the development of preeclampsia. Despite intensive researches the cause of disorder is still unknown. Insufficient NO synthesis may have a key role in pathogenesis. Endothelial NO synthesis (eNOS) is the primary isoenzyme expressed in human placenta, its known disturbances are discussed. Deficiency of substrate (arginine), cofactor (tetrahydrobiopterin, BH4) and calcium can decrease the NO synthesis. Serum levels of free fatty acids (FFA), asymmetric dimethylarginine, reactive oxygen species and glucose may increase in preeclamptic pregnancy. These substances decrease NO production by different ways. The reduced affinity of eNOS to the cofactor BH4 may lead to insufficient NO, but increased superoxide production in preeclamptic placentas. Polymorphisms of eNOS gene (D298E, -786T→C) were associated with preeclamptic complications (not adequately documented). Data suggest that smoking has protective role against preeclampsia. The mechanism is not clear, even the actions of smoking on eNOS are ambivalent. The expression of eNOS is decreased, while the phosphorylation of the activator Ser1177 and also the deactivator Thr495 are increased by cigarette smoke. The oxidative stress directly decreases NO levels. Smoking lowers serum FFA levels, thus the activity of eNOS may be increased. CO produced during smoking mimics the effect of NO and can compensate its absence partially.

A Potential Antitumor Drug (Arginine Deiminase) Reengineered for Efficient Operation under Physiological Conditions

Arginine deiminase (ADI, EC 3.5.3.6) is a potential antitumor drug for the treatment of arginine-auxotrophic tumors such as hepatocellular carcinomas (HCCs) and melanomas, and studies on human lymphatic leukemia cell lines have confirmed that ADI has antiangiogenic activity. Recent studies showed that a combination of taxane and ADI-PEG20, which induces caspase-independent apoptosis, is more effective than taxane monotherapy for prostate cancer. The main limitation of ADI from Pseudomonas plecoglossicida (PpADI) and of many other ADI enzymes lies in their pH-dependent activity profile. PpADI has a pH optimum at 6.5 and a pH shift from 6.5 to 7.5 results in an ∼80 % activity drop (the pH of human plasma is 7.35 to 7.45). In 2010, we reported a proof of concept for ADI engineering by directed evolution that resulted in variant M2 (K5T/D44E/H404R). M2 has a pH optimum of pH 7.0, a fourfold higher k(cat) value than the wild-type PpADI (pH 7.4, 0.5 M phosphate buffer), and an increased K(m) value for substrate arginine. In our latest work, variants M5 (K5T/D38H/D44E/A128T/H404R) and M6 (K5T/D38H/D44E/A128T/E296K/H404R) were generated by directed evolution by employing PBS buffer (pH 7.4), which mimics physiological conditions. The S(0.5) value of parent M3 (K5T/D44E/A128T/H404R) decreased from 2.01 to 1.48 mM (M5) and 0.81 mM (M6). The S(0.5) value of M6 (0.81 mM) is lower than that of wild-type PpADI (1.30 mM); the k(cat) values improved from 0.18 s(-1) (wild-type PpADI) to 17.56 s(-1) (M5, 97.6-fold) and 11.64 s(-1) (M6, 64.7-fold).

Pulmonary hypertension in thalassemia

Pulmonary hypertension (PH) is common in thalassemia and contributes to mortality. Advancing age and a history of splenectomy are major risk factors in this population. The etiology of PH is multifactorial, involving a complex interaction of platelets, the coagulation system, erythrocytes, and endothelial cells along with inflammatory and vascular mediators. The long-term effect of splenectomy, red cell membrane pathology, coagulation abnormalities, low nitric oxide (NO) bioavailability, excess arginase activity, platelet activation, oxidative stress, iron overload, and chronic hemolysis play a role. The process of hemolysis disables the arginine-NO pathway through the simultaneous release of erythrocyte arginase and cell-free hemoglobin. Both NO and its obligate substrate arginine are rapidly consumed. The biological consequences of hemolysis on NO bioavailability ultimately translate into the clinical manifestations of PH. Guidelines for the management of PH in thalassemia have not yet been established; however, clinical trials are ongoing in an effort to guide future therapy.

