Subunit-subunit interactions in trimeric arginase. Generation of active monomers by mutation of a single amino acid

Discovery of a Ni2+-dependent heterohexameric metformin hydrolase

The biguanide drug metformin is a first-line blood glucose-lowering medication for type 2 diabetes, leading to its presence in the global environment. However, little is known about the fate of metformin by microbial catabolism. Here, we characterize a Ni2+-dependent heterohexameric enzyme (MetCaCb) from the ureohydrolase superfamily, catalyzing the hydrolysis of metformin into guanylurea and dimethylamine. Either subunit alone is catalytically inactive, but together they work as an active enzyme highly specific for metformin. The crystal structure of the MetCaCb complex shows the coordination of the binuclear metal cluster only in MetCa, with MetCb as a protein binder of its active cognate. An in-silico search and functional assay discover a group of MetCaCb-like protein pairs exhibiting metformin hydrolase activity in the environment. Our findings not only establish the genetic and biochemical foundation for metformin catabolism but also provide additional insights into the adaption of the ancient enzymes toward newly occurred substrate.

Human arginase I: a potential broad-spectrum anti-cancer agent

With high rates of morbidity and mortality, cancer continues to pose a serious threat to public health on a global scale. Considering the discrepancies in metabolism between cancer and normal cells, metabolism-based anti-cancer biopharmaceuticals are gaining importance. Normal cells can synthesize arginine, but they can also take up extracellular arginine, making it a semi-essential amino acid. Arginine auxotrophy occurs when a cancer cell has abnormalities in the enzymes involved in arginine metabolism and relies primarily on extracellular arginine to support its biological functions. Taking advantage of arginine auxotrophy in cancer cells, arginine deprivation, which can be induced by introducing recombinant human arginase I (rhArg I), is being developed as a broad-spectrum anti-cancer therapy. This has led to the development of various rhArg I variants, which have shown remarkable anti-cancer activity. This article discusses the importance of arginine auxotrophy in cancer and different arginine-hydrolyzing enzymes that are in various stages of clinical development and reviews the need for a novel rhArg I that mitigates the limitations of the existing therapies. Further, we have also analyzed the necessity as well as the significance of using rhArg I to treat various arginine-auxotrophic cancers while considering the importance of their genetic profiles, particularly urea cycle enzymes.

Human arginase 1, a Jack of all trades?

Arginine, a conditionally essential amino acid, plays a crucial role in several metabolic and signalling pathways. Arginine metabolism in the body can be significantly increased under stress or during certain pathological conditions. Depletion of circulating arginine by administering arginine-hydrolysing enzyme has been shown to mitigate varied pathophysiological conditions ranging from cancer, inflammatory conditions, and microbial infection. This review provides an overview of such intriguing expanse of potential applications of recombinant human arginase 1 for different pathological conditions and its status of development.

Not just a cousin of the naked mole-rat: Damaraland mole-rats offer unique insights into biomedicine

Evolutionary medicine has been a fast-growing field of biological research in the past decade. One of the strengths of evolutionary medicine is to use non-traditional model organisms which often exhibit unusual characteristics shaped by natural selection. Studying these unusual traits could provide valuable insight to understand biomedical questions, since natural selection likely discovers solutions to those complex biological problems. Because of many unusual traits, the naked mole-rat (NMR) has attracted attention from different research areas such as aging, cancer, and hypoxia- and hypercapnia-related disorders. However, such uniqueness of NMR physiology may sometimes make the translational study to human research difficult. Damaraland mole-rat (DMR) shares multiple characteristics in common with NMR, but shows higher degree of similarity with human in some aspects of their physiology. Research on DMR could therefore offer alternative insights and might bridge the gap between experimental findings from NMR to human biomedical research. In this review, we discuss studies of DMR as an extension of the current set of model organisms to help better understand different aspects of human biology and disease. We hope to encourage researchers to consider studying DMR together with NMR. By studying these two similar but evolutionarily distinct species, we can harvest the power of convergent evolution and avoid the potential biased conclusions based on life-history of a single species.

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.

A synthetic peptide as an allosteric inhibitor of human arginase I and II

Arginine metabolism mediated by arginases plays a critical role in cell and tissue function. The arginine hydrolysis is deeply involved in the urea cycle, which helps the kidney excrete ammonia from blood. Upregulation of arginases affects microenvironment stability due to the presence of excess urea in blood. To regulate the arginase activities properly, a synthetic peptide based on the structure of human arginase I was designed and assessed. Preliminary data shows it inhibits human arginase I and II with an IC₅₀ of 2.4 ± 0.3 and 1.8 ± 0.1 mmol, respectively. Our kinetic analysis indicates the inhibition is not competitive with substrate – suggesting an allosteric mechanism. This result provides a step towards specific inhibitors design.

