Showing papers on "Urea published in 1990"

pH dependence of the urea and guanidine hydrochloride denaturation of ribonuclease A and ribonuclease T1.

Abstract: To investigate the pH dependence of the conformational stability of ribonucleases A and T1, urea and guanidine hydrochloride denaturation curves have been determined over the pH range 2-10. The maximum conformational stability of both proteins is about 9 kcal/mol and occurs near pH 4.5 for ribonuclease T1 and between pH 7 and 9 for ribonuclease A. The pH dependence suggests that electrostatic interactions among the charged groups make a relatively small contribution to the conformational stability of these proteins. The dependence of delta G on urea concentration increases from about 1200 cal mol-1 M-1 at high pH to about 2400 cal mol-1 M-1 at low pH for ribonuclease A. This suggests that the unfolded conformations of RNase A become more accessible to urea as the net charge on the molecule increases. For RNase T1, the dependence of delta G on urea concentration is minimal near pH 6 and increases at both higher and lower pH. An analysis of information of this type for several proteins in terms of a model developed by Tanford [Tanford, C. (1964) J. Am. Chem. Soc. 86, 2050-2059] suggests that the unfolded states of proteins in urea and GdnHCl solutions may differ significantly in the extent of their interaction with denaturants. Thus, the conformations assumed by unfolded proteins may depend to at least some extent on the amino acid sequence of the protein.

Microbial immobilization of ammonium and nitrate in cultivated soils

Abstract: The microbial immobilization of ammonium and nitrate was measured by 13 N organic measurements after the application of labelled urea, (NH 4 ) 2 SO 4 , KNO 3 (KN) or NH 4 NO 3 with or without glucose in four different soils. In the soils incubated without glucose, the microbial immobilization of the added ammonium varied between 1.5 and 4 mg N kg −1 soil. No immobilization occurred at the expense of NO 3 when KN was applied. When glucose was added at the rate 500 mg C kg −1 soil, the immobilization was very active between the first and the third day, at 10°C. The maximal amounts of 13 N immobilized were much higher for the [ 15 N]urea, 15 (NH 4 ) 2 SO 4 , 15 NH 4 NO 3 and 15 NO 3 K. treatments than for the NH 4 15 NO 3 application. This preferential immobilization of NH 4 was also observed in pure cultures of bacteria isolated from one of the soils and attributed to the inhibition of nitrate uptake by ammonium. The immobilization ratio, immobilized N: decomposed C, was calculated for glucose, accounting for pool substitution effects and immobilization due to native C. It was independent of the form of N applied and similar between soils, c 45–48 mg N g −1 C.

Bladder surface glycosaminoglycans is a human epithelial permeability barrier.

Abstract: Transitional epithelium of the bladder has been known to be impermeable. The data reported herein suggest the principal barrier to permeability may be glycosaminoglycans (GAG) of the surface of the bladder. We examined the ability of surface GAG to prevent a small molecule, urea, from moving across the epithelium in humans. It appears that GAG provide a physical barrier which prevents small molecules from reaching the underlying tight junctions and cell membranes and, hence, are a major permeability barrier. Normal volunteers (27) had 100 milliliters of a 200 grams per liter urea solution placed into their bladders for 45 minutes. Net flow of urea from the bladder lumen was 5.1 per cent. Volunteers who were capable of completing the study (19) had protamine sulfate (5 milligrams per milliliter) instilled in the bladder for 15 minutes, then removed and a second urea study done. Urea loss was significantly higher at 22 per cent (p less than 0.02). A solution of heparin (2,000 units per milliliter) was instilled for 15 minutes followed by a third urea study and urea loss was reversed to 9 per cent. All volunteers experienced significant urinary urgency and discomfort after protamine treatment which were reduced by heparin.

