Showing papers on "Urea published in 2003"

The molecular basis for the chemical denaturation of proteins by urea

Abstract: Molecular dynamics simulations of the protein chymotrypsin inhibitor 2 in 8 M urea at 60 degrees C were undertaken to investigate the molecular basis of chemical denaturation. The protein unfolded rapidly under these conditions, but it retained its native structure in a control simulation in water at the same temperature. The overall process of unfolding in urea was similar to that observed in thermal denaturation simulations above the protein's T(m) of 75 degrees C. The first step in unfolding was expansion of the hydrophobic core. Then, the core was solvated by water and later by urea. The denatured structures in both urea and at high temperature contained residual native helical structure, whereas the beta-structure was completely disrupted. The average residence time for urea around hydrophilic groups was six times greater than around hydrophobic residues and in all cases greater than the corresponding water residence times. Water self-diffusion was reduced 40% in 8 M urea. Urea altered water structure and dynamics, thereby diminishing the hydrophobic effect and encouraging solvation of hydrophobic groups. In addition, through urea's weakening of water structure, water became free to compete with intraprotein interactions. Urea also interacted directly with polar residues and the peptide backbone, thereby stabilizing nonnative conformations. These simulations suggest that urea denatures proteins via both direct and indirect mechanisms.

A New Fluoride Selective Fluorescent as Well as Chromogenic Chemosensor Containing a Naphthalene Urea Derivative

Abstract: A new naphthalene derivative containing a urea group at the 1,8-position of naphthalene was synthesized and showed a unique absorption and fluorescence peak with a fluoride ion Calculations suggested that a new peak was attributed to the increased anion character of urea nitrogen due to the strong interaction of the fluoride and N-H protons

Effects of temperature and fertilizer on activity and community structure of soil ammonia oxidizers.

Abstract: We investigated the effect of temperature on the activity of soil ammonia oxidizers caused by changes in the availability of ammonium and in the microbial community structure. Both short (5 days) and long (6.5, 16 and 20 weeks) incubation of an agricultural soil resulted in a decrease in ammonium concentration that was more pronounced at temperatures between 10 and 25 degrees C than at either 4 degrees C or 30-37 degrees C. Consistently, potential nitrification was higher between 10 and 25 degrees C than at either 4 degrees C or 37 degrees C. However, as long as ammonium was not limiting, release rates of N2O increased monotonously between 4 and 37 degrees C after short-term temperature adaptation, with nitrification accounting for about 35-50% of the N2O production between 4 and 25 degrees C. In order to see whether temperature may also affect the community structure of ammonia oxidizers, we studied moist soil during long incubation at low and high concentrations of commercial fertilizer. The soil was also incubated in buffered (pH 7) slurry amended with urea. Communities of ammonia oxidizers were assayed by denaturant gradient gel electrophoresis (DGGE) of the amoA gene coding for the alpha subunit of ammonia monooxygenase. We found that a polymerase chain reaction (PCR) system using a non-degenerated reverse primer (amoAR1) gave the best results. Community shifts occurred in all soil treatments after 16 weeks of incubation. The community shifts were obviously influenced by the different fertilizer treatments, indicating that ammonium was a selective factor for different ammonia oxidizer populations. Temperature was also a selective factor, in particular as community shifts were also observed in the soil slurries, in which ammonium concentrations and pH were better controlled. Cloning and sequencing of selected DGGE bands indicated that amoA sequences belonging to Nitrosospira cluster 1 were dominant at low temperatures (4-10 degrees C), but were absent after long incubation at low fertilizer treatment. Sequences of Nitrosospira cluster 9 could only be detected at low ammonium concentrations, whereas those of Nitrosospira cluster 3 were found at most ammonium concentrations and temperatures, although individual clones of this cluster exhibited trends with temperature. Obviously, ammonia oxidizers are able to adapt to soil conditions by changes in the community structure if sufficient time (several weeks) is available.

Nitrogen metabolism and recycling in Holstein heifers.

