Multi-step analysis of Hg2+ ion inhibition of jack bean urease

Biochemical and thermodynamic properties of de novo synthesized urease from Vicia sativa seeds with enhanced industrial applications

Urease is one of the most significant enzymes in the industry. The objective of this research was to isolate and partially purify urease from Vicia sativa seeds with urease characterization. With a 6.4 % yield, the purification fold was 9.0. By using chromatography, it was determined that the isolated urease had a molecular weight of 55 kDa. The maximum urease activity was found following a 60-s incubation period at 40 °C and pH 8. The activity of urease was significantly boosted by a mean of calcium, barium, DL-dithiothreitol, Na2EDTA, and citrate (16.9, 26.6, 18.6, 13.6, and 31 %), respectively. But nickel and mercury caused inhibitory effects and completely inhibited urease activity, indicating the presence of a thiol (-SH) group in the enzyme active site. The Arrhenius plot was used to analyze the thermodynamic constants of activation, Ea, ΔH*, ΔG*, and ΔS*. The results showed that the values were 30 kJ/mol, 93.14 kJ/mol, 107.17 kJ/mol/K, and -40.80 J/mol/K, respectively. The significance of urease extraction from various sources may contribute to our understanding of the metabolism of urea in plants. The current report has novelty as it explained for the first time the kinetics and thermodynamics of hydrolysis of urea and inactivation of urease from V. sativa seeds.

Characterization of the mercury-binding proteins in tuna and salmon sashimi: Implications for health risk of mercury in food

Fish consumption is one of the major ways through which humans receive exposure to mercury (Hg). The existing forms of Hg in food, particularly Hg bound to proteins, may affect the absorption of Hg by humans and subsequently its potentially toxic effects. However, the knowledge regarding Hg-binding proteins in edible fish muscle is scarce. In the present study, salmon and tuna fish muscles, collected from seven different regions and countries, were analyzed using metallomics- and proteomics-based techniques. The concentration of Hg in sashimi samples ranged from 4.4 to 317.4 ng/g. Size exclusion chromatography (SEC) coupled with inductively coupled plasma mass spectrometer (ICP-MS) showed that beta-actin was a novel Hg-binding protein from the fish muscles, and this protein could also bind bismuth (Bi), silver (Ag), and copper (Cu). Hg bound to beta-actin accounted for approximately 30.2-37.6% of the total Hg in the tuna muscles and was significantly correlated to total Hg in the fish muscles (r = 0.98, p < 0.01) and in the fraction of soluble proteins (r = 0.94, p < 0.01). These findings suggest that proteins act as the main Hg accumulation sites in edible fish; thus, increasing human exposure to Hg following gastrointestinal digestion.

Self-Referenced Ratiometric Detection of Sulfatase Activity with Dual-Emissive Urease-Encapsulated Gold Nanoclusters

In this study, on the basis of the biomineralization capability of urease, a facile, one-step, and green synthetic method has been proposed for the fabrication of gold nanoclusters (AuNCs). The prepared urease-encapsulated AuNCs (U-AuNCs) exhibited strong red fluorescence emission (λ_em = 630 nm) with a quantum yield as high as 17%. Interestingly, at a low concentration, the U-AuNC solution was found to be a dual-emissive system with the blue emission of the dityrosine (diTyr) residues of urease and the red emission of the embedded AuNCs. Further experiments demonstrated that p-nitrophenol (PNP) can selectively suppress the 410 nm emission of the diTyr residues of U-AuNCs without affecting the red emission of the U-AuNCs. The fluorescence quenching mechanism between U-AuNCs and PNP was systematically studied, and the leading role of the inner filter effect (IFE) was identified. Additionally, based on the sulfatase-catalyzed hydrolysis of p-nitrophenyl sulfate (PNPS) to release PNP, a self-referenced ratiometric detection method for sulfatase, which plays a crucial role in sulfur cycling, degradation of sulfated glycosaminoglycans and glycolipids, and extracellular remodeling of sulfated glycosaminoglycans, was developed by using dual-emissive U-AuNCs as the signal readout, in which the diTyr residues served as the probe and the AuNCs functioned as the internal reference. This IFE-based ratiometric sensing strategy showed a good linear relationship over the range of 0.01-1 U/mL ( R² = 0.997). The detection limit for sulfatase activity was 0.01 U/mL. The developed protocol was successfully used to detect sulfatase activity in human serum samples. The simplicity, rapidity, low cost, high credibility, good reproducibility, and excellent selectivity of the detection platform serve as an inspiration for further applications of fluorescent AuNCs in chemo/biosensing.

