Showing papers on "Urea published in 1985"

Control of ureogenesis

Abstract: Control of urea synthesis was studied in rat hepatocytes incubated with physiological mixtures of amino acids in which arginine was replaced by equimolar amounts of ornithine. The following observations were made. Intramitochondrial carbamoyl phosphate was always below 0.1 mM. Only when ornithine was absent and when, in addition, the concentration of amino acids was higher than four times their plasma concentration, intramitochondrial carbamoyl phosphate rose up to about 3 mM; under these conditions ammonia accumulated in the medium. The relationship between ornithine-cycle flux and the concentration of the cycle intermediates at varying amino acid concentration indicated that under near-physiological conditions the ornithine-cycle enzymes are far from being saturated with their subsidiaries. Moderate concentrations of norvaline had no effect on the rate of urea synthesis unless the cells were severely depleted of ornithine. Activation of carbamoyl-phosphate synthetase (ammonia) by addition of N-carbamoylglutamate only slightly stimulated urea production at all amino acid concentrations. However, in the presence of the activator the curve relating ornithine-cycle flux to the steady-state ammonia concentration was shifted to lower concentrations of ammonia. The intramitochondrial concentration of carbamoyl phosphate in rat liver in vivo was below 0.1 mM. This value is far below the concentration required for substantial inhibition of carbamoyl-phosphate synthetase. It is concluded that in vivo the function of activity changes in carbamoyl-phosphate synthetase, via the well-documented alterations in the intramitochondrial concentration of N-acetylglutamate, is to buffer the intrahepatic ammonia concentration rather than to affect urea production per se. At constant concentration of ammonia the rate of urea production is entirely controlled by the activity of carbamoyl-phosphate synthetase.

Transformations of nitrogen fertilizers in soil

Abstract: The effects of 15N-labelled urea, (NH4)2SO4 and KNO3 on immobilization, mineralization, nitrification and ammonium fixation were examined under aerobic conditions in an acid tropical soil (pH 4.0) and in a neutral temperate soil (pH 6.8). Urea, (NH4)2SO4 and KNO3 slightly increased net mineralization of soil organic nitrogen in both soils. There was also an apparent Added Nitrogen Interaction (ANI) i.e. added labelled NH4-N stood proxy for unlabelled NH4-N that would otherwise have been immobilized. So far as immobilization and nitrification were concerned, urea and (NH4)2SO4 behaved very similarly in each soil. Immobilization of NO3-N was negligible in both soils. Some of the added labelled NH4-N was rapidly fixed, more by the temperate soil than by the tropical soil. This labelled fixed NH4-N decreased during incubation, in contrast to labelled organic N, which did not decline.

Effects of form of nitrogen, season, and urea application rate on ammonia volatilisation from pastures

Abstract: Ammonia (NH3) volatilisation from various nitrogenous (N) fertilisers broadcast at different times of year on to pasture at a rate of 3 g/m2 N (30 kg/ha N) was measured by an enclosure method. The average percent losses of NH3-N were: urea, 11.9; diammonium phosphate, 5.3; ammonium sulphate, 1.0; and calcium ammonium nitrate, 0.8. Loss from sulphur-impregnated urea was equal to, or slightly less than that from urea. The loss was positively related to the maximum soil surface pH produced by each of the fertilisers. No seasonal pattern was observed, except that NH3 loss tended to be lower in August–September. The loss from urea increased from 13 to 33% of N applied as the application rate increased from 3 to 20 g/m2 N (30–200 kg/ha N).