Abstract: To study the effect of dietary N level on urea kinetics and recycling, four Holstein heifers (267 +/- 3.6 kg) were used in a Youden square design. Isocaloric diets with a N content of 1.44, 1.89, 2.50, 2.97, and 3.40% were fed at approximately 1.8 times maintenance intake. Increasing the N content of the diet increased urinary N excretion (P < 0.001) and N balance (P < 0.01), but did not affect the fecal N excretion (P = 0.21). Increasing the level of dietary N, increased urea production (P < 0.001) and excretion (P < 0.001), but no effect (P = 0.24) could be detected in the amount of N recycled to the gut. Urea recycled with the saliva, however, increased (P < 0.001) both in absolute and relative terms, with increasing dietary N. No difference could be detected on the amount of recycled N that was used for anabolism or returned to the ornithine cycle, but less (P = 0.001) N originating from urea was excreted in feces as dietary N increased. Ruminal ammonia concentration increased (P < 0.001) with increasing N intake, but total tract neutral detergent fiber digestibility was depressed only on the lowest N intake diet. No difference (P = 0.30) was detected in ruminal microbial yield among diets, but more (P < 0.003) N was derived from blood urea at low N intakes, and the efficiency of use of the recycled N decreased (P < 0.001) with increasing levels of dietary N. Adaptive changes to low-N diets were a decrease (P < 0.003) in the renal clearance of urea and an increase (P < 0.001) in the gastrointestinal clearance of urea. Urea transporters were present in the rumen wall of the heifers and differentially expressed depending on dietary N content, but their role in the transfer of urea into the rumen remains uncertain. Different mechanisms of N salvage and recycling were involved when animals were fed low-N diets that ensured a supply of endogenous N to the gastrointestinal tract and, due to the reduced contribution of dietary N, an increased efficiency of the N recycled was observed.

Urea transport by nitrogen-regulated tonoplast intrinsic proteins in Arabidopsis.

Abstract: Urea is the major nitrogen (N) form supplied as fertilizer in agricultural plant production and also an important N metabolite in plants. Because urea transport in plants is not well understood, the aim of the present study was to isolate urea transporter genes from the model plant Arabidopsis. Using heterologous complementation of a urea uptake-defective yeast (Saccharomyces cerevisiae) mutant allowed to isolate AtTIP1;1, AtTIP1;2, AtTIP2;1, and AtTIP4;1 from a cDNA library of Arabidopsis. These cDNAs encode channel-like tonoplast intrinsic proteins (TIPs) that belong to the superfamily of major intrinsic proteins (or aquaporins). All four genes conferred growth of a urea uptake-defective yeast mutant on 2 mm urea in a phloretin-sensitive and pH-independent manner. Uptake studies using 14C-labeled urea into AtTIP2;1-expressing Xenopus laevis oocytes demonstrated that AtTIP2;1 facilitated urea transport also in a pH-independent manner and with linear concentration dependency. Expression studies showed that AtTIP1;2, AtTIP2;1, and AtTIP4;1 genes were up-regulated during early germination and under N deficiency in roots but constitutively expressed in shoots. Subcellular localization of green fluorescent protein-fused AtTIPs indicated that AtTIP1;2, AtTIP2;1, and AtTIP4;1 were targeted mainly to the tonoplast and other endomembranes. Thus, in addition to their role as water channels, TIP transporters may play a role in equilibrating urea concentrations between different cellular compartments.

Urea thermolysis and NOx reduction with and without SCR catalysts

Abstract: Urea–selective catalytic reduction (SCR) has been a leading contender for removal of nitrogen oxides (deNOx) from diesel engine emissions Despite its advantages, the SCR technology faces some critical detriments to its catalytic performance such as catalyst surface passivation (caused by deposit formation) and consequent stoichiometric imbalance of the urea consumption Deposit formation deactivates catalytic performance by not only consuming part of the ammonia produced during urea decomposition but also degrading the structural and thermal properties of the catalyst surface We have characterized the urea thermolysis with and without the urea-SCR catalyst using both spectroscopic (DRIFTS and Raman) and thermal techniques (thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC)) to identify the deposit components and their corresponding thermal properties Urea thermolysis exhibits two decomposition stages, involving ammonia generation and consumption, respectively The decomposition after the second stage leads to the product of melamine complexes, (HNCNH)x(HNCO)y, that hinder catalytic performance The presence of catalyst accompanied with a good spray of the urea solution helps to eliminate the second stage In this work, kinetics of the direct reduction of NOx by urea is determined and the possibility of using additives to the urea solution in order to rejuvenate the catalyst surface and improve its performance will be discussed

