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

New Insights into the Determinants of Specificity in Human Type I Arginase: Generation of a Mutant That Is Only Active with Agmatine as Substrate

Arginase catalyzes the hydrolysis of L-arginine into L-ornithine and urea. This enzyme has several analogies with agmatinase, which catalyzes the hydrolysis of agmatine into putrescine and urea. However, this contrasts with the highlighted specificity that each one presents for their respective substrate. A comparison of available crystal structures for arginases reveals an important difference in the extension of two loops located in the entrance of the active site. The first, denominated loop A (I129-L140) contains the residues that interact with the alpha carboxyl group or arginine of arginase, and the loop B (D181-P184) contains the residues that interact with the alpha amino group of arginine. In this work, to determine the importance of these loops in the specificity of arginase, single, double, and triple arginase mutants in these loops were constructed, as well as chimeras between type I human arginase and E. coli agmatinase. In previous studies, the substitution of N130D in arginase (in loop A) generated a species capable of hydrolyzing arginine and agmatine. Now, the specificity of arginase is completely altered, generating a chimeric species that is only active with agmatine as a substrate, by substituting I129T, N130Y, and T131A together with the elimination of residues P132, L133, and T134. In addition, Quantum Mechanic/Molecular Mechanic (QM/MM) calculations were carried out to study the accommodation of the substrates in in the active site of this chimera. With these results it is concluded that this loop is decisive to discriminate the type of substrate susceptible to be hydrolyzed by arginase. Evidence was also obtained to define the loop B as a structural determinant for substrate affinity. Concretely, the double mutation D181T and V182E generate an enzyme with an essentially unaltered k_cat value, but with a significantly increased K_m value for arginine and a significant decrease in affinity for its product ornithine.

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.

Infrared Multiple Photon Dissociation Spectroscopy of Cationized Canavanine: Side-Chain Substitution Influences Gas-Phase Zwitterion Formation†

Infrared multiple photon dissociation spectroscopy was performed on protonated and cationized canavanine (Cav), a non-protein amino acid oxy-analog of arginine. Infrared spectra in the XH stretching region (3000 - 4000 cm^-1) were obtained at the Centre Laser Infrarouge d'Orsay (CLIO) facility. Comparison of the experimental infrared spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory indicates that canavanine is in a canonical neutral form in CavH⁺, CavLi⁺, and CavNa⁺; therefore, these cations are charge-solvated structures. The infrared spectrum of CavK⁺ is consistent with a mixture of Cav in canonical and zwitterionic forms leading to both charge-solvated and salt-bridged cationic structures. The Cav moiety in CavCs⁺ is shown to be zwitterionic, forming a salt-bridged structure for the cation. Infrared spectra in the fingerprint region (1000 - 2000 cm^-1) obtained at the FELIX Laboratory in Nijmegen, Netherlands support these assignments. These results show that that a single oxygen atom substitution in the side chain reduces the stability of the zwitterion compared to that of the protein amino acid arginine (Arg), which has been shown previously to adopt a zwitterionic structure in ArgNa⁺ and ArgK⁺. This difference can be explained in part due to the decreased basicity of Cav (PA = 1001 kJ/mol) as compared to arginine (PA = 1051 kJ/mol), but not entirely, as lysine, which has nearly the same proton affinity as Cav, (~993 kJ/mol) forms only canonical structures with Na⁺, K⁺, and Cs⁺. A major difference between the zwitterionic forms of ArgM⁺ and CavM⁺ is that the protonation site is on the side chain for Arg and on the N-terminus for Cav. This results in systematically weaker salt bridges in the Cav zwitterions. In addition, the presence of another hydrogen-bonding acceptor atom in the side chain contributes to the stability of the canonical structures for the smaller alkali cations.

Evidence of metabolic microevolution of the limpet Nacella concinna to naturally high heavy metal levels in Antarctica