Fate and efficiency of nitrogen fertilizers applied to wetland rice. II. Punjab, India

Abstract: Two modified urea products (urea supergranules [USG] and sulfur-coated urea [SCU]) were compared with conventional urea and ammonium sulfate as sources of nitrogen (N), applied at 58 kg N ha−1 and 116 kg N ha−1, for lowland rice grown in an alkaline soil of low organic matter and light texture (Typic Ustipsamment) having a water percolation rate of 109 mm day−1. The SCU and USG were applied at transplanting, and the whole dose of nitrogen was15N-labeled; the SCU was prepared in the laboratory and was not completely representative of commercial SCU. The SCU was broadcast and incorporated, whereas the USG was point-placed at a depth of 7–8 cm. The urea and ammonium sulfate applications were split: two-thirds was broadcast and incorporated at transplanting, and one-third was broadcast at panicle initiation. All fertilizers except the last one-third of the urea and ammonium sulfate were labeled with15N so that a fertilizer-N balance at flowering and maturity stages of the crop could be constructed and the magnitude of N loss assessed. At all harvests and N rates, rice recovered more15N from SCU than from the other sources. At maturity, the crop recovered 38 to 42% of the15N from SCU and only 23 to 31% of the15N from the conventional fertilizers, urea and ammonium sulfate, whose recovery rates were not significantly different. In contrast, less than 9% of the USG-N was utilized. Fertilizer nitrogen uptake was directly related to the yield response from the different sources. Most of the fertilizer N was taken up by the time the plants were flowering although recovery did increase up to maturity in some treatments. Analysis of the soil plus roots revealed that less than 1% of the added15N was in the mineral form. Between 20 and 30% of the15N applied as urea, SCU, and ammonium sulfate was recovered in the soil plus roots, mainly in the 0–15 cm soil layer. Only 16% of the15N applied as USG was recovered in the soil, and this15N was distributed throughout the soil profile to a depth of 70 cm, which was the lowest depth of sampling. Calculations of the15N balance showed that 46 to 50% of the urea and ammonium sulfate was unaccounted for and considered lost from the system. Only 27 to 38% of the15N applied as SCU was not recovered at maturity, but 78% of the USG application was unaccounted for. The extensive losses and poor plant recovery of USG at this site are discussed in relation to the high percolation rate, which is atypical of many ricegrowing areas.

The distribution and metabolism of urea in the eastern Canadian Arctic

Abstract: Urea concentrations, uptake, and excretion were measured at various locations in northern Baffin Bay and surrounding waters during the summer of 1980. Concentrations were variable ( 2.00 mg-at. N m−3) but followed patterns of decreasing concentration with depth in the euphotic zone and with distance from land. Urea accounted for > 50% of the total dissolved nitrogen in the upper mixed layer at most stations. Urea uptake rates showed generally the same distributional patterns as did concentrations and on the average accounted for 32% of the total nitrogen (NO3− + NH4+ + urea) productivity in the eupholic zone. Ammonium, and frequently NO3−, were utilized in preference to urea. Dual isotope (14C and 15N-urea) labelling experiments suggested that most urea-C was respired as CO2 while 50 to 80% of the urea-N was incorporated by the phytoplankton. Excretion measurements suggested that the four dominant macrozooplankton species (Calanus hyperboreus, C. finmarchicus, C. glacialis, and Metridia sp.) supplied only −3% of the urea-N but –40% of the NH4+-N requirements of the primary producers.

Ring cleavage and degradative pathway of cyanuric acid in bacteria.

Abstract: The degradative pathway of cyanuric acid [1,3,5-triazine-2,4,6(1H,3H,5H)-trione] was examined in Pseudomonas sp. strain D. The bacterium grew with cyanuric acid, biuret, urea or NH4+ as sole source of nitrogen, and each substrate was entirely metabolized concomitantly with growth. Enzymes from strain D were separated by chromatography on DEAE-cellulose and three reactions were examined. Cyanuric acid (1 mol) was converted stoichiometrically into 1.0 mol of CO2 and 1.1 mol of biuret, which was conclusively identified. Biuret (1 mol) was converted stoichiometrically into 1.1 mol of NH4+, about 1 mol of CO2 and 1.0 mol of urea, which was conclusively identified. Urea (1 mol) was converted into 1.9 mol of NH4+ and 1.0 mol of CO2. The reactions proceeded under aerobic or anoxic conditions and were presumed to be hydrolytic. Data indicate that the same pathway occurred in another pseudomonad and a strain of Klebsiella pneumoniae.