Ammonia volatilization from urea as influenced by soil temperature, soil water content, and nitrification and hydrolysis inhibitors

Abstract: Residue cover influences temperature and water gradients in the soil profile. Changes in the physical environment of the soil influence NH₃ volatilization from urea-containing fertilizers. Field and laboratory experiments were conducted to investigate the influence of residue-cover-induced changes in soil water and temperature on NH₃ volatilization as impacted by urea treatment with a nitrification and urease inhibitor. Fertilizer treatments were urea, urea plus dicyandiamide (DCD), urea plus N-(n-butyl)thiophosphoric triamide (NBPT), and urea plus NBPT and DCD. Following fertilizer application, the soil was either left bare or covered with corn (Zea mays L.) residue. Every 3 h over a 4-d period, water potential, soil temperature, CO₂ production, and NH₃ volatilization were measured. The influence of fertilizer treatments on soil pH was determined in a laboratory incubation experiment conducted over 8 d under controlled environmental conditions. Treatments were similar to the field experiment, with NH₃ volatilization, pH, and CO₂ production measured daily. The NH₃-volatilization rate in the field was highest 2 d after urea application at a time that corresponded with daily maximum soil temperature and decreasing soil water content. Residue cover reduced NH₃ volatilization. Volatilization of NH₃ as a result of urea application was not increased when urea was treated with DCD. Ammonia volatilization as a result of urea treatment with NBPT was reduced by 100 times over untreated urea. During an incubation experiment, soil pH increased from 6.5 to 7.2 in the urea-NBPT, and from 6.5 to 9.0 in the urea and urea-DCD treatments. Associated with the pH increase in the urea-NBPT treatment was a reduction in CO₂ production when compared with the untreated soil.

Variation in urea excretion in the gulf toadfish Opsanus beta

Abstract: Under long-term (24 h) control measurements, significant urea was excreted (15 to 90% of excreted nitrogen) byOpsanus beta (Goode and Bean) collected in 1989 from Biscayne Bay, Florida, USA. Urea excretion rates and plasma urea concentrations were not affected by antibiotic treatments which decreased intestinal microbe populations. These results suggest that nitrogen recycling by gut microbe urease is probably not significant in this species. Urea excretion rates increased significantly following 8 h of air-exposure and in response to high levels of NH4Cl. These results suggest that urea is synthesized and excreted by the toadfish primarily in situations that limit ammonia excretion. Thus, the ornithineurea cycle in the toadfish liver primarily maintains low concentrations of ammonia within the fish. High levels of variability in urea excretion rates and variation in response to air-exposure suggest that urea synthetic rates are affected by immediate past environmental conditions.

BUN and Creatinine

Abstract: Urea and creatinine are nitrogenous end products of metabolism. Urea is the primary metabolite derived from dietary protein and tissue protein turnover. Creatinine is the product of muscle creatine catabolism. Both are relatively small molecules (60 and 113 daltons, respectively) that distribute throughout total body water. In Europe, the whole urea molecule is assayed, whereas in the United States only the nitrogen component of urea (the blood or serum urea nitrogen, i.e., BUN or SUN) is measured. The BUN, then, is roughly one-half (28/60 or 0.446) of the blood urea.The normal range of urea nitrogen in blood or serum is 5 to 20 mg/dl, or 1.8 to 7.1 mmol urea per liter. The range is wide because of normal variations due to protein intake, endogenous protein catabolism, state of hydration, hepatic urea synthesis, and renal urea excretion. A BUN of 15 mg/dl would represent significantly impaired function for a woman in the thirtieth week of gestation. Her higher glomerular filtration rate (GFR), expanded extracellular fluid volume, and anabolism in the developing fetus contribute to her relatively low BUN of 5 to 7 mg/dl. In contrast, the rugged rancher who eats in excess of 125 g protein each day may have a normal BUN of 20 mg/dl.The normal serum creatinine (sCr) varies with the subject's body muscle mass and with the technique used to measure it. For the adult male, the normal range is 0.6 to 1.2 mg/dl, or 53 to 106 μmol/L by the kinetic or enzymatic method, and 0.8 to 1.5 mg/dl, or 70 to 133 μmol/L by the older manual Jaffe reaction. For the adult female, with her generally lower muscle mass, the normal range is 0.5 to 1.1 mg/dl, or 44 to 97 μmol/L by the enzymatic method.

