A reappraisal of protein turnover values in neonates fed human milk or formula

We investigated the effect of human milk feeding on the nitrogen metabolism of appropriate-for-gestational age infants of birth weight 1.5-2.0 kg. Eight infants received pooled mature human milk. The remaining 20 were divided into two equal groups, who received one of two low-protein, milk-based formulae. The formulae were identical in composition except for the protein source, which was either casein- or whey-predominant. The three diet groups received similar total nitrogen (390 mg N.kg-1.d-1) and energy (500 kJ.kg-1.d-1) intakes. The human-milk-fed group, however, received a significantly higher intake of nonprotein and urea nitrogen and a significantly lower true protein nitrogen. Nitrogen metabolism was studied using a modified constant infusion of [15N]glycine, mixed with the feeding every 2-3 h. Urine was collected in approximately 3-h aliquots and analysed for total ammonia and urea nitrogen. Excretion of the 15N label was measured in urinary urea and ammonia. No differences were seen between the three diet groups in total [15N]urea or [15N]ammonia urinary excretion. However, the concentration of 15N in urinary urea in the human-milk-fed group was lower than in the two formula-fed groups. This reduction in concentration appeared due to a higher dietary intake of urea among the human-milk-fed group, and the consequent dilution of the label in the urine. As a result, protein turnover rates calculated from the [15N]urea end product were artificially raised in the milk-fed group, and were significantly higher than those in the formula groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

A comparison of the estimates of whole-body protein turnover in parenterally fed neonates obtained using three different end products

Protein turnover rates in neonates have been calculated largely by measuring urinary [15N]urea enrichment following administration of [15N]glycine. Although ammonia has been increasingly recognized as an end product of nitrogen metabolism, in neonates it yields a different estimate of protein turnover than does urea. Comparisons of ammonia and urea end products in parenterally fed neonates have not previously been reported. A third and independent way of estimating protein turnover, developed for adults, is to use breath 13CO2 as an end product following administration of [1-13C]leucine. We therefore carried out simultaneous measurements of protein turnover in 10 parenterally fed neonates, using the three end products. The infants were clinically stable, weighed 2.6 +/- 0.2 kg, and received 3.1 +/- 0.2 g.kg-1.d-1 of amino acid, 2.2 +/- 0.1 g.kg-1.d-1 of lipids, and an energy intake of 90 +/- 4 kcal.kg-1.d-1 (1 kcal = 4.186 kJ). The turnover estimates derived from the 13CO2 and [15N]urea end products were very similar. The [15N]ammonia end product produced values approximately 66% (p less than 0.01) of the other two. We conclude that the ammonia and urea end products probably originate in different precursor pools. The similarity of the urea and breath carbon dioxide results helps validate the use of the urea end product in studying the nitrogen metabolism of parenterally fed neonates. Ideally in future studies two or more end products should be used, since they provide information about different aspects of the neonates' protein metabolism.     

Estimating fermentative amino acid catabolism in the small intestine of growing pigs

Fermentative catabolism (FAAC) of dietary and endogenous amino acids (AA) in the small intestine contributes to loss of AA available for protein synthesis and body maintenance functions in pigs. A continuous isotope infusion study was performed to determine whole body urea flux, urea recycling and FAAC in the small intestine of ileal-cannulated growing pigs fed a control diet (CON, 18.6% CP; n=6), a high fibre diet with 12% added pectin (HF, 17.7% CP; n=4) or a low-protein diet (LP, 13.4% CP; n=6). ¹⁵N-ammonium chloride and ¹³C-urea were infused intragastrically and intravenously, respectively, for 4 days. Recovery of ammonia at the distal ileum was increased by feeding additional fibre when compared with the CON (P<0.05) but was not affected by dietary protein (0.24, 0.39 and 0.14 mmol nitrogen/kg BW/day for CON, HF and LP, respectively; P<0.05). Lowering protein intake reduced urea flux (25.3, 25.7 and 10.3 mmol nitrogen/kg BW/day; P<0.01), urinary urea excretion (14.4, 15.0 and 6.2 mmol N/kg BW/day; P<0.001) and urea recycling (12.1, 11.3 and 3.23 mmol nitrogen/kg BW/day; P<0.01) compared with CON. There was a rapid reduction in ¹⁵N-ammonia enrichment in digesta along the small intestine suggesting rapid absorption of ammonia before the distal ileum and lack of uniformity of enrichment in the digesta ammonia pool. A two-pool model was developed to determine possible value ranges for nitrogen flux in the small intestine assuming rapid absorption of ammonia. Maximum estimated FAAC based on this model was significantly lower when dietary protein content was decreased (32.9, 33.4 and 17.4 mmol nitrogen/kg BW/day; P<0.001). There was no impact of dietary fibre on estimates of small intestine nitrogen flux (P>0.05) compared with CON. The two-pool model developed in the present study allows for estimation of FAAC but still has limitations. Quantifying FAAC in the small intestine of pigs, as well as other non-ruminants and humans, offers a number of challenges but warrants further investigation.     