Effects of Ni Deficiency on Some Nitrogen Metabolites in Cowpeas ( Vigna unguiculata L. Walp)

Abstract: Cowpeas grown in nutrient solutions, from which Ni had been removed by a ligand exchange technique, accumulated urea in most tissues. Urea levels were highest (up to 3.1 percent dry weight) in necrotic leaf tips. Urea accumulation in Ni-deficient cowpea tissues amounted to about 1 percent of the total N. The accumulation of urea was presumably associated with the catabolism of N compounds in older tissues and the redistribution of N catabolites within the plant during the reproductive growth. The exclusion of N salts from the nutrient media at a late stage of growth, either with or without added Ni, led to a general amelioration of urea accumulation and a lower level of the related amino acid, arginine, in root and stem tissue. Plant leaves that contained toxic levels of urea and displayed necrotic symptoms had tissue Ni levels ranging from less than 0.01 to 0.15 μg Ni per gram dry weight. Nickel concentrations in tissue from plants not treated with Ni, were initially very low, but increased as the cowpeas matured. Apparently, there was a source of Ni contamination in the Ni-deficient growth media which provided a source of Ni for uptake by the plants during growth. Ureide levels were low and unaffected by Ni deprivation. No evidence for free purines or uric acid accumulation in plant tissues could be found. It is hypothesized that Ni (and urease) participates in the normal N metabolism of these plants during the reproductive phase of growth.

Hepatic urea synthesis and pH regulation: role of CO2, HCO3−, pH and the activity of carbonic anhydrase

Abstract: In isolated perfused rat liver, urea synthesis from ammonium ions was dependent on extracellular HCO3- and CO2 concentrations when the HCO3-/CO2 ratio in the influent perfusate was constant (pH 7.4). Urea synthesis was half-maximal at HCO3- = 4 mM, CO2 = 0.19 mM and was maximal at HCO3- and CO2 concentrations above 20 mM and 0.96 mM, respectively. At physiological HCO3- (25 mM) and CO2 (1.2 mM) concentrations in the influent perfusate, acetazolamide, the inhibitor of carbonic anhydrase, inhibited urea synthesis from ammonium ions (1 mM) by 50-60% and led to a 70% decrease in citrulline tissue levels. Acetazolamide concentrations required for maximal inhibition of urea synthesis were 0.01-0.1 mM. At subphysiological HCO3- and CO2 concentrations, inhibition of urea synthesis by acetazolamide was increased up to 90%. Inhibition of urea synthesis by acetazolamide was fully overcome in the presence of unphysiologically high HCO3- and CO2 concentrations, indicating that the inhibitory effect of acetazolamide is due to an inhibition of carbonic-anhydrase-catalyzed HCO3- supply for carbamoyl-phosphate synthetase, which can be bypassed when the uncatalyzed intramitochondrial HCO3- formation from portal CO2 is stimulated in the presence of high portal CO2 concentrations. With respect to HCO3- supply of mitochondrial carbamoyl-phosphate synthetase, urea synthesis can be separated into a carbonic-anhydrase-dependent (sensitive to acetazolamide at 0.5 mM) and a carbonic-anhydrase-independent (insensitive to acetazolamide) portion. Carbonic-anhydrase-independent urea synthesis linearly increased with the portal 'total CO2 addition' (which was experimentally determined to be CO2 addition plus 0.036 HCO3- addition) and was independent of the perfusate pH. At a constant 'total CO2 addition', carbonic-anhydrase-dependent urea synthesis was strongly affected by perfusate pH and increased about threefold when the perfusate pH was raised from 6.9 to 7.8. It is concluded that the pH dependent regulation of urea synthesis is predominantly due to mitochondrial carbonic anhydrase-catalyzed HCO3- supply for carbamoyl phosphate synthesis, whereas there is no control of urea synthesis by pH at the level of the five enzymes of the urea cycle. Because HCO3- provision for carbamoyl phosphate synthetase increases with increasing portal CO2 concentrations even in the absence of carbonic anhydrase activity, susceptibility of ureogenesis to pH decreases with increasing portal CO2 concentrations. This may explain the different response of urea synthesis to chronic metabolic and chronic respiratory acidosis in vivo.

