Crystallization and oligomeric structure of rat liver arginase

Illuminating the structure-function landscape of an evolutionary nonconserved motif in the arginases of Helicobacter gastric pathogens

The bimetallic enzyme arginase catalyses the conversion of L-arginine to L-ornithine and urea. In Helicobacter pylori (a known human gastric pathogen), this enzyme is an important virulence factor. In spite of the conservation of the catalytic and the metal-binding residues, the H. pylori homolog possesses a 13-residue motif (-153 ESEEKAWQKLCSL165 -) present in the middle of the protein sequence, whose role was recently elucidated. Despite several reviews available on arginases, no report has thoroughly illustrated the underlying basis for the importance of the above motif of the H. pylori enzyme in structure and function. In this review, we systematically describe a mechanistic basis for its importance in structure and function based on the known data. This motif of the H. pylori enzyme is present exclusively in the arginases of other Helicobacter gastric pathogens, where the critical residues are conserved, implying that the nonconserved stretch has been selected during the evolution of the enzyme in these gastric pathogens in a specific manner to perform its role in the structure and function. The combined information can be useful for understanding the function of arginases in other Helicobacter gastric pathogens. Additionally, this knowledge can be utilised to screen and design new small molecule inhibitors, specific to the arginases of these pathogens.

Sequence Divergence in the Arginase Domain of Ornithine Decarboxylase/Arginase in Fusobacteriacea Leads to Loss of Function in Oral Associated Species

A number of species within the Fusobacteriaceae family of Gram-negative bacteria uniquely encode for an ornithine decarboxylase/arginase (ODA) that ostensibly channels l-ornithine generated by hydrolysis of l-arginine to putrescine formation. However, two aspartate residues required for coordination to a catalytically obligatory manganese cluster of arginases are substituted for a serine and an asparagine. Curiously, these natural substitutions occur only in a clade of Fusobacterium species that inhabit the oral cavity. Herein, we expressed and isolated full-length ODA from the opportunistic oral pathogen Fusobacterium nucleatum along with the individual arginase and ornithine decarboxylase components. The crystal structure of the arginase domain reveals that it adopts the classical α/β arginase-fold, but metal ions are absent in the active site. As expected, the ureohydrolase activity with l-arginine was not detected for wild-type ODA or the isolated arginase domain. However, engineering of the complete metal coordination environment through site-directed mutagenesis restored Mn²⁺ binding capacity and arginase activity, although the catalytic efficiency for l-arginine was low (60-100 M^-1 s^-1). Full-length ODA and the isolated ODC component were able to decarboxylate both l-ornithine and l-arginine to form putrescine and agmatine, respectively, but k_cat/K_M of l-ornithine was ∼20-fold higher compared to l-arginine. We discuss environmental conditions that may have led to the natural selection of an inactive arginase in the oral associated species of Fusobacterium.

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.

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.

Comprehensive molecular analysis of arginase-encoding genes in common wheat and its progenitor species

Arginase (ARG) contributes to nitrogen remobilization by conversion of arginine to ornithine and urea. However, wheat ARG genes have not yet been identified. Here we isolated and characterized ARG genes from wheat and its progenitor species and found that a single copy was present in wheat progenitors. Three common wheat ARG genes of TaARG-2AS, TaARG-2BS, and TaARG-2DS were experimentally assigned to the short arms of the group 2 chromosomes. We found an in-frame stop codon in TaARG-2AS, but not in the other two genes. The highest expression was detected in stems and sheaths for TaARG-2BS and in leaves for TaARG-2DS. Both genes have similar expression trend in different developmental stages, peaking at booting and grain filling stages. TaARG-2BS transcript was induced by high salinity and drought, whereas TaARG-2DS was induced by drought only, but neither of them were induced by low temperature. In addition, both genes showed analogous expression pattern upon powdery mildew (PM) infection in the resistant line Pm97033, with TaARG-2BS induced greatly at 72 h post PM infection. In contrast, no obvious transcripts were accumulated for TaARG-2DS in the PM susceptible line Wan7107. Monocot ARGs have more conserved mitochondrion-targeting signals and are more evolutionarily conserved than dicot ARGs.