Extracellular urease from Arthrobacter creatinolyticus MTCC 5604: scale up, purification and its cytotoxic effect thereof

Urease is a potent metalloenzyme with diverse applications. This paper describes the scale up and purification of an extracellular urease from Arthrobacter creatinolyticus MTCC 5604. The urease production was scaled-up in 3.7 L and 20 L fermentor. A maximum activity of 27 and 27.8 U/mL and a productivity of 0.90 and 0.99 U/mL/h were obtained at 30 h and 28 h in 3.7 and 20 L fermentor, respectively. Urease was purified to homogeneity with 49.85-fold purification by gel filtration and anion exchange chromatography with a yield of 36% and a specific activity of 1044.37 U/mg protein. The enzyme showed three protein bands with molecular mass of 72.6, 11.2 and 6.1 kDa on SDS-PAGE and ~ 270 kDa on native PAGE. The cytotoxic effect of urease was assessed in vitro using cancer cell lines (A549 and MG-63) and normal cell line (HEK 293). Urease showed its inhibitory effects on cancer cell lines through the generation of toxic ammonia, which in turn increased the pH of the surrounding medium. This increase in extracellular pH, enhanced the cytotoxic effect of weak base chemotherapeutic drugs, doxorubicin (50 µM) and vinblastine (100 µM) in the presence of urease (5 U/mL) and urea (0-4 mM) significantly.

Interfacing enzymes with silicon nanocrystals through the thiol-ene reaction

This study reports the preparation of functional bioinorganic hybrids, through application of the thiol-ene reaction, that exhibit catalytic activity and photoluminescent properties from enzymes and freestanding silicon nanocrystals. Thermal hydrosilylation of 1,7-octadiene and alkene-terminated poly(ethylene oxide)methyl ether with hydride-terminated silicon nanocrystals afforded nanocrystals functionalized with alkene residues and poly(ethylene oxide) moieties. These silicon nanocrystals were conjugated with representative enzymes through the photochemical thiol-ene reaction to afford bioinorganic hybrids that are dispersible and photostable in buffer, and that exhibit photoluminescence (λ_max = 630 nm) and catalytic activity. They were characterized using Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), dynamic light scattering analysis (DLS), absorption spectroscopy, steady-state and time-resolved photoluminescence spectroscopy, and pertinent enzyme activity assays. The general derivatization approach presented for interfacing enzymes with biocompatible silicon nanocrystals has far reaching implications for many applications ranging from sensors to therapeutic agents. The bioinorganic hybrids presented herein have potential applications in the chemical detection of nitrophenyl esters and urea. They can also be employed in enzyme-based theranostics as they combine long-lived silicon nanocrystal photoluminescence with substrate-specific enzymatic activity.

Communication between hydrogel beads via chemical signalling

In this work, we demonstrate chemical communication between millimetre-sized soft hydrogel beads in an aqueous environment. Silver cations (Ag⁺) and the Ag⁺ chelator dithiothreitol (DTT) are used as signalling molecules. By exploiting their interplay, we conduct a series of 'conversations' between millimetre-sized beads. The communication process is monitored by tracking the response and behaviour of a central bead. This bead is loaded with the enzyme urease and has the ability to undergo a change in colour associated with a change in pH. Competitive communication between three beads, whereby the central bead receives two competing signals from two senders, is shown. We believe that our hydrogel-based system demonstrates an advance in the communication capabilities of small soft matter objects.

