Showing papers on "Urea published in 1993"

Cloning and characterization of the vasopressin-regulated urea transporter

Abstract: UREA is the principal end product of nitrogen metabolism in mammals1. Movement of urea across cell membranes was originally thought to occur by lipid-phase permeation, but recent studies have revealed the existence of specialized transporters with a low affinity for urea (Km > 200 mM)2. Here we report the isolation of a complementary DNA from rabbit renal medulla that encodes a 397-amino-acid membrane glycoprotein, UT2, with the functional characteristics of the vasopressin-sensitive urea transporter previously described in in vitro-perfused inner medullary collecting ducts3,4. UT2 is not homologous to any known protein and displays a unique pattern of hydrophobicity. Because of the central role of this transporter in fluid balance1,3–7 and nitrogen metabolism8, the study of this protein will provide important insights into the urinary concentrating mechanism and nitrogen balance.

Roles of urease in plant cells

Abstract: Publisher Summary This chapter discusses the role of urease in plant cells Urease is important for efficient nitrogen assimilation Considerable amounts of plant nitrogen flow through urea (urease substrate), which can be recycled only by urease action This recapture can have significant quality impact on protein rich crops It appears to have an important role in germination of protein poor seeds The urease substrate urea is derived from arginine and ureides Arginine is the richest nitrogen repository among the amino acids of seed storage proteins On the other hand, ureides are not only significant sources of nitrogen in nucleic acid turnover but are also a predominant transport from of fixed nitrogen in soybean and other tropical legumes Urease-negative plants accumulate substantial, nonutilizable urea in both maternal and embryonic tissue During germination of urease-negative seeds, further urea accumulates as a dead end in nitrogen metabolism Abundant seed ureases, such as, Sumner's jackbean urease, may play a chemical defense role All of the ureases, both from bacteria and plants, resemble each other in primary structure and in their requirement for accessory genes

Compounds containing alkoxysilane and amino groups

Abstract: N-alkoxysilylalkyl-aspartic acid esters are prepared by the reaction of equimolar quantities of amino-alkyl alkoxysilanes with maleic or fumaric acid esters. These N-alkoxysilylalkyl-aspartic acid esters are particularly useful as reactants in the preparation of prepolymers containing alkoxysilane and urea groups.

Aggregation and denaturation of apomyoglobin in aqueous urea solutions.

Abstract: The effects of urea on apomyoglobin solubility have been investigated. Apomyoglobin precipitation was found to be a thermodynamically reversible process independent of the pathway of aggregation. A liquid-solid phase diagram was constructed for the precipitation of apomyoglobin as a function of urea and protein concentration. Apomyoglobin solubility decreases by an order of magnitude between 0 and 1.5 M urea, reaching a minimum near 2.4 M urea and increasing at higher urea concentrations (the denaturation midpoint is at approximately 2.6 M urea). This decrease in protein solubility is opposite to that expected based on amino acid solubilities, since both polar and nonpolar molecules become more soluble with increasing urea concentration. Solubility minima for proteins have been rationalized in terms of folding intermediates. However, our structural studies show no evidence for folding intermediates in apomyoglobin under the experimental conditions, apart from small predenaturation changes. Our data are consistent with an alternative hypothesis, namely, that the primary aggregating species are denatured protein molecules, rather than intermediate states. Consistent with recent thermodynamic and statistical mechanical models, the solubility minimum may be described as the result of two competing effects of urea: (1) urea denatures the protein, and (2) urea makes the solvent more favorable for the native and any denatured state. At low urea concentration, solubility decreases with increasing urea concentration due to the domination of the solubility behavior by the increase in the population of aggregation-competent (denatured) protein molecules. However, at high urea concentration, the increasingly favorable nature of the solvent dominates, resulting in increasing solubility with urea concentration. The phase diagram provides guidance for the best experimental conditions (pathway) to use to avoid aggregation during the refolding of denaturant-unfolded protein.

Protein internal flexibility and global stability: Effect of urea on hydrogen exchange rates of bovine pancreatic trypsin inhibitor

Abstract: The hydrogen isotope exchange kinetics of buried NH protons in bovine pancreatic trypsin inhibitor (BPTI) was measured in 8 M urea at 30 degrees C and pH 3.5. The data were analyzed by the two-process model in which slower exchanging protons utilize an unfolding mechanism and more rapidly exchanging protons exchange from the folded state. Urea accelerates the set of protons exchanging by the unfolding mechanism, all of which have approximately the same exchange rate constants in urea. For protons in this set, the ratio of exchange rate constants in the presence and absence of urea is used to estimate delta delta G(0-->8M urea) = 6.6 kcal/mol. For the set of protons exchanging from the folded state, 8 M urea either has no effect or slows exchange. Slowing of exchange by urea implies binding of urea to sites at or near the exchanging proton. Some buried protons exchanging from the folded state have diminished rates in 8 M urea, meaning that urea is accessible to these buried sites. Several unassigned side-chain NH's of arginine or lysine are highly protected from exchange by urea, suggesting that they are the location of urea binding sites on the surface of the molecule.

