Showing papers on "Urea published in 1972"

Assay of urease activity in soils

Abstract: A simple and precise method of assaying urease activity in soils is described. It involves determination of the ammonium released by urease activity when soil is incubated with tris(hydroxymethyl)aminomethane (THAM) buffer, urea solution, and toluene at 37°C for 2 h, ammonium release being determined by a rapid procedure involving treatment of the incubated soil sample with 2.5 M KC1 containing a urease inhibitor (Ag 2 SO 4 ) and steam distillation of an aliquot of the resulting soil suspension with MgO for 3.3 min. Studies reported showed that the optimal buffer pH and substrate (urea) concentration for assay of soil urease activity using THAM buffer are 9.0 and 0.02 M, respectively, and that the method described is satisfactory for assay of urease activity in ammonium-fixing soils.

Dynamic aspects of ammonia and urea metabolism in sheep

Abstract: 1. To obtain a quantitative model for nitrogen pathways in sheep, a study of ammonia and urea metabolism was made by using isotope dilution techniques with [15N]ammonium sulphate and [15N]urea and [14C]urea.2. Single injection and continuous infusion techniques of isotope dilution were used for measuring ammonia and urea entry rates.3. Sheep were given 33 g of chaffed lucerne hay every hour; the mean dietary N intake was 23.4 g/d.4. It was estimated that 59% of the dietary N was digested in the reticulo-rumen; 29% of the digested N was utilized as amino acids by the micro-organisms, and 71% was degraded to ammonia.5. Of the 14.2 g N/d entering the ruminal ammonia pool, 9.9 g N/d left and did not return to the pool, the difference of 4.3 g N/d represented recycling, largely within the rumen itself (through the pathways: ruminal ammonia → microbial protein → amino acids → ammonia).6. Urea was synthesized in the body at a rate of 18.4 g N/d from 2.0 g N/d of ammonia absorbed through the rumen wall and 16.4 g N/d apparently arising from deamination of amino acids and ammonia absorbed from the lower digestive tract.7. In the 24 h after intraruminal injection of [15N]ammonium salt, 40–50% of the N entering the plasma urea pool arose from ruminal ammonia; 26% of the 15N injected was excreted in urinary N.8. Although 5.1g N/d as urea was degraded apparently in the digestive tract, only 1.2g N/d appeared in ruminal ammonia; it is suggested that the remainder may have been degraded in the lower digestive tract.9. A large proportion of the urea N entering the digestive tract is apparently degraded and absorbed and the ammonia incorporated in the pools of nitrogenous compounds that turn over only slowly. This may be a mechanism for the continuous supply to the liver of ammonia for these syntheses.10. There was incorporation of 15N into bacterial fractions isolated from rumen contents after intraruminal and intravenous administration of [15N]ammonium salts and [15N]urea respectively.11. A model for N pathways in sheep is proposed and, for this diet, many of the pool sizes and turn-over rates have been either deduced or estimated directly.

The Uptake of Urea by Natural Populations of Marine PHYTOPLANKTON1

Abstract: Nitrogen-15 isotopes were used to study the uptake of nitrate, ammonium, and urea by natural phytoplankton populations in 36 samples collected at nine stations off the coast of southern California. The percentage of the total phytoplankton nitrogen productivity accounted for by urea varied from 60% and for the entire study averaged 28%. The percentage of total available nitrogenous nutrient (ambient nitrate, ammonium, and urea plus the 15N additions) utilized per day varied among the stations from a minimum of 5% at station 4 (12 km off San Diego) to a maximum of 46% at station 19 (off White Point). The average C:N uptake ratio was 12.4.

Production and excretion of urea by the fetal lamb

Abstract: The urea production rate of the fetal lamb has been measured in 10 pregnant ewes studied who were free of operative stress. The measurement of the fetal urea production rate was made by utilizing an independent measurement of placental urea clearance by the infusion of urea tagged with radioactive carbon (14C) into the fetal circulation and the endogenous concentration difference for urea between umbilical arterial blood and maternal arterial blood. It was shown that the concentration difference of urea between umbilical arterial and maternal arterial blood is significantly elevated during the first four days of recovery from surgery, with a mean value of 0.06 mg/ml plasma water on the first day. By the fifth postoperative day this concentration difference has decreased to a mean value of 0.034, which does not change from the fifth to the twelfth postoperative days. This study demonstrates that urea is excreted across the placenta at a rate of approximately 0.54 mg/min/kg fetal weight. By comparison with the urea excretion rate of adult sheep, this is a high rate of fetal urea excretion. It was demonstrated that the urea excretion rate across the placenta could be increased threefold by an infusion of ammonium lactate into the fetus. Thus, the endogenous urea excretion rate of 0.54 mg/min/kg fetal weight does not represent the maximum rate of urea production for the fetus. The finding of a high urea production rate in fetal life is new and contrary to the common belief that nitrogen catabolism plays a minor role in fetal life. These studies demonstrate that as much as 25% of the fetal oxygen consumption could be accounted for by the catabolism of amino acids.