A Simple Method for Estimating Respiratory Solute Dilution in Exhaled Breath Condensates

Abstract: Exhaled breath condensates have been widely used to detect inflammatory mediators in the fluid that covers airway surfaces of patients with inflammatory lung disorders. This approach is much less invasive than bronchoalveolar lavage, but respiratory droplets are markedly diluted by large and variable amounts of water vapor. We estimated the dilution of respiratory droplets by comparing concentrations of nonvolatile, reference indicators (total nonvolatile cations, urea or conductivity) in 18 normal subjects with normal plasma concentrations by assuming similar concentrations in the respiratory fluid and plasma. The volatile cation, NH4+ (most of which is delivered as NH3 gas from the mouth), represented 93 +/- 3% (SEM) of the condensate cations. More than 99% of the NH4+ was removed by lyophilization, making it possible to use conductivity to estimate total nonvolatile ionic concentrations and facilitating analysis of urea. Conductivity was significantly correlated with electrolyte and urea concentrations. Estimates of dilution based on total cations, conductivity, and urea were not significantly different (cations: 20,472 +/- 2,516; conductivity: 21,019 +/- 2,427; and urea: 18,818 +/- 2,402). These observations suggest that the conductivity of lyophilized samples can be used as an inexpensive, simple, and reliable method for estimating dilution of nonvolatile, hydrophilic mediators in condensates.

Thermal and Hydrolytic Decomposition of Urea for Automotive Selective Catalytic Reduction Systems: Thermochemical and Practical Aspects

Abstract: An energetic analysis of the thermal decomposition of solid urea and urea solutions is presented, and the results are discussed in view of urea selective catalytic reduction (SCR) for automotive DeNOx systems. Various types of decomposition reactors are possible which differ in their effectiveness to produce ammonia from urea. For reasons of simplicity, the decomposition is usually performed by atomizing urea solutions directly into the hot exhaust. However, this technique suffers from short residence times, leading to incomplete decomposition into ammonia and isocyanic acid and causing a significant performance loss of the SCR catalyst. The thermal decomposition out of the main exhaust stream allows much increased residence times for the process of urea decomposition. A reactor utilizing a partial stream of the exhaust seems particularly promising, especially if such a reactor includes a hydrolyzing catalyst, leading to ammonia practically free from isocyanic acid.

Mammalian Urea Transporters

Abstract: ▪ Abstract Urea plays a key role in the urine-concentrating mechanism. Physiologic and molecular data demonstrate that urea transport in kidney and red blood cells occurs by specific urea transporter proteins. Two gene families for facilitated urea transporters, UT-A and UT-B, and several urea transporter cDNA isoforms have been cloned from human, rat, mouse, and several non-mammalian species. Polyclonal antibodies have been generated to many of the urea transporter proteins, and several novel findings have resulted from their use in integrative animal studies. For example, (a) vasopressin increases the phosphorylation of UT-A1 in rat inner medullary collecting duct; (b) UT-A1 protein abundance is increased in the rat inner medulla during conditions in which urine-concentrating ability is reduced; and (c) urea transporters are expressed in non-renal tissues, and UT-A protein abundance is up-regulated in uremia in both liver and heart. In addition to the facilitated urea transporters, functional evidence e...

The gastric biology of Helicobacter pylori.

Abstract: ▪ Abstract Helicobacter pylori is a neutralophilic, gram-negative, ureolytic organism that is able to colonize the human stomach but does not survive in a defined medium with a pH <4.0 unless urea is present. In order to live in the gastric environment, it has developed a repertoire of acid resistance mechanisms that can be classified into time-independent, acute, and chronic responses. Time-independent acid resistance depends on the structure of the organism's inner and outer membrane proteins that have a high isoelectric point, thereby reducing their proton permeability. Acute acid resistance depends on the constitutive synthesis of a neutral pH optimum urease that is an oligomeric Ni2+-containing heterodimer of UreA and UreB subunits. Gastric juice urea is able to rapidly access intrabacterial urease when the periplasmic pH falls below ∼6.2 owing to pH-gating of a urea channel, UreI. This results in the formation of NH3, which then neutralizes the bacterial periplasm to provide a pH of ∼6.2 and an inne...

