Steady-state kinetic studies of arginase with an improved direct spectrophotometric assay

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.

Unusual hepatic mitochondrial arginase in an Indian air-breathing teleost, Heteropneustes fossilis: purification and characterization

A functional urea cycle with both cytosolic (ARG I) and mitochondrial (ARG II) arginase activity is present in the liver of an ureogenic air-breathing teleost, Heteropneustes fossilis. Antibodies against mammalian ARG II showed no cross-reactivity with the H. fossilis ARG II. ARG II was purified to homogeneity from H. fossilis liver. Purified ARG II showed a native molecular mass of 96 kDa. SDS-PAGE showed a major band at 48 kDa. The native enzyme, therefore, appears to be a homodimer. The pI value of the enzyme was 7.5. The purified enzyme showed maximum activity at pH 10.5 and 55 °C. The K(m) of purified ARG II for l-arginine was 5.25±1.12 mM. L-Ornithine and N(ω)-hydroxy-L-arginine showed mixed inhibition with K(i) values 2.16±0.08 and 0.02±0.004 mM respectively. Mn(+2) and Co(+2) were effective activators of arginase activity. Antibody raised against purified H. fossilis ARG II did not cross-react with fish ARG I, and mammalian ARG I and ARG II. Western blot with the antibodies against purified H. fossilis hepatic ARG II showed cross reactivity with a 96 kDa band on native PAGE and a 48 kDa band on SDS-PAGE. The molecular, immunological and kinetic properties suggest uniqueness of the hepatic mitochondrial ARG II in H. fossilis.

Polyamine metabolism in Leishmania: from arginine to trypanothione

Polyamines (PAs) are essential metabolites in eukaryotes, participating in a variety of proliferative processes, and in trypanosomatid protozoa play an additional role in the synthesis of the critical thiol trypanothione. The PAs are synthesized by a metabolic process which involves arginase (ARG), which catalyzes the enzymatic hydrolysis of L-arginine (L-Arg) to L-ornithine and urea, and ornithine decarboxylase (ODC), which catalyzes the enzymatic decarboxylation of L-ornithine in putrescine. The S-adenosylmethionine decarboxylase (AdoMetDC) catalyzes the irreversible decarboxylation of S-adenosylmethionine (AdoMet), generating the decarboxylated S-adenosylmethionine (dAdoMet), which is a substrate, together with putrescine, for spermidine synthase (SpdS). Leishmania parasites and all the other members of the trypanosomatid family depend on spermidine for growth and survival. They can synthesize PAs and polyamine precursors, and also scavenge them from the microenvironment, using specific transporters. In addition, Trypanosomatids have a unique thiol-based metabolism, in which trypanothione (N1-N8-bis(glutathionyl)spermidine, T(SH)(2)) and trypanothione reductase (TR) replace many of the antioxidant and metabolic functions of the glutathione/glutathione reductase (GR) and thioredoxin/thioredoxin reductase (TrxR) systems present in the host. Trypanothione synthetase (TryS) and TR are necessary for the protozoa survival. Consequently, enzymes involved in spermidine synthesis and its utilization, i.e. ARG, ODC, AdoMetDC, SpdS and, in particular, TryS and TR, are promising targets for drug development.

Purification, properties and alternate substrate specificities of arginase from two different sources: Vigna catjang cotyledon and buffalo liver

Arginase was purified from Vigna catjang cotyledons and buffalo liver by chromatographic separations using Bio-Gel P-150, DEAE-cellulose and arginine AH Sepharose 4B affinity columns. The native molecular weight of an enzyme estimated on Bio-Gel P-300 column for Vigna catjang was 210 kDa and 120 kDa of buffalo liver, while SDS-PAGE showed a single band of molecular weight 52 kDa for cotyledon and 43 kDa for buffalo liver arginase. The kinetic properties determined for the purified cotyledon and liver arginase showed an optimum pH of 10.0 and pH 9.2 respectively. Optimal cofactor Mn(++) ion concentration was found to be 0.6 mM for cotyledon and 2 mM for liver arginase. The Michaelis-Menten constant for cotyledon arginase and hepatic arginase were found to be 42 mM and 2 mM respectively. The activity of guanidino compounds as alternate substrates for Vigna catjang cotyledon and buffalo liver arginase is critically dependent on the length of the amino acid side chain and the number of carbon atoms. In addition to L-arginine cotyledon arginase showed substrate specificity towards agmatine and L-canavanine, whereas the liver arginase showed substrate specificity towards only L-canavanine.

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.

Catalysis by metal-activated hydroxide in zinc and manganese metalloenzymes

The metal-activated hydroxide ion is a critical nucleophile in metalloenzymes that catalyze hydrolysis or hydration reactions. The most common metal used is zinc; occasionally, other transition metals such as manganese are required. Human carbonic anhydrase II and rat liver arginase serve as well-studied paradigms of zinc and manganese metalloenzymes, respectively. Comparative structure-function relationships between these two metalloenzymes highlight parallels in the chemistry of metal-activated hydroxide: (a) the protein environment of metal-bound hydroxide modulates its reactivity; (b) a hydrogen bond with metal-bound hydroxide holds it in the proper orientation for catalysis; (c) nonmetal substrate-binding sites are implicated in both enzyme mechanisms; and (d) regeneration of metal-bound hydroxide ion from a metal-bound water molecule requires proton transfer to bulk solvent mediated by a histidine proton shuttle residue. Interestingly, the electrostatics of catalysis differ between the two enzymes, in that the first step of catalysis requires formation of a negatively charged transition state in the carbonic anhydrase II mechanism, whereas a neutral transition state is approached in the arginase mechanism. This electrostatic feature may contribute to the differences in the chemistry, the metal binding sites, and the metal specificity between these two enzymes.