Apparent compartmentation of body nitrogen in one human subject: its consequences in measuring the rate of whole-body protein synthesis with 15N.

Abstract: 1. The rate of protein synthesis in the whole body was measured in one fed subject with seven 15 N-labelled amino acids (intravenous and oral doses) and two 15 N protein mixtures (oral doses only). The rates were determined individually from the urinary excretion of ammonia and total urea over a 12 h experimental period. 2. Except with oral glycine and alanine, the synthesis rates given by ammonia and urea were appreciably different within each study when calculated on the assumption of a single pool of metabolic nitrogen in the body. In general, intravenous administration of the tracers gave higher rates with urea and the oral route gave higher rates with ammonia. 3. The differences between intravenous and oral doses of 15 N could be reduced significantly by calculating synthesis rates from either the arithmetic or harmonic average of flux rates given by ammonia and urea. The averages correspond to estimates of the total flux in a two-pool model of metabolic nitrogen when it is assumed either that both pools receive an equal amount of tracer (arithmetic) or that both have the same rate of nitrogen turnover (harmonic). 4. By so reducing the effect of physical separation of nitrogen in the body, the metabolic aspects of compartmentation of the tracer could be examined. The results show that the absolute value obtained for protein synthesis depends on the source of labelled nitrogen. The data are discussed in this empirical context.

The Osmoprotective Properties of Urine for Bacteria: The Protective Effect of Betaine and Human Urine Against Low pH and High Concentrations of Electrolytes, Sugars, and Urea

Abstract: Growth of Escherichia coli was inhibited in a defined minimal medium by high concentrations of electrolytes and sugars in direct relation to their osmotic strength. Choline, betaine, proline, and human urine increased resistance to these substances. In contrast, the toxic effect of urea was not altered directly by betaine or urine, but was reduced in the presence of other osmolytes. The osmolyte protective effect was augmented by betaine. The osmoprotective effect of betaine and urine was confirmed with 40 strains of enteric bacteria. Urine from 19 healthy subjects contained osmoprotective activity greater than that observed with betaine. A methanol extract of urine was found to be highly protective. Although betaine was present in the extract, it could not account for all the protective activity. Urine contains additional low-molecular-weight osmoprotective agents.

Decreased urea synthesis in cafeteria-diet-induced obesity in the rat.

Abstract: Feeding rats with a cafeteria diet resulted in increases in total body weight and in epididymal-adipose-tissue weight Those rats excreted significantly less N than did controls The amount of N ingested by cafeteria-diet-fed rats was kept equal to that of controls This decrease in N excretion is explained by a decrease in urinary excretion of urea This may be due to the following facts The rate of synthesis of urea from precursors by isolated hepatocytes from cafeteria-diet-fed rats was lower than in controls In cafeteria-diet-fed rats the activities of all the enzymes of the urea cycle are decreased The major percentage decreases are those of carbamoylphosphate synthetase (EC 63416) and of argininosuccinate synthetase (EC 6345), the enzymes probably involved in the regulation of the overall rate of the cycle When rats are switched to normal chow diet, the enzyme activities return to normal values The uptake of amino acids by liver of cafeteria-diet-fed rats is lower than in controls These results contrast with those obtained previously by using other models of obesity in rat (ie genetic or hypothalamic), in which N excretion was increased

Nitrification in forest soil of different pH as affected by urea, ammonium sulphate and potassium sulphate

Abstract: Nitrification was inhibited by ammonium sulphate and potassium sulphate added to soil from the organic horizon (pH 4.7) of a Myrtillus-type pine forest. Urea did not inhibit nitrification. Soil pH was slightly decreased by the salts but increased by urea. The salts increased soil electrical conductivity more than urea did. The inhibition of nitrification following salt treatments was probably due to a decrease in soil pH and not to osmotic effects. In acid conditions, the salts had a less inhibitory effect on CO 2 production than on nitrification, indicating that nitrifying bacteria were more sensitive than other organisms to the salts.