Vasopressin activates collecting duct urea transporters and water channels by distinct physical processes

Abstract: The present studies were performed to investigate the kinetics of regulation of water channels and urea carriers in the rat terminal (IMCD) in response to vasopressin (AVP). The time courses of osmotic water permeability (Pf) and urea permeability (P(urea)) were measured in isolated perfused rat terminal IMCD segments following AVP stimulation and subsequently following AVP washout. Under control conditions, Pf and P(urea) kinetics were similar. Both transport processes exhibited complex patterns of activation with a period of rapid permeability increase followed by a period of slower increase. Both transporters also exhibited complex patterns of reversal following AVP washout, with a rapid permeability decrease (5 min) followed by a slower decrease toward the baseline value. The measurements were repeated in the presence of a lumen > bath osmotic gradient, a condition associated with a decreased rate of apical endocytosis in collecting ducts. The lumen > bath gradient did not alter the kinetics of Pf increase after AVP addition, but completely blocked the decrease in Pf normally seen with washout of AVP. In contrast, the lumen > bath osmotic gradient did not affect the decrease in urea permeability after AVP washout, but blocked the rapid phase of urea permeability increase following AVP addition. Thus imposition of a lumen > bath osmotic gradient resulted in separation of the time courses of P(urea) and Pf changes associated with AVP addition and washout. This finding indicates that the physical processes responsible for AVP-mediated alteration of urea transporter and water channel activity in the apical membrane are distinct.

Device for catalytic NOx reduction

Abstract: For the effective catalytic reduction of NOx from oxygen-containing exhaust gases using urea, a device is proposed which contains a hydrolysis catalyst (3 and 4), which is composed of fine flow channels, which admit part-streams through diversions and through holes or slots, which part-streams are oriented approximately perpendicularly to the main stream. By this means, a uniform distribution of the urea solution and very rapid heating of the solution is to be effected. The urea solution can be quantitatively converted in this manner into ammonia and carbon dioxide without formation of harmful byproducts.

Ethyl Carbamate Formation: Reaction of Urea and Citrulline with Ethanol in Wine Under Low to Normal Temperature Conditions

Abstract: A two year storage study determined the effect of time and temperature on the formation of ethyl carbamate from the reaction of ethanol with urea and with citrulline. Under fairly normal storage temperatures (13.1°C to 23.9°C) and not uncommon concentrations of urea (1 to 10 mg/L), a significant amount of ethyl carbamate will accumulate over a two-year period. The formation rates of ethyl carbamate due to urea concentration only, for a table wine of about 12% ethanol, were per mg urea per day 0.0004, 0.0017, and 0.0061 µg ethyl carbamate at 13.3°C, 18.6°C, and 23.9°C, respectively. Other factors, such as pH and wine type, are not as significant to the accumulation. From this data we were able to verify that some other compounds(s), other than urea and citrulline, are contributing to the formation of ethyl carbamate. Best estimates of potential ethyl carbamate that may be formed should include consideration of temperature of storage and effects on rate of formation by urea, citrulline, and other contributing factors.

Hydrolysis of urea by Ureaplasma urealyticum generates a transmembrane potential with resultant ATP synthesis.

Abstract: When urea is added to Ureaplasma urealyticum, it is hydrolysed internally by a cytosolic urease Under our measuring conditions, and at an external pH of 60, urea hydrolysis caused an ammonia chemical potential equivalent to almost 80 mV and, simultaneously, an increase in proton electrochemical potential (delta p) of about 24 mV with resultant de novo ATP synthesis Inhibition of the urease with the potent inhibitor flurofamide abolished both the chemical potential and the increase of delta p such that ATP synthesis was reduced to approximately 5% of normally obtained levels Uncouplers of electrochemical gradients had little or no effect on these systems The electrochemical parameters and ATP synthesis were measured similarly at three other external pH values Any change in delta p was primarily via membrane potential (delta psi), and the level of de novo ATP synthesis was related to the increase in delta p generated upon addition of urea and more closely to the ammonia chemical potential Although the organisms lack an effective mechanism for internal pH homeostasis, they maintained a constant delta pH The data reported are consistent with, and give evidence for, the direct involvement of a chemiosmotic mechanism in the generation of around 95% of the ATP by this organism Furthermore, the data suggest that the ATP-generating system is coupled to urea hydrolysis by the cytosolic urease via an ammonia chemical potential