Organic nitrogen in rain and aerosol at Cape Grim, Tasmania, Australia

Abstract: During the Southern Hemispheric spring of 2000 (during the months of November and early December), rain, bulk and size-separated aerosol samples were collected at the Cape Grim Baseline Air Pollution Station located on the island of Tasmania, Australia and analyzed for total organic nitrogen (N), urea, and dissolved free amino acids. Rain and bulk aerosol samples contained organic N at concentrations representing, on average, between 19 and 25% of total N. Urea was not detected in the six rain samples analyzed. However, urea represented ∼24% of the organic N contained in nonbaseline aerosol samples, and ∼2% of the organic N contained within baseline samples. Trajectory analysis combined with meteorological data indicated that high concentrations of urea within aerosols were mainly due to Tasmanian sources, likely animal emissions, although the application of urea fertilizers cannot be dismissed as a source. In nonbaseline samples the highest concentrations of urea were associated with the coarse mode aerosol (greater than 1 μm), although urea was also found in the fine mode aerosol (less than 1 μm), potentially indicating gas-to-particle conversion of urea. Aerosol samples collected in marine air masses contained urea within an intermediate fraction centered at ∼1 μm suggesting the sea surface microlayer as a source. Dissolved free amino acids contributed ∼53% of the organic N in rain, but were not a significant proportion of the total organic N fraction in either nonbaseline or baseline aerosol samples. Due to their presence in rain, amino acids likely exist in aerosols as unhydrolyzed proteins. In cascade impactor samples highly influenced by marine sources, profiles for amino N were inversely related to urea N, possibly indicating live species and the sea surface microlayer as a source for organic N.; ;

AtDUR3 Encodes a New Type of High-Affinity Urea/H + Symporter in Arabidopsis

Abstract: Urea is the major nitrogen form supplied as fertilizer in agricultural plant production but also an important nitrogen metabolite in plants. We report the cloning and functional characterization of AtDUR3, a high-affinity urea transporter in plants. AtDUR3 contains 14 putative transmembrane-spanning domains and represents an individual member in Arabidopsis that belongs to a superfamily of sodium-solute symporters. Heterologous expression in urea uptake–defective yeast as well as two-electrode voltage clamp and uptake studies using 14C-labeled urea in AtDUR3-expressing oocytes demonstrated that AtDUR3 mediates urea transport. In both heterologous systems, urea transport was stimulated at low pH. In oocytes, inward currents indicated that urea is cotransported with protons. By contrast, a supply of Na+ ions could not stimulate urea transport. Transport of 14C-labeled urea by AtDUR3 in oocytes exhibited saturation kinetics with a Km of ∼3 μM. AtDUR3 was expressed in shoots and roots and upregulated during early germination and under nitrogen deficiency in roots. We propose a role of AtDUR3 in urea uptake by plant cells at low external urea concentrations.

Effects of urea infusion and ruminal degradable protein concentration on microbial growth, digestibility, and fermentation in continuous culture

Abstract: The effects of urea and rumen-degradable protein (RDP) on microbial growth, digestibility, and fermentation were examined using dual-flow continuous culture. The experimental design was a 4 x 4 Latin square with a 2 x 2 factorial arrangement of treatments. Factors were urea infusion (0.4 g/L of artificial saliva) and RDP concentration, and the treatments were as follows: 1) low RDP (8% of dietary dry matter) without urea (LDNU), 2) high RDP (11% of dietary dry matter) without urea (HDNU), 3) low RDP (8% of dietary dry matter) with urea (LDU), and 4) high RDP (11% of dietary dry matter) with urea (HDU). The LDNU (i.e., negative control) and HDNU treatments were formulated to be nitrogen limiting. Results indicated that infusion of urea increased all digestibility measurements (P 0.05). Isobutyrate concentration decreased (P = 0.05) with increased RDP. Increased dietary RDP increased crude protein degradation and soluble protein concentration (P < 0.05), but NH3 nitrogen, trichloroacetic acid-soluble nitrogen, and peptide nitrogen were unaffected by changing RDP levels. Microbial growth efficiency was 19.9, 24.9, 28.0, and 32.2 g N/g organic matter truly digested for LDNU, HDNU, LDU, and HDU, respectively, and was significantly improved both by urea infusion (P = 0.002) and increased RDP concentration (P = 0.021). The interactions of urea and RDP (P < 0.05) were explained by the high digestibility of neutral detergent fiber, nonstructural carbohydrate, and especially hemicellulose, with the HDNU treatment. The results of this study indicated that hemicellulose-degrading bacteria were able to effectively compete with nonstructural carbohydrate-degrading bacteria for available peptide and amino acid nitrogen. Further, the extent of protein degradation was dependent on the availability of NH3 nitrogen in the system.