.kappa.-Carrageenan gels: effect of sucrose, glucose, urea, and guanidine hydrochloride on the rheological and thermal properties

Abstract: The effect of sucrose, glucose, guanidine hydrochloride, or urea on the dynamic elastic modulus E' and mechanical loss tangent and on the differential scanning calorimetry (DSC) curves of agarose gels were examined. E' increased with increasing concentration of sugars up to a certain amount, but the excessive addition of sugars decreased E'. The DSC endothermic peak accompanying the transition from gel to sol shifted to higher temperatures, while the heat absorbed on forming 1 mol of junction zones increased and then decreased with increasing sugar concentration. Guanidine hydrochloride or urea decreased E' and shifted the gel to sol transition temperature to lower temperatures. The mechanism by which these chemical reagents weaken or strengthen the gel-forming ability was discussed

The interdependence of ammonia volatilization and denitrification as nitrogen loss processes in flooded rice fields in the Philippines.

Abstract: The relative importance of ammonia volatilization and denitrification as loss processes following the application of urea to flooded rice by the traditional method was assessed at four sites with different characteristics in the Philippines. The effect of reducing ammonia loss on denitrification and total N loss was also studied. The total N loss was determined by a 15N-balance method and ammonia volatilization was assessed by a bulk aerodynamic method following the application of urea to small plots (4.8×5.2 m). As run-off was prevented and leaching losses were negligible, the denitrification loss was assessed as the difference between total N loss and ammonia loss. When urea was broadcast into the floodwater at transplanting, the ammonia loss varied from 10% to 56% of the applied N. Loss was smallest at Aguilar where wind speeds were low and the greatest at Mabitac where floodwater pH values and temperatures were high and the winds were strong. The ammonia loss was reduced at all sites by incorporating the urea into the soil by harrowing. However, the reduction achieved varied markedly between sites, with the largest reduction (from 56% to 7% loss of the applied N) being observed at Mabitac. The total N lost from the basal application into the floodwater ranged from 59% to 71% of the applied N. Incorporating the urea by harrowing reduced the total N loss at two sites, increased the total N loss at the third site, and had no effect at the fourth site. The denitrification losses ranged widely (from 3% to 50% of the applied N) when urea was broadcast into the floodwater at the four sites. The denitrification loss was low when the ammonia loss was high (Mabitac) and high when the ammonia loss was low (Aguilar). Reducing ammonia losses by incorporating the urea into the flooded soil resulted in increased denitrification losses at three of the sites and appeared to have no effect on denitrification at the fourth site. The results show that reducing the ammonia loss by incorporating urea into the soil does not necessarily result in reduced total N loss, and suggest that the efficiency of fertilizer N will be improved only when both N-loss processes are controlled simultaneously.

Ammonia volatilization from five nitrogen compounds used as fertilizers following surface application to soils.

Abstract: SUMMARY Measurements were made of the volatilization of ammonia from mono-ammonium phosphate (MAP), di-ammonium phosphate (DAP), ammonium sulphate (AS), ammonium nitrate (AN) and urea, applied to the surface of five contrasting soils. The compounds were applied as solids, at a rate equivalent to 100 kg N ha−1, to samples of moist soil packed into columns (48 mm diameter) and placed individually in jars through which a stream of air was passed for a period of 8 d. Volatilization ranged from nil to 53% of the N applied, with both the nature of the compound and soil type having large effects. Taking all combinations into account, there was a close relationship between the extent of volatilization, expressed as a percentage of the ammonium or urea N, and the pH attained after 24 h by the corresponding mixtures of soil and compound. Using the results of these and other experiments, the proportion of fertilizer N volatilized as ammonia is estimated to be about 3.4% over the UK as a whole.