What we know about plant arginases?

Nitrogen is one of the essential element required for plant growth and development. In plants, most of the nitrogen is stored in arginine. Hence, metabolism of arginine to urea by arginase and its further hydrolysis to ammonia by urease is involved in nitrogen recycling to meet the metabolic demands of growing plants. In this respect, plant arginases differ from that of animals. Animals excrete urea while plants recycle the urea. However, the studies on the biochemical and biophysical characteristics of plant arginase are limited when compared to animal arginase(s). In this review, the structural and biochemical characteristics of various plant arginases are discussed. Moreover, the significance of arginase in nitrogen recycling is explained and recent literature on function and activation of plant arginases in response to various environmental (biotic and abiotic) insults is also presented.

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.

Insights on the participation of Glu256 and Asp204 in the oligomeric structure and cooperative effects of human arginase type I

Arginase (EC 3.5.3.1) catalyzes the hydrolysis of L-arginine to L-ornithine and urea, and requires a bivalent cation, especially Mn²⁺ for its catalytic activity. It is a component of the urea cycle and regulates the intracellular levels of l-arginine, which makes the arginase a target for treatment of vascular diseases and asthma. Mammalian arginases contain an unusual S-shaped motif located at the intermonomeric interface. Until now, the studies were limited to structural role of the motif. Then, our interest was focused on functional aspects and our hypothesis has been that the motif is essential for maintain the oligomeric state, having Arg308 as a central axis. Previously, we have shown that the R308A mutant is monomeric and re-associates to the trimeric-cooperative state in the presence of low concentrations of guanidine chloride. We have now mutated Asp204 that interacts with Arg308 in the neighbor subunit, and also we mutated Glu256, proposed as important for oligomerization. Concretely, the human arginase I mutants D204A, D204E, E256A, E256Q and E256D were generated and examined. No differences were observed in the kinetic parameters at pH 9.5 or in tryptophan fluorescence. However, the D204A and E256Q variants were monomeric. On the other hand, D204E and E256D proved to be trimeric and kinetically cooperative at pH 7.5, whereas hyperbolic kinetics was exhibited by E256A, also trimeric. The results obtained strongly support the importance of the interaction between Arg255 and Glu256 in the cooperative properties of arginase, and Asp204 would be relevant to maintain the oligomeric state through salt bridges with Arg255 and Arg308.

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.

New pyrazolopyrimidine derivatives as Leishmania amazonensis arginase inhibitors

Arginase performs the first enzymatic step in polyamine biosynthesis in Leishmania and represents a promising target for drug development. Polyamines in Leishmania are involved in trypanothione synthesis, which neutralize the oxidative burst of reactive oxygen species (ROS) and nitric oxide (NO) that are produced by host macrophages to kill the parasite. In an attempt to synthesize arginase inhibitors, six 1-phenyl-1H-pyrazolo[3,4-d]pyrimidine derivatives with different substituents at the 4-position of the phenyl group were synthesized. All compounds were initially tested at 100 µM concentration against Leishmania amazonensis ARG (LaARG), showing inhibitory activity ranging from 36 to 74%. Two compounds, 1 (R=H) and 6 (R=CF₃), showed arginase inhibition >70% and IC₅₀ values of 12 µM and 47 µM, respectively. Thus, the kinetics of LaARG inhibition were analyzed for compounds 1 and 6 and revealed that these compounds inhibit the enzyme by an uncompetitive mechanism, showing K_is values, and dissociation constants for ternary complex enzyme-substrate-inhibitor, of 8.5 ± 0.9 µM and 29 ± 5 µM, respectively. Additionally, the molecular docking studies proposed that these two uncompetitive inhibitors interact with different LaARG binding sites, where compound 1 forms more H-bond interactions with the enzyme than compound 6. These compounds showed low activity against L. amazonensis free amastigotes obtained from mice lesions when assayed with as much as 30 µM. The maximum growth inhibition reached was between 20 and 30% after 48 h of incubation. These results suggest that this system can be promising for the design of potential antileishmanial compounds.

Molecular characterization of recombinant arginase of Leishmania donovani

Arginase catalyzes the first committed step in the biosynthesis of polyamines that enable cell growth and hence potential drug target for the treatment of leishmaniasis. The arginase from Leishmania donovani (LdARG) was cloned, overexpressed and characterized. Analysis of the deduced amino acid sequence of LdARG with homologous enzyme from other trypanosomatids arginases identified a non-conserved 12 residues long segment VWGLIERTFLSA from position 161-172. This counter segment in L. mexicana arginase exhibits a different conformation compared with human arginase I. The pH and temperature optima of LdARG were 9.0 and 37 °C, respectively. Biochemical studies revealed that the K_M for the substrate L-arginine was 24.76 ± 0.06 mM. Molecular modeling of LdARG studies revealed that the glutamic acid residue at position 288 plays a role in substrate binding. The importance of this glutamic acid residue was validated by constructing a mutant variant of LdARG (E288Q-LdARG) by replacing glutamic acid with glutamine through site-directed mutagenesis. The K_M value of mutant variant for L-arginine was found to be 107 ± 0.18 mM. The increase in K_M value of E288Q-LdARG as compared to LdARG suggested that substrate binding was significantly affected which could be exploited further. Studies on biochemical and structural characterization of recombinant LdARG will help in evaluating this enzyme as a potential drug target for visceral leishmaniasis.