Insight into the role of a unique SSEHA motif in the activity and stability of Helicobacter pylori arginase

Arginase is a binuclear Mn(2+) -metalloenzyme of urea cycle that hydrolyzes arginine to ornithine and urea. Unlike other arginases, the Helicobacter pylori enzyme is selective for Co(2+) and has all conserved motifs except (88) SSEHA(92) (instead of GGDHS). To examine the role of this motif in the activity and stability, steady-state kinetics, mutational analysis, thermal denaturation, and homology modeling were carried out. With a series of single and double mutants, we show that mutations of Ser88 and Ala92 to its analogous residues in other arginases individually enhance the catalytic activity. This is supported by the modeling studies, where the motif plays a role in alteration at the active site structure compared to other arginases. Mutational analysis further shows that both Glu90 and His91 are important for the activity, as their mutations lead to significant decrease in the catalytic efficiency but they appear to act in two different ways; Glu90 has a more catalytic role as its mutant displays binding of the two metal ions per monomer of the protein, but His91 plays a critical role in retaining the metal ion at the active site as its mutation exhibits a loss of one metal ion. Thermal denaturation studies demonstrated that Ser88 and His91 both play crucial roles in the stability of the protein as their mutants showed a decrease in the T(m) by ∼10-11°. Unlike wild type, the metal ions have larger role in providing the stability to the mutant proteins. Thus, our data demonstrate that the motif not only plays an important role in the activity but also critical in the stability of the protein.

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.

Design of Amino Acid Aldehydes as Transition-State Analogue Inhibitors of Arginase

Arginase is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea. Chiral L-amino acids bearing aldehyde side chains have been synthesized in which the electrophilic aldehyde C=O bond is isosteric with the C=N bond of L-arginine. This substitution is intended to facilitate nucleophilic attack by the metal-bridging hydroxide ion upon binding to the arginase active site. Syntheses of the amino acid aldehydes have been accomplished by reduction, oxidation, and Wittig-type reaction with a commercially available derivative of L-glutamic acid. Amino acid aldehydes exhibit inhibition in the micromolar range, and the X-ray crystal structure of arginase I complexed with one of these inhibitors, (S)-2-amino-7-oxoheptanoic acid, has been determined at 2.2 A resolution. In the enzyme-inhibitor complex, the inhibitor aldehyde moiety is hydrated to form the gem-diol: one hydroxyl group bridges the Mn(2+)(2) cluster and donates a hydrogen bond to D128, and the second hydroxyl group donates a hydrogen bond to E277. The binding mode of the neutral gem-diol may mimic the binding of the neutral tetrahedral intermediate and its flanking transition states in arginase catalysis.

Inhibitor Coordination Interactions in the Binuclear Manganese Cluster of Arginase,

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

Human arginase II: crystal structure and physiological role in male and female sexual arousal

Arginase is a binuclear manganese metalloenzyme that catalyzes the hydrolysis of l-arginine to form l-ornithine and urea. The X-ray crystal structure of a fully active, truncated form of human arginase II complexed with a boronic acid transition state analogue inhibitor has been determined at 2.7 A resolution. This structure is consistent with the hydrolysis of l-arginine through a metal-activated hydroxide mechanism. Given that human arginase II appears to play a role in regulating l-arginine bioavailability to NO synthase in human penile corpus cavernosum smooth muscle, the inhibition of human arginase II is a potential new strategy for the treatment of erectile dysfunction [Kim, N. N., Cox, J. D., Baggio, R. F., Emig, F. A., Mistry, S., Harper, S. L., Speicher, D. W., Morris, S. M., Ash, D. E., Traish, A. M., and Christianson, D. W. (2001) Biochemistry 40, 2678-2688]. Since NO synthase is found in human clitoral corpus cavernosum and vagina, we hypothesized that human arginase II is similarly present in these tissues and functions to regulate l-arginine bioavailability to NO synthase. Accordingly, hemodynamic studies conducted with a boronic acid arginase inhibitor in vivo are summarized, suggesting that the extrahepatic arginase plays a role in both male and female sexual arousal. Therefore, arginase II is a potential target for the treatment of male and female sexual arousal disorders.