Measurement of 15N enrichment in multiple amino acids and urea in a single analysis by gas chromatography/mass spectrometry

Abstract: A precise and accurate procedure to measure the 15N isotopic enrichment of 18 common plasma amino acids and singly (15N1) and doubly (15N2) labeled urea in a single analysis by selected ion monitoring electron impact ionization gas chromatography/mass spectrometry analysis is presented. The choice of tert-butyldimethylsilyl derivatives allowed the enrichments in the amide and amino nitrogens of glutamine to be resolved. The ions monitored contained all the nitrogen atoms from the parent compounds except for arginine, which lost one guanidino nitrogen. Isotope ratios were determined with a coefficient of variation (within-assay precision) of 0.35% (range, 0.1-1.0%) on replicate measures averaged over all components; thus, the standard deviation associated with a nominal [m + 1]/[m + 0] isotope ratio of 0.2000 was 0.0007. The average error between measured and theoretical [m + 1]/[m + 0] isotope ratios was +0.0001 +/- 0.0086 for samples at natural abundance isotopic composition. The utility of the procedure is demonstrated by monitoring the incorporation of 15N into 18 plasma amino acids and urea during a 6 h oral administration of 15NH4Cl to a human volunteer. Highest levels of enrichment were achieved in arginine and urea, followed by glutamine. Approximately 80% of the label in glutamine was in the amino nitrogen. Excess 15N enrichment was observed in all plasma amino acids monitored with the exception of the essential amino acids phenylalanine, lysine and histidine. This method will facilitate the measurement of isotopic enrichment of multiple amino acids by a single analysis when it is necessary to monitor multiple stable-isotopically labeled amino acids in studies of amino acid and protein metabolic kinetics.

Reconstituted tobacco product

Abstract: A process for the manufacture of a reconstituted tobacco product wherein the process comprises extraction of tobacco furnish with an aqueous solvent in the presence of at least one ammonium salt or urea or a urea derivative or a mixture of urea or a urea derivative and at least one ammonium salt to provide an aqueous solvent extract and a fibrous material and forming the fibrous material into a sheet-like product using the papermaking process. The aqueous extract is then applied to the formed sheet-like material and further processing provides a reconstituted tobacco product having improved smoke quality.

Granular urea-based fertilizer

Abstract: An improved homogenous granular fertilizer composition is disclosed; the composition is comprised primarily of urea, N-(n-butyl)thiophosphoric triamide (NBPT), and dicyandiamide (DCD). The composition of this invention can be applied to a field crop in a single surface application and will nevertheless supply sufficient nitrogen to the plants throughout their growth and maturing cycles. The new improved composition increases the nitrogen uptake by plants, enhances crop yields, and minimizes the loss of both ammonium nitrogen and nitrate nitrogen from the soil. The NBPT is incorporated into the homogenous granular fertilizer composition of this invention by blending a concentrated solution of NBPT in a solvent selected from the group consisting of liquid amides, 2-pyrrolidone, and N-alkyl 2-pyrrolidones directly into molten urea prior to its granulation; the DCD may be added to the urea melt as a solid or in dissolved form along with the NBPT.

Rumen digestion and urinary excretion of purine derivatives in response to urea supplementation of sodium-treated straw fed to sheep

Abstract: The present study examined the effect of urea-N supplementation of a N-deficient diet on digestion and metabolism in the rumen. Five Rasa Aragonesa ewes, each fitted with a rumen cannula, were offered alkali-treated barley straw ad lib. alone or supplemented continuously via the cannula with four levels of urea-N (3, 6, 9 and 12 g/d). Rumen NH3 concentrations increased in response to urea infusion (6-128 mg/l; P < 0.001). At the highest level of rumen NH3 concentration there was a significant increase, compared with the unsupplemented treatment, in dry matter (DM) intake (846-1206 g/d; P < 0.001) and apparent digestibility of DM (0.38-0.43), organic matter (0.38-0.45) and neutral-detergent fibre (0.41-0.49; P < 0.01). Rumen outflow rates of particulate matter and potential DM disappearances, assessed using nylon bags, were not affected by the experimental treatments, although fractional rate of DM disappearance increased significantly with increasing levels of urea infusion (2.4-4.6 per h). Urinary excretion of total purine derivatives increased with N supplementation, although the response was exclusively due to an increase in allantoin excretion (26.9-66.4 mg/kg live weight (W)0.75 per d; P < 0.001). Xanthine, hypoxanthine and uric acid excretion rates were constant, averaging 1.8 (SE 0.17); 5.4 (SE 0.21) and 7.2 (SE 0.36) mg/kg W0.75 per d respectively. The maintenance of a minimum rumen NH3 concentration (approximately 50 mg/l) was necessary to avoid significant reductions in DM intake and fermentation rate. Higher levels, however, may further increase microbial N flow at the duodenum, as suggested by the response in urinary allantoin excretion over the range of rumen NH3 concentrations.