Adaptations to a subterranean environment and longevity revealed by the analysis of mole rat genomes

Subterranean mammals spend their lives in dark, unventilated environments that are rich in carbon dioxide and ammonia and low in oxygen. Many of these animals are also long-lived and exhibit reduced aging-associated diseases, such as neurodegenerative disorders and cancer. We sequenced the genome of the Damaraland mole rat (DMR, Fukomys damarensis) and improved the genome assembly of the naked mole rat (NMR, Heterocephalus glaber). Comparative genome analyses, along with the transcriptomes of related subterranean rodents, revealed candidate molecular adaptations for subterranean life and longevity, including a divergent insulin peptide, expression of oxygen-carrying globins in the brain, prevention of high CO2-induced pain perception, and enhanced ammonia detoxification. Juxtaposition of the genomes of DMR and other more conventional animals with the genome of NMR revealed several truly exceptional NMR features: unusual thermogenesis, an aberrant melatonin system, pain insensitivity, and unique processing of 28S rRNA. Together, these genomes and transcriptomes extend our understanding of subterranean adaptations, stress resistance, and longevity.

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.

Structural, enzymatic and biochemical studies on Helicobacter pylori arginase

Arginase is an enzyme involved in the last step of the urea cycle, where it catalyses the hydrolysis of l-arginine to generate l-ornithine and urea. Compared to the well-characterised arginases from animals, yeast and other bacteria, Helicobacter pylori arginase, or RocF, is unique in at least three aspects. Firstly, it has been identified as an important factor in evasion of the host's immune system and thus contributes to persistent infection by the bacterium. Secondly, the optimal catalytic conditions of RocF are different from those of other arginases. Finally, sequence alignment indicates that RocF possesses considerable differences at its N- and C-terminal from other arginases and harbours an insertion of 13 residues in the middle of the sequence. To better understand these unique biochemical and enzymatic properties, we therefore have embarked on determining the structure of RocF. In this study, the crystal structure of RocF was solved with the molecular replacement method. Based on the structure and systematic mutagenesis studies, we confirmed that the inserted residues form a helix that was not observed in other arginases and was able to raise the arginase activity by 30% probably by change the conformation of the substrate binding pocket. Six residues were involved in Mn(2+) binding, all of which were essential for arginase activity. The C-terminal motif is not sufficient in establishing the oligomeric state of RocF, and no disulphide bonds were observed in RocF.

Analysis of novel ARG1 mutations causing hyperargininemia and correlation with arginase I activity in erythrocytes

Hyperargininemia (HA) is an autosomal recessive disease that typically has a clinical presentation that is distinct from other urea cycle disorders. It is caused by the deficient activity of the enzyme arginase I, encoded by the gene ARG1. We screened for ARG1 mutations and measured erythrocyte enzyme activity in a series of 16 Brazilian HA patients. Novel mutations, in addition to previously described missense mutations, were analysed for their effect on the structure, stability and/or function of arginase I (ARG1) using bioinformatics tools. Three previously reported mutations were found (p.R21X; p.I11T and p.W122X), and five novel mutations were identified (p.G27D; p.G74V; p.T134I; p.R308Q; p.I174fs179). The p.T134I mutation was the most frequent in the Brazilian population. Patients carrying the p.R308Q mutation had higher residual ARG1 decreased activity, but presented no distinguishable phenotype compared to the other patients. Bioinformatics analyses revealed that missense mutations (1) affect the ARG1 active site, (2) interfere with the stability of the ARG1 folded conformation or (3) alter the quaternary structure of the ARG1. Our study reinforced the role of Arg308 residue for assembly of the ARG1 homotrimer. The panel of heterogeneous ARG1 mutations that cause HA was expanded, nevertheless a clear genotype-phenotype correlation was not observed in our series.