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

The structure of the trimeric, manganese metalloenzyme, rat liver arginase, has been previously determined at 2.1-A resolution (Kanyo, Z. F., Scolnick, L. R., Ash, D. E., and Christianson, D. W., (1996) Nature 383, 554-557). A key feature of this structure is a novel S-shaped oligomerization motif at the carboxyl terminus of the protein that mediates approximately 54% of the intermonomer contacts. Arg-308, located within this oligomerization motif, nucleates a series of intramonomer and intermonomer salt links. In contrast to the trimeric wild-type enzyme, the R308A, R308E, and R308K variants of arginase exist as monomeric species, as determined by gel filtration and analytical ultracentrifugation, indicating that mutation of Arg-308 shifts the equilibrium for trimer dissociation by at least a factor of 10(5). These monomeric arginase variants are catalytically active, with k(cat)/K(m) values that are 13-17% of the value for wild-type enzyme. The arginase variants are characterized by decreased temperature stability relative to the wild-type enzyme. Differential scanning calorimetry shows that the midpoint temperature for unfolding of the Arg-308 variants is in the range of 63.6-65.5 degrees C, while the corresponding value for the wild-type enzyme is 70 degrees C. The three-dimensional structure of the R308K variant has been determined at 3-A resolution. At the high protein concentrations utilized in the crystallizations, this variant exists as a trimer, but weakened salt link interactions are observed for Lys-308.

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

Arginase is a binuclear Mn(2+) metalloenzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. X-ray crystal structures of arginase complexed to substrate analogues N(omega)-hydroxy-L-arginine and N(omega)-hydroxy-nor-L-arginine, as well as the products L-ornithine and urea, complete a set of structural "snapshots" along the reaction coordinate of arginase catalysis when interpreted along with the X-ray crystal structure of the arginase-transition-state analogue complex described in Kim et al. [Kim, N. N., Cox, J. D., Baggio, R. F., Emig, F. A., Mistry, S., Harper, S. L., Speicher, D. W., Morris, Jr., S. M., Ash, D. E., Traish, A. M., and Christianson, D. W. (2001) Biochemistry 40, 2678-2688]. Taken together, these structures render important insight on the structural determinants of tight binding inhibitors. Furthermore, we demonstrate for the first time the structural mechanistic link between arginase and NO synthase through their respective complexes with N(omega)-hydroxy-L-arginine. That N(omega)-hydroxy-L-arginine is a catalytic intermediate for NO synthase and an inhibitor of arginase reflects the reciprocal metabolic relationship between these two critical enzymes of L-arginine catabolism.

In situ characterization of Helicobacter pylori arginase

The properties of Helicobacter pylori arginase activity in metabolically competent cells and lysates were investigated with the aim of obtaining a better understanding of the nitrogen metabolism of the bacterium. One-dimensional 1H- and 13C-nuclear magnetic resonance spectroscopy, spectrophotometry, radio tracer analysis and protein purification techniques were employed to characterize in situ the first step in the utilization of l-arginine by the bacterium. Arginase activity was associated with the cell-envelope fraction obtained by centrifugation of lysates. A Km of 22+/-3 mM was determined for the enzyme activity, and differences of Vmax were observed between strains. Divalent cations stimulated arginase activity, and the most potent activators were Co2+>Ni2+>Mn2+. The activity was highly specific for l-arginine and did not catabolize analogs recognized by other arginases of prokaryote and eukaryote origin. The Ki of several inhibitors was measured and served also to characterize the enzyme activity. The presence of bicarbonate enhanced the hydrolysis of l-arginine in cell suspensions, but not in lysates or semi-purified enzyme preparations. Amino acid sequence analyses revealed important differences between the deduced structures of H. pylori arginase and those of other organisms. This finding was consistent with experimental data which showed that H. pylori arginase has unique properties.

