Mechanistic and metabolic inferences from the binding of substrate analogues and products to arginase

Boron in cancer therapeutics: An overview

Boron has become a crucial weapon in anticancer research due to its significant intervention in cell proliferation. Being an excellent bio-isosteric replacement of carbon, it has modulated the anticancer efficacy of various molecules in the development pipeline. It has elicited promising results through interactions with various therapeutic targets such as HIF-1α, steroid sulfatase, arginase, proteasome, etc. Since boron liberates alpha particles, it has a wide-scale application in Boron Neutron Capture therapy (BNCT), a radiotherapy that demonstrates selectivity towards cancer cells due to high boron uptake capacity. Significant advances in the medicinal chemistry of boronated compounds, such as boronated sugars, natural/unnatural amino acids, boronated DNA binders, etc., have been reported over the past few years as BNCT agents. In addition, boronated nanoparticles have assisted the field of bio-nano medicines by their usage in radiotherapy. This review exclusively focuses on the medicinal chemistry aspects, radiotherapeutic, and chemotherapeutic aspects of boron in cancer therapeutics. Emphasis is also given on the mechanism of action along with advantages over conventional therapies.

Arginase inhibitory activities of guaiane sesquiterpenoids from Curcuma comosa rhizomes

Arginases are bimanganese enzymes involved in many human illnesses, and thus are targets for disease treatments. The screening of traditional medicinal plants demonstrated that an ethanol extract of Curcuma comosa rhizomes showed significant human arginase I and II inhibitory activity, and further fractionation led to the isolation of three known guaiane sesquiterpenoids, alismoxide (1), 7α,10α-epoxyguaiane-4α,11-diol (2) and guaidiol (3). Tests of their inhibitory activities on human arginases I and II revealed that 1 exhibited selective and potent competitive inhibition for human arginase I (IC50 = 30.2 μM), whereas the other compounds lacked inhibitory activities against human arginases. To the best of our knowledge, this is the first demonstration of human arginase I inhibitory activity by a sesquiterpenoid. Thus, 1 is a primary and specific inhibitory molecule against human arginase I.

Arginase: Biological and Therapeutic Implications in Diabetes Mellitus and Its Complications

Arginase is a ubiquitous enzyme in the urea cycle (UC) that hydrolyzes L-arginine to urea and L-ornithine. Two mammalian arginase isoforms, arginase1 (ARG1) and arginase2 (ARG2), play a vital role in the regulation of β-cell functions, insulin resistance (IR), and vascular complications via modulating L-arginine metabolism, nitric oxide (NO) production, and inflammatory responses as well as oxidative stress. Basic and clinical studies reveal that abnormal alterations of arginase expression and activity are strongly associated with the onset and development of diabetes mellitus (DM) and its complications. As a result, targeting arginase may be a novel and promising approach for DM treatment. An increasing number of arginase inhibitors, including chemical and natural inhibitors, have been developed and shown to protect against the development of DM and its complications. In this review, we discuss the fundamental features of arginase. Next, the regulatory roles and underlying mechanisms of arginase in the pathogenesis and progression of DM and its complications are explored. Furthermore, we review the development and discuss the challenges of arginase inhibitors in treating DM and its related pathologies.

Arginase: An emerging and promising therapeutic target for cancer treatment

Arginase is a key hydrolase in the urea cycle that hydrolyses L-arginine to urea and L-ornithine. Increasing number of studies in recent years demonstrate that two mammalian arginase isoforms, arginase 1 (ARG1) and arginase 2 (ARG2), were aberrantly upregulated in various types of cancers, and played crucial roles in the regulation of tumor growth and metastasis through various mechanisms such as regulating L-arginine metabolism, influencing tumor immune microenvironment, etc. Thus, arginase receives increasing focus as an attractive target for cancer therapy. In this review, we provide a comprehensive overview of the physiological and biological roles of arginase in a variety of cancers, and shed light on the underlying mechanisms of arginase mediating cancer cells growth and development, as well as summarize the recent clinical research advances of targeting arginase for cancer therapy.

Integrated molecular response of exposure to traffic-related pollutants in the US trucking industry

Exposure to traffic-related pollutants, including diesel exhaust, is associated with increased risk of cardiopulmonary disease and mortality; however, the precise biochemical pathways underlying these effects are not known. To investigate biological response mechanisms underlying exposure to traffic related pollutants, we used an integrated molecular response approach that included high-resolution metabolomic profiling and peripheral blood gene expression to identify biological responses to diesel exhaust exposure. Plasma samples were collected from 73 non-smoking males employed in the US trucking industry between February 2009 and October 2010, and analyzed using untargeted high-resolution metabolomics to characterize metabolite associations with shift- and week-averaged levels of elemental carbon (EC), organic carbon (OC) and particulate matter with diameter ≤ 2.5 μm (PM2.5). Metabolic associations with EC, OC and PM2.5 were evaluated for biochemical processes known to be associated with disease risk. Annotated metabolites associated with exposure were then tested for relationships with the peripheral blood transcriptome using multivariate selection and network correlation. Week-averaged EC and OC levels, which were averaged across multiple shifts during the workweek, resulted in the greatest exposure-associated metabolic alterations compared to shift-averaged exposure levels. Metabolic changes associated with EC exposure suggest increased lipid peroxidation products, biomarkers of oxidative stress, thrombotic signaling lipids, and metabolites associated with endothelial dysfunction from altered nitric oxide metabolism, while OC exposures were associated with antioxidants, oxidative stress biomarkers and critical intermediates in nitric oxide production. Correlation with whole blood RNA gene expression provided additional evidence of changes in processes related to endothelial function, immune response, inflammation, and oxidative stress. We did not detect metabolic associations with PM2.5. This study provides an integrated molecular assessment of human exposure to traffic-related air pollutants that includes diesel exhaust. Metabolite and transcriptomic changes associated with exposure to EC and OC are consistent with increased risk of cardiovascular diseases and the adverse health effects of traffic-related air pollution.