The gastropod Nacella concinna is the most conspicuous macroinvertebrate of the intertidal zone of the Antarctic Peninsula and adjacent islands. Naturally high levels of copper and cadmium in coastal marine ecosystems are accumulated in N. concinna tissues. We aimed to study the effects of metal cations on N. concinna arginase in the context of possible adaptive microevolution. Gills and muscle had the highest argininolytic activity, which was concentrated in the cytosol in both tissues. Gills had the highest levels of arginase and may be involved in the systemic control of l-arginine levels. The relatively high argininolytic activity of the N. concinna muscular foot, with K_M=25.3±3.4mmolL^-1, may be involved in the control of l-arginine levels during phosphagen breakdown. N. concinna arginases showed the following preferences for metal cations: Ni²⁺>Mn²⁺>Co²⁺>Cu²⁺ in muscle and Mn²⁺>Cu²⁺ in gills. Cu²⁺ activation is a unique characteristic of N. concinna arginases, as copper is a potent arginase inhibitor. Cu²⁺ partly neutralized N. concinna arginase inhibition by Cd²⁺, worked synergistically in muscle arginase activation by Co²⁺ and neutralized muscle arginase activation by Ni²⁺. Mn²⁺ was able to activate muscle arginase in the presence of Fe³⁺ and Pb²⁺. The selection of arginases that are activated by Cu²⁺ and resistant to inhibition by Cd²⁺ in the presence of Cu²⁺ over evolutionary timescales may have favored N. concinna occupation of copper- and cadmium-rich niches.

Arginase-1 deficiency

Arginase-1 (ARG1) deficiency is a rare autosomal recessive disorder that affects the liver-based urea cycle, leading to impaired ureagenesis. This genetic disorder is caused by 40+ mutations found fairly uniformly spread throughout the ARG1 gene, resulting in partial or complete loss of enzyme function, which catalyzes the hydrolysis of arginine to ornithine and urea. ARG1-deficient patients exhibit hyperargininemia with spastic paraparesis, progressive neurological and intellectual impairment, persistent growth retardation, and infrequent episodes of hyperammonemia, a clinical pattern that differs strikingly from other urea cycle disorders. This review briefly highlights the current understanding of the etiology and pathophysiology of ARG1 deficiency derived from clinical case reports and therapeutic strategies stretching over several decades and reports on several exciting new developments regarding the pathophysiology of the disorder using ARG1 global and inducible knockout mouse models. Gene transfer studies in these mice are revealing potential therapeutic options that can be exploited in the future. However, caution is advised in extrapolating results since the lethal disease phenotype in mice is much more severe than in humans indicating that the mouse models may not precisely recapitulate human disease etiology. Finally, some of the functions and implications of ARG1 in non-urea cycle activities are considered. Lingering questions and future areas to be addressed relating to the clinical manifestations of ARG1 deficiency in liver and brain are also presented. Hopefully, this review will spark invigorated research efforts that lead to treatments with better clinical outcomes.

Insight on the interaction of an agmatinase-like protein with Mn(2+) activator ions

Agmatinase is an enzyme that catalyzes the hydrolysis of agmatine, a compound that is associated with numerous functions in the brain of mammalian organisms such as neurotransmitter, anticonvulsant, antinociceptive, anxiolytic and antidepressant-like actions. To date the only characterized agmatinases with significant enzymatic activity were extracted from bacterial organisms. These agmatinases are closely related to another ureahydrolase, arginase; both have binuclear Mn(2+) centers in their active sites. An agmatinase-like protein (ALP) from rat brain was identified that bears no sequence homology to known agmatinases (E. Uribe, M. Salas, S. Enriquez, M.S. Orellana, N. Carvajal, Arch. Biochem. Biophys. 461(2007) 146-150). Since all known ureahydrolases contain histidines in their binuclear Mn(2+) site each of the five histidine residues in ALP was individually replaced by alanines to identify those that may be involved in metal ion binding. Reactivation assays and thermal stability measurements indicated that His206 is likely to interact with a Mn(2+) bound to a high affinity site. In contrast, His65 and possibly His435 are important for binding of a second Mn(2+) to a lower affinity site. Metal ion binding to that site is not only leading to an increase in reactivity but also enzyme stability. Thus, similar to bacterial agmatinases and some of the antibiotic-degrading, Zn(2+)-dependent metallo-β-lactamases ALP appears to be active in the mono and binuclear form, with binding of the second metal ion increasing both reactivity and stability.

Catalytic mechanisms of metallohydrolases containing two metal ions

At least one-third of enzymes contain metal ions as cofactors necessary for a diverse range of catalytic activities. In the case of polymetallic enzymes (i.e., two or more metal ions involved in catalysis), the presence of two (or more) closely spaced metal ions gives an additional advantage in terms of (i) charge delocalisation, (ii) smaller activation barriers, (iii) the ability to bind larger substrates, (iv) enhanced electrostatic activation of substrates, and (v) decreased transition-state energies. Among this group of proteins, enzymes that catalyze the hydrolysis of ester and amide bonds form a very prominent family, the metallohydrolases. These enzymes are involved in a multitude of biological functions, and an increasing number of them gain attention for translational research in medicine and biotechnology. Their functional versatility and catalytic proficiency are largely due to the presence of metal ions in their active sites. In this chapter, we thus discuss and compare the reaction mechanisms of several closely related enzymes with a view to highlighting the functional diversity bestowed upon them by their metal ion cofactors.