Temperature and low concentration effects of the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) on ammonia volatilization from urea

Abstract: The effect of the urease inhibitor N-(n-butyl) thiophosphoric triamide (nBTPT) on urea hydrolysis and NH3 volatilization was studied in laboratory experiments. At ambient laboratory temperatures. NH3 volatilization rates were shown to be significantly reduced by an nBTPT concentration in urea as low as 0.005% (w/w). After 8 days, at ambient laboratory temperatures, NH3 loss from urea was 37.7%. whereas losses from the 0.005 and 0.05% nBTPT products were 13.6 and 1.8%. respectively. Higher concentrations of nBTPT were required at 32°C than at 18°C to achieve equivalent suppression of NH, loss. A direct relationship was not observed between inhibition of urea hydrolysis and the concomitant reduction in NH3 loss. After 4 days at ambient laboratory temperatures, 65% of untreated urea had been hydrolyzcd. whereas hydrolysis from the 0.01% product was 42%. Over the same period, NH3 losses amounted to 12.3 and 1.8%, respectively. In the soils evaluated, effective inhibition of urea hydrolysis and NH3 volatilization was obtained with very low concentrations of nNBPT, but higher concentrations were necessary at higher temperatures and in soils that had been amended with plant residues. The effect of nBTPT on loss may not have been due solely to delayed urea hydrolysis per se; rather, this delay may have also resulted in greater diffusion of urea into the soil and thus lowering the NH3 concentrations at the microsites on the soil surface.

Inhibition of browning by sulfur amino acids. 1. Heated amino acid-glucose systems

Abstract: Amino acids interact with carbohydrates to form Maillard browning products. Such reactions reduce the nutritional value of foods containing amino acids and carbohydrates and may lead to the formation of compounds that are mutagenic and clastogenic or chromosome-damaging. A need therefore exists to inhibit these heat-induced interactions. To demonstrate whether SH-containing sulfur amino acids minimize nonenzymatic browning, beta-alanine, N alpha-acetyl-L-lysine, glycylglycine, and a mixture of amino acids were each heated with glucose in the absence and presence of the following potential inhibitors: N-acetyl-L-cysteine, L-cysteine, reduced glutathione, sodium bisulfite, and urea. Inhibition was measured as a function of temperature, time of heating, and concentration of reactants. The extent of browning was estimated by absorbance measurements at 420 nm. Inhibition was independent of the amino group containing reactant. The minimum concentrations for optimum inhibition, in moles of inhibitor per mole of D-glucose, were as follows: sodium bisulfite, 0.02; L-cysteine, 0.05; N-acetyl-L-cysteine, 0.2; reduced glutathione, 0.2; urea, 8. An "index of prevention" (IP) was used to calculate the inhibition at the optimum mole ratio range, where IP = 100 - [molar absorptivity value (MAV) of the amine compound + glucose + inhibitor] X 100/(MAV of the amine compound + glucose). The calculated values were about 90% in all cases. Possible mechanisms of browning prevention are discussed.

Counteracting effects of urea and betaine in mammalian cells in culture.

Abstract: Urea and methylamines, such as betaine, are among the major organic osmotic effectors accumulated by organisms under hyperosmotic (high NaCl) stress; the mammalian renal medulla also accumulates such compounds in antidiuresis. Studies on isolated proteins show that urea generally destabilizes protein structure, whereas methylamines are generally stabilizers capable of offsetting the effects of urea. The counteracting-osmolytes hypothesis predicts that cells exposed to high urea concentrations require methylamines for optimal function. In this study, urea, betaine, and other solutes (NaCl, glycerol, sorbitol) were added to growth medium of cultured mammalian cells under conditions in which most solutes entered the cells. For two renal [Madin-Darby canine kidney (MDCK) and PAP-HT25] and one nonrenal (Chinese hamster ovary) cell line, urea (greater than 100 mM) or betaine (greater than 50-100 mM) alone inhibited cell growth and survival, measured as colony-forming efficiency. However, the addition of betaine (up to 120 mM) to media with urea (50-300 mM) greatly increased colony-forming efficiency above that with urea alone. A similar, although less marked effect, was seen on colony sizes with MDCK cells. These results support the counteracting-osmolytes hypothesis.

Metabolic fate of [13N]ammonia in human and canine blood.