Identification of the key amino acid residues in Sabellastarte arginine kinase for distinguishing chiral guanidino substrates ( d- and l-arginine)

Arginine kinase (AK, EC 2.7.3.3) is a phosphagen kinase widely distributed in lower and higher invertebrates. Sabellastarte indica AKs (AK1 and AK2) are the only AKs that use d-arginine in addition to l-arginine as a substrate. S. indica AKs are therefore clearly unrelated to typical AK. The purpose of this work was to identify the amino acid residues responsible for distinguishing the chirality of d- and l-arginine. The three-dimensional structure of Sabellastarte AK2 was predicted using the SWISS-MODEL automated modeling server. Two amino acids, L64 in the N-terminal flexible loop and N320 in the C-terminal flexible loop, were found to be closest to the substrate arginine. We introduced several mutations around the predicted binding site of the substrate arginine. In the L64I mutant, the affinity for l-arginine was greatly increased (9.5-fold that of the wild-type), whereas its affinity for d-arginine was increased 2.9-fold, indicating that the L64I mutant enzyme was more specific for l-arginine. On the other hand, the L64V mutant showed a 1.7-fold decrease in affinity for l-arginine, but unchanged affinity for d-arginine. Thus the L64V mutant enzyme was more specific for d-arginine. Since the replacement of amino acid residue L64 by I or V significantly affected the affinity for l-arginine or d-arginine, it can be concluded that amino acid 64 is a key residue for distinguishing d- and l-arginine. Seven mutants at position 320 (N320H, Q, D, E, R, K and A) had considerably reduced enzymatic activity (0.05–52% of the wild-type). The reduction in enzyme activity was more significant when d-arginine was the substrate rather than l-arginine, except for N320D, suggesting that the carbonyl oxygen of the Asn320 side chain forms a hydrogen bond with the α-amino nitrogen attached to the asymmetric carbon of the d-arginine substrate. In addition, we found that even a residue remote from the guanidino substrate-binding site, such as G54 and Y89 in Sabellastarte AK2, significantly affected guanidino substrate specificity.

Mechanism of substrate recognition and transport by an amino acid antiporter

In extremely acidic environments, enteric bacteria such as Escherichia coli rely on the amino acid antiporter AdiC to expel protons by exchanging intracellular agmatine (Agm(2+)) for extracellular arginine (Arg(+)). AdiC is a representative member of the amino acid-polyamine-organocation (APC) superfamily of membrane transporters. The structure of substrate-free AdiC revealed a homodimeric assembly, with each protomer containing 12 transmembrane segments and existing in an outward-open conformation. The overall folding of AdiC is similar to that of the Na(+)-coupled symporters. Despite these advances, it remains unclear how the substrate (arginine or agmatine) is recognized and transported by AdiC. Here we report the crystal structure of an E. coli AdiC variant bound to Arg at 3.0 A resolution. The positively charged Arg is enclosed in an acidic binding chamber, with the head groups of Arg hydrogen-bonded to main chain atoms of AdiC and the aliphatic portion of Arg stacked by hydrophobic side chains of highly conserved residues. Arg binding induces pronounced structural rearrangement in transmembrane helix 6 (TM6) and, to a lesser extent, TM2 and TM10, resulting in an occluded conformation. Structural analysis identified three potential gates, involving four aromatic residues and Glu 208, which may work in concert to differentially regulate the upload and release of Arg and Agm.