Strategies for optimizing the serum persistence of engineered human arginase I for cancer therapy

Systemic L-arginine depletion following intravenous administration of l-arginine hydrolyzing enzymes has been shown to selectively impact tumors displaying urea cycle defects including a large fraction of hepatocellular carcinomas, metastatic melanomas and small cell lung carcinomas. However, the human arginases display poor serum stability (t(1/2)=4.8h) whereas a bacterial arginine deiminase evaluated in phase II clinical trials was reported to be immunogenic, eliciting strong neutralizing antibody responses. Recently, we showed that substitution of the Mn(2+) metal center in human Arginase I with Co(2+) (Co-hArgI) results in an enzyme that displays 10-fold higher catalytic efficiency for L-Arg hydrolysis, 12-15 fold reduction in the IC(50) towards a variety of malignant cell lines and, importantly a t(1/2)=22h in serum. To investigate the utility of Co-hArgI for L-Arg depletion therapy in cancer we systematically investigated three strategies for enhancing the persistence of the enzyme in circulation: (i) site specific conjugation of Co-hArgI engineered with an accessible N-terminal Cys residue to 20kDa PEG-maleimide (Co-hArgI-C(PEG-20K)); (ii) engineering of the homotrimeric Co-hArgI into a linked, monomeric 110kDa polypeptide (Co-hArgI x3) and (iii) lysyl conjugation of 5kDa PEG-N-hydroxysuccinimide (NHS) ester (Co-hArgI-K(PEG-5K)). Surprisingly, even though all three formulations resulted in proteins with a predicted hydrodynamic radius larger than the cut-off for renal filtration, only Co-hArgI amine conjugated to 5kDa PEG remained in circulation for sufficiently long durations. Using Co-hArgI-K(PEG-5K) labeled with an end-terminal fluorescein for easy detection, we demonstrated that following intraperitoneal administration at 6mg/kg weight, a well tolerated dose, the circulation t(1/2) of the protein in Balb/c mice is 63±10h. Very low levels of serum L-Arg (<5μM) could be sustained for over 75h after injection, representing a 9-fold increase in pharmacodynamic efficacy relative to similarly prepared Mn(2+)-containing hArgI conjugated to 5kDa PEG-NHS ester (Mn-hArgI-K(PEG-5K)). The favorable pharmacokinetic and pharmacodynamic properties of Co-hArgI-K(PEG-5K) reported here, coupled with its human origin which should reduce the likelihood of adverse immune responses, make it a promising candidate for cancer therapy.

The redox state of the glutathione/glutathione disulfide couple mediates intracellular arginase activation in HCT-116 colon cancer cells

BACKGROUND

Emerging studies have implicated arginase hyperactivity in the dysregulation of nitric oxide synthesis, which can lead to the development of vascular disease and the promotion of tumor cell growth. Recently, we showed that cysteine, in the presence of molecular iron, promotes arginase activity by driving the Fenton reaction. However, the exact mechanism of arginase activation in the cell induced by oxidative stress is unknown.

AIM

The aim of the present study is to examine whether intracellular arginase is regulated by the cellular redox status of glutathione.

METHOD

To test this hypothesis, the glutathione/glutathione disulfide redox couple was altered in colon cancer cells with the thiol-specific oxidant, diamide, or the glutathione inhibitor, buthionine-(S,R)-sulfoximine, and the activity of the arginase in the cells was assessed.

RESULTS

Treatment of cells with diamide, a thiol-specific oxidant, resulted in a dose-dependent decrease in the glutathione/glutathione disulfide ratio that was associated with the loss of glutathione and a coincident increase in arginase activity and arginase-1 levels in drug-treated cells compared with untreated cells. These results show that oxidation-induced redox changes of glutathione are of sufficient magnitude to control the activity of arginase in the cells. Thus, the physiologic modulation of the glutathione/glutathione disulfide ratio could prove to be a fundamental parameter for the control of arginase activity in pathological conditions of increased oxidative stress.

CONCLUSION

This is the first evidence supporting the ex vivo regulation of arginase activity through the redox modulation of intracellular glutathione. The potential adaptive and pathological consequences of glutathione redox regulation of arginase activity are discussed.

Crystal structure of arginase from Plasmodium falciparum and implications for L-arginine depletion in malarial infection

The 2.15 A resolution crystal structure of arginase from Plasmodium falciparum, the parasite that causes cerebral malaria, is reported in complex with the boronic acid inhibitor 2(S)-amino-6-boronohexanoic acid (ABH) (K(d) = 11 microM). This is the first crystal structure of a parasitic arginase. Various protein constructs were explored to identify an optimally active enzyme form for inhibition and structural studies and to probe the structure and function of two polypeptide insertions unique to malarial arginase: a 74-residue low-complexity region contained in loop L2 and an 11-residue segment contained in loop L8. Structural studies indicate that the low-complexity region is largely disordered and is oriented away from the trimer interface; its deletion does not significantly compromise enzyme activity. The loop L8 insertion is located at the trimer interface and makes several intra- and intermolecular interactions important for enzyme function. In addition, we also demonstrate that arg- Plasmodium berghei sporozoites show significantly decreased liver infectivity in vivo. Therefore, inhibition of malarial arginase may serve as a possible candidate for antimalarial therapy against liver-stage infection, and ABH may serve as a lead for the development of inhibitors.