Abstract: Nitrogen-13- (({sup 13}N)) ammonia is a widely used tracer for PET myocardial blood flow studies. Quantification of blood flow using tracer kinetic principles requires accurate determination of ({sup 13}N)ammonia activity in blood. Since (13N) ammonia is rapidly metabolized, the arterial input function may be contaminated by labeled metabolites. We, therefore, characterized the {sup 13}N-labeled metabolites in blood after intravenous (i.v.) injection of 20 mCi ({sup 13}N)ammonia in nine healthy volunteers. Utilizing a series of ion exchange resins, {sup 13}N-labeled compounds were separated into four groups: ammonia, neutral amino acids, acidic amino acids, and urea. Analysis of the metabolic fate of ({sup 13}N)ammonia indicates that over 90% of the blood activity within the first two minutes after injection is present as ({sup 13}N)ammonia. However, there is considerable contamination of the blood activity at 3-5 min by ({sup 13}N)glutamine (amide) and urea, which collectively represent 18%-50% of the blood activity. Thus, correction of the arterial input function for {sup 13}N-metabolites is required to accurately quantify the arterial input function of ({sup 13}N)ammonia in myocardial blood flow studies.

ESR study of sodium dodecyl sulfate and dodecyltrimethylammonium bromide micellar solutions: effect of urea

Abstract: Electron spin resonance spectroscopy of cationic ((4-(trimethylammonio)-2,2,6,6-tetramethylpiperidinyl)-1-oxyiodide Temp-TMA + ) and nonionic spin probes ((4-octanoyl-2,2,6,6-tetramethylpiperidinyl)-1-oxy, C 8 -TEMPO, and x-doxylstearic acid with the nitroxide group in position x=5, 12, and 16, along the stearic acid chain, 5-DSA, 12-DSA, and 16-DSA has been studied in sodium dodecyl sulfate and in dodecyltrimethylammonium bromide solutions as a function of surfactant and urea concentration

Defining adequacy of CAPD with urea kinetics.

Abstract: The purpose of this paper is to explore the validity of applying urea kinetic indices to CAPD. According to the peak concentration hypothesis, the values of Kt/V required for adequate dialysis are lower for CAPD than for hemodialysis because of the continuous steady state nature of CAPD. Pilot clinical studies were undertaken in 19 patients to correlate the (Kt/V)urea index with clinical assessment of adequacy based on a 12 parameter score. The data shows that the correlation between serum urea nitrogen (Kt/V)urea and protein catabolic rate (PCR) are in keeping with the theoretical predictions of the urea kinetic model. PCR and dietary protein are well correlated. Also, PCR and Kt/V had a high degree of positive correlation. Serum creatinine was inversely correlated with (Kt/V)creatinine. In 74% of the patients, the clinical assessment of adequacy was in agreement with the (Kt/V)urea domains of adequacy established from the peak concentration hypothesis and the urea kinetic model. The lack of correlation in the remaining 26% is being investigated.

Intracellular Vacuolization Caused by the Urease of Helicobacter pylori

Abstract: Tissue culture cells were exposed to supernatants of Helicobacter pylori for 24 h at 37 degrees C in the presence of various quantities of urea. In the normal human stomach the concentration of urea is less than or equal to 4 mmol/l, and in the presence of this low concentration up to 10% of Vero cells showed intracellular vacuolization. In the presence of 7.5 mmol/l urea, 25% of the cells showed vacuolization. With 30 mmol/l urea, the final pH was 7.6, indicating that vacuolization was not due to change of pH. The first report of vacuolization of tissue culture cells by H. pylori was in a system without added urea but with concentrated bacterial supernatant; 30% of H. pylori strains demonstrated a cytotoxic effect. In those experiments fetal calf serum was used; it contains 6 mmol/l urea but was used at a concentration of 10%. A urease inhibitor, acetohydroxamic acid, caused a 75% drop in the number of cells showing vacuolization, and ammonia caused vacuolization. Thus the urea of H. pylori probably causes this vacuolization.

