Possible involvement of manganese in the catalytic mechanism of bovine liver arginase

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.

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.

Hormonal and nutritional modulation of hepatic arginase activity in growing cattle

The hormonal and nutritional modulation of hepatic arginase activity (HARG) was characterized in growing cattle in two studies. In the first study, 20 steers (initial weight, 182 +/- 2 kg) were assigned in equal numbers to either Synovex-S (SYN) (ear implant), recombinant bovine somatotropin (Somavubove; SbV; 0.1 mg/kg intramuscularly daily), SYN + SbV, or nothing (control). Steers were individually fed, for 56 d, a concentrate (80% diet dry matter [DM]) and silage (20% diet DM) diet providing 20 g of crude protein (CP) and 252 kcal metabolizable energy (ME) per kg body weight0.75. On Day 57, samples of liver were obtained at slaughter and subsequently assayed for HARG by the incubation of a tissue homogenate for 2 hr with 250 mM arginine, with and without Mn2+ and heat activation, and the measurement of the resulting urea. HARG was uniformly increased by divalent cation (Mn2+) and heating. SYN had no effect on HARG, whereas SbV treatment resulted in an overall 34% decrease in HARG. Plasma urea nitrogen (PUN) was decreased by SbV but not consistently affected by SYN. In the second study, 16 steers (avg. initial weight, 284 +/- 5 kg) were initially fed a concentrate basal diet consisting of 11% CP and 1.96 Mcal ME for 21 d. Steers were then assigned to one of four dietary treatments (6.4 kg DM/hr per day) in a factorial arrangement of high and low CP (8 and 14%) and two levels of energy (1.96 and 2.67 Mcal ME/kg of diet DM) for 210 d and slaughtered. HARG and PUN were higher in steers fed 14% CP but were lower at each level of CP fed at the higher level of ME. The data suggest that hormonal repartitioning compounds and diet composition may modulate nitrogen metabolism by affecting the activity of arginase in the liver as well as by affecting the total content of arginase in association with changes in organ size.

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.

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 and inhibition by some aminoacids of lactating rat mammary gland arginase

Some kinetic and regulatory properties of lactating rat mammary gland arginase were studied. At pH 7.4, i.e. at near-physiological conditions, there was evidence of inhibition by excess of substrate, with a Km value of 9.5 mM, slightly lower than the value of 18 mM observed at pH 9.8 (maximum enzyme activity). A study was also made of the effects of proline, ornithine, lysine and certain branched-chain aminoacids on enzyme activity: lactating rat mammary gland arginase was strongly and competitively inhibited by lysine, ornithine and valine, with Ki values of 1.2 mM, 1.1 mM and 3.6 mM, respectively. Other aminoacids (proline, isoleucine and leucine) also inhibited lactating rat mammary gland arginase, although to a lesser extent.

An enzymatic route to L-ornithine from arginine--activation, selectivity and stabilization of L-arginase

The non-proteinogenic amino acid L-ornithine (L-Orn) can be conveniently obtained by enzymatic hydrolysis of arginine (Arg) with arginase (EC 3.5.3.1). Arginase from calf liver (Vmax = 459 mumol/(min.mg), Km = 25.5 mM) is inhibited competitively by L-Orn (Ki = 480 mM). The enzyme was found to be completely enantioselective (E-value > 100) so that D,L-Arg can be split into D-Arg and L-Orn. Operational stability at 25 degrees C (deactivation rate constant kdeact = 3.8 x 10(-3) h-1; tau 1/2 = 182 h) is sufficient for use in a continuous process but is significantly smaller than temperature stability (kdeact = 4.1 x 10(-4) h-1; tau 1/2 = 1682 h); mechanical stress through stirring and unsteady Mn2+ supply owing to oxidation in the continuous process are believed to cause the difference. Addition of ascorbic acid stabilizes calf liver arginase at temperatures higher than 25 degrees C (at 60 degrees C, delta delta G not equal to = 2.9 kJ/mol).

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.