a Comparison of Chemical, Isotopic, and Enzymatic Methods for Measuring Nitrogen Assimilation of Marine PHYTOPLANKTON1

Abstract: Shipboard cultures of natural seawater enriched with nitrate, ammonium, or urea were used to compare different techniques for measuring nitrogen assimilation by phytoplankton and to measure the rates of uptake of these nutrients when more than one was present. Increases in particulate nitrogen, decreases of nitrogenous nutrients, and the uptake by phytoplankton (using the 15N-isotope technique) balanced rather well and for the nitrate-enriched culture were in good agreement with the estimate of nitrogen assimilation from nitrite reductase activity. Glutamic dehydrogenase activity was present with both NADH and NADPH in all three cultures; either singly or as a sum it agreed poorly with ammonium uptake estimated by the 15N-isotope technique. The presence of ammonium almost totally suppressed the uptake of nitrate and urea over a wide range of concentrations. The suppression of nitrate uptake in the presence of urea was nearly the same as the suppression of urea uptake in the presence of nitrate. Comparisons between the isotopic and enzymatic techniques were also made in seawater samples. Nitrite reductase activity was usually much greater than the rate of nitrate uptake estimated by the 15N-isotope technique. Comparisons with chlorophyll a and ATP analysis indicate that chlorophyll-containing cells were primarily responsible for the nitrite reductase activity. The rate of pyridine nucleotide oxidation with both NADII and NADPII was more consistent with the ATP than with the chlorophyll a content of seawater samples, suggesting that this enzymatic activity was not restricted to the phytoplankton.

Utilization of Urea by Some Marine PHYTOPLANKTERS1

Abstract: Three diatoms from the Sargasso Sea and two diatoms and a haptophyte flagellate from an inshore area near Woods IIolc exhibited similar growth rates on urea, nitrate, and ammonia. A coccolithophore Emiliana (Coccolithus) huxleyi from the Sargasso Sea did not grow on urea. The half-saturation constant (K) for urea determined for one inshore diatom, Stephanopyxis costata ( Skeletonema costatum), was 8.5 pg-atom urea-N liter-l. At the urea concentrations of its habitats, the calculated division rate for this species, growing on urea as the sole nitrogen source, was 2.2 days, similar to that of diatoms growing in inshore habitats. It appears, from data on natural urea concentrations and urea uptake rates by S. costata, that urea can be ,a significant N source for at least one common inshore phytoplankter. According to growth rate studies, at least three inshore and three offshore marine phytoplankton can grow as rapidly on urea as on nitrate or ammonia,

Urea transport in the proximal tubule and the descending limb of Henle

Abstract: Urea transport in proximal convoluted tubule (PCT) and descending limb of Henle (DLH) was studied in perfused segments of rabbit nephrons in vitro. Active transport of urea was ruled out in a series of experiments in which net transport of fluid was zero. Under these conditions the collected urea concentration neither increased nor decreased when compared to the mean urea concentration in the perfusion fluid and the bath. Permeability coefficient for urea (P(urea)) was calculated from the disappearance of urea-(14)C added to perfusion fluid. Measurements were obtained under conditions of zero net fluid movement: DLH was perfused with isosmolal ultrafiltrate (UF) of the same rabbit serum as the bath, while PCT was perfused with equilibrium solution (UF diluted with raffinose solution for fluid [Na] = 127 mEq/liter). Under these conditions P(urea) per unit length was 3.3+/-0.4 x 10(-7) cm(2)/sec (5.3+/-0.6 x 10(-5) cm/sec assuming I.D. = 20mu) in PCT and 0.93+/-0.4 x 10(-7) cm(2)/sec (1.5+/-0.5 x 10(-5) cm/sec) in DLH. When compared to previously published results, these values show that the PCT is 2.5 times less permeable to urea than to Na, while the DLH is as impermeable to urea as to Na. These results further indicate that the DLH is less permeable to both Na and urea than the PCT. The reflection coefficient for urea, sigma(urea), was calculated as the ratio of induced solution efflux when 95 mOsm/liter of urea was added to the bath, as compared to net fluid movement induced by addition to the bath of equivalent amount of raffinose, sigma(urea) in DLH is 0.95+/-0.4 as compared to 0.91+/-0.05 in PCT. sigma(urea) in DLH is approximately equal to sigma(Na); however, sigma(urea) in PCT is higher than sigma(Na) (0.68). Several types of studies were conducted to examine the role of urea and urea plus sodium chloride in concentrating the fluid in the DLH. From the obtained results it was concluded that the intraluminal fluid of DLH is primarily concentrated by abstraction of water without significant net entry of solute. These results are discussed with respect to possible significance in the overall operation of the countercurrent system.