The vasopressin-regulated urea transporter in renal inner medullary collecting duct

Abstract: The terminal part of the inner medullary collecting duct (terminal IMCD) is unique among collecting duct segments in part because its permeability to urea is regulated by vasopressin. The urea permeability can rise to extremely high levels (greater than 100 x 10(-5) cm/s) in response to vasopressin. Recent studies in isolated perfused IMCD segments have established that the rapid movement of urea across the tubule epithelium occurs via a specialized urea transporter, presumably an intrinsic membrane protein, present in both the apical and basolateral membranes. This urea transporter has properties similar to those of the urea transporters in mammalian erythrocytes and in toad urinary bladder, namely, inhibition by phloretin, inhibition by urea analogues, saturation kinetics in equilibrium-exchange experiments, and regulation by vasopressin. The urea transport pathway is distinct from and independent of the vasopressin-regulated water channel. The increase in transepithelial urea transport in response to vasopressin is mediated by adenosine 3',5'-cyclic monophosphate and is associated with an increase in the urea permeability of the apical membrane. However, little is known about the physical events associated with the activation or insertion of urea transporters in the apical membrane. Because of the importance of this transporter to the urinary concentrating mechanism, efforts toward understanding its molecular structure and the molecular basis of its regulation appear to be justified.

In vivo osmoregulation of aldose reductase mRNA, protein, and sorbitol in renal medulla.

Abstract: Sorbitol accumulates in renal medullary cells by synthesis from glucose in a reaction catalyzed by aldose reductase. Medullary sodium and urea are high and vary with urinary concentration. Sorbitol varies similarly, consistent with its role as a compatible intracellular organic osmolyte. We measured renal medullary sodium, urea, sorbitol, aldose reductase (protein and activity), and aldose reductase mRNA in rats treated to change medullary sodium and urea. In untreated Brattleboro rats all measurements were low and increased after 7 days of treatment with arginine vasopressin. In contrast, when normal rats were water deprived for 3 days, urea increased out of proportion to sodium, and sorbitol, aldose reductase, and aldose reductase mRNA were unchanged. After 2 h of diuresis, normal rats had lower medullary sodium and urea and reduced mRNA and sorbitol; however aldose reductase did not change. These data are consistent with previous results from cultured cells in which altered extracellular sodium, but not urea, leads to rapid changes in aldose reductase mRNA and slow changes (days) in aldose reductase. In addition, acute decreases in extracellular sodium increase leakage of sorbitol from cells. We also confirm previous results showing medullary glycerophosphorylcholine correlates best with urea, whereas the sum of all compatible osmolytes correlates best with sodium.

A comparison of the sulfur and oxygen analogs of phosphoric triamide urease inhibitors in reducing urea hydrolysis and ammonia volatilization

Abstract: A variety of compounds have been tested as urease inhibitors with the goal of providing a means of reducing ammonia volatilization losses from urea fertilizers when they are applied to the soil surface. Four phosphoric triamide compounds were studied in laboratory experiments to assess their effect on urea hydrolysis, soil ammonium levels, and ammonia volatilization. The compounds N(n-butyl) thiophosphoric triamide (nBTPT), cyclohexyl thiophosphoric triamide (CHTPT), and their oxygen analogs [N-(n-butyl) phosphoric triamide (nBPT) and cyclohexyl phosphoric triamide (CHPT), respectively] were mixed with urea at 0.1% and 0.01% w/w ratios, and the products were applied to the soil surface. A forced-draft apparatus was used to measure ammonia loss. The urea treatment lost 47% of applied N as ammonia in 14 d. The inhibitors applied at 0.1% w/w showed losses of 7%–10% in 14 d; at 0.01%, losses ranged from 13%–30% in the same period. At the 0.1% level, no significant difference was found among the inhibitors in terms of ammonia loss or urea hydrolysis trends. At the 0.01% concentrations, the oxygen analogs showed better urea urease inhibition than did the thio compounds, and their ammonia losses were half those of their sulfur analogs.

Effects of methanol and urea on optical resolution of phenylthiohydantoin-DL-amino acids by micellar electrokinetic chromatography with sodium N-dodecanoyl-L-valinate.