Targeting Metalloenzymes by Boron-Containing Metal-Binding Pharmacophores

Metalloenzymes have critical roles in a wide range of biological processes and are directly involved in many human diseases; hence, they are considered as important targets for therapeutic intervention. The specific characteristics of metal ion(s)-containing active sites make exploitation of metal-binding pharmacophores (MBPs) critical to inhibitor development targeting metalloenzymes. This Perspective focuses on boron-containing MBPs, which display unique binding modes with metalloenzyme active sites, particularly via mimicking native substrates or tetrahedral transition states. The design concepts regarding boron-containing MBPs are highlighted through the case analyses on five distinct classes of clinically relevant nucleophilic metalloenzymes from medicinal chemistry perspectives. The challenges (e.g., selectivity) faced by some boron-containing MBPs and possible strategies (e.g., bioisosteres) for metalloenzyme inhibitor transformation are also discussed.

Nucleophilic Substitution at the Guanidine Carbon Center via Guanidine Cyclic Diimide Activation

Despite the electron-deficient nature of the guanidine carbon centers, nucleophilic reactions at these sites have been underdeveloped because of the resonance stabilization of the guanidine group. We propose a guanidine C-N bond substitution strategy entailing the formation of guanidine cyclic diimide (GCDI) structures, which effectively destabilize the resonance structure of the guanidine group. In the presence of acid additives, the guanidine carbon center of GCDIs undergoes nucleophilic substitution reactions with various amines and alcohols.

Diversity, properties and functions of bacterial arginases

The metalloenzyme arginase hydrolyzes L-arginine to produce L-ornithine and urea. In bacteria, arginase has important functions in basic nitrogen metabolism and redistribution, production of the key metabolic precursor L-ornithine, stress resistance and pathogenesis. We describe the regulation and specific functions of the arginase pathway as well as summarize key characteristics of related arginine catabolic pathways. The use of arginase-derived ornithine as a precursor molecule is reviewed. We discuss the biochemical and transcriptional regulation of arginine metabolism, including arginase, with the latter topic focusing on the RocR and AhrC transcriptional regulators in the model organism Bacillus subtilis. Finally, we consider similarities and contrasts in the structure and catalytic mechanism of the arginases from Bacillus caldovelox and Helicobacter pylori. The overall aim of this review is to provide a panorama of the diversity of physiological functions, regulation, and biochemical features of arginases in a variety of bacterial species.

Neutrophils drive endoplasmic reticulum stress-mediated apoptosis in cancer cells through arginase-1 release

Human neutrophils constitutively express high amounts of arginase-1, which depletes arginine from the surrounding medium and downregulates T-cell activation. Here, we have found that neutrophil arginase-1, released from activated human neutrophils or dead cells, induced apoptosis in cancer cells through an endoplasmic reticulum (ER) stress pathway. Silencing of PERK in cancer cells prevented the induction of ER stress and apoptosis. Arginase inhibitor Nω-hydroxy-nor-arginine inhibited apoptosis and ER stress response induced by conditioned medium from activated neutrophils. A number of tumor cell lines, derived from different tissues, were sensitive to neutrophil arginase-1, with pancreatic, breast, ovarian and lung cancer cells showing the highest sensitivity. Neutrophil-released arginase-1 and arginine deprivation potentiated the antitumor action against pancreatic cancer cells of the ER-targeted antitumor alkylphospholipid analog edelfosine. Our study demonstrates the involvement of neutrophil arginase-1 in cancer cell killing and highlights the importance and complex role of neutrophils in tumor surveillance and biology.

Structure of the E. coli agmatinase, SPEB

Agmatine amidinohydrolase, or agmatinase, catalyzes the conversion of agmatine to putrescine and urea. This enzyme is found broadly across kingdoms of life and plays a critical role in polyamine biosynthesis and the regulation of agmatine concentrations. Here we describe the high-resolution X-ray crystal structure of the E. coli agmatinase, SPEB. The data showed a relatively high degree of pseudomerohedral twinning, was ultimately indexed in the P31 space group and led to a final model with eighteen chains, corresponding to three full hexamers in the asymmetric unit. There was a solvent content of 38.5% and refined R/Rfree values of 0.166/0.216. The protein has the conserved fold characteristic of the agmatine ureohydrolase family and displayed a high degree of structural similarity among individual protomers. Two distinct peaks of electron density were observed in the active site of most of the eighteen chains of SPEB. As the activity of this protein is known to be dependent upon manganese and the fold is similar to other dinuclear metallohydrolases, these peaks were modeled as manganese ions. The orientation of the conserved active site residues, in particular those amino acids that participate in binding the metal ions and a pair of acidic residues (D153 and E274 in SPEB) that play a role in catalysis, are similar to other agmatinase and arginase enzymes and is consistent with a hydrolytic mechanism that proceeds via a metal-activated hydroxide ion.