Factors Related to Urea Hydrolysis in Soils

Abstract: The use of urea as a nitrogen source for crop production has increased in recent years. Because the hydrolysis of urea to ammonia and carbon dioxide is mediated by the enzyme urease, efficient use of urea requires an understanding of factors that influence urea hydrolysis in soil. The purpose of this research was to relate the rate of urea hydrolysis to chemical, physical, and biological properties for soils in pasture and in cultivation. For each of 22 soils in Kansas, Missouri, and Oklahoma, samples were collected from paired fields, one in pasture and one in cultivation. The samples were taken from both 0 to 2.5 cm and 0 to 15 cm, and urea hydrolysis rates were measured in field-moist and air-dried samples. Urea hydrolysis rates in air-dried soils ranged from 1 to 149 µg urea g soil⁻¹ h⁻¹ and from 1 to 117 µg urea g soil⁻¹ h⁻¹ in field moist soils. For each soil, the greatest urea hydrolysis rate was in the 0 to 2.5-cm depth of pasture samples. The urea hydrolysis rate for each soil was greater in pasture samples than in samples from cultivated fields. For samples from cultivated fields, urea hydrolysis rates in both field-moist and air-dry samples were positively correlated with total nitrogen (TN) and organic carbon (OC). Only in the field moist soils were urea hydrolysis rates correlated with TN and OC in the soils from pastures. Urea hydrolysis rates in the 0 to 2.5-cm depth of pastures exhibited the greatest change from field-moist to air-dry conditions. Rates generally decreased with drying, but did remain constant or increased with drying in some cases. Both the greatest rates and the greatest variation in urea hydrolysis rates between field-moist and air-dry soils were observed in the surface 0 to 2.5 cm samples from pastures.

The contribution of endogenous urea to faecal ammonia in man, determined by 15N labelling of plasma urea.

Abstract: 1. To establish the role of endogenous urea as a source of faecal ammonia, the plasma urea of two healthy men was labelled with 15 N at a constant level for several days and its 15 N enrichment was compared with that of faecal ammonia and total nitrogen. 2. Faeces collected after one complete gastrointestinal transit from the onset of plasma labelling had ammonia 15 N enrichments which were only 8.5 ± 1.2% and total nitrogen enrichments which were 6.8 ± 0.7% of the plasma urea 15 N enrichment. 3. These results show that endogenous urea is not the main precursor of faecal ammonia, which is probably derived by bacterial deamination from the protein of dietary residues, intestinal secretions and shed epithelial cells. The minor contribution of endogenous urea to faecal ammonia suggests that the lumen of the large bowel is not the main site of endogenous urea hydrolysis. The similar labelling of faecal total nitrogen and ammonia nitrogen supports other evidence that these faecal nitrogen fractions are in a constant state of exchange.

Stimulation by nickel of soil microbial urease activity and urease and hydrogenase activities in soybeans grown in a low-nickel soil

Abstract: The concentration of nickel in some soils may be insufficient to meet the requirements of enzymes such as urease in soybeans and hydrogenase in Rhizobium. In an initial evaluation of nickel availability, several soils were examined for nickel content and microbial urease activity. Total and extractable nickel were determined by atomic emission spectrometry. Purified glucose and urea were added to soils to stimulate microbial growth and urease activity, the latter of which was monitored by the rate of decomposition of14C urea. Nickel also was added to some samples to determine if the indigenous supply was limiting. In one low-nickel soil (total Ni 13 ppm) urease activity increased 150% in response to additional nickel, while other soils (total Ni 22–3491 ppm) failed to respond to nickel. However, additional nickel did stimulate urease activity (up to 109%) in 3 out of 10 soils to which purified CaCO3 was added. Presumably the rise in pH associated with this treatment decreased nickel availability. Additions of Co, Mn, Fe, or Cu had no consistent effect on urease activity, thus indicating that the response to Ni was specific. Nickel fertilization increased leaf urease and nodule hydrogenase activity of soybeans grown in low-nickel soil, however, yield was not improved. These results may have practical implications in the nutrition of plants and micro-organisms that metabolize H2 and urea.