Electrochemical Oxidation of Uric Acid and Xanthine at the Pyrolytic Graphite Electrode Mechanistic Interpretation of Electrochemistry

Abstract: The quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between pH 1–7, the most commonly encountered pH range in biological systems. Basically, uric acid is oxidized in a primary oxidation of the bond to give a readily reducible and highly reactive bis‐imine. The bis‐imine can be detected as a cathodic peak by fast sweep cyclic voltammetry. Complete hydration of the bis‐imine gives rise to uric acid‐4,5‐diol that breaks down to alloxan and urea at pH 1, allantoin and a small amount of urea at pH 7, and at intermediate pH values mixtures of alloxan, allantoin, urea, and occasionally traces of parabanic acid are formed. Xanthine undergoes a oxidation that proceeds by two stages. The first involves oxidation of the bond to give uric acid which, being more easily oxidized than xanthine, is immediately further oxidized to the uric acid bisimine. This then hydrates and fragments in essentially the same way as described for uric acid, the same products being observed. Mechanisms are developed which explain the observed electrochemistry and over‐all formation of products from this reaction. The nature and yields of products formed electrochemically and the effect of pH on the reaction parallels the biological (enzymatic) oxidation of these compounds.

Factors Influencing Ammonia Losses From Urea Applied to Northwestern Forest Soils

Abstract: Laboratory studies indicated that volatilization losses of NH₃ increased with increasing air movement, temperature, and pH of mineral soils and forest floors. At an air flow rate of 3 liters/min (equivalent to calculated air velocity of approximately 0.01 km/hour) and temperature of 19°C, losses were high—ranging from 6 to 30% of the urea N applied to bare mineral soils and 27 to 46% for mineral soils covered by forest floors. Ammonia losses were less for mixtures of NH₄Cl and urea crystals than for urea crystals or pellets alone. Losses were not related to urea pellet size.

Pelvic urine composition as a determinant of inner medullary solute concentration and urine osmolarity.

Abstract: To clarify the question whether solute and water fluxes between pelvic urine and the renal papilla contribute to the medullary accumulation of osmotically active substances and thus to final urine concentration, we measured the osmolarity of urine samples from the papillary tip of rat kidneys during superfusion of the exposed papillae with solutions of widely varying osmotic concentrations. When the osmolarity of the superfusion fluid consisted half of urea and half of sodium chloride, urine osmolarity was observed to change parallel to the bath solution over a certain concentration range (800–2000 mosm/l). The changes of urine concentration occurred within 90 min after the start of the papillary superfusion. Similar results were obtained when the sodium chloride concentration was kept constant at 300 mosm/l and the urea concentration varied to yield bath concentrations up to 3000 mosm/l. A rise of urine concentration by papillary superfusion above 2000 mosm/l was achieved when exogenous arginine-vasopressin was infused intravenously suggesting that the failure of urine concentration to equilibrate with the bath concentration was due to a limited water permeability of the collecting ducts. These results suggest that solute and water fluxes between pelvic urine and the renal papilla are a necessary prerequisite to achieve maximal osmotic urine concentrations. In addition, such fluxes may explain the variability of medullary tissue concentrations under various diuretic states.

Urea and other nitrogenous nutrients in la jolla bay during february, march, and april 1970

Abstract: Samples collected from La Jolla Bay twice weekly for 2'12 months were analyzed for nitrate, nitrite, ammonium, and urea in addition to other chemical, physical, imd biolog­ ical parameters. On the basis of an infestation of blue sharks (P1'ionace glauca) , periods before, during, and after the infestation were defined. Statistical analyses indicated that: 1) Urea concentrations were highest during the period of shark infestation. 2) There was strong positive correlation between phaeo-pigment/chlorophyll ratios and ammonium concentrations during the infestation but none between the pigment ratios and either the ammonium concentrations for the other two periods or the urea concen­ trations for any of the three periods. 3) There was no correlation between ammonium and urea concentrations before, a strong positive correlation during, and no correlation after the shark infestation. 4) Urea was the only nitrogenous nutrient for which the concentrations above and below the thermocline were not different. 5) Comparisons between two stations 1.5 km distant indicate that on a horizontal scale, the patch struc­ ture for urea is smaller than that of the other nitrogenous nutrients although the median urea concentration in the water column was not different at the two stations. The temporal similarity and the more complex patch structure for urea (as seen in 4 and 5 above) suggest that the blue sharks were responsible for the higher urea con­ centrations during the infestation. Although the median ammonium concentrations before and during the infestation were not different, the strong positive correlation between ammonium and urea concentrations during the infestation hint that the sources or rates of supply and utilization for both nutrients may have been closely related. The strong positive correlation between phaeo-pigment/chlorophyll ratios and ammonium concentrations during the infestation may imply that the source of ammonium was her­ bivore excretion.