Altering the binuclear manganese cluster of arginase diminishes thermostability and catalytic function

Arginase is a thermostable (Tm = 75 degrees C) binuclear manganese metalloenzyme which hydrolyzes l-arginine to form l-ornithine and urea. The three-dimensional structures of native metal-depleted arginase, metal-loaded H101N arginase, and metal-depleted H101N arginase have been determined by X-ray crystallographic methods to probe the roles of the manganese ion in site A (Mn2+A) and its ligand H101 in catalysis and thermostability. We correlate these structures with thermal stability and catalytic activity measurements reported here and elsewhere [Cavalli, R. C., Burke, C. J., Kawamoto, S., Soprano, D. R., and Ash, D. E. (1994) Biochemistry 33, 10652-10657]. We conclude that the substitution of a wild-type histidine ligand to Mn2+A compromises metal binding, which in turn compromises protein thermostability and catalytic function. Therefore, a fully occupied binuclear manganese metal cluster is required for optimal catalysis and thermostability.

EXAFS comparison of the dimanganese core structures of manganese catalase, arginase, and manganese-substituted ribonucleotide reductase and hemerythrin

The solution structures of the binuclear Mn centers in arginase, Mn catalase, and the Mn-substituted forms of the Fe enzymes ribonucleotide reductase and hemerythrin have been determined using X-ray absorption spectroscopy (XAS). X-ray absorption near edge structure (XANES) spectra for these proteins were compared to those obtained for Mn(II) models. The Mn model spectra show an inverse correlation between the XANES peak maximum and the root-mean-square (RMS) deviation in metal-ligand bond lengths. For these complexes, the XANES maxima appear to be more effective than the 1s --> 3d areas as an indicator of metal-site symmetry. Arginase and Mn-substituted ribonucleotide reductase have symmetric nearest neighbor environments with low RMS deviation in bond length, while Mn catalase and Mn-substituted hemerythrin appear to have a larger RMS bond length deviation. The 1s --> 3d areas for arginase and Mn-substituted ribonucleotide reductase are consistent with six coordinate Mn, while the 1s --> 3d areas for Mn catalase and Mn-substituted hemerythrin are larger, suggesting that one or both of the Mn ions are five-coordinate in these proteins. Extended x-ray absorption fine structure (EXAFS) spectra were used to determine the Mn2 core structure for the four proteins. In order to quantitate the number of histidine residues bound to the Mn2 centers, EXAFS data for the crystallographically characterized model hexakis-imidazole Mn(II) dichloride tetrahydrate were used to calibrate the Mn-imidazole multiple scattering interactions. These calibrated parameters allowed the outer shell EXAFS to be fit to give a lower limit on the number of bound histidine residues. The EXAFS spectra for Mn-substituted ribonucleotide reductase and arginase are nearly identical, with symmetric Mn-nearest neighbor environments and outer shell scattering consistent with a lower limit of one histidine per Mn2 core. In contrast, the EXAFS data for Mn catalase and Mn-substituted hemerythrin show two distinct Mn-nearest neighbor shells, modeled as Mn-O at ca. 2.1 A and Mn-N at ca. 2.3 A, and outer shell carbon scattering consistent with a lower limit of ca. 2-3 His residues per Mn2 core. Only Mn catalase shows clear evidence for Mn...Mn scattering. The observed Mn...Mn distance is 3.53 A, which is significantly longer than the approximately 3.3 A distances that are typically observed for Mn(II)2 cores with two single atom bridges, but which is typical of the distances seen in Mn(II)2 cores having one single atom bridge (e.g., aqua or hydroxo) together with one or two carboxylate bridges. The absence of EXAFS-detectable Mn...Mn interactions for the other three proteins suggests either that there are no single atom bridges in these cases or that the Mn...Mn interactions are more disordered.

Structure of a unique binuclear manganese cluster in arginase

Each individual excretes roughly 10 kg of urea per year, as a result of the hydrolysis of arginine in the final cytosolic step of the urea cycle. This reaction allows the disposal of nitrogenous waste from protein catabolism, and is catalysed by the liver arginase enzyme. In other tissues that lack a complete urea cycle, arginase regulates cellular arginine and ornithine concentrations for biosynthetic reactions, including nitric oxide synthesis: in the macrophage, arginase activity is reciprocally coordinated with that of NO synthase to modulate NO-dependent cytotoxicity. The bioinorganic chemistry of arginase is particularly rich because this enzyme is one of very few that specifically requires a spin-coupled Mn2+-Mn2+ cluster for catalytic activity in vitro and in vivo. The 2.1 angstrom-resolution crystal structure of trimeric rat liver arginase reveals that this unique metal cluster resides at the bottom of an active-site cleft that is 15 angstroms deep. Analysis of the structure indicates that arginine hydrolysis is achieved by a metal-activated solvent molecule which symmetrically bridges the two Mn2+ ions.