Effect of intraruminal infusion of urea on the response in voluntary food intake by sheep

Abstract: Six mature Merino sheep received three treatments in a randomized block design experiment. The treatments were: chopped oaten hay diet at 90% of ad libitum intake without urea (L); the ciet offered at the same level as for L with urea infused into the rumen at 11.5 g kg-1 dry matter intake (LU); and the diet offered at 90% of the ad libitum intake achieved with urea infused at 11.5 g kg-1 dry matter intake (HU). Sheep given HU consumed 37% more (P < 0.01) organic matter (OM) than those fed L or LU, but the apparent digestibility of OM did not vary (59.2-61.8%) between treatments. The addit onal food consumption was associated with c. 20% increase (P < 0.05) in the weight of OM in the reticulorumen and significantly higher (by 10-35%; P < 0.05) fractional outflow rates of most dietary and microbial constituents of digesta. The fractional digestion rate of potentially digestible plant cell walls was not affected by urea, but the flow of microbial non-ammonia nitrogen from the abomasum was enhanced (L, 7.0; LU, 8.2; HU, 12.5 g day-1; P < 0.05). The results of this study suggest that the stimulatory effect of urea upon food intake was associated with the provision of additional microbial protein for digestion in the intestines, rather than changes in the rate or extent of organic matter fermentation in the reticulorumen.

Water and urea transport in renal microvillus membrane vesicles.

Abstract: Light scattering was used to measure the water and urea permeability of brush border membrane vesicles (BBMV) isolated from rabbit renal cortex. In stop-flow experiments, exposure of BBMV to a 200 mM inwardly directed mannitol gradient gave a monophasic time course of decreasing BBMV volume corresponding to an osmotic water permeability (Pf) of 1.1 +/- 0.1 X 10(-2) cm/s at 37 degrees C. The temperature dependence of Pf was biphasic with delta H = 2 kcal/mol for T less than 33 degrees C and delta H = 14 kcal/mol for T greater than 33 degrees C. A 200 mM inwardly directed urea gradient gave a biphasic time course of BBMV volume due to rapid water efflux (approximately 50 ms) followed by slower urea influx (1-5 s) with urea permeability (Purea) of 2.4 +/- 0.2 X 10(-6) cm/s. Preincubation of BBMV with increasing [urea] reversibly inhibited both urea flux (Kd = 1,200 mM) and thiourea flux (Kd = 370 mM) according to a single-site inhibition model, suggesting a saturable urea carrier. Comparison of BBMV Pf and Purea with proximal tubule transepithelial water and urea transport rates suggests that the permeability of the tubular apical membrane (BBMV) is high enough to support a transcellular route for both osmotic water and urea transport.

Enzymic Degradation of Allantoate in Developing Soybeans

Abstract: A Mn(2+)-dependent enzymic breakdown of allantoate has been detected in crude and partially purified extracts of developing soybeans. The products detected were CO(2), NH(3), glyoxylate, labile glyoxylate derivatives, and low levels of urea. Urea is initially produced at less than 10% the rate of urease-independent CO(2) release indicating that the activity is not allantoate amidinohydrolase (i.e. urea is not directly cleaved off allantoate). The urease-independent CO(2) releasing activity has an apparent K(m) of 1.0 millimolar for allantoate. Ethylenediaminetetraacetate, borate, and acetohydroxamate (all at 10 millimolar) inhibit the enzymic production of NH(3), CO(2), and labile glyoxylate derivatives from allantoate. However, the potent urease inhibitor, phenyl phosphordiamidate does not inhibit CO(2) and NH(3) release indicating that the action of acetohydroxamate is not due to its inhibition of urease. That the allantoatedegrading activity was more than 5-fold greater in seed coats than in embryos is consistent with the data of Rainbird et al. (Plant Physiol 1984 74: 329-334) which indicate that available ureides are metabolized before reaching the embryo. 2-Ethanolthio, 2'ureido, acetic acid (NH(2)COHNCHCO(2)HSCH(2)CH(2)OH), the first allantoate-derived product detected by HPLC analysis, is an addition produced of mercaptoethanol with an unidentified enzymically produced ureido intermediate that is not derived from ureidoglycolate or oxalurate.