Antunes I, Dömling A, H. Elsinga P. Arginase as a Potential Biomarker of Disease Progression: A Molecular Imaging Perspective

Arginase is a widely known enzyme of the urea cycle that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. The action of arginase goes beyond the boundaries of hepatic ureogenic function, being widespread through most tissues. Two arginase isoforms coexist, the type I (Arg1) predominantly expressed in the liver and the type II (Arg2) expressed throughout extrahepatic tissues. By producing L-ornithine while competing with nitric oxide synthase (NOS) for the same substrate (L-arginine), arginase can influence the endogenous levels of polyamines, proline, and NO•. Several pathophysiological processes may deregulate arginase/NOS balance, disturbing the homeostasis and functionality of the organism. Upregulated arginase expression is associated with several pathological processes that can range from cardiovascular, immune-mediated, and tumorigenic conditions to neurodegenerative disorders. Thus, arginase is a potential biomarker of disease progression and severity and has recently been the subject of research studies regarding the therapeutic efficacy of arginase inhibitors. This review gives a comprehensive overview of the pathophysiological role of arginase and the current state of development of arginase inhibitors, discussing the potential of arginase as a molecular imaging biomarker and stimulating the development of novel specific and high-affinity arginase imaging probes.

Boronic acid-based arginase inhibitors in cancer immunotherapy

Arginase is an enzyme that converts l-arginine to l-ornithine and urea in the urea cycle. There are two isoforms of arginase in mammals: ARG-1 and ARG-2. l-Arginine level changes occur in patients with various types of affliction. An overexpression of arginase leads to the depletion of arginine and then to inhibition of the growth of T and NK cells, and in effect to the tumor escape of the immune response. Based on those observations, an inhibition of arginase is proposed as a method to improve anti-tumor immune responses (via an activation and proliferation of T and NK cells). Boronic acid derivatives as arginase inhibitors are leading, potential therapeutic agents for the treatment of several diseases. All these compounds are derived from the original 2-(S)-amino-6-boronohexanoic acid (ABH), the first boronic acid arginase inhibitor proposed by Christianson et al. This article focuses on the review of such sub-class of arginase inhibitors and highlights their SAR and PK properties. It covers molecules published until early 2020, including patent applications.

Crystal structure of an Nω-hydroxy-L-arginine hydrolase found in the D-cycloserine biosynthetic pathway

DcsB, one of the enzymes encoded in the D-cycloserine (D-CS) biosynthetic gene cluster, displays a high sequence homology to arginase, which contains two manganese ions in the active site. However, DcsB hydrolyzes N^ω-hydroxy-L-arginine, but not L-arginine, to supply hydroxyurea for the biosynthesis of D-CS. Here, the crystal structure of DcsB was determined at a resolution of 1.5 Å using anomalous scattering from the manganese ions. In the crystal structure, DscB generates an artificial dimer created by the open and closed forms. Gel-filtration analysis demonstrated that DcsB is a monomeric protein, unlike arginase, which forms a trimeric structure. The active center containing the binuclear manganese cluster differs between DcsB and arginase. In DcsB, one of the ligands of the Mn_A ion is a cysteine, while the corresponding residue in arginase is a histidine. In addition, DcsB has no counterpart to the histidine residue that acts as a general acid/base during the catalytic reaction of arginase. The present study demonstrates that DcsB has a unique active site that differs from that of arginase.

Characterization of a Dimeric Arginase From Zymomonas mobilis ZM4

Many organisms have genes to protect themselves from toxic conditions such as high ethanol and/or ammonia concentrations. When a high ethanol condition is induced to Zymomonas mobilis ZM4, a representative ethanologenic organism, this bacterium overexpresses several genes to overcome this ethanol stress. Among them, we characterized a gene product annotated as an arginase (zmARG) from Z. mobilis ZM4. Even though all of the arginase-determining sequence motifs are not strictly conserved in zmARG, this enzyme converts L-arginine to urea and L-ornithine in the presence of a divalent manganese ion. The revealed high-resolution crystal structure of zmARG shows that it has a typical globular α/β arginase fold with a protruded C-terminal helix. Two zinc ions reside in the active site, where one metal ion is penta-coordinated and the other has six ligands, discerning this zmARG from the reported arginases with two hexa-liganded metal ions. zmARG forms a dimeric structure in solution as well as in the crystalline state. The dimeric assembly of zmARG is formed mainly by interaction formed between the C-terminal α-helix of one molecule and the α/β hydrolase fold of another molecule. The presented findings demonstrate the first reported dimeric arginase formed by the C-terminal tail and has two metal ions coordinated by different number of ligands.

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.