The activity of Plasmodium falciparum arginase is mediated by a novel inter-monomer salt-bridge between Glu295-Arg404

A recent study implicated a role for Plasmodium falciparum arginase in the systemic depletion of arginine levels, which in turn has been associated with human cerebral malaria pathogenesis. Arginase (EC 3.5.3.1) is a multimeric metallo-protein that catalyses the hydrolysis of arginine to ornithine and urea by means of a binuclear spin-coupled Mn(2+) cluster in the active site. A previous report indicated that P. falciparum arginase has a strong dependency between trimer formation, enzyme activity and metal co-ordination. Mutations that abolished Mn(2+) binding also caused dissociation of the trimer; conversely, mutations that abolished trimer formation resulted in inactive monomers. By contrast, the monomers of mammalian (and therefore host) arginase are also active. P. falciparum arginase thus appears to be an obligate trimer and interfering with trimer formation may therefore serve as an alternative route to enzyme inhibition. In the present study, the mechanism of the metal dependency was explored by means of homology modelling and molecular dynamics. When the active site metals are removed, loss of structural integrity is observed. This is reflected by a larger equilibration rmsd for the protein when the active site metal is removed and some loss of secondary structure. Furthermore, modelling revealed the existence of a novel inter-monomer salt-bridge between Glu295 and Arg404, which was shown to be associated with the metal dependency. Mutational studies not only confirmed the importance of this salt-bridge in trimer formation, but also provided evidence for the independence of P. falciparum arginase activity on trimer formation.

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.

Evolution of the arginase fold and functional diversity

Novel structural superfamilies can be identified among the large number of protein structures deposited in the Protein Data Bank based on conservation of fold in addition to conservation of amino acid sequence. Since sequence diverges more rapidly than fold in protein Evolution, proteins with little or no significant sequence identity are occasionally observed to adopt similar folds, thereby reflecting unanticipated evolutionary relationships. Here, we review the unique alpha/beta fold first observed in the manganese metalloenzyme rat liver arginase, consisting of a parallel eight-stranded beta-sheet surrounded by several helices, and its evolutionary relationship with the zinc-requiring and/or iron-requiring histone deacetylases and acetylpolyamine amidohydrolases. Structural comparisons reveal key features of the core alpha/beta fold that contribute to the divergent metal ion specificity and stoichiometry required for the chemical and biological functions of these enzymes.

In vitro evidence of the inhibitory capacity of chloroquine on arginase activity in sickle erythrocytes

Increased levels of erythrocyte arginase activity are believed to play a key role in the pathogenesis of sickle cell disease (SCD). Because increased arginase activity has been implicated in the exacerbation of pulmonary hypertension in SCD, this enzyme is considered an important therapeutic target for identifying new drugs to treat and manage SCD and other inflammatory disorders. Although chloroquine (CQ) is prescribed as an anti-malarial and anti-rheumatoid drug, the mechanism of its anti-inflammatory activity is largely unknown. The present study found that CQ inhibited arginase in a dose-dependent manner, and also displayed a linear competitive inhibition on sickle erythrocyte arginase. The apparent K(M) values in the presence of inhibitor were considerably reduced at both physiological and slightly acidic pHs. Slope replots of the double reciprocal plots at pH 6.8 and 7.4 also indicated simple competitive inhibitory mechanism of CQ, and Ki values for CQ were within micromolar levels. To our knowledge, this is the first example of an anti-malarial and anti-inflammatory agent displaying competitive inhibition kinetics on arginase. The outcome of this study may provide a template for the rational design of specific agents either alone or in combination with CQ for the inhibition of arginase activity.

Inducible NO synthase dependent S-nitrosylation and activation of arginase1 contribute to age-related endothelial dysfunction

Endothelial function is impaired in aging because of a decrease in NO bioavailability. This may be, in part, attributable to increased arginase activity, which reciprocally regulates NO synthase (NOS) by competing for the common substrate, L-arginine. However, the high Km of arginase (>1 mmol/L) compared with NOS (2 to 20 micromol/L) seemingly makes direct competition for substrate unlikely. One of the mechanisms by which NO exerts its effects is by posttranslational modification through S-nitrosylation of protein cysteines. We tested the hypothesis that arginase1 activity is modulated by this mechanism, which serves to alter its substrate affinity, allowing competition with NOS for L-arginine. We demonstrate that arginase1 activity is altered by S-nitrosylation, both in vitro and ex vivo. Furthermore, using site-directed mutagenesis we demonstrate that 2 cysteine residues (C168 and C303) are able to undergo nitrosylation. S-Nitrosylation of C303 stabilizes the arginase1 trimer and reduces its Km value 6-fold. Finally, arginase1 nitrosylation is increased (and thus its Km decreased) in blood vessels from aging rats, likely contributing to impaired NO bioavailability and endothelial dysfunction. This is mediated by inducible NOS, which is expressed in the aging endothelium. These findings suggest that S-nitrosylated arginase1 can compete with NOS for L-arginine and contribute to endothelial dysfunction in the aging cardiovascular system.