Determination of the Catalytic Pathway of a Manganese Arginase Enzyme Through Density Functional Investigation

The catalytic mechanism of dimanganese-containing arginase enzyme has been investigated by DFT calculations. Two exchange-correlation functionals, B3 LYP and MPWB1 K, have been used to construct the potential energy profiles for the hydrolysis of an arginine substrate performed by an arginase active site model system. Two reaction mechanisms have been investigated, one involving a water molecule (mechanism 1) and the other involving a hydroxide ion (mechanism 2) as nucleophilic agent. Results obtained in the gas phase and in the protein environment have indicated that mechanism 1 involving a water molecule entails structural features as well as an activation energy for the rate-determining step that are inconsistent with experimental data available for the arginase enzyme. On the other hand, when a hydroxide ion is present at the Mn2 site, a lower activation energy and a structural arrangement closer to the experimental indication are obtained.

Active Site Cysteine Is Protonated in the PAD4 Michaelis Complex: Evidence from Born−Oppenheimer Ab Initio QM/MM Molecular Dynamics Simulations

The protein arginine deiminase 4 (PAD4) catalyzes the citrullination of the peptidylarginine and plays a critical role in rheumatoid arthritis (RA) and gene regulation. Understanding its catalytic mechanism is not only of fundamental importance but also of significant medical interest for the rational design of new inhibitors. By employing on-the-fly Born-Oppenheimer ab initio QM/MM molecular dynamics simulations, we have demonstrated that it is unlikely for the active site cysteine and histidine to exist as a thiolate-imidazolium ion pair in the PAD4 Michaelis reactant complex. Instead, a substrate-assisted proton transfer mechanism for the deimination reaction step has been characterized: both Cys645 and His471 in the PAD4 active site are neutral prior to the reaction; the deprotonation of Cys645 by the substrate arginine occurs in concert with the nucleophilic addition of the Cys thiolate to Czeta of the substrate, and leads to a covalent tetrahedral intermediate; then, the Czeta-Neta1 bond cleaves and the resulted ammonia is displaced by a solvent water molecule. The initial deprotonation and nucleophilic attack step is found to be rate-determining. The computed free energy barrier with B3LYP(6-31G*) QM/MM MD simulations and umbrella sampling is 20.9 kcal.mol(-1), consistent with the experimental kinetic data. During the deimination, His471 plays an important role in stabilizing the transition state through the formation of the hydrogen bond with the guanidinium group. Our current studies further demonstrated the viability and strength of the ab initio QM/MM molecular dynamics approach in simulating enzyme reactions.

Synthesis of Nitric Oxide by the NOS-like Protein from Deinococcus radiodurans: A Direct Role for Tetrahydrofolate

Genes encoding for proteins with high sequence homology to the heme-containing, oxygenase domain of mammalian nitric oxide synthase (NOS) have been identified in a number of bacteria. Many of these species of bacteria do not contain the genes that encode for the synthetic machinery to produce tetrahydrobiopterin (H(4)B), a cofactor of NOS required for NO synthesis. These bacteria have the genes for the synthesis of tetrahydrofolate (H(4)F) which contains the redox-active pteridine ring of H(4)B. These observations led us to investigate whether H(4)F could be used for the synthesis of NO by NOS-like enzymes from bacteria that cannot make H(4)B. The NOS gene from one such bacterium, Deinococcus radiodurans, was cloned and expressed (deiNOS) in Escherichia coli and then purified and characterized. The K(D) of deiNOS for the NOS substrate arginine (0.9 +/- 0.1 mM) drops by over 2 orders of magnitude in the presence of H(4)F (7.4 +/- 0.1 microM). Further, NO is synthesized from the NOS substrate N-hydroxy-l-arginine (NHA) by deiNOS in the presence of H(4)F. Stopped-flow spectroscopic data reveal that H(4)F accelerates the rate of decay of the ferrous-oxy/ferric-superoxo species in substrate turnover. These data strongly suggest that H(4)F may be used by D. radiodurans to replace H(4)B as a redox-active cofactor for nitric oxide synthesis.