Modification of phenol–formaldehyde resol resins by lignin, starch, and urea

Abstract: Lignin-based chemicals, starch, and urea were used as modifiers for phenol–formaldehyde resol resins. The effects of the addition stage of the modifiers used in the synthesis of the resins and the type of modification reagent on the structures of the resins and their molar masses and reactivities were investigated. The modifications with corn starch and lignin promoted condensation; this was verified by increased molar masses and high ratios of methylene bridges to the sum of free ortho and para aromatic groups with respect to the corresponding reference resin without a modification reagent. The later the modifier was added to the resin condensation mixture, the more methylene bridges were formed with respect to the amounts of free ortho and para aromatic groups. In addition, when urea or wheat starch was added in the later condensation stage, the final condensation also reached high stages. The modifications with lignosulfonate and starch, as well as the early addition of urea, enhanced p–p′ bridge structures. The lowest condensation stage and, therefore, the highest reactivity were found when wheat starch was added with the starting reagents. The curing heat of the wheat-starch-modified resins decreased according to the deferred addition point of starch. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 88: 582–588, 2003

Kinetic Study of Enzymatic Urea Hydrolysis in the pH Range 4-9

Abstract: The enzymatic hydrolysis of urea by jack bean urease was investigated at 25 °C over the pH range 4-9. Reaction rate data were found to be well described by a modified Michaelis-Menten equation with a pH-dependent rate coefficient and a product inhibition term. The influence of pH on activity was interpreted in terms of perturbation of the enzyme distribution among three differently protonated forms. Kinetic analysis yielded a Michaelis constant of 3.21 mmol I -1 and indicated that the inhibition mechanism was of the fully non-competitive type, with K p = 12.2 mmol l -1 . The estimated activation energy was 35.3 kJ mol -1 . The resulting kinetic expression was tested by comparing model predictions with the experimental behaviour observed in unbuffered media and over a long-term period.

The dominant interaction between peptide and urea is electrostatic in nature: a molecular dynamics simulation study.

Abstract: The conformational equilibrium of a blocked valine peptide in water and aqueous urea solution is studied using molecular dynamics simulations. Pair correlation functions indicate enhanced concentration of urea near the peptide. Stronger hydrogen bonding of urea–peptide compared to water–peptide is observed with preference for helical conformation. The potential of mean force, computed using umbrella sampling, shows only small differences between urea and water solvation that are difficult to quantify. The changes in solvent structure around the peptide are explained by favorable electrostatic interactions (hydrogen bonds) of urea with the peptide backbone. There is no evidence for significant changes in hydrophobic interactions in the two conformations of the peptide in urea solution. Our simulations suggest that urea denatures proteins by preferentially forming hydrogen bonds to the peptide backbone, reducing the barrier for exposing protein residues to the solvent, and reaching the unfolded state. © 2003 Wiley Periodicals, Inc. Biopolymers: 359–369, 2003

Utilization of carbonate and ammonia‐based treatments to eliminate Escherichia coli O157:H7 and Salmonella Typhimurium DT104 from cattle manure