Abstract: Effects of methanol and urea on enantiomeric resolution of phenylthiohydantoin (PTH)-DL-amino acids by micellar electrokinetic chromatography with sodium N-dodecanoyl-L-valinate (SDVal) were investigated. PTH-DL-Met that could not be optically resolved with an SDVal solution alone was resolved by using an SDVal/methanol solution. Four PTH-DL-amino acids were separated from each other and each pair of enantiomers was also optically resolved, although seriously tailed peaks were observed for each solute. The addition of urea to SDVal/methanol solutions improved these peak shapes.

Plasma chemistry references values in psittaciformes

Abstract: Reference values for 17 plasma chemical variables in African greys. Amazons, cockatoos and macaws were established for use in avian clinical practice. The inner limits are given for the percentiles P(2.5) and P(97.5) with a probability of 90%. The following variables were studied: urea, creatinine, uric acid, urea/uric acid ratio, osmolality, sodium, potassium, calcium, glucose, aspartate aminotransferase, alanine aminotransferase, gamma glutamyltransferase, lactate dehydrogenase, creatine kinase, bile acids, total protein, albumin/globulin ratio. Differences between methods used and values found in this study and those reported previously are discussed.

pH Regulation of Urease Levels in Streptococcus salivarius

Abstract: Potential mechanisms for regulation of urease levels in Streptococcus salivarius were examined, including: induction by urea, nitrogen or carbon source repression, and effects of pH and CO2 (because CO2 enrichment enhanced urease detection on urea agar plates). Regulation by either pH or CO2 was confirmed by comparison of the urease accumulation pattern during anaerobic growth under CO2 with that under N2. Under CO2, there was an initial buffering plateau at pH 6.2 and a rate of Streptococcus salivarius urease accumulation three-fold that under N2, with a pH 7.6 plateau. With both gas phases there was also an increase in the rate of urease appearance coincident with the decrease in medium pH following the pH plateau. The effects of pH, CO2, and HCO3- on urease levels and on growth were separately assessed by culture in media containing 0, 25, 100 mmol/L KHCO3 buffered at different pH levels. There was an inverse relationship between the logarithm of the urease level after 24-hour growth and the pH during growth-the urease specific activity was 100-fold higher at pH 5.5, compared with pH 7.0 and above. HCO3-/CO2 (100 mmol/L) had little effect on urease levels, but was essential for growth at pH 5.5. There was no significant urease induction by urea, or repression by ammonia or glucose. There was also evidence of pH regulation of urease levels in some staphylococci, Klebsiella pneumonia, and Corynebacterium renale, but not in Actinomyces naeslundii and several other species. We conclude that the external pH is a major factor regulating urease levels in S. salivarius and possibly some other species-a mechanism equivalent to urease repression by OH-.

Metabolic response to egg white and cottage cheese protein in normal subjects

Abstract: In type II diabetic subjects, we previously demonstrated differences in the serum insulin, C-peptide, and glucagon response to ingestion of seven different protein sources when administered with 50 g of glucose. The response was smallest with egg white and greatest with cottage cheese protein. In the present study, we compared the responses to 50 g of the above two proteins ingested without glucose in normal male subjects. We also determined the proportion of each ingested protein converted to urea nitrogen. The incremental area response integrated over 8 hours for serum insulin, C-peptide, glucagon, alpha-amino-nitrogen (AAN), and urea nitrogen were all approximately 50% less following egg white. This was associated with a 50% smaller conversion of protein to urea. Overall, 70% of the cottage cheese but only 47% of the egg white protein could be accounted for by urea formation. Most likely the smaller hormonal response to egg white is due to poor digestibility of this protein.

Urinary and serum urea as indicators of protein metabolism in very low birthweight infants fed varying human milk protein intakes.