The inhibitory effect of various indolyl amino acid derivatives on arginase activity in macrophages

Numerous indolyl amino acids and their derivatives inhibited arginase activity. The inhibition was found to be non-competitive, - at least partly - allosteric, and independent on manganese ions in the active site, and it cannot be explained by the dissociation of arginase homotrimers. Indole alone is weakly inhibitory; however, the presence of three-carbon side chains and their net charges is favorable for the inhibition. The binding of the inhibitory compounds caused only minor changes in the steric structure of arginase: a slight increase in alpha-helix content was detected by circular dichroism together with a decrease in parallel pleated sheet and beta-turn sections. A slight alteration in the tertiary structure was also found using tryptophane fluorescence studies, but buried apolar side chains were not transposed to the protein surface. Computer studies that were performed did not provide additional structural information.

Urea cycle of Fasciola gigantica: Purification and characterization of arginase

The ornithine-urea cycle has been investigated in Fasciola gigantica. Agrinase had very high activity compared to the other enzymes. Carbamoyl phosphate synthetase and ornithine carbamoyltransferase had very low activity. A moderate enzymatic activity was recorded for argininosuccinate synthetase and argininosuccinate lyase. The low levels of F. gigantica urea cycle enzymes except to the arginase suggest the urea cycle is operative but its role is of a minor important. The high level of arginase activity may benefit for the hydrolysis of the exogenous arginine to ornithine and urea. Two arginases Arg I and Arg II were separated by DEAE-Sepharose column. Further purification was restricted to Arg II with highest activity. The molecular weight of Arg II, as determined by gel filtration and SDS-PAGE, was 92,000. The enzyme was capable to hydrolyze l-arginine and to less extent l-canavanine at arginase:canavanase ratio (>10). The enzyme exhibited a maximal activity at pH 9.5 and Km of 6 mM. The optimum temperature of F. gigantica Arg II was 40 degrees C and the enzyme was stable up to 30 degrees C and retained 80% of its activity after incubation at 40 degrees C for 15 min and lost all of its activity at 50 degrees C. The order of effectiveness of amino acids as inhibitors of enzyme was found to be lysine>isoleucine>ornithine>valine>leucine>proline with 67%, 43%, 31%, 25%, 23% and 15% inhibition, respectively. The enzyme was activated with Mn2+, where the other metals Fe2+, Ca2+, Hg2+, Ni2+, Co2+ and Mg2+ had inhibitory effects.

Structural metal dependency of the arginase from the human malaria parasite Plasmodium falciparum

The human malaria parasite Plasmodium falciparum possesses a single gene with high similarity to the metalloproteins arginase and agmatinase. The recombinant protein reveals strict specificity for arginine, and it has been proposed that its function in ornithine production is as a precursor for polyamine biosynthesis. The specific activity of the plasmodial arginase was found to be 31 micromol min(-1) mg(-1) protein and the k(cat) was calculated as 96 (s-1) . The Km value for arginine and Ki value for ornithine were determined as 13 mM and 19 mM, respectively. The active arginase is a homotrimer of ca. 160 kDa. Dialysis of the arginase against EDTA results in monomers of approximately 48 kDa; however, the quaternary structure can be restored by addition of Mn 2+ . Mutagenic analyses of all the amino acid residues proposed to be involved in metal binding led to complex dissociation, except for the His-193-Ala mutant, which was also inactive but retained the trimeric structure. Substitution of His-233, which has been suggested to be in charge of proton shuttling within the active site, disrupted the trimeric structure and thereby the activity of the Pf arginase. Northern blot analysis identified a stage-specific expression pattern of the plasmodial arginase in the ring/young trophozoite stage, which guarantees the provision of ornithine for essential polyamine biosynthesis.

Potential involvement of NOS and arginase in age-related behavioural impairments

The present study investigated age-related changes in nitric oxide synthase (NOS) and arginase, which shares a substrate with NOS, in the hippocampus and parahippocampal region and the relationship between NOS/arginase and age-associated behavioural impairments. Aged rats (24 months old) displayed reduced exploratory activity, enhanced anxiety, poorer spatial learning and memory, and impaired object recognition memory relative to the young adults (4 months old). There were significant increases in total NOS activity in the aged hippocampus and perirhinal, postrhinal and temporal cortices and a dramatic decrease in endothelial NOS expression in the aged postrhinal cortex. Activity and protein expression of inducible NOS were not detected in any region from either group and a significant increase in total arginase activity was found in the aged perirhinal cortex. Multiple regression analysis revealed significant correlations between NOS/arginase and behavioural measures in both groups. The present findings provide further support for a contribution of nitric oxide to the normal aging process and suggest a potential involvement of arginase in aging and learning and memory.