Unique hepatic cytosolic arginase evolved independently in ureogenic freshwater air-breathing teleost, Heteropneustes fossilis

Hepatic cytosolic arginase (ARG I), an enzyme of the urea cycle operating in the liver of ureotelic animals, is reported to be present in an ammoniotelic freshwater air-breathing teleost, Heteropneustes fossilis which has ureogenic potential. Antibodies available against mammalian ARG I showed no cross reactivity with the H. fossilis ARG I. We purified unique ARG I from H. fossilis liver. Purified ARG I is a homotrimer with molecular mass 75 kDa and subunit molecular mass of 24 kDa. The pI value of the enzyme was 8.5. It showed maximum activity at pH 10.5 and 55°C. The Km of purified enzyme for L-arginine was 2.65±0.39 mM. L-ornithine and N(ω)-hydroxy-L-arginine showed inhibition of the ARG I activity, with Ki values 0.52±0.02mM and 0.08±0.006mM, respectively. Antibody raised against the purified fish liver ARG I showed exclusive specificity, and has no cross reactivity against fish liver ARG II and mammalian liver ARG I and ARG II. We found another isoform of arginase bound to the outer membrane of the mitochondria which was released by 150-200 mM KCl in the extraction medium. This isoform was immunologically different from the soluble cytosolic and mitochondrial arginase. The results of present study support that hepatic cytosolic arginase evolved in this ureogenic freshwater teleost, H. fossilis. Phylogenetic analysis confirms an independent evolution event that occurred much after the evolution of the cytosolic arginase of ureotelic vertebrates.

Expression, purification, and biochemical properties of arginase from Bacillus subtilis 168

The arginine-degrading and ornithine-producing enzymes arginase has been used to treat arginine-dependent cancers. This study was carried out to obtain the microbial arginase from Bacillus subtilis, one of major microorganisms found in fermented foods such as Cheonggukjang. The gene encoding arginase was isolated from B. subtilis 168 and cloned into E. coli expression plasmid pET32a. The enzyme activity was detected in the supernatant of the transformed and IPTG induced cell-extract. Arginase was purified for homogeneity from the supernatant by affinity chromatography. The specific activity of the purified arginase was 150 U/mg protein. SDS-PAGE analysis revealed the molecular size to be 49 kDa (Trix·Tag, 6×His·Tag added size). The optimum pH and temperature of the purified enzyme with arginine as the substrate were pH 8.4 and 45°C, respectively. The Km and Vmax values of arginine for the enzyme were 4.6 mM and 133.0 mM/min/mg protein respectively. These findings can contribute in the development of functional fermented foods such as Cheonggukjang with an enhanced level of ornithine and pharmaceutical products by providing the key enzyme in arginine-degradation and ornithine-production.

Transcriptional profiling of gastric epithelial cells infected with wild type or arginase-deficient Helicobacter pylori

Background

Helicobacter pylori causes acute and chronic gastric inflammation induced by proinflammatory cytokines and chemokines secreted by cells of the gastric mucosa, including gastric epithelial cells. Previous studies have demonstrated that the bacterial arginase, RocF, is involved in inhibiting T cell proliferation and CD3ζ expression, suggesting that arginase could be involved in a more general dampening of the immune response, perhaps by down-regulation of certain pro-inflammatory mediators.

Results

Global transcriptome analysis was performed on AGS gastric epithelial cells infected for 16 hours with a wild type Helicobacter pylori strain 26695, an arginase mutant (rocF-) or a rocF⁺ complemented strain. H. pylori infection triggered altered host gene expression in genes involved in cell movement, death/growth/proliferation, and cellular function and maintenance. While the wild type strain stimulates host inflammatory pathways, the rocF- mutant induced significantly more expression of IL-8. The results of the microarray were verified using real-time PCR, and the differential levels of protein expression were confirmed by ELISA and Bioplex analysis. MIP-1B was also significantly secreted by AGS cells after H. pylori rocF- mutant infection, as determined by Bioplex. Even though not explored in this manuscript, the impact that the results presented here may have on the development of gastritis, warrant further research to understand the underlying mechanisms of the relationship between H. pylori RocF and IL-8 induction.