Abstract: Aims: The objective of this study was to investigate alkaline treatments of cattle manure to kill coliforms, Escherichia coli O157:H7 and Salmonella Typhimurium DT104 based on their inhibition by carbonate ion and ammonia. Methods and Results: Pure cultures of S. Typhimurium DT104 and E. coli O157:H7 strains were treated with sodium carbonate and ammonia to determine threshold inhibitory concentrations. Fresh cattle manure samples were inoculated with the same strains and their survival was determined after addition of sodium hydroxide, ammonium sulphate, sodium carbonate and/or urea. Control of CO -2 3 and NH 3 concentrations in manure by pH adjustment to 9.5 with sodium hydroxide to more than 5 and 30 mmol l -1 , respectively, killed more than 106 cells g -1 in 7 days. Addition of sodium carbonate enhanced the killing effect of NaOH by increasing the CO -2 3 and NH 3 concentrations. Addition of 100 mmol l -1 urea, produced high levels of CO -2 3 and NH 3 and decreased all bacterial counts by at least 10 6 cells g -1 after 7 days. Conclusions: Reduction of food-borne pathogens in manure can be achieved by a combination of high concentrations of CO -2 3 and NH 3 which are pH-dependent parameters. Significance and Impact of Study: Addition of urea could provide a simple manure treatment by combining both antimicrobial factors.

Urease activity in cultures and field populations of the toxic dinoflagellate Alexandrium

Abstract: Nitrogen availability is an important factor controlling phytoplankton abundance and species composition in marine waters. In addition to inorganic nitrogen, some phytoplankton species can use dissolved organic nitrogen sources such as urea for growth. Herein we demonstrate that axenic laboratory cultures of the toxic dinoflagellate, Alexandrium fundyense strain CB301A and A. catenella strain TN9A were able to grow on urea as a sole nitrogen source in the presence of nickel. This nickel dependence suggests that these Alexandrium species hydrolyze urea into ammonia with the enzyme urease rather than adenosine triphosphate urea amidolyase. Cells grown on urea had lower toxin content (15%-30%) than f/2-grown cells. In A. fundyense the urease enzyme appears to be nitrogen-regulated. In culture experiments, enzyme activity was highest in nitrate-starved and urea-grown (replete) cells, whereas activity was undetectable in f/2-grown (replete) and phosphate-starved cells. Urease activity in ammonia-grown (replete) cells was also depressed. Urease activity also appeared to increase with decreasing nitrate-limited growth rate in semicontinuous cultures. May and June cruises in the Gulf of Maine followed the yearly bloom of A. fundyense. On average, inorganic nitrogen concentrations in May were higher than in June, whereas cell abundances, urea concentrations, and urease activity in May were lower than in June. The latter measurements relied on an immunomagnetic bead separation to isolate living A. fundyense cells from mixed phytoplankton samples for analysis. The differences between May and June suggest that urea may be important for Alexandrium nutrition as inorganic nitrogen concentrations in surface water decline.

Gaseous nitrogen loss from temperate perennial grass and clover dairy pastures in south-eastern Australia

Abstract: The use of nitrogen (N) fertiliser on dairy pastures in south-eastern Australia has increased exponentially over the past 15 years. Concerns have been raised about the economic and environmental impact of N loss through volatilisation and denitrification. Emissions of NH3, N2, and N2O were measured for 3 years in the 4 different seasons from a grazed grass/clover pasture, with or without 200 kg N fertiliser/ha, applied as ammonium nitrate and urea. Nitrogen-fertilised treatments lost significantly more N than the control treatments in all cases. More NH3 was lost from urea-fertilised treatments than from either the control or ammonium nitrate treatments, whereas ammonium nitrate treatments lost significantly more N through denitrification than the control or urea treatments in all seasons, except for summer. More NH3 was lost in summer than in the other seasons, whereas denitrification and N2O losses were highest in winter and lowest in summer. The total annual NH3 loss from the control, ammonium nitrate, and urea treatments averaged 17, 32, and 57 kg N/ha.year, respectively. Annual denitrification losses were estimated at around 6, 15, and 13 kg N/ha.year for the control, ammonium nitrate, and urea treatments, respectively. Total gaseous N losses were estimated to be 23, 47, and 70 kg N/ha.year from the control, ammonium nitrate, and urea treatments respectively. Although the use of ammonium nitrate fertiliser would significantly reduce NH3 volatilisation losses in summer, this fertiliser costs 45% more per unit N than urea, so there is no economic justification for recommending its use over urea for the other seasons. However, the use of urea during the cooler, wetter months may result in significantly less denitrification loss. The results are discussed in terms of potential management strategies to improve fertiliser efficiency and reduce adverse effects on the environment.