Biological Evaluation and Molecular Docking of Protocatechuic Acid from Hibiscus sabdariffa L. as a Potent Urease Inhibitor by an ESI-MS Based Method

Studies on enzyme inhibition remain a crucial area in drug discovery since these studies have led to the discoveries of new lead compounds useful in the treatment of several diseases. In this study, protocatechuic acid (PCA), an active compound from Hibiscus sabdariffa L. has been evaluated for its inhibitory properties against jack bean urease (JBU) as well as its possible toxic effect on human gastric epithelial cells (GES-1). Anti-urease activity was evaluated by an Electrospray Ionization-Mass Spectrometry (ESI-MS) based method, while cytotoxicity was assayed by the MTT method. PCA exerted notable anti-JBU activity compared with that of acetohydroxamic acid (AHA), with IC50 values of 1.7 and 3.2 µM, respectively. PCA did not show any significant cytotoxic effect on (GES-1) cells at concentrations ranging from 1.12 to 3.12 µM. Molecular docking study revealed high spontaneous binding ability of PCA to the active site of urease. Additionally, the anti-urease activity was found to be related to the presence of hydroxyl moieties of PCA. This study presents PCA as a natural urease inhibitor, which could be used safely in the treatment of diseases caused by urease-producing bacteria.

Epiberberine, a natural protoberberine alkaloid, inhibits urease of Helicobacter pylori and jack bean: Susceptibility and mechanism

In our previous study, Rhizoma Coptidis extract was found to exert more potent inhibitory effect than its major component berberine towards urease from Helicobacter pylori (HPU) and jack bean (JBU). In continuation of our work, the present study was designed to further comparatively investigate the urease inhibitory activities of five major protoberberine alkaloids in Rhizoma Coptidis, namely berberine, palmatine, coptisine, epiberberine, jateorhizine to identify the bioactive constituent, and illuminate the potential mechanism of action. Results indicated that the five protoberberine alkaloids acted as concentration-dependent inactivators of urease with IC₅₀ values ranging between 3.0 and 5087μM for HPU and 2.3->10,000μM for JBU, respectively. Notably, epiberberine (EB) was found to be the most potent inhibitor against both ureases with IC₅₀ values of 3.0±0.01μM for HPU and 2.3±0.01μM for JBU, which was more effective than the standard urease inhibitor, acetohydroxamic acid (83±0.01μM for HPU and 22±0.01μM for JBU, respectively). Further kinetic analysis revealed that the type of EB inhibition against HPU was slow-binding and uncompetitive, with K_i of 10.6±0.01μM, while slow-binding and competitive against JBU with K_i of 4.6±0.01μM. Addition of thiol reagents, such as l-cysteine, glutathione and dithiothreitol, significantly abolished the inhibition, while Ni²⁺ competitive inhibitors, boric acid and sodium fluoride, synergetically inhibited urease with EB, indicating the obligatory role of the active site sulfhydryl group for the inhibition. In addition, binding of EB with the urease proved to be reversible, as about 65% and 90% enzymatic activity of HPU and JBU, respectively, could be restored by dithiothreitol application. These findings highlighted the potential role of Rhizoma Coptidis protoberberine alkaloids, especially EB, as a lead urease inhibitor in the treatment of diseases associated with ureolytic bacteria. Thus, EB had good potential for further development into a promising therapeutic approach for the treatment of urease-related diseases.

Motion Control of Urea-Powered Biocompatible Hollow Microcapsules

The quest for biocompatible microswimmers powered by compatible fuel and with full motion control over their self-propulsion is a long-standing challenge in the field of active matter and microrobotics. Here, we present an active hybrid microcapsule motor based on Janus hollow mesoporous silica microparticles powered by the biocatalytic decomposition of urea at physiological concentrations. The directional self-propelled motion lasts longer than 10 min with an average velocity of up to 5 body lengths per second. Additionally, we control the velocity of the micromotor by chemically inhibiting and reactivating the enzymatic activity of urease. The incorporation of magnetic material within the Janus structure provides remote magnetic control on the movement direction. Furthermore, the mesoporous/hollow structure can load both small molecules and larger particles up to hundreds of nanometers, making the hybrid micromotor an active and controllable drug delivery microsystem.