Raman spectroscopic investigation of the dynamics of urea–water complexes

Abstract: The hydration of urea via hydrogen‐bond formation was found to have a significant effect on the observed Raman band shapes. An analysis indicates that, on the average, one molecule of water is carried along with a solute molecule during its reorientational motion (τR≊2.5 ps). Vibrational dephasing of the ν4 vibration (τω≊0.2 ps) is independent of concentration and appears to decrease slightly with increasing temperature. No evidence of urea dimer formation was detected.

AMMONIA AND UREA EXCRETION IN LAHONTAN CUTTHROAT TROUT (ONCORHYNCHUS CLARKI HENSHAWI) ADAPTED TO THE HIGHLY ALKALINE PYRAMID LAKE (pH 9.4)

Abstract: Earlier studies have reported that acute exposure to alkaline pH strongly inhibits ammonia excretion in freshwater rainbow trout, but the Lahontan cutthroat trout thrives in Pyramid Lake, Nevada, at pH 9.4. We investigated the rates and mechanisms of ammonia and urea excretion in this species in Pyramid Lake water to determine whether special strategies are employed to excrete nitrogenous wastes in an environment unfavourable for ammonia excretion. The majority of nitrogen wastes (N-wastes) were excreted as ammonia (56 % through the gills, 10 % through the kidney), while urea excretion accounted for 34 % (32 % gills, 2 % kidney). Ammonia excretion was dependent on the NH3 partial pressure gradient (deltaPNH3) across the gills and independent of Na+ influx and acidification of the gill water boundary layer. Acute exposure to more alkaline water (pH 10) decreased ammonia excretion by 52 %, while exposure to neutral water (pH 7.6) increased ammonia excretion by 200 %. When fish were held in a ‘closed system’ for 8 h, ammonia excretion decreased as water ammonia levels increased over the first 6 h. However, after 6 h a marked increase in ammonia excretion occurred which may have been associated with an increase in the PNH3 gradient and/or activation of a carrier-mediated transporter. We conclude that Lahontan cutthroat trout, adapted to pH 9.4 water, maintain N-waste excretion by modifying mechanisms common to other teleosts. These modifications include lower rates of ammonia excretion, a higher ratio of urea excretion to ammonia excretion, a higher rate of renal ammonia excretion, greater plasma pH and greater total ammonia level (increased plasma PNH3), which facilitate the diffusive excretion of NH3 across the gills, and a lack of dependence of ammonia excretion on Na+ influx.

Urea-formaldehyde resin composition and method of manufacture thereof

Abstract: A urea-formaldehyde resin useful as a binder for making a variety of products, and of a method for making the resin. The resin is prepared by reacting formaldehyde, urea, triethanolamine, and optionally ammonia in a two-step process, first under alkaline conditions and then under acidic conditions. The urea-formaldehyde resin thus produced has good resistance to hydrolysis, cures quickly without smoking, and is characterized by low formaldehyde release.

Design of receptors for urea derivatives based on the pyrido[3,2-g]indole subunit

Abstract: The Fischer cyclization of appropriate 8-quinolinylhydrazones was employed to prepare a series of cavity-shaped hosts consisting of a central pyridine ring appended in either the 2,6- or 3,5-positions by two pyrido[3,2-g]indole subunits. The pyridine and pyridoindole moieties were further connected by dimethylene or trimethylene bridges which control the shape of the cavity. Hosts having a dimethylene bridge evidenced a strong affinity for urea derivatives in chloroform or dichloromethane solution. Binding constants were measured by NMR titration, and a structure-binding model was developed involving four strong hydrogen bonds.An X-ray analysis of the trimethylene-bridged host revealed a cavity which was too small to accomodate a urea guest.A decrease in the IR stretching frequency of the urea carbonyl in the complex was taken as a sign of diminished π-character of the C=O bond