Interaction of urea with poly(2‐hydroxyethyl methacrylate) hydrogels

Abstract: The interaction of poly(2-hydroxyethyl methacrylate) [poly(HEMA)] and other similar hydrogels with dilute urea solution has been studied by a variety of techniques, including swelling experiments, fluorescence quenching, near infrared spectroscopy and fundamental band infrared spectroscopy. The results obtained indicate that the anomalous swelling behavior of poly(HEMA) gels in the presence of such dilute urea solutions is probably not due to the disruption of a secondary hydrophobic bond structure as has been generally believed. Although poly(HEMA) gels do contain sites that can participate in hydrophobic bonding, the evidence gathered indicates that dilute urea solutions have no effect on such bonds. A plausible model that does fit all the data involves the interaction of urea with a secondary structure composed of hydrogen-bonded hydroxyl groups, stabilized by the exclusion of water molecules from the regions containing the bonds.

The effect of antidiuretic hormone on solute flows in mammalian collecting tubules

Abstract: These experiments were intended to evaluate the antidiuretic hormone (ADH)-dependent reflection coefficients of urea, sucrose, and NaCl in cortical and outer medullary collecting tubules isolated from mammalian kidney. In one group of experiments, the ADH-dependent osmotic water flows, when the perfusing solutions contained hypotonic NaCl solutions, were indistinguishable from control observations when either urea or sucrose replaced, in part, NaCl in isotonic bathing solutions (cortical collecting tubules). Similarly, both in cortical and outer medullary collecting tubules exposed to ADH, there was zero net osmotic volume flow when a portion of the NaCl in the bathing and/or perfusing solutions was replaced by either sucrose or urea, so long as the perfusing and bathing solutions were isosmolal. Taken together, these observations suggest that the ADH-dependent reflection coefficients of NaCl, urea, and sucrose, in these tubules, were identical. Since the effective hydrodynamic radii of urea and sucrose are, respectively, 1.8 and 5.2 A, it is likely that sigma(i), for urea, sucrose, and NaCl, was unity. In support of this, the diffusion permeability coefficient (P(Di) cm sec(-1)) of urea was indistinguishable from zero. Since the limiting sites for urea penetration were the luminal interfaces of the tubules, these data are consistent with the view that ADH increases diffusional water flow across such interfaces.

The uptake of urea by marine phytoplankton1

Abstract: SUMMARY Half-saturation constants for urea uptake by 4 clones of neritic diatoms capable of utilizing urea were determined from short-term uptake studies with 15N-labeled urea. K8 values obtained were similar to those determined, earlier for ammonium, and since ammonium and urea concentrations are similar in the marine environment, it was concluded that these species are capable of utilizing ecologically significant concentrations of urea. Two of 3 species unable to grow on urea showed patterns of short-term uptake not unlike those of species capable of utilizing urea, which implies that, their assimilatory rather than uptake processes are defective with, regard to urea utilization. The third species initially took 15N (supplied as urea) into the cells but subsequently released it back into the medium.

Formation of urea and guanidine by irradiation of ammonium cyanide.

Abstract: Aqueous solutions of ammonium cyanide yield urea, cyanamide and guanidine when exposed to sunlight or an unfiltered 254 nm ultraviolet source. The prebiotic significance of these results is discussed.

Degradation of poly(methyl 2‐cyanoacrylates)

Abstract: Methyl 2-cyanoacrylate-2-14C, -3-14C and -14CN tagged monomers were im planted subcutaneously in mongrel dogs and their urine assayed for radioactivity. The results show that the urea from the -3-14C was radioactive and radioactive acidic components, as well as neutral moieties, were present. Urea recovered from the urine of animals in which the -2-14C and -14CN methyl 2-cyanoacrylate were implanted was only very slightly radioactive. The data are consistent with the hypothesis that hydrolytic chain scission and ester hydrolysis may be involved in the degradation of 2-cyanoacrylates in vivo.

Reversible Transformation by Urea of Contact-Inhibited Fibroblasts

Abstract: 200 mM urea elicits alterations of cell social behavior in vitro. In the presence of urea, contact inhibition of movement and growth is reduced compared to untreated fibroblastic cell populations. This reduction of contact inhibition is rapidly reversible, but reversal requires some cycloheximide-sensitive cellular process. Cells treated with urea are agglutinable by concanavalin A, suggesting that urea might remove some cell-surface component(s). A nondialyzable factor can, in fact, be detected in the supernatant medium of urea-treated cells. This active constituent appears to be heat-labile and trypsin-sensitive, and can itself restore normal contact-inhibitory behavior to cells maintained in the urea-transformed state by cycloheximide. This system may permit identification and characterization of surface components that are involved in regulating normal cell social behavior.