Pharmacokinetics and Pharmacodynamics of Promising Arginase Inhibitors

Up-regulation of arginase activity in several chronic disease conditions, including cancer and hypertension, may suggest new targets for treatment. Recently, the number of new arginase inhibitors with promising therapeutic effects for asthma, cancer, hypertension, diabetes mellitus, and erectile dysfunction has shown a remarkable increase. Arginase inhibitors may be chemical substances, such as boron-based amino acid derivatives, α-difluoromethylornithine (DMFO), and Nω-hydroxy-nor-L-arginine (nor-NOHA) or, of plant origin such as sauchinone, salvianolic acid B (SAB), piceatannol-3-O-β-D-glucopyranoside (PG) and obacunone. Despite their promising therapeutic potential, little is known about pharmacokinetics and pharmacodynamics of some of these agents. Several studies were conducted in different animal species and in vitro systems and reported significant differences in pharmacokinetics and pharmacodynamics of arginase inhibitors. Therefore, extra caution should be considered before extrapolating these studies to human. Physicochemical and pharmacokinetic profiles of some effective arginase inhibitors make it challenging to formulate stable and effective formulation. In this article, existing literature on the pharmacokinetics and pharmacodynamics of arginase inhibitors were reviewed and compared together with emphasis on possible drug interactions and solutions to overcome pharmacokinetics challenges and shortage of arginase inhibitors in clinical practice.

Crystal structure of Schistosoma mansoni arginase, a potential drug target for the treatment of schistosomiasis

The X-ray crystal structure of arginase from Schistosoma mansoni (SmARG) and the structures of its complexes with several amino acid inhibitors have been determined at atomic resolution. SmARG is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea, and this enzyme is upregulated in all forms of the parasite that interact with the human host. Current hypotheses suggest that parasitic arginases could play a role in host immune evasion by depleting pools of substrate l-arginine that would otherwise be utilized for NO biosynthesis and NO-dependent processes in the immune response. Although the amino acid sequence of SmARG is only 42% identical with that of human arginase I, residues important for substrate binding and catalysis are strictly conserved. In general, classical amino acid inhibitors such as 2(S)-amino-6-boronohexanoic acid (ABH) tend to bind more weakly to SmARG than to human arginase I despite identical inhibitor binding modes in each enzyme active site. The identification of a patch on the enzyme surface capable of accommodating the additional Cα substitutent of an α,α-disubstituted amino acid inhibitor suggests that such inhibitors could exhibit higher affinity and biological activity. The structures of SmARG complexed with two different α,α-disubstituted derivatives of ABH are presented and provide a proof of concept for this approach in the enhancement of enzyme-inhibitor affinity.

Development of novel arginase inhibitors for therapy of endothelial dysfunction

Endothelial dysfunction and resulting vascular pathology have been identified as an early hallmark of multiple diseases, including diabetes mellitus. One of the major contributors to endothelial dysfunction is a decrease in nitric oxide (NO) bioavailability, impaired NO signaling, and an increase in the amount of reactive oxygen species (ROS). In the endothelium NO is produced by endothelial nitric oxide synthase (eNOS), for which l-arginine is a substrate. Arginase, an enzyme critical in the urea cycle also metabolizes l-arginine, thereby directly competing with eNOS for their common substrate and constraining its bioavailability for eNOS, thereby compromising NO production. Arginase expression and activity is upregulated in many cardiovascular diseases including ischemia reperfusion injury, hypertension, atherosclerosis, and diabetes mellitus. More importantly, since the 1990s, specific arginase inhibitors such as N-hydroxy-guanidinium or N-hydroxy-nor-l-arginine, and boronic acid derivatives, such as, 2(S)-amino-6-boronohexanoic acid, and S-(2-boronoethyl)-l-cysteine, that can bridge the binuclear manganese cluster of arginase have been developed. These highly potent and specific inhibitors can now be used to probe arginase function and thereby modulate the redox milieu of the cell by changing the balance between NO and ROS. Inspired by this success, drug discovery programs have recently led to the identification of α–α-disubstituted amino acid based arginase inhibitors [such as (R)-2-amino-6-borono-2-(2-(piperidin-1-yl)ethyl)hexanoic acid], that are currently under early investigation as therapeutics. Finally, some investigators concentrate on identification of plant derived compounds with arginase inhibitory capability, such as piceatannol-3′-O-β-d-glucopyranoside (PG). All of these synthesized or naturally derived small molecules may represent novel therapeutics for vascular disease particularly that associated with diabetes.

Arginase: the emerging therapeutic target for vascular oxidative stress and inflammation

Oxidative stress and inflammation in the vascular wall are essential mechanisms of atherosclerosis and vascular dysfunctions associated with risk factors such as metabolic diseases, aging, hypertension, etc. Evidence has been provided that activation of the vascular endothelial cells in the presence of the risk factors promotes oxidative stress and vascular inflammatory responses, leading to acceleration of atherosclerotic vascular disease. Increasing number of studies from recent years demonstrates that uncoupling of endothelial nitric oxide synthase (eNOS), whereby the enzyme eNOS produces detrimental amount of superoxide anion O2− instead the vasoprotective nitric oxide (NO^⋅), plays a critical role in vascular dysfunction under various pathophysiological conditions and in aging. The mechanisms of eNOS-uncoupling seem multiple and complex. Recent research provides emerging evidence supporting an essential role of increased activity of arginases including arginase-I and arginase-II in causing eNOS-uncoupling, which results in vascular oxidative stress and inflammatory responses, and ultimately leading to vascular diseases. This review article will summarize the most recent findings on the functional roles of arginases in vascular diseases and/or dysfunctions and the underlying mechanisms in relation to oxidative stress and inflammations. Moreover, regulatory mechanisms of arginases in the vasculature are reviewed and the future perspectives of targeting arginases as therapeutic options in vascular diseases are discussed.