Skeletal muscle glycogen phosphorylase is irreversibly inhibited by mercury: molecular, cellular and kinetic aspects

Muscle glycogen phosphorylase (GP) plays an important role in muscle functions. Mercury has toxic effects in skeletal muscle leading to muscle weakness or cramps. However, the mechanisms underlying these toxic effects are poorly understood. We report that GP is irreversibly inhibited by inorganic (Hg(2+)) and organic (CH3Hg(+)) mercury (IC50=380 nM and kinact=600 M(-1) s(-1) for Hg(2+) and IC50=43 μM and kinact=13 M(-1) s(-1) for CH3Hg(+)) through reaction of these compounds with cysteine residues of the enzyme. Our data suggest that the irreversible inhibition of GP could represent one of the mechanisms that contribute to mercury-dependent muscle toxicity.

Thermodynamic Study of Human Serum Albumin upon Interaction with Ytterbium (III)

Complexation reaction between Yb3+and human serum albumin is examined using isothermal titration calorimetry (ITC). The extension solvation theory was used to reproduce the enthalpies of HAS + Yb3+interactions over the whole range of Yb3+concentrations. The binding parameters recovered from this model were attributed to the structural change of HSA. The results show that Yb3+ions bind to HSA with three equivalent affinity sites. It was found that in the high concentrations of the ytterbium ions, the HSA structure was destabilized.

Kinetics and mechanism of jack bean urease inhibition by Hg2+

BACKGROUND

Jack bean urease (EC 3.5.1.5) is a metalloenzyme, which catalyzes the hydrolysis of urea to produce ammonia and carbon dioxide. The heavy metal ions are common inhibitors to control the rate of the enzymatic urea hydrolysis, which take the Hg2+ as the representative. Hg2+ affects the enzyme activity causing loss of the biological function of the enzyme, which threatens the survival of many microorganism and plants. However, inhibitory kinetics of urease by the low concentration Hg2+ has not been explored fully. In this study, the inhibitory effect of the low concentration Hg2+ on jack bean urease was investigated in order to elucidate the mechanism of Hg2+ inhibition.

RESULTS

According to the kinetic parameters for the enzyme obtained from Lineweaver-Burk plot, it is shown that the Km is equal to 4.6±0.3 mM and Vm is equal to 29.8±1.7 μmol NH3/min mg. The results show that the inhibition of jack bean urease by Hg2+ at low concentration is a reversible reaction. Equilibrium constants have been determined for Hg2+ binding with the enzyme or the enzyme-substrate complexes (Ki =0.012 μM). The results show that the Hg2+ is a noncompetitive inhibitor. In addition, the kinetics of enzyme inhibition by the low concentration Hg2+ has been studied using the kinetic method of the substrate reaction. The results suggest that the enzyme first reversibly and quickly binds Hg2+ and then undergoes a slow reversible course to inactivation. Furthermore, the rate constant of the forward reactions (k+0) is much larger than the rate constant of the reverse reactions (k-0). By combining with the fact that the enzyme activity is almost completely lost at high concentration, the enzyme is completely inactivated when the Hg2+ concentration is high enough.

CONCLUSIONS

These results suggest that Hg2+ has great impacts on the urease activity and the established inhibition kinetics model is suitable.

Enzymatic detection of As(III) in aqueous solution using alginate immobilized pumpkin urease: optimization of process variables by response surface methodology

Urease immobilized on alginate was utilized to detect and quantify As(3+) in aqueous solution. Urease from the seeds of pumpkin (vegetable waste) was purified to apparent homogeneity by heat treatment and gel filtration (Sephadex G-200). Further enzyme was entrapped in 3.5% alginate beads. Urea hydrolysis by enzyme revealed a clear dependence on the concentration and interaction time of As(3+). The process variables effecting the quantitation of As(3+) was investigated using central composite design with Minitab 15 software. The predicted results were found in good agreement (R(2)=96.71%) with experimental results indicating the applicability of proposed model. The multiple regression analysis and ANOVA showed that enzyme activity decreased with increase of As(3+) concentration and interaction time. 3D plot and contour plot between As(3+) concentration and interaction time was helpful to predict residual activity of enzyme for a particular As(3+) at a particular time.