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.

Determination of arginase activity in macrophages: a micromethod

We propose a modification of Schimke's method for urea determination as a valuable micromethod for measuring arginase in activated macrophages. The method exhibits the following advantages: (a) it uses small amounts of samples (approximately 25,000 macrophages per assay); (b) it does not interfere with other related metabolites that are also present in the activated macrophage such as citrulline or arginine; (c) saturating concentrations of the substrate arginine can be used; and (d) it is much more sensitive than Schimke's method and can detect small amounts of urea, in the order of 0.02 mumol.

A novel colorimetric method for assaying arginase activity

Arginase catalyzes the conversion of arginine to urea and ornithine in the liver of ureotelic animals. Higher activity of this enzyme is found in tumors as well as in the sera of patients with hepatic diseases. We have developed a simple colorimetric method for its determination. This is based on the determination of residual arginine, after its conversion with p-nitrophenyl glyoxal (PNPG) at pH 9.0 in the presence of sodium ascorbate. The reaction product obeys Beer's law in the range of 0.01-0.20 mmol/L arginine with an arginine-equivalent molar extinction coefficient of 0.65 x 10(4) M-1 cm-1. The decrease in absorbance in the presence of arginase correlates with the enzyme activity. Color development as well as termination of enzyme activity is achieved by addition of a single reagent, thereby obviating the use of many chemicals necessary in other methods. The sensitivity of this method is equivalent to those of currently available procedures but has the added advantages of greater convenience.

Arginase distribution in tissues of domestic animals

1. A new colorimetric method was used for determination of arginase in different tissues of some domestic animals. 2. In all species studied liver was the richest source of arginase. 3. Significant differences were observed in the specific activity of arginase in livers from different species. 4. In all species, besides liver, kidney and brain also contained significant levels of arginase. 5. In the dog, in addition to the three organs mentioned above, lung, heart, spleen and skeletal muscle showed some arginase activity. 6. In sheep and cattle significant arginase activity was observed in the rumen. No differences were observed between epithelial and muscular layers of different parts of digestive system in all species studied. 7. These results are discussed in terms of the possible role of arginase in different tissues of animals.

Characterisation of arginase from the extreme thermophile 'Bacillus caldovelox'

A thermostable arginase (L-arginine amidinohydrolase, EC 3.5.3.1) was purified from the extreme thermophile 'Bacillus caldovelox' (DSM 411) by a procedure including DEAE-Sepharose chromatography, and gel filtration, anion exchange and hydrophobic-interaction fast-protein liquid chromatography, with substantial retention of the metal ion cofactor. The purified enzyme is a hexamer with a subunit Mr of 31,000 +/- 2000 and contains greater than or equal to 1 Mn atom per subunit. Maximum activation on incubation with Mn2+ is 29%. Activity is optimal at pH 9 and at 60 degrees C the Km for arginine is 3.4 mM and Ki(ornithine) is 0.55 mM. Incubation in 0.1 M Mops/NaOH buffer (pH 7) causes rapid inactivation at 60 degrees C (t1/2 (half life) = 4.5 min) and individually 0.1 mM Mn2+ or 1 mg/ml BSA (bovine serum albumin) increase the t1/2 of arginase activity 4-fold, but combined they produce greater than 1000-fold increase and a t1/2 = 105 min at 95 degrees C. Aspartic acid and other species that bind Mn2+ can replace BSA, and it is suggested that arginase can be inactivated by free Mn2+. A strong chelating agent causes inactivation without subunit dissociation, but arginase dissociates rapidly at pH 2.5. Reassociation occurs at pH 9 and is unusual in that it does not require Mn2+.

pH-sensitive control of arginase by Mn(II) ions at submicromolar concentrations

The manganese dependence of arginase was reinvestigated with extracts of mouse liver to see whether more physiological properties were displayed than have been reported for the purified enzyme. In a preincubation with Mn(II) ions at 37 degrees C the enzyme underwent a slow and reversible activation. At least 90-95% of the activation achieved was dependent on Mn2+. However, no Mn2+ was required for catalytic activity in the assay. The activation showed little dependence upon pH over the range 6.5-9.5, whereas the catalytic activity increased 12-fold in apparent accord with the titration curve of an ionizable group of pKa 7.9. The Mn2+ dependence of arginase activation obeyed Michaelis-Menten kinetics, with Kd varying from 0.3 microM at pH 6.8 to 0.08 microns at pH 7.7. Free Mn2+ concentrations were established in these assays with a trimethylenediaminetetraacetate-Mn buffer. Vmax increased about three-fold over this range. The calculated arginase activity at 0.05 microM Mn2+ increases about nine-fold over this physiological pH range. An enzyme model is proposed to explain these findings. The activity of arginase at "physiological" [Mn2+] and the pronounced pH dependence conferred upon it are consistent with a recently revised role for the urea cycle in the control of bicarbonate and pH in the body. It appears possible that arginase loses Mn2+ sensitivity during the usual purification.

Assay and kinetics of arginase

A sensitive colorimetric assay for arginase was developed. Urea produced by arginase was hydrolyzed to ammonia by urease, the ammonia was converted to indophenol, and the absorbance was measured at 570 nm. The assay is useful with low concentrations of arginase (0.5 munit or less than 1 ng rat liver arginase) and with a wide range of arginine concentrations (50 microM to 12.5 mM). Michaelis-Menten kinetics and a Km for arginine of 1.7 mM were obtained for Mn2+-activated rat liver arginase; the unactivated enzyme did not display linear behavior on double-reciprocal plots. The kinetic data for unactivated arginase indicated either negative cooperativity or two types of active sites on the arginase tetramer with different affinities for arginine. The new assay is particularly well suited for kinetic studies of activated and unactivated arginase.