Secondary amines containing one aromatic nitro group: preparation, nitrosation, sustained nitric oxide release, and the synergistic effects of released nitric oxide and an arginase inhibitor on vascular smooth muscle cell proliferation

Atherosclerosis, a leading cause of death worldwide, is associated with the excessive proliferation of vascular smooth muscle cells. Nitrogen monoxide, more commonly known as nitric oxide, inhibits this uncontrolled proliferation. Herein we report the preparation of two families of nitric oxide donors; beginning with the syntheses of secondary amine precursors, obtained through the reaction between 2 equiv of various monoamines with 2,4 or 2,6-difluoronitrobenzene. The purified secondary amines were nitrosated then subjected to a Griess reagent test to examine the slow and sustained nitric oxide release rate for each compound in both the absence and presence of reduced glutathione. The release rate profiles of these two isomeric families of NO-donors were strongly dependent on the number of side chain methylene units and the relative orientations of the nitro groups with respect to the N-nitroso moieties. The nitrosated compounds were then added to human aortic smooth muscle cell cultures, individually and in tandem with S-2-amino-6-boronic acid (ABH), a potent arginase inhibitor. Cell viability studies indicated a lack of toxicity of the amine precursors, in addition to anti-proliferative effects exhibited by the nitrosated compounds, which were enhanced in the presence of ABH.

Arginase and arginine dysregulation in asthma

In recent years, evidence has accumulated indicating that the enzyme arginase, which converts L-arginine into L-ornithine and urea, plays a key role in the pathogenesis of pulmonary disorders such as asthma through dysregulation of L-arginine metabolism and modulation of nitric oxide (NO) homeostasis. Allergic asthma is characterized by airway hyperresponsiveness, inflammation, and remodeling. Through substrate competition, arginase decreases bioavailability of L-arginine for nitric oxide synthase (NOS), thereby limiting NO production with subsequent effects on airway tone and inflammation. By decreasing L-arginine bioavailability, arginase may also contribute to the uncoupling of NOS and the formation of the proinflammatory oxidant peroxynitrite in the airways. Finally, arginase may play a role in the development of chronic airway remodeling through formation of L-ornithine with downstream production of polyamines and L-proline, which are involved in processes of cellular proliferation and collagen deposition. Further research on modulation of arginase activity and L-arginine bioavailability may reveal promising novel therapeutic strategies for asthma.

Identification of two arginases generated by alternative splicing in the silkworm, Bombyx mori

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of arginine to ornithine and urea. Here, we have cloned two arginase cDNAs from the silkworm, Bombyx mori. The analysis of exon/intron structures showed that the two mRNAs named bmarg-r and bmarg-f were generated from a single gene by alternative usage of exons. The bmarg-r and bmarg-f were predicted to encode almost the same amino acid sequences, except that the latter had additional ten N-terminal residues. Recombinant bmARG-r and bmARG-f in Escherichia coli cell lysates were roughly similar to each other in enzymatic characteristics, which did not show large difference from those of arginases assayed by using tissue extracts. Differential RT-PCR experiments and tissue distribution analyses of arginase activity indicated that the bmarg-r gene is expressed in the male reproductive organs, especially in the glandula lacteola and vesicular seminalis, from which it is secreted to the seminal fluid and transferred to the female during copulation, whereas the bmarg-f gene is expressed in the larval and adult nonreproductive organs including the fat body and muscle, where the produced arginase proteins are considered to stay in the cells. Thus, the two silkworm arginase isoforms may have a difference in whether or not the product is excreted out of the cells in which it is synthesized.

Inhibition profile of Leishmania mexicana arginase reveals differences with human arginase I

Arginase (ARG), the enzyme that catalyzes the conversion of arginine to ornithine and urea, is the first and committed step in polyamine biosynthesis in Leishmania. The creation of a conditionally lethal Δarg null mutant in Leishmania mexicana has established that ARG is an essential enzyme for the promastigote form of the parasite and that the enzyme provides an important defense mechanism for parasite survival in the eukaryotic host. Furthermore, human ARGI (HsARGI) has also been implicated as a key factor in parasite proliferation. Thus, inhibitors of ARG offer a rational paradigm for drug design. To initiate a search for inhibitors of the L. mexicana ARG (LmARG), recombinant LmARG and HsARGI enzymes were purified from Escherichia coli. Both LmARG and HsARGI were specific for l-arginine and exhibited no activity with either d-arginine or agmatine as possible substrates. LmARG exhibited a K(m) of 25±4mM for l-arginine, a pH optimum ∼9.0, and was dependent upon the presence of a divalent cation, preferentially manganese. A K(m) of 13.5 ± 2mM for l-arginine was calculated for the HsARGI. A collection of 37 compounds was evaluated against both enzymes. Twelve of these compounds were identified as being either strong inhibitors of both LmARG and HsARGI or differential inhibitors between the two enzymes. Of the 12 compounds, six were selected for further analysis and the type and extent of inhibition determined.