Conclusions

We conclude that H. pylori arginase modulates multiple host signaling and metabolic pathways of infected gastric epithelial cells. Arginase may play a critical role in anti-inflammatory host responses that could contribute to the ability of H. pylori to establish chronic infections.

Expression, purification and characterization of arginase from Helicobacter pylori in its apo form

Arginase, a manganese-dependent enzyme that widely distributed in almost all creatures, is a urea cycle enzyme that catalyzes the hydrolysis of L-arginine to generate L-ornithine and urea. Compared with the well-studied arginases from animals and yeast, only a few eubacterial arginases have been characterized, such as those from H. pylori and B. anthracis. However, these enzymes used for arginase activity assay were all expressed with LB medium, as low concentration of Mn²⁺ was detectable in the medium, protein obtained were partially Mn²⁺ bonded, which may affect the results of arginase activity assay. In the present study, H. pylori arginase (RocF) was expressed in a Mn²⁺ and Co²⁺ free minimal medium, the resulting protein was purified through affinity and gel filtration chromatography and the apo-form of RocF was confirmed by flame photometry analysis. Gel filtration indicates that the enzyme exists as monomer in solution, which was unique as compared with homologous enzymes. Arginase activity assay revealed that apo-RocF had an acidic pH optimum of 6.4 and exhibited metal preference of Co²⁺>Ni²⁺>Mn²⁺. We also confirmed that heat-activation and reducing regents have significant impact on arginase activity of RocF, and inhibits S-(2-boronoethyl)-L-Cysteine (BEC) and Nω-hydroxy-nor-Arginine (nor-NOHA) inhibit the activity of RocF in a dose-dependent manner.

Structures of metal-substituted human histone deacetylase 8 provide mechanistic inferences on biological function

The metal-dependent histone deacetylases (HDACs) adopt an alpha/beta protein fold first identified in rat liver arginase. Despite insignificant overall amino acid sequence identity, these enzymes share a strictly conserved metal binding site with divergent metal specificity and stoichiometry. HDAC8, originally thought to be a Zn(2+)-metallohydrolase, exhibits increased activity with Co(2+) and Fe(2+) cofactors based on k(cat)/K(M) (Gantt, S. L., Gattis, S. G., and Fierke, C. A. (2006) Biochemistry 45, 6170-6178). Here, we report the first X-ray crystal structures of metallo-substituted HDAC8, Co(2+)-HDAC8, D101L Co(2+)-HDAC8, D101L Mn(2+)-HDAC8, and D101L Fe(2+)-HDAC8, each complexed with the inhibitor M344. Metal content of protein samples in solution is confirmed by inductively coupled plasma mass spectrometry. For the crystalline enzymes, peaks in Bijvoet difference Fourier maps calculated from X-ray diffraction data collected near the respective elemental absorption edges confirm metal substitution. Additional solution studies confirm incorporation of Cu(2+); Fe(3+) and Ni(2+) do not bind under conditions tested. The metal dependence of the substrate K(M) values and the K(i) values of hydroxamate inhibitors that chelate the active site metal are consistent with substrate-metal coordination in the precatalytic Michaelis complex that enhances catalysis. Additionally, although HDAC8 binds Zn(2+) nearly 10(6)-fold more tightly than Fe(2+), the affinities for both metal ions are comparable to the readily exchangeable metal concentrations estimated in living cells, suggesting that HDAC8 could bind either or both Fe(2+) or Zn(2+) in vivo.

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.

Mutational analysis of substrate recognition by human arginase type I − agmatinase activity of the N130D variant

Upon mutation of Asn130 to aspartate, the catalytic activity of human arginase I was reduced to approximately 17% of wild-type activity, the Km value for arginine was increased approximately 9-fold, and the kcat/Km value was reduced approximately 50-fold. The kinetic properties were much less affected by replacement of Asn130 with glutamine. In contrast with the wild-type and N130Q enzymes, the N130D variant was active not only on arginine but also on its decarboxylated derivative, agmatine. Moreover, it exhibited no preferential substrate specificity for arginine over agmatine (kcat/Km values of 2.48 x 10(3) M(-1) x s(-1) and 2.14 x 10(3) M(-1) x s(-1), respectively). After dialysis against EDTA and assay in the absence of added Mn2+, the N130D mutant enzyme was inactive, whereas about 50% full activity was expressed by the wild-type and N130Q variants. Mutations were not accompanied by changes in the tryptophan fluorescence properties, thermal stability or chromatographic behavior of the enzyme. An active site conformational change is proposed as an explanation for the altered substrate specificity and low catalytic efficiency of the N130D variant.