15N]glycine metabolism in normal and cirrhotic subjects

Following a single oral dose of 10 mg/kg of [15N]glycine, plasma [15N]glycine kinetics and urinary 15N excretion were measured in 12 cirrhosis patients and in 6 control subjects. Cirrhosis patients were divided into two groups of 6 patients with and without a history of hepatic encephalopathy designated as group II and group I, respectively. Thirty minutes after oral administration of labeled glycine, the plasma concentration of [15N]glycine was significantly higher in both cirrhosis groups than that in the control group (P less than 0.05 and P less than 0.01). The elimination constant of plasma [15N]glycine slightly decreased in group II, but not significantly. Urinary 15N excretion did not differ among the three groups, but the rate of urinary ammonia 15N in urinary 15N was significantly increased in group II (P less than 0.05). The whole-body protein flux did not differ among the three groups, but whole-body protein breakdown was significantly increased in group II cirrhosis patients (P less than 0.05). These findings indicated that the kinetics of glycine were substantially altered in severe cirrhosis patients. Because hepatic uptake and oxidation of glycine was well maintained even in group II, increased endogenous protein breakdown seemed to be responsible for hyperglycinemia and also for the negative nitrogen balance seen in this group.     

The fate of nitrogen from 15N-labeled nitrate after single intravenous administration of Na15NO3 in sheep

The metabolic fate of nitrogen from 15N-labeled sodium nitrate has been investigated in four healthy Polish Merino ewes. 15N-labeled sodium nitrate was administered intravenously at the dosage of 400 micromol.kg(-1) body weight. Blood plasma and urine concentrations of nitrate, ammonia, and urea and 15N enrichment of ammonia and urea were estimated over a 50-h period following 15N-nitrate administration. Nitrate (NO3-) was slowly eliminated from the blood plasma, and the presence of NO3(-) in the blood plasma above the nitrate "background" was observed for 50 h. 15N enrichment of blood plasma urea already appeared at 15 min and reached the maximum 6 h after 15N-nitrate administration. The urinary excretion of nitrate occured during 50 h after 15N-nitrate injection; the total urine excretion of NO3(-) was 23.63+/-2.39% of the administered dose. The mean urinary recoveries of nitrogen as 15N-urea and 15N-ammonia were 14.76+/-1.32% and 0.096+/-0.015% of the administered 15N-nitrate dose, respectively. It should be pointed out that in total only 38.49% of the administered nitrate-N was excreted in urine (as nitrate, ammonia and urea nitrogen) during 50 h. The results obtained indicate that sheep are able to store nitrate nitrogen in their body. The fate of the remaining approximately 60% of the 15NO3(-) administered dose is unknown. The results obtained do not allow one to conclude what fraction of the unrecovered approximately 60% of the 15NO3(-) dose was utilized by gastrointestinal microorganisms, and (or) metabolized, or stored in sheep tissues.     

Stimulation of protein synthesis and breakdown by vaccination.

Six normal volunteers were vaccinated against typhoid-cholera. 15N-Glycine was injected the morning after vaccination. The injection was repeated three to six days and 10 days later. All subjects ate the same diet on each occasion. Excretion of 15N in urinary ammonia and total urinary excretion of nitrogen, ammonia, and creatinine were determined after each injection of isotope. Urinary excretion of 15N was used to calculate rates of whole-body protein turnover. Total urinary nitrogen and ammonia excretions showed no appreciable change on all three days. Creatinine excretion was significantly higher the day after vaccination than on the other two days (p < 0.05). Rates of protein turnover were also significantly higher on this day: a 37% increase in synthesis and 55% increase in degradation were noted. These results show that during the reaction to vaccination there was a stimulation of whole-body protein metabolism that is similar to that produced by sepsis.     

Dietary protein requirements and body protein metabolism in endurance-trained men

The effects of regular submaximal exercise on dietary protein requirements, whole body protein turnover, and urinary 3-methylhistidine were determined in six young (26.8 +/- 1.2 yr) and six middle-aged (52.0 +/- 1.9 yr) endurance-trained men. They consumed 0.6, 0.9, or 1.2 g.kg-1.day-1 of high-quality protein over three separate 10-day periods, while maintaining training and constant body weight. Nitrogen measurements in diet, urine, and stool and estimated sweat and miscellaneous nitrogen losses showed that they were all in negative nitrogen balance at a protein intake of 0.6 g.kg-1.day-1. The estimated protein requirement was 0.94 +/- 0.05 g.kg-1.day-1 for the 12 men, with no effect of age. Whole body protein turnover, using [15N]glycine as a tracer, and 3-methylhistidine excretion were not different in the two groups, despite lower physical activity of the middle-aged men. Protein intake affected whole body protein flux and synthesis but not 3-methylhistidine excretion. These data show that habitual endurance exercise was associated with dietary protein needs greater than the current Recommended Dietary Allowance of 0.8 g.kg-1.day-1. However, whole body protein turnover and 3-methylhistidine excretion were not different from values reported for sedentary men.     