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.

Arginase contributes to endothelial cell oxidative stress in response to plasma from women with preeclampsia

AIMS

Preeclampsia is a hypertensive disorder characterized by vascular oxidative stress. Decreased availability of the vasodilator nitric oxide (NO) has been postulated to be involved in the pathophysiology of this disorder. Arginase, an enzyme that competes with nitric oxide synthase (NOS) for l-arginine, not only reduces NO formation but also increases superoxide production by NOS. In placenta of preeclamptic women, arginase upregulation has been shown to be increased and contributes to superoxide formation via uncoupling of NOS. However, the role of arginase in the maternal vasculature is not clear. We hypothesized that arginase would be upregulated in the maternal vasculature of women with preeclampsia and contribute to oxidative stress within the endothelium.

METHODS AND RESULTS

We observed increased arginase expression in the maternal vasculature of women with preeclampsia compared with normotensive pregnant women. Furthermore, human umbilical vein endothelial cells treated with 2% plasma from preeclamptic women show increased arginase II expression and activity that was reduced by a peroxynitrite scavenger. Also, both 3-morpholino sydnonimine and exogenous peroxynitrite increased arginase expression and activity. Preeclamptic plasma treatment increased superoxide and peroxynitrite levels. Superoxide levels were significantly reduced after arginase and NOS inhibition with [(S)-(2-boronoethyl)-l-cysteine] and N(omega)-nitro-l-arginine methyl ester, respectively, but peroxynitrite levels were in fact increased after arginase inhibition. Moreover, in the presence of preeclamptic plasma, l-arginine supplementation increased peroxynitrite formation during arginase inhibition.

CONCLUSION

Increased arginase expression in preeclampsia can induce uncoupling of NOS as a source of superoxide in the maternal vasculature in preeclampsia. However, l-arginine supplementation in the face of oxidative stress could lead to a further increase in peroxynitrite.

Arginase: a key enzyme in the pathophysiology of allergic asthma opening novel therapeutic perspectives

Allergic asthma is a chronic inflammatory airways' disease, characterized by allergen-induced early and late bronchial obstructive reactions, airway hyperresponsiveness (AHR), airway inflammation and airway remodelling. Recent ex vivo and in vivo studies in animal models and asthmatic patients have indicated that arginase may play a central role in all these features. Thus, increased arginase activity in the airways induces reduced bioavailability of L-arginine to constitutive (cNOS) and inducible (iNOS) nitric oxide synthases, causing a deficiency of bronchodilating and anti-inflammatory NO, as well as increased formation of peroxynitrite, which may be involved in allergen-induced airways obstruction, AHR and inflammation. In addition, both via reduced NO production and enhanced synthesis of L-ornithine, increased arginase activity may be involved in airway remodelling by promoting cell proliferation and collagen deposition in the airway wall. Therefore, arginase inhibitors may have therapeutic potential in the treatment of acute and chronic asthma. This review focuses on the pathophysiological role of arginase in allergic asthma and the emerging effectiveness of arginase inhibitors in the treatment of this disease.

The vascular effects of different arginase inhibitors in rat isolated aorta and mesenteric arteries

BACKGROUND AND PURPOSE

Arginase and nitric oxide (NO) synthase share the common substrate L-arginine, and arginase inhibition is proposed to increase NO production by increasing intracellular levels of L-arginine. Many different inhibitors are used, and here we have examined the effects of these inhibitors on vascular tissue.

EXPERIMENTAL APPROACH

Each arginase inhibitor was assessed by its effects on isolated rings of aorta and mesenteric arteries from rats by: (i) their ability to preserve the tolerance to repeated applications of the endothelium-dependent agonist acetylcholine (ACh); and (ii) their direct vasorelaxant effect.

KEY RESULTS

In both vessel types, tolerance (defined as a reduced response upon second application) to ACh was reversed with addition of L-arginine, (S)-(2-boronethyl)-L-cysteine HCl (BEC) or N(G)-Hydroxy-L-arginine (L-NOHA). On the other hand, N(omega)-hydroxy-nor-L-arginine (nor-NOHA) significantly augmented the response to ACh, an effect that was partially reversed with L-arginine. No effect on tolerance to ACh was observed with L-valine, nor-valine or D,L, alpha-difluoromethylornithine (DFMO). BEC, L-NOHA and nor-NOHA elicited endothelium-independent vasorelaxation in both endothelium intact and denuded aorta while L-valine, DFMO and nor-valine did not.

CONCLUSIONS AND IMPLICATIONS

BEC and L-NOHA, but not nor-NOHA, L-valine, DFMO or nor-valine, significantly reversed tolerance to ACh possibly conserving L-arginine levels and therefore increasing NO bioavailability. However, both BEC and L-NOHA caused endothelium-independent vasorelaxation in rat aorta, suggesting that these inhibitors have a role beyond arginase inhibition alone. Our data thus questions the interpretation of many studies using these antagonists as specific arginase inhibitors in the vasculature, without verification with other methods.