KLYP956 Is a Non-Imidazole-Based Orally Active Inhibitor of Nitric-Oxide Synthase Dimerization

Nitric-oxide synthases (NOS) generate nitric oxide (NO) through the oxidation of l-arginine. Inappropriate or excessive production of NO by NOS is associated with the pathophysiology of various disease states. Efforts to treat these disorders by developing arginine mimetic, substrate-competitive NOS inhibitors as drugs have met with little success. Small-molecule-mediated inhibition of NOS dimerization represents an intriguing alternative to substrate-competitive inhibition. An ultra-high-throughput cell-based screen of 880,000 small molecules identified a novel quinolinone with inducible NOS (iNOS) inhibitory activity. Exploratory chemistry based on this initial screening hit resulted in the synthesis of KLYP956, which inhibits iNOS at low nanomolar concentrations. The iNOS inhibitory potency of KLYP956 is insensitive to changes in concentrations of the substrate arginine, or the cofactor tetrahydrobiopterin. Mechanistic analysis suggests that KLYP956 binds the oxygenase domain in the vicinity of the active site heme and inhibits iNOS and neuronal NOS (nNOS) by preventing the formation of enzymatically active dimers. Oral administration of KLYP956 [N-(3-chlorophenyl)-N-((8-fluoro-2-oxo-1,2-dihydroquinolin-4-yl)methyl)-4-methylthiazole-5-carboxamide] inhibits iNOS activity in a murine model of endotoxemia and blocks pain behaviors in a formalin model of nociception. KLYP956 thus represents the first nonimidazole-based inhibitor of iNOS and nNOS dimerization and provides a novel pharmaceutical alternative to previously described substrate competitive inhibitors.

Density functional theory (DFT) and combined quantum mechanical/molecular mechanics (QM/MM) studies on the oxygen activation step in nitric oxide synthase enzymes

In this review paper, we will give an overview of recent theoretical studies on the catalytic cycle(s) of NOS (nitric oxide synthase) enzymes and in particular on the later stages of these cycles where experimental work is difficult due to the short lifetime of intermediates. NOS enzymes are vital for human health and are involved in the biosynthesis of toxic nitric oxide. Despite many experimental efforts in the field, the catalytic cycle of this important enzyme is still surrounded by many unknowns and controversies. Our theoretical studies were focused on the grey zones of the catalytic cycle, where intermediates are short-lived and experimental detection is impossible. Thus combined QM/MM (quantum mechanics/molecular mechanics) as well as DFT (density functional theory) studies on NOS enzymes and active site models have established a novel mechanism of oxygen activation and the conversion of L-arginine into N(omega)-hydroxo-arginine. Although NOS enzymes show many structural similarities to cytochrome P450 enzymes, it has long been anticipated that therefore they should have a similar catalytic cycle where molecular oxygen binds to a haem centre and is converted into an Fe(IV)-oxo haem(+*) active species (Compound I). Compound I, however, is elusive in the cytochrome P450s as well as in NOS enzymes, but indirect experimental evidence on cytochrome P450 systems combined with theoretical modelling have shown it to be the oxidant responsible for hydroxylation reactions in cytochrome P450 enzymes. By contrast, in the first catalytic cycle of NOS it has been shown that Compound I is first reduced to Compound II before the hydroxylation of arginine. Furthermore, substrate arginine in NOS enzymes appears to have a dual function, namely first as a proton donor in the catalytic cycle to convert the ferric-superoxo into a ferric-hydroperoxo complex and secondly as the substrate that is hydroxylated in the process leading to N(omega)-hydroxo-arginine.

Antioxidant therapy: Reducing malaria severity?*

Ackerman, Hans C. MD, DPhil, MSc; Beaudry, Steven D. BS; Fairhurst, Rick M. MD, PhD Author Information

AtNOS/AtNOA1 Is a Functional Arabidopsis thaliana cGTPase and Not a Nitric-oxide Synthase