Abstract: Urea concentrations in serum and urine were measured in 28 growing, very low birth weight, appropriate-for-gestational age infants fed varying human milk protein intakes (range 1.7 to 3.9 g/kg/day). We found a high correlation between serum urea values at the end of the study and mean protein intake (rs = 0.85, p less than 0.001) and between urinary urea concentrations in eight-hour urine collections and protein intake (rs = 0.81, p less than 0.001). All serum and urine urea values were below 1.6 and 18 mmol/l, respectively, at protein intakes less than 3 g/kg/day. Higher protein intakes caused higher serum and urinary urea concentrations. We also found a strong correlation between the individual serum and urinary urea values at the end of the study (rs = 0.90, p less than 0.001). The presented data are consistent with the growth data previously reported and indicate that inadequate or excessive protein intakes can be detected by measurement of urea concentrations in serum and/or urine. If urine urea samples alone can be used for estimating optimal protein intake, painful blood sampling procedures could be obviated.

Control of hepatic nitrogen metabolism and glutathione release by cell volume regulatory mechanisms.

Abstract: 1. Urea synthesis was studied in isolated perfused rat liver during cell volume regulatory ion fluxes following exposure of the liver to anisotonic perfusion media. Lowering of the osmolarity in influent perfusate from 305 mOsm/l to 225 mOsm/l (by decreasing influent [NaCl] by 40 mmol/l) led to an inhibition of urea synthesis from NH4Cl (0.5 mmol/l) by about 60% and a decrease of hepatic oxygen uptake by 0.43 +/- 0.03 mumol g-1 min-1 [from 3.09 +/- 0.13 mumol g-1 min-1 to 2.66 +/- 0.12 mumol g-1 min-1 (n = 9)]. The effects on urea synthesis and oxygen uptake were observed throughout hypotonic exposure (225 mOsm/l). They persisted although volume regulatory K+ efflux from the liver was complete within 8 min and were fully reversible upon reexposure to normotonic perfusion media (305 mOsm/l). A 42% inhibition of urea synthesis from NH4Cl (0.5 mmol/l) during hypotonicity was also observed when the perfusion medium was supplemented with glucose (5 mmol/l). Urea synthesis was inhibited by only 10-20% in livers from fed rats, and was even stimulated in those from starved rats when an amino acid mixture (twice the physiological concentration) plus NH4Cl (0.2 mmol/l) was infused. 2. The inhibition of urea synthesis from NH4Cl (0.5 mmol/l) during hypotonicity was accompanied by a threefold increase of citrulline tissue levels, a 50-70% decrease of the tissue contents of glutamate, aspartate, citrate and malate, whereas 2-oxoglutarate, ATP and ornithine tissue levels, and the [3H]inulin extracellular space remained almost unaltered. Further, hypotonic exposure stimulated hepatic glutathione (GSH) release with a time course roughly paralleling volume regulatory K+ efflux. NH4Cl stimulated lactate release from the liver during hypotonic but not during normotonic perfusion. In the absence of NH4Cl, hypotonicity did not significantly affect the lactate/pyruvate ratio in effluent perfusate. With NH4Cl (0.5 mmol/l) present, the lactate/pyruvate ratio increased from 4.3 to 8.2 in hypotonicity, whereas simultaneously the 3-hydroxybutyrate/acetoacetate ratio slightly, but significantly decreased. 3. Addition of lactate (2.1 mmol/l) and pyruvate (0.3 mmol/l) to influent perfusate did not affect urea synthesis in normotonic perfusions, but completely prevented the inhibition of urea synthesis from NH4Cl (0.5 mmol/l) induced by hypotonicity. Restoration of urea production in hypotonic perfusions by addition of lactate and pyruvate was largely abolished in the presence of 2-cyanocinnamate (0.5 mmol/l). Addition of 3-hydroxybutyrate (0.5 mmol/l), but not of acetoacetate (0.5 mmol/l) largely reversed the hypotonicity-induced inhibition of urea synthesis from NH4Cl.(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanism of urea-induced whey protein gelation

Abstract: Whey protein isolate (WPI) at an 11.0 % concentration spontaneously formed a gel at 25°C in 6 M urea. As the pH was increased from 7 to 10, gel formation, from a viscous sol to an elastic network, was accelerated. The spontaneous formation of gels in 6 M urea resulted from protein-protein cross-linkages via oxidation of thiol groups and SH-disulfide interchange reactions