Structure and function of arginases

The arginases catalyze the divalent cation dependent hydrolysis of L-arginine to produce L-ornithine and urea. Although traditionally considered in terms of its role as the final enzyme of the urea cycle, the enzyme is found in a variety of nonhepatic tissues. These findings suggest that the enzyme may have other functions in addition to its role in nitrogen metabolism. High-resolution crystal structures have been determined for recombinant rat liver (type I) arginase and for recombinant human kidney (type II) arginase, their variants, and complexes with products and inhibitors. Each identical subunit of the trimeric enzyme contains an active site that lies at the bottom of a 15 A deep cleft. The 2 essential Mn(II) ions are located at the bottom of this cleft, separated by approximately 3.3 A and bridged by oxygens derived from 2 aspartic acid residues and a solvent-derived hydroxide. This metal bridging hydroxide is proposed to be the nucleophile that attacks the guanidinium carbon of substrate arginine. On the basis of this proposed mechanism, boronic acid inhibitors of the enzyme have been synthesized and characterized kinetically and structurally. These inhibitors display slow-onset inhibition at the pH optimum of the enzyme, and are found as tetrahedral species at the active site, as determined by X-ray diffraction. The potent inhibition of arginases I and II by these compounds has not only delineated key enzyme-substrate interactions, but has also led to a greater understanding of the role of arginase in nonhepatic tissues.

Insights into the interaction of human liver arginase with tightly and weakly bound manganese ions by chemical modification and site-directed mutagenesis studies

Diethyl pyrocarbonate (DEPC) caused a loss in the ability of inactive subunits of wild-type and H141F mutant human liver arginase (EC 3.5.3.1) to be reactivated by Mn(2+). The effect was reversed by hydroxylamine and involved a residue with a pK(a) of 6.5+/-0.1. Half activation with Mn(2+) was sufficient for total resistance of H141F and full activation was not impeded by a previous incubation of the half-active species with DEPC. The H101N and H126N mutants expressed 60 and 82% of the wild-type activity, respectively, without changes in K(m) for arginine or K(i) for lysine inhibition. After dialysis against EDTA, H126N was inactive in the absence of added Mn(2+) and contained <0.1 Mn(2+)/subunit, whereas H101N was half active and contained 1.2+/-0.1 Mn(2+)/subunit. Results support the concept that a weakly bound metal ion is needed only for conversion of active species to a more active active state.

Mouse model for human arginase deficiency

Deficiency of liver arginase (AI) causes hyperargininemia (OMIM 207800), a disorder characterized by progressive mental impairment, growth retardation, and spasticity and punctuated by sometimes fatal episodes of hyperammonemia. We constructed a knockout mouse strain carrying a nonfunctional AI gene by homologous recombination. Arginase AI knockout mice completely lacked liver arginase (AI) activity, exhibited severe symptoms of hyperammonemia, and died between postnatal days 10 and 14. During hyperammonemic crisis, plasma ammonia levels of these mice increased >10-fold compared to those for normal animals. Livers of AI-deficient animals showed hepatocyte abnormalities, including cell swelling and inclusions. Plasma amino acid analysis showed the mean arginine level in knockouts to be approximately fourfold greater than that for the wild type and threefold greater than that for heterozygotes; the mean proline level was approximately one-third and the ornithine level was one-half of the proline and ornithine levels, respectively, for wild-type or heterozygote mice--understandable biochemical consequences of arginase deficiency. Glutamic acid, citrulline, and histidine levels were about 1.5-fold higher than those seen in the phenotypically normal animals. Concentrations of the branched-chain amino acids valine, isoleucine, and leucine were 0.4 to 0.5 times the concentrations seen in phenotypically normal animals. In summary, the AI-deficient mouse duplicates several pathobiological aspects of the human condition and should prove to be a useful model for further study of the disease mechanism(s) and to explore treatment options, such as pharmaceutical administration of sodium phenylbutyrate and/or ornithine and development of gene therapy protocols.