The cloning, expression and crystallisation of a thermostable arginase

The gene for the thermostable arginase from the thermophilic bacterium 'Bacillus caldovelox' has been cloned and sequenced. Expression of recombinant arginase at high levels has been achieved in E. coli using an inducible T7 RNA polymerase-based system. A facile purification procedure incorporating a heat-treatment step yielded 0.2 g of recombinant arginase per litre of induced culture. The kinetic properties of the purified recombinant protein are essentially identical to the native enzyme. The recombinant protein has been crystallised and one crystal form is isomorphous to crystals of the native protein.

Comparative properties of arginases

Arginase is a primordial enzyme, widely distributed in the biosphere and represented in all primary kingdoms. It plays a critical role in the hepatic metabolism of most higher organisms as a cardinal component of the urea cycle. Additionally, it occurs in numerous organisms and tissues where there is no functioning urea cycle. Many extrahepatic tissues have been shown to contain a second form of arginase, closely related to the hepatic enzyme but encoded by a distinct gene or genes and involved in a host of physiological roles. A variety of functions has been proposed for the "extrahepatic" arginases over the last three decades. In recent years, interest in arginase has been stimulated by a demonstrated involvement in the metabolism of the ubiquitous and multifaceted molecule nitric oxide. Molecular biology has begun to furnish new clues to the disparate functions of arginases in different environments and organisms. Comparative studies of arginase sequences are also beginning to elucidate the comparative evolution of arginases, their molecular structures and the nature of their catalytic mechanism. Further studies have sought to clarify the involvement of arginase in human disease. This review presents an outline of the current state of arginase research by giving a comparative overview of arginases and their associated properties.

Transcriptional regulation of genes for ornithine cycle enzymes

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Interaction of arginase with metal ions: studies of the enzyme from human liver and comparison with other arginases

As determined by atomic absorption, fully activated human liver arginase contained 1.1 +/- 0.1 Mn2+/subunit. Upon dissociation to inactive subunits (< 0.01 Mn2+/subunit), there was decreased intensity and a red shift in the tryptophan fluorescence emission spectra of the enzyme, and the resulting species were markedly sensitive to thermal and proteolytic inactivation by trypsin. Arginine and lysine specifically protected the subunits from heat inactivation. Subunit activation by Mn2+ followed hyperbolic kinetics (Kd = 0.08 +/- 0.01 microM). In addition to Mn2+, Ni2+ and Co2+ converted inactive subunits into active monomers, and favoured their association to the oligomeric state of the enzyme (M(r) = 120,000 +/- 2000). The replacement of Mn2+ by Ni2+ or Co2+ resulted in significant changes in Vmax without any change in the Km values for the substrates (arginine or canavanine) or the Ki value for lysine inhibition. The results support our previous suggestion (Carvajal et al., 1994) that Mn2+ is not essential for substrate binding to arginase, and substantiates the conclusion that species differences may exist in the interaction of arginase with metal ions.

Kinetics of manganese reconstitution and thiol group exposition in dialyzed rat mammary gland arginase

1. Rat mammary gland arginase is a metallo-enzyme dependent on Mn2+, which can be only partially substituted by Cd2+. 2. Reconstitution of the activity of dialyzed arginase by manganese is a two-phase process; the second phase is independent of the cation concentration, with a half-time recovery (t1/2) of 10.77 min. 3. The apparent Km for Mn2+ is 280 microM and 10.5 microM for enzyme dialyzed for 24 and 72 hr, respectively. 4. Treatment with 5 mM EDTA at pH 6 totally inhibits enzyme activity, which is reconstituted by Mn2+. 5. Results obtained with iodoacetamide treatment suggest the existence of sulphydryl groups accessible only when the enzyme is dialyzed.