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.

Arginase and vascular aging

Vascular and associated ventricular stiffness is one of the hallmarks of the aging cardiovascular system. Both an increase in reactive oxygen species production and a decrease in nitric oxide (NO) bioavailability contribute to the endothelial dysfunction that underlies this vascular stiffness, independent of other age-related vascular pathologies such as atherosclerosis. The activation/upregulation of arginase appears to be an important contributor to age-related endothelial dysfunction by a mechanism that involves substrate (L-arginine) limitation for NO synthase (NOS) 3 and therefore NO synthesis. Not only does this lead to impaired NO production but also it contributes to the enhanced production of reactive oxygen species by NOS. Although arginase abundance is increased in vascular aging models, it appears that posttranslational modification by S-nitrosylation of the enzyme enhances its activity as well. The S-nitrosylation is mediated by the induction of NOS2 in the endothelium. Furthermore, arginase activation contributes to aging-related vascular changes by mechanisms that are not directly related to changes in NO signaling, including polyamine-dependent vascular smooth muscle proliferation and collagen synthesis. Taken together, arginase may represent an as yet elusive target for the modification of age-related vascular and ventricular stiffness contributing to cardiovascular morbidity and mortality.

Arginine homeostasis in allergic asthma

Allergic asthma is a chronic disease characterized by early and late asthmatic reactions, airway hyperresponsiveness, airway inflammation and airway remodelling. Changes in l-arginine homeostasis may contribute to all these features of asthma by decreased nitric oxide (NO) production and increased formation of peroxynitrite, polyamines and l-proline. Intracellular l-arginine levels are regulated by at least three distinct mechanisms: (i) cellular uptake by cationic amino acid (CAT) transporters, (ii) metabolism by NO-synthase (NOS) and arginase, and (iii) recycling from l-citrulline. Ex vivo studies using animal models of allergic asthma have indicated that attenuated l-arginine bioavailability to NOS causes deficiency of bronchodilating NO and increased production of procontractile peroxynitrite, which importantly contribute to allergen-induced airway hyperresponsiveness after the early and late asthmatic reaction, respectively. Decreased cellular uptake of l-arginine, due to (eosinophil-derived) polycations inhibiting CATs, as well as increased consumption by increased arginase activity are major causes of substrate limitation to NOS. Increasing substrate availability to NOS by administration of l-arginine, l-citrulline, the polycation scavenger heparin, or an arginase inhibitor alleviates allergen-induced airway hyperresponsiveness by restoring the production of bronchodilating NO. In addition, reduced l-arginine levels may contribute to the airway inflammation associated with the development of airway hyperresponsiveness, which similarly may involve decreased NO synthesis and increased peroxynitrite formation. Increased arginase activity could also contribute to airway remodelling and persistent airway hyperresponsiveness in chronic asthma via increased synthesis of l-ornithine, the precursor of polyamines and l-proline. Drugs that increase the bioavailability of l-arginine in the airways - particularly arginase inhibitors - may have therapeutic potential in allergic asthma.

Immobilization of arginase and its application in an enzymatic chromatographic column: Thermodynamic studies of nor-NOHA/arginase binding and role of the reactive histidine residue

A biochromatographic approach is developed to measure for the first time changes in enthalpy, heat capacity change and protonation for the binding of nor-NOHA to arginase in a wide temperature range. For this, the arginase enzyme was immobilized on a chromatographic support. It was established that this novel arginase column was stable during an extended period of time. The affinity of nor-NOHA to arginase is high and changes slightly with the pH, because the number of protons linked to binding is low. The determination of the enthalpy change at different pH values suggested that the protonated group in the nor-NOHA-arginase complex exhibits a heat protonation of approximately -33 kJ/mol. This value agrees with the protonation of an imidazole group. Our result confirmed that active-site residue Hist 141 is protonated as imidazolium cation. Hist 141 can function as a general acid to protonate the leaving amino group of L-ornithine during catalysis. The thermodynamic data showed that nor-NOHA-arginase binding, for low temperature (<15 degrees C), is enthalpically unfavourable and being dominated by a positive entropy change. This result suggests that dehydration at the binding interface and charge-charge interactions contribute to the nor-NOHA-arginase complex formation. The temperature dependence of the free energy of binding is weak because of the enthalpy-entropy compensation caused by a large heat capacity change, DeltaC(p)=-2.43 kJ/mol/K, of arginase. Above 15 degrees C, the thermodynamic data DeltaH and DeltaS became negative due to van der Waals interactions and hydrogen bonding which are engaged at the complex interface confirming strong enzyme-inhibitor hydrogen bond networks. As well, by the use of these thermodynamic data and known correlations it was clearly demonstrated that the binding of nor-NOHA to arginase produces slight conformational changes in the vicinity of the active site. Our work indicated that our biochromatographic approach could soon become very attractive for studying other enzyme-ligand binding.