Proton Affinity of Canavanine and Canaline, Oxyanalogues of Arginine and Ornithine, from the Extended Kinetic Method

The absolute proton affinities of the nonprotein amino acids canavanine and canaline have been determined using the extended kinetic method in an electrospray ionization quadrupole ion trap instrument. Canavanine results from the substitution of an oxygen atom for the delta-CH2 group in the side chain of the protein amino acid arginine, whereas canaline results from a similar substitution at the delta-CH2 group in the side chain of ornithine. Absolute proton affinities of 1001+/-9 and 950+/-7 kJ/mol are obtained for canavanine and canaline, respectively. For canaline, this proton affinity is in excellent agreement with theoretical predictions obtained using the hybrid density functional theory method B3LYP/6-311++G**//B3LYP/6-31+G*. For canavanine, theory predicts a somewhat larger proton affinity of 1015 kJ/mol. Oxygen atom substitution in these nonprotein amino acids results in a decrease in their proton affinities of 40-50 kJ/mol compared to arginine and ornithine.

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.

Helicobacter pylori arginase inhibits T cell proliferation and reduces the expression of the TCR zeta-chain (CD3zeta)

Helicobacter pylori infects approximately half the human population. The outcomes of the infection range from gastritis to gastric cancer and appear to be associated with the immunity to H. pylori. Patients developing nonatrophic gastritis present a Th1 response without developing protective immunity, suggesting that this bacterium may have mechanisms to evade the immune response of the host. Several H. pylori proteins can impair macrophage and T cell function in vitro through mechanisms that are poorly understood. We tested the effect of H. pylori extracts and live H. pylori on Jurkat cells and freshly isolated human normal T lymphocytes to identify possible mechanisms by which the bacteria might impair T cell function. Jurkat cells or activated T lymphocytes cultured with an H. pylori sonicate had a reduced proliferation that was not caused by T cell apoptosis or impairment in the early T cell signaling events. Instead, both the H. pylori sonicate and live H. pylori induced a decreased expression of the CD3zeta-chain of the TCR. Coculture of live H. pylori with T cells demonstrated that the wild-type strain, but not the arginase mutant rocF(-), depleted L-arginine and caused a decrease in CD3zeta expression. Furthermore, arginase inhibitors reversed these events. These results suggest that H. pylori arginase is not only important for urea production, but may also impair T cell function during infection.

Molecular cloning of the gene for edeine B1 amidinohydrolase in addition to the agmatinase activity in Bacillus subtilis

A gene with a high-nucleotide sequence homology to the edeine B1 amidinohydrolase gene of Bacillus brevis was identified in the database of the Bacillus subtilis genome. The gene was isolated, expressed in Escherichia coli, and the gene product was analyzed with regard to the characteristics of its enzyme activity. A 32-kDa protein encoded by the ywhG gene showed a 69.8% amino acid sequence-homology to the edeine B1 amidinohydrolase of B. brevis. Among various guanidino-compounds, edeine B1 and agmatine were both efficiently hydrolyzed by the protein encoded by the ywhG gene, although edeine B1 was a more potent substrate than agmatine in this assay system. These data indicate that the protein encoded by the ywhG gene is an agmatinase that is essential for polyamine biosynthesis in B. subtilis.

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.

The proton affinity of proline analogs using the kinetic method with full entropy analysis

The proton affinity of proline analogs, L-azetidine-2-carboxylic acid (Aze), L-proline (Pro), and L-pipecolic acid (Pip), have been measured using the Armentrout modification of the extended kinetic method in a quadrupole ion trap instrument. Experimental values of 223.0 +/- 1.5, 224.9 +/- 1.6, and 225.6 +/- 1.6 kcal/mol have been determined for the 298K proton affinities of Aze, Pro, and Pip respectively. High level theoretical calculations using both MP2 and B3LYP methods at a variety of basis sets were carried out in order to give theoretical predictions for the 298 K proton affinity and gas phase basicity of all three analogs. Recommended values for the gas phase basicity and proton affinity for proline based on our work and other recent determinations are 216 +/- 2 and 224 +/- 2 kcal/mol.