AtNOS1 was previously identified as a potential nitric-oxide synthase (NOS) in Arabidopsis thaliana, despite lack of sequence similarity to animal NOSs. Although the dwarf and yellowish leaf phenotype of Atnos1 knock-out mutant plants can be rescued by treatment with exogenous NO, doubts have recently been raised as to whether AtNOS1 is a true NOS. Moreover, depending on the type of physiological responses studied, Atnos1 is not always deficient in NO induction and/or detection, as previously reported. Here, we present experimental evidence showing that AtNOS1 is unable to bind and oxidize arginine to NO. These results support the argument that AtNOS1 is not a NOS. We also show that the renamed NO-associated protein 1 (AtNOA1) is a member of the circularly permuted GTPase family (cGTPase). AtNOA1 specifically binds GTP and hydrolyzes it. Complementation experiments of Atnoa1 mutant plants with different constructs of AtNOA1 show that GTP hydrolysis is necessary but not sufficient for the physiological function of AtNOA1. Mutant AtNOA1 lacking the C-terminal domain, although retaining GTPase activity, failed to complement Atnoa1, suggesting that this domain plays a crucial role in planta. cGTPases appear to be RNA-binding proteins, and the closest homolog of AtNOA1, the Bacillus subtilis YqeH, has been shown to participate in ribosome assembly and stability. We propose a similar function for AtNOA1 and discuss it in the light of its potential role in NO accumulation and plant development.

Nitric Oxide and Arginine Dysregulation: A Novel Pathway to Pulmonary Hypertension in Hemolytic Disorders

Secondary pulmonary hypertension (PH) is emerging as one of the leading causes of mortality and morbidity in patients with hemolytic anemias such as sickle cell disease (SCD) and thalassemia. Impaired nitric oxide (NO) bioavailability represents the central feature of endothelial dysfunction, and is a major factor in the pathophysiology of PH. Inactivation of NO correlates with hemolytic rate and is associated with the erythrocyte release of cell-free hemoglobin, which consumes NO directly, and the simultaneous release of the arginine-metabolizing enzyme arginase, which limits bioavailability of the NO synthase substrate arginine during the process of intravascular hemolysis. Rapid consumption of NO is accelerated by oxygen radicals that exists in both SCD and thalassemia. A dysregulation of arginine metabolism contributes to endothelial dysfunction and PH in SCD, and is strongly associated with prospective patient mortality. The central mechanism responsible for this metabolic disorder is enhanced arginine turnover, occurring secondary to enhanced plasma arginase activity. This is consistent with a growing appreciation of the role of excessive arginase activity in human diseases, including asthma and pulmonary arterial hypertension. New treatments aimed at improving arginine and NO bioavailability through arginase inhibition, suppression of hemolytic rate, oral arginine supplementation, or use of NO donors represent potential therapeutic strategies for this common pulmonary complication of hemolytic disorders.

049 Suppression of macrophage NO Synthesis by wound fluid

Background: Essential parameters of wound healing are regulated by the wound environment., as reflected by the bathing wound fluid (WF). WF increases fibroblast proliferation, collagen synthesis, and also inhibits Nitric oxide (NO) synthesis in wound-derived fibroblasts. The aim of this study was to investigate whether WF influences NO metabolism in other wound cells, namely macrophages. Methods: The mouse hybridoma macrophage cell line RAW 267.4 was employed. Cells were seeded at 2 × 105/well into 24 well plates or at 2 × 106 in Petri dishes. Cells were stimulated with 1 μg/ml LPS and 10 U/ml IFN-gamma to induce inducible nitric oxide synthase (iNOS). iNOS expression was investigated by Northern blotting after 6 h, enzyme secretion by western blotting after 20 h and NO production by nitrite synthesis in cell culture supernatants after 24 h. WF was harvested at different days postwounding from PVA sponges implanted into male Lewis rats. WF was added at 10% v/v to the cell media. Cells were incubated in the presence of 60 mM L-valine, an arginase inhibitor, to rule out substrate (arginine) depletion by arginase as a mechanism. Results are expressed as mean ± SD, n = 5. Results: Following inflammatory cytokine stimulation, RAW cells produced 57.36 μmol nitrite/ml/24 h. Nitrite synthesis was significantly inhibited to 23.89 ± 13.3 by day 7 and to 6.79 ± 1.28 by day 10 WF, respectively. iNOS expression was almost completely blocked by addition of 10% WF from day 10 as assessed by Northern and western blotting. WF from the later days (day 10 and 14) had larger inhibitory effect than WF from early days (day 2, 5, and 7). Although omitting L-arginine from the media abolished iNOS expression, incubation of WF in the presence of 60 mM L-valine did not restore the inhibitory effect suggesting that arginase is not involved. Conclusion: There is a factor in WF which strongly inhibits NO synthesis in RAW cells; the effect becomes more marked over time after wounding. The effect appears to be transcriptionally regulated since mRNA expression is impaired. Further characterization of this inhibitory factor is under way.