Probing the role of the hyper-reactive histidine residue of arginase

Rat liver arginase (arginase I) is potently inactivated by diethyl pyrocarbonate, with a second-order rate constant of 113M(-1)s(-1) for the inactivation process at pH 7.0, 25 degrees C. Partial protection from inactivation is provided by the product of the reaction, l-ornithine, while nearly complete protection is afforded by the inhibitor pair, l-ornithine and borate. The role of H141 has been probed by mutagenesis, chemical modulation, and X-ray diffraction. The hyper-reactivity of H141 towards diethyl pyrocarbonate can be explained by its proximity to E277. A proton shuttling role for H141 is supported by its conformational mobility observed among the known arginase structures. H141 is proposed to serve as an acid/base catalyst, deprotonating the metal-bridging water molecule to generate the metal-bridging hydroxide nucleophile, and by protonating the amino group of the product to facilitate its departure.

Insights into the interaction of human arginase II with substrate and manganese ions by site-directed mutagenesis and kinetic studies. Alteration of substrate specificity by replacement of Asn149 with Asp

To examine the interaction of human arginase II (EC 3.5.3.1) with substrate and manganese ions, the His120Asn, His145Asn and Asn149Asp mutations were introduced separately. About 53% and 95% of wild-type arginase activity were expressed by fully manganese activated species of the His120Asn and His145Asn variants, respectively. The K(m) for arginine (1.4-1.6 mM) was not altered and the wild-type and mutant enzymes were essentially inactive on agmatine. In contrast, the Asn149Asp mutant expressed almost undetectable activity on arginine, but significant activity on agmatine. The agmatinase activity of Asn149Asp (K(m) = 2.5 +/- 0.2 mM) was markedly resistant to inhibition by arginine. After dialysis against EDTA, the His120Asn variant was totally inactive in the absence of added Mn(2+) and contained < 0.1 Mn(2+).subunit(-1), whereas wild-type and His145Asn enzymes were half active and contained 1.1 +/- 0.1 Mn(2+).subunit(-1) and 1.3 +/- 0.1 Mn(2+).subunit(-1), respectively. Manganese reactivation of metal-free to half active species followed hyperbolic kinetics with K(d) of 1.8 +/- 0.2 x 10(-8) M for the wild-type and His145Asn enzymes and 16.2 +/- 0.5 x 10(-8) m for the His120Asn variant. Upon mutation, the chromatographic behavior, tryptophan fluorescence properties (lambda(max) = 338-339 nm) and sensitivity to thermal inactivation were not altered. The Asn149-->Asp mutation is proposed to generate a conformational change responsible for the altered substrate specificity of arginase II. We also conclude that, in contrast with arginase I, Mn(2+) (A) is the more tightly bound metal ion in arginase II.

Studies on the interaction of Escherichia coli agmatinase with manganese ions: structural and kinetic studies of the H126N and H151N variants

The H126N and H151N variants of Escherichia coli agmatinase (EC 3.5.3.11) were produced by site-directed mutagenesis, and their kinetic and structural properties were examined. About 51% and 30% of wild-type activity were expressed by fully manganese activated species of the H126N and H151N variants, respectively. Mutations were not accompanied by changes in the K(m) value for arginine (1.2+/-0.3 mM), K(i) value for putrescine inhibition (3.2+/-0.4 mM), molecular weight (M(r) 67,000+/-2000), tryptophan fluorescence properties (lambda(max) = 342 nm) or CD spectra of the enzyme. However, the interaction with the required manganese ions was significantly altered, as indicated by the effects of dialysis of the enzymes against metal-free buffer. We conclude that replacement of His151 with asparagine results in the loss of a catalytically essential Mn(2+) upon dialysis and concomitant reversible inactivation of the H151N mutant, and that the affinity of a more weakly bound Mn(2+) is decreased in the H126N variant.

Crystal Structure of Agmatinase Reveals Structural Conservation and Inhibition Mechanism of the Ureohydrolase Superfamily

Agmatine is the product of arginine decarboxylation and can be hydrolyzed by agmatinase to putrescine, the precursor for biosynthesis of higher polyamines, spermidine, and spermine. Besides being an intermediate in polyamine metabolism, recent findings indicate that agmatine may play important regulatory roles in mammals. Agmatinase is a binuclear manganese metalloenzyme and belongs to the ureohydrolase superfamily that includes arginase, formiminoglutamase, and proclavaminate amidinohydrolase. Compared with a wealth of structural information available for arginases, no three-dimensional structure of agmatinase has been reported. Agmatinase from Deinococcus radiodurans, a 304-residue protein, shows approximately 33% of sequence identity to human mitochondrial agmatinase. Here we report the crystal structure of D. radiodurans agmatinase in Mn(2+)-free, Mn(2+)-bound, and Mn(2+)-inhibitor-bound forms, representing the first structure of agmatinase. It reveals the conservation as well as variation in folding, oligomerization, and the active site of the ureohydrolase superfamily. D. radiodurans agmatinase exists as a compact homohexamer of 32 symmetry. Its binuclear manganese cluster is highly similar but not identical to the clusters of arginase and proclavaminate amidinohydrolase. The structure of the inhibited complex reveals that inhibition by 1,6-diaminohexane arises from the displacement of the metal-bridging water.