Chemical modification and site-directed mutagenesis of human liver arginase: evidence that the imidazole group of histidine-141 is not involved in substrate binding

Native and wild-type recombinant human liver arginases (EC 3.5.3.1) were photoinactivated by Rose bengal, and protection was afforded by the competitive inhibitor l-lysine. The dissociation constant for the enzyme-protector complex was essentially equal to the corresponding K(i) value. Upon mutation of His141 by phenylalanine, the enzyme activity was reduced to 6-10% of wild-type activity, with no changes in K(m) for arginine or K(i) for l-lysine or l-ornithine. The subunit composition of active enzyme was not altered by mutation, but the mutant H141F was markedly more sensitive to trypsin inactivation and completely insensitive to inactivation by diethyl pyrocarbonate (DEPC) and photoinactivation. Species with histidine groups blocked with DEPC were also insensitive to photoinactivation. We conclude that His141, which is the target for both inactivating procedures, is not involved in substrate binding, but plays a critical, albeit not essential role in the hydrolysis of enzyme-bound substrate.

Manganese-dependent inhibition of human liver arginase by borate

Full activation of human liver arginase (EC 3.5.3.1), by incubation with 5 mM Mn2+ for 10 min at 60 degrees C, resulted in increased Vmax and a higher sensitivity of the enzyme to borate inhibition, with no change in the K(m) for arginine. Borate behaved as an S-hyperbolic I-hyperbolic non-competitive inhibitor and had no effect on the interaction of the enzyme with the competitive inhibitors L-ornithine (Ki = 2 +/- 0.5 mM), L-lysine (Ki = 2.5 +/- 0.4 mM), and guanidinium chloride (Ki = 100 +/- 10 mM). The pH dependence of the inhibition was consistent with tetrahedral B(OH)4- being the inhibitor, rather than trigonal B(OH)3. We suggest that arginase activity is associated with a tightly bound Mn2+ whose catalytic action may be stimulated by addition of a more loosely bound Mn2+, to generate a fully activated enzyme form. The Mn2+ dependence and partial character of borate inhibition are explained by assuming that borate binds in close proximity to the loosely bound Mn2+ and interferes with its stimulatory action. Although borate protects against inactivation of the enzyme by diethyl pyrocarbonate (DEPC), the DEPC-sensitive residue is not considered as a ligand for borate binding, since chemically modified species, which retain about 10% of enzymatic activity, were also sensitive to the inhibitor.

Evidence that histidine-163 is critical for catalytic activity, but not for substrate binding to Escherichia coli agmatinase

Agmatinase (agmatine ureohydrolase, EC 3.5.3.11) from Escherichia coli was inactivated by diethyl pyrocarbonate (DEPC) and illumination in the presence of Rose bengal. Protection against photoinactivation was afforded by the product putrescine, and the dissociation constant of the enzyme-protector complex (12 mM) was essentially equal to the K(i) value for this compound acting as a competitive inhibitor of agmatine hydrolysis. Upon mutation of His163 by phenylalanine, the agmatinase activity was reduced to 3-5% of wild-type activity, without any change in K(m) for agmatine or K(i) for putrescine inhibition. The mutant was insensitive to DEPC and dye-sensitized inactivations. We conclude that His163 plays an important role in the catalytic function of agmatinase, but it is not directly involved in substrate binding.

Manganese is essential for catalytic activity of Escherichia coli agmatinase

Purified Escherichia coli agmatinase (EC 3.5.3.11) expressed the same activity in the absence or presence of added Mn2+ (0-5mM). However, it was strongly inhibited by Co2+, Ni2+, and Zn2+ and almost half inactivated by EDTA. Partial inactivation by EDTA yielded enzyme species containing 0.85 +/- 0.1 Mn2+/subunit, and it was accompanied by a decrease in intensity of fluorescence emission and a red shift from the emission maximum of 340 nm to 346 nm, indicating the movement of tryptophane residues to a more polar environment. The activity and fluorescence properties of fully activated agmatinase were restored by incubation of dialysed species with Mn2+. Manganese-free species, obtained by treatment with EDTA and guanidinium chloride (3 M), were active only in the presence of added Mn2+. Results obtained, which represent the first demonstration of the essentiality of Mn2+ for agmatinase activity, are discussed in connection with a possible binuclear metal center in the enzyme.

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.