Factorial design for the optimization of enzymatic detection of cadmium in aqueous solution using immobilized urease from vegetable waste

Free as well as alginate immobilized urease was utilized for detection and quantitation of cadmium (Cd2+) in aqueous samples. Urease from the seeds of pumpkin (Cucumis melo), being a vegetable waste, was extracted and purified to apparent homogeneity (Sp. Activity 353 U/mg protein; A280/A260=1.12) by heat treatment at 48+/-0.1 degrees C and gel filtration through Sephadex G-200. The homogeneous enzyme preparation was immobilized in 3.5% alginate leading to 86% immobilization and no leaching of the enzyme was found over a period of 15 days at 4 degrees C. Urease catalyzed urea hydrolysis by both soluble and immobilized enzyme revealed a clear dependence on the concentration of Cd2+. The inhibition caused by Cd2+ was non-competitive (Ki=1.41 x 10(-5) M). The time dependent inhibition both in the presence and in absence of Cd2+ ion revealed a biphasic inhibition in the activity. A Response Surface Methodology (RSM) for the parametric optimization of this process was performed using two-level-two-full factorial (2(2)), central composite design (CCD). The regression coefficient, regression equation and analysis of variance (ANOVA) was obtained using MINITAB 15 software. The predicted values thus obtained were closed to the experimental value indicating suitability of the model. In addition to this 3D response surface plot and isoresponse contour plot were helpful to predict the results by performing only limited set of experiments.

Insights into the role and structure of plant ureases

The broad distribution of ureases in leguminous seeds, as well as the accumulation pattern of the protein during seed maturation, are suggestive of an important physiological role for this enzyme. Since the isolation and characterization of jack bean urease by Sumner in 1926, many investigations have been dedicated to the structural and biological features of this enzyme; nevertheless, many questions still remain. It has been reported that ureases from plants (jack bean and soybean seeds) display biological properties unrelated to their ureolytic activity, notably a high insecticidal activity against Coleoptera (beetles) and Hemiptera (bugs), suggesting that ureases might be involved in plant defense. Besides the insecticidal activity, canatoxin, a jack bean urease isoform, causes convulsions and death in mice and rats, induces indirect hemagglutination (hemilectin activity) and promotes exocytosis in several cell types. Not only plant ureases but also some microbial ureases (found in Bacillus pasteurii and Helicobacter pylori) are able to induce activation of platelets in a process mediated by lipoxygenase-derived metabolites. This review summarizes the biological and structural properties of plant ureases, compares them with those displayed by bacterial ureases, and discusses the significance of these findings.

Jack bean urease: The effect of active-site binding inhibitors on the reactivity of enzyme thiol groups

In view of the complexity of the role of the active site flap cysteine in the urease catalysis, in this work we studied how the presence of typical active-site binding inhibitors of urease, phenylphosphorodiamidate (PPD), acetohydroxamic acid (AHA), boric acid and fluoride, affects the reactivity of enzyme thiol groups, the active site flap thiol in particular. For that the inhibitor-urease complexes were prepared with excess inhibitors and had their thiol groups titrated with DTNB. The effects observed were analyzed in terms of the structures of the inhibitor-urease complexes reported in the literature. We found that the effectiveness in preventing the active site cysteine from the modification by disulfides, varied among the inhibitors studied, even though they all bind to the active site. The variations were accounted for by different extents of geometrical distortion in the active site that the inhibitors introduced upon binding, leaving the flap either open in AHA-, boric acid- and fluoride-inhibited urease, like in the native enzyme or closed in PPD-inhibited urease. Among the inhibitors, only PPD was found to be able to thoroughly protect the flap cysteines from the further reaction with disulfides, this apparently resulting from the closed conformation of the flap. Accordingly, in practical terms PPD may be regarded as the most suitable inhibitor for active-site protection experiments in inhibition studies of urease.

Quinone-induced inhibition of urease: Elucidation of its mechanisms by probing thiol groups of the enzyme