Genomic organisation and transcription characterisation of the gene encoding Leishmania (Leishmania) amazonensis arginase and its protein structure prediction

The genomic organisation of the gene encoding Leishmania (Leishmania) amazonensis arginase as well as its flanking regions were characterised. The size of the transcribed RNA was determined, allowing us to map the genomic sites signalling for RNA trans-splicing and putative polyadenylation regions. The general organisation was compared with genes encoding other proteins already described in organisms of the Trypanosomatid family. The complete nucleotide sequence of the arginase open reading frame was obtained and the three-dimensional structure of the enzyme was inferred by a computational analysis of the deduced amino acid sequence, based on the established crystal structure described for Rattus norvergicus arginase. The human liver arginase sequence was analysed in the same way and the comparison of the presumed structure of both the Leishmania and human enzymes identified some differences that may be exploited in chemotherapeutic studies.

Functional consequences of the G235R mutation in liver arginase leading to hyperargininemia

Hyperargininemia is a rare autosomal disorder that results from a deficiency in hepatic type I arginase. This deficiency is the consequence of random point mutations that occur throughout the gene. The G235R patient mutation has been proposed to affect the catalytic activity and structural integrity of the protein [D. E. Ash, L. R. Scolnick, Z. F. Kanyo, J. G. Vockley, S. D. Cederbaum, and D. W. Christianson (1998) Mol. Genet. Metab. 64, 243-249]. The G235R (patient) and G235A (control) arginase mutants of rat liver arginase have been generated to probe the effects of these point mutations on the structure and function of hepatic type I arginase. Both mutant arginases were trimeric by gel filtration, but the control G235A mutant had 56% of wild-type activity and the G235R mutant had less than 0.03% activity compared to the wild-type enzyme. The G235R mutant contained undetectable levels of tightly bound manganese as determined by electron paramagnetic resonance, while the G235A mutant had a Mn(II) stoichiometry of 2 Mn/subunit. Molecular modeling indicates that the introduction of an arginine residue at position 235 results in a major rearrangement of the metal ligands that compromise Mn(II) binding.

The V122I cardiomyopathy variant of transthyretin increases the velocity of rate-limiting tetramer dissociation, resulting in accelerated amyloidosis

The transthyretin (TTR) amyloid diseases are of keen interest, because there are >80 mutations that cause, and a few mutations that suppress, disease. The V122I variant is the most common amyloidogenic mutation worldwide, producing familial amyloidotic cardiomyopathy primarily in individuals of African descent. The substitution shifts the tetramer-folded monomer equilibrium toward monomer (lowers tetramer stability) and lowers the kinetic barrier associated with rate-limiting tetramer dissociation (pH 7; relative to wild-type TTR) required for amyloid fibril formation. Fibril formation is also accelerated because the folded monomer resulting from the tetramer-folded monomer equilibrium rapidly undergoes partial denaturation and self-assembles into amyloid (in vitro) when subjected to a mild denaturation stress (e.g., pH 4.8). Incorporation of the V122I mutation into a folded monomeric variant of transthyretin reveals that this mutation does not destabilize the tertiary structure or alter the rate of amyloidogenesis relative to the wild-type monomer. The increase in the velocity of rate-limiting tetramer dissociation coupled with the lowered tetramer stability (increasing the mol fraction of folded monomer present at equilibrium) may explain why V122I confers an apparent absolute anatomic risk for cardiac amyloid deposition.

Allosteric inhibition of rat liver and kidney arginase by copper and mercury ions

Two isozyme forms of arginase are found in the rat. All arginases are metalloenzymes which require manganese for activity. Many arginases are activated by cobalt and nickel ions and inhibited by heavy metal ions. The purpose of this study was to compare the effect of other heavy metal ions on the rat liver isozyme (arginase I) and the rat kidney isozyme (arginase II). The activation and inhibition of arginase I and II by metal ions were different. However, both isozymes were strongly inhibited by cupric and mercuric ions. The inhibition of arginase I by cupric and mercuric ions was increased greatly by preincubation of the enzyme with the metal ions. However, preincubation of arginase II by cupric and mercuric ions had little effect on the inhibition of the enzyme. Under certain conditions the kinetics of the inhibition of both arginases I and II by cupric and mercuric ions was nonlinear allosteric.

Glu-256 is a main structural determinant for oligomerisation of human arginase I

One determinant that could play a role in the quaternary structure of human arginase is the pair of salt links between the strictly conserved residues R255 from one monomer and E256 from every adjacent subunit. In this work, the ionic interaction between monomers was disrupted by expressing a human arginase where Glu-256 had been substituted by Gln. Biochemical analyses of the mutant protein showed that: (i) it shares the wild-type kinetic parameters of the arginine substrate; (ii) E256Q arginase behaves as a monomer by gel filtration; (iii) it is drastically inactivated by dialysis in the presence of EDTA, an inhibitory effect which is reversed by addition of Mn(2+); and (iv) the mutant enzyme loses thermal stability. The lack of oligomerisation for E256Q arginase and the conservation of E256 throughout evolution of the protein family suggest that this residue is involved in the quaternary structure of arginases.