Arginine and Arginase

See related article, pages 923–932 In a remarkable article published in this issue of Circulation Research , Ryoo et al1 propose endothelial arginase II as a novel target for the treatment of atherosclerosis. Actually, they already had done so 2 years ago, based on work mainly on cultured human aortic endothelial cells,2 which indeed appeared to provide an unexpected way to explain endothelial dysfunction in terms of NO production.3 This new road to endothelial dysfunction proposed that oxidized low-density lipoproteins (LDLs), which we all agree are a main culprit in the initiation of the atherosclerotic process, increase the activity of arginase II, which, in turn, decreases NO production presumably by shunting the common substrate arginine away from endothelial NO synthase (eNOS) (Figure). Figure. Model of endothelial dysfunction in the hypercholesterolemic mouse as proposed by Ryoo and colleagues.1,2 Left, In the normal mouse aortic endothelium, l-arginine (L-Arg) is transformed by eNOS to NO, which exerts its well-documented beneficial effects (most are not shown for the sake of clarity), including inhibition of the oxidation of LDLs to OxLDL. The byproduct of the reaction, l-citrulline (L-Cit), inhibits arginase II (AaII), which is constrained to the microtubules (MT). Right, In the aortic endothelium of the ApoE−/− and the wild-type hypercholesterolemic mice, the accumulation of OxLDL dislocates arginase II from the microtubules and augments its activity. Arginase II competes with endothelial NO synthase for the common substrate l-arginine, leading to uncoupling of NO synthase and the production of superoxide anions (O![Graphic][1] ,2]), which further enhance the production of OxLDL. The latter also facilitates dissociation of eNOS from the caveolae and reduces the genomic expression of the enzyme, leading to further reduction of the production of NO. This model does not account for the biological effects, if … [1]: F1/embed/inline-graphic-1.gif

Arginase blockade protects against hepatic damage in warm ischemia-reperfusion

Background Liver ischemia reperfusion (I/R) injury is associated with profound arginine depletion due to arginase release from injured hepatocytes. Nitric oxide (NO), shown to have protective effects in I/R, is produced by nitric oxide synthase (NOS) from the substrate arginine. The purpose of this study was to determine if nor-NOHA, a novel arginase inhibitor, would be able to increase circulating arginine levels and decrease hepatic damage following warm I/R. Methods C57BL/6 mice underwent partial liver warm I/R and were treated intraperitoneally with either nor-NOHA (100 mg/kg) or saline. Serum and tissue samples were collected to measure liver enzyme levels, amino acids, and inflammatory mediators. The agent nor-NOHA (100 mg/kg) was administered 15 min before ischemia and immediately after reperfusion. Serum amino acid analysis was performed using HPLC. Results Arginase activity after hepatic I/R peaked at 3–6 h after reperfusion and resulted in a 10-fold drop in circulating arginine levels. Treatment with nor-NOHA inhibited arginase activity and reversed the arginine depletion after I/R while simultaneously increasing serum nitric oxide. In addition, circulating citrulline, a product of NOS activity, was increased in nor-NOHA-treated animals compared to controls. Inhibition of arginase also resulted in protection from hepatic I/R-induced damage in association with markedly lower hepatic TNF, IL-6, and inducible NOS mRNA levels compared to controls. Conclusion Arginase blockade represents a potentially novel strategy to combat liver injury under conditions of arginine deficiency. This protection may be mediated through the arginine–NO pathway.