Urea stabilized with a lactone in various pharmaceutical and cosmetic preparations

Abstract: A pharmaceutical preparation containing 1 to 50% urea and 0.05 to 30% of a lactone is disclosed. A preferred lactone is gamma-butyrolactone which is preferably present in an amount of about 0.05 to 0.1 part by weight per 1 part by weight of urea. The lactone stabilizes the urea with respect to various parameters including changes in pH. The pharmaceutical preparation may be any preparation with stabilized urea such as a skin lotion or cream used to moisturize skin, or a post surgical preparation because urea aids in debridement, dissolves coagulum and promotes epithelialization. Alternatively the preparation may be an injectable solution used in the treatment for mobilization of edema fluid, as urea is used for an osmotic diuretic.

Computer experiments on aqueous solutions. VII. Potential energy function for urea dimer and molecular dynamics calculation of 8 mol % aqueous solution of urea

Abstract: Molecular dynamics calculation (MD) has been carried out for an aqueous solution of urea at 298.15 K and with experimental density value at ordinary pressure by the use of constant temperature technique developed previously. The total number of molecules is 216, of which 17 are urea. The mole fraction of urea in the solution is thus 0.078. For water–water and water–urea interactions, the MCY potential and previously determined potential have been used. A new urea–urea pair potential is determined by ab initio LCAO SCF calculations for more than 750 different dimer configurations with an STO‐3G basis set and subsequent multiparameter optimization of the MO data to a 12‐6‐3‐1 potential energy function. The MD calculation is extended up to 76 000 time steps and final 44 000 time steps (17.6 ps) are used to calculate both static and dynamic properties. Among other information, the following results are important and interesting: (1) Urea molecules exhibit appreciable self‐association, (2) such association is ...

A method for determination of the capacity of urea synthesis in the rat.

Abstract: The relationship between total blood alpha-amino nitrogen concentration and urea synthesis rate was investigated with alanine as nitrogen source in 24 rats. Alanine was given as prime-continuous doses for 70 min so that constant amino acid concentration was attained between 5.5 and 34 mmol/1. Urea synthesis rate was assessed as accumulation in body water, corrected for intestinal hydrolysis. There was a positive correlation between nitrogen balance and alpha-amino nitrogen concentration. Urea synthesis rate in relation to amino acid concentration suggested barrier-limited substrate inhibition kinetics and data were examined accordingly by non-linear regression analysis. The estimated kinetic constants (mean ± standard deviation) were: Vmax: 19.2 ± 3.3 μrnol (min. 100 g BW)-1, Km: 1.74 ± 0.5 mmol/1, K- 6.84 ± 1.9 mmol/1, and the barrier : 5.4 ± 0.13 mmol/1. Because of the substrate inhibition, saturation cannot be attained, but the maximum synthesis rate, i.e. the capacity of urea nitrogen synthesis (CUNS)...

Increased Intestinal Hydrolysis of Urea in Patients with Alcoholic Cirrhosis

Abstract: Fourteen patients with biopsy-proven alcoholic liver cirrhosis in a clinically stable phase but with compromised liver function entered the study, together with 10 control persons. All had normal creatinine clearance, and none received antibiotics or hormones. They ingested a diet containing 1 g of protein/kg body weight daily during the study. The fractional intestinal loss of newly synthesized urea, determined by a 14C-urea tracer method, was increased from 0.17 ± 0.08 in controls to 0.26 ± 0.08 in cirrhotics (mean ± SD, P<0.02). Urea nitrogen synthesis rate, determined as urinary excretion rate, corrected for accumulation in the total body water and for fractional intestinal loss, was the same in controls and cirrhotics (26.1 ± 3.8 and 22.1 ± 6.8 mmol/h, respectively). The patients with cirrhosis had a significantly greater nitrogen balance than the control group (12.5 ± 7.0 mmol/h versus 7.0 ±5.9 mmol/h; P < 0.05). Furthermore, there was a positive correlation between intestinal loss and blood urea ni...