In this work we studied the reaction of four quinones, 1,4-benzoquinone (1,4-BQ), 2,5-dimethyl-1,4-benzoquinone (2,5-DM-1,4-BQ), tetrachloro-1,4-benzoquinone (TC-1,4-BQ) and 1,4-naphthoquinone (1,4-NQ) with jack bean urease in phosphate buffer, pH 7.8. The enzyme was allowed to react with different concentrations of the quinones during different incubation times in aerobic conditions. Upon incubation the samples had their residual activities assayed and their thiol content titrated. The titration carried out with use of 5,5'-di-thiobis(2-nitrobenzoic) acid was done to examine the involvement of urease thiol groups in the quinone-induced inhibition. The quinones under investigation showed two distinct patterns of behaviour, one by 1,4-BQ, 2,5-DM-1,4-BQ and TC-1,4-BQ, and the other by 1,4-NQ. The former consisted of a concentration-dependent inactivation of urease where the enzyme-inhibitor equilibrium was achieved in no longer than 10min, and of the residual activity of the enzyme being linearly correlated with the number of modified thiols in urease. We concluded that arylation of the thiols in urease by these quinones resulting in conformational changes in the enzyme molecule is responsible for the inhibition. The other pattern of behaviour observed for 1,4-NQ consisted of time- and concentration-dependent inactivation of urease with a nonlinear residual activity-modified thiols dependence. This suggests that in 1,4-NQ inhibition, in addition to the arylation of thiols, operative are other reactions, most likely oxidations of thiols provoked by 1,4-NQ-catalyzed redox cycling. In terms of the inhibitory strength, the quinones studied formed a series: 1,4-NQ approximately 2,5-DM-1,4-BQ<1,4-BQ<TC-1,4-BQ.

Double mode of inhibition-inducing interactions of 1,4-naphthoquinone with urease: Arylation versus oxidation of enzyme thiols

In their inhibition-inducing interactions with enzymes, quinones primarily utilize two mechanisms, arylation and oxidation of enzyme thiol groups. In this work, we investigated the interactions of 1,4-naphthoquinone with urease in an effort to estimate the contribution of the two mechanisms in the enzyme inhibition. Jack bean urease, a homohexamer, contains 15 thiols per enzyme subunit, six accessible under non-denaturing conditions, of which Cys592 proximal to the active site indirectly participates in the enzyme catalysis. Unlike by 1,4-benzoquinone, a thiol arylator, the inactivation of urease by 1,4-naphthoquinone under aerobic conditions was found to be biphasic, time- and concentration-dependent with a non-linear residual activity-modified thiols dependence. DTT protection studies and thiol titration with DTNB suggest that thiols are the sites of enzyme interactions with the quinone. The inactivated enzyme had approximately 40% of its activity restored by excess DTT supporting the presence of sulfenic acid resulting from the oxidation of enzyme thiols by ROS. Furthermore, the aerobic inactivation was prevented in approximately 30% by catalase, proving the involvement of hydrogen peroxide in the process. When H2O2 was directly applied to urease, the enzyme showed susceptibility to this inactivation in a time- and concentration-dependent manner with the inhibition constant of H2O2 Ki = 3.24 mM. Additionally, anaerobic inactivation of urease was performed and was found to be weaker than aerobic. The results obtained are consistent with a double mode of 1,4-naphthoquinone inhibitory action on urease, namely through the arylation of the enzyme thiol groups and ROS generation, notably H2O2, resulting in the oxidation of the groups.

Inhibition of jack bean urease by p-benzoquinone: elucidation of the role of thiols and reversibility of the process

p-Benzoquinone (pBQ) was studied as an inhibitor of jack bean urease in 20 mM phosphate buffer, pH 7.0, 1 mM EDTA, 25 degrees C. The inhibition was carried out by the use of a preincubation procedure in the absence of substrate. The influence of the inhibitor concentration and the preincubation time on the enzyme activity was elucidated. It was found that increase in pBQ concentration resulted in a linear decrease of urease activity. The dependence of the enzyme activity on the preincubation time showed that the rate of inhibition rapidly decreased at the beginning of the process in order to achieve the constant value. The inhibition became time independent in the studied time range. This observation is characteristic of a slow binding mechanism of inhibition. The protective experiment proved that the urease active site is involved in the binding of pBQ. High effectiveness of thiol protectors against pBQ inhibition indicates the strategic role of the active site sulfhydryl group in the blocking process. There were two methods used for reactivation of pBQ-inhibited urease. The dilution of the urease-pBQ complex in urea solution did not result in a regain of enzyme activity. Alternatively, the addition of dithiothreitol into the urease-pBQ mixture caused the instant and efficient reactivation of the enzyme. The experiments showed that the nature of the urease-pBQ complex is irreversible but the application of a specific thiol reagent can release the active enzyme from the complex.