Validation of lactose[15N,15N]ureide as a tool to study colonic nitrogen metabolism

In vitro experiments have shown that fermentation of carbohydrates prevents accumulation of nitrogen in the colon. Variable results have been obtained on modulation of dietary intakes in vivo. Lactose[15N,15N]-labeled ureide has been proposed as a tool to study colonic nitrogen metabolism. However, on oral administration of the marker, different urinary excretion patterns of the 15N label have been found. In this study, 50 mg lactose[15N,15N]ureide was directly instilled in the colon through an orocecal tube to investigate the colonic handling of this molecule in a direct way. In basal conditions, 42% (range, 37-48%) of labeled nitrogen administered as lactose[15N,15N]ureide was retrieved in urine after 72 h. A substantial variability in total urinary excretion of the label was found, but the urinary excretion pattern of the label was similar in all volunteers. When inulin, a fermentable carbohydrate, was administered together with the labeled marker, a significant decrease in urinary excretion of 15N after 72 h was found, to 29% (range, 23-34%). The effect of a smaller dose of inulin (250 mg) on colonic handling of lactose[15N,15N]ureide (50 mg), was investigated in another group of volunteers, and this time, fecal excretion of the marker was also evaluated. The results seem to indicate that fermentation of inulin causes an increased fecal excretion of the marker, thereby reducing urinary excretion but not retention in the human nitrogen pool. This instillation study shows that lactose[15N,15N]ureide is a tool with good properties to investigate the effect of different types of carbohydrates on nitrogen metabolism in the proximal colon in vivo.     

Effect of exercise on protein metabolism in humans as explored with stable isotopes

Exercising for 3.75 h on a treadmill at 50% VO2 max in the fed state induced an increased excretion of 71 mg nitrogen/kg over the 18 h after exercise. However, measurements of the time course of changes in 13CO2 excretion from ingested [1-13C]leucine indicated that all of this increased nitrogen production occurs during the exercise period. Because of the reduced renal clearance and slow turnover of the urea pool, urea excretion lags behind urea production. Measurements of nitrogen flux from the plateau labeling of urinary ammonia achieved by repeated oral doses of 15N-labeled glycine indicated that the nitrogen loss resulted from an increase in protein degradation and a decrease in protein synthesis. Further studies with [1-13C]leucine indicated that a 2-h treadmill exercise induced an increase in the nitrogen loss from 5.4 to 16 mg . kg-1 . h-1 measured with a primed constant infusion of [1-13C]leucine. This resulted from a fall in whole-body protein synthesis. Glucose given at the rate of 0.88 g . kg-1 . h-1 depressed the rate of whole-body protein degradation and appeared to suppress the exercise-induced increase in nitrogen excretion. When leucine oxidation rates were measured at increasing work rates, a linear relationship between percentage of VO2 max and leucine oxidation was observed up to 89% VO2 max when 54% of the flux of leucine was oxidized. These changes may involve nonmuscle as well as muscle tissue. Thus the source of the increased nitrogen losses is probably liver. In muscle, protein degradation is actually decreased judged by methylhistidine excretion, whereas in liver, protein degradation may be increased. Also the fall in whole-body protein synthesis may reflect changes in nonmuscle tissues because in running rats protein synthesis in muscle is maintained. As far as leucine metabolism is concerned, because the increase in leucine oxidation occurs when leucine and its keto acid concentration falls, exercise must specifically activate the 2-oxoacid dehydrogenase.     

Box-modeling of 15N/14N in mammals

The ¹⁵N/¹⁴N signature of animal proteins is now commonly used to understand their physiology and quantify the flows of nutrient in trophic webs. These studies assume that animals are predictably ¹⁵N-enriched relative to their food, but the isotopic mechanism which accounts for this enrichment remains unknown. We developed a box model of the nitrogen isotope cycle in mammals in order to predict the ¹⁵N/¹⁴N ratios of body reservoirs as a function of time, N intake and body mass. Results of modeling show that a combination of kinetic isotope fractionation during the N transfer between amines and equilibrium fractionation related to the reversible conversion of N-amine into ammonia is required to account for the well-established ≈4‰ ¹⁵N-enrichment of body proteins relative to the diet. This isotopic enrichment observed in proteins is due to the partial recycling of ¹⁵N-enriched urea and the urinary excretion of a fraction of the strongly ¹⁵N-depleted ammonia reservoir. For a given body mass and diet δ¹⁵N, the isotopic compositions are mainly controlled by the N intake. Increase of the urea turnover combined with a decrease of the N intake lead to calculate a δ¹⁵N increase of the proteins, in agreement with the observed increase of collagen δ¹⁵N of herbivorous animals with aridity. We further show that the low δ¹⁵N collagen values of cave bears cannot be attributed to the dormancy periods as it is commonly thought, but inversely to the hyperphagia behavior. This model highlights the need for experimental investigations performed with large mammals in order to improve our understanding of natural variations of δ¹⁵N collagen.     

Ammonia production from amino acids and urea in the caecal contents of the chicken

1. Ammonia production from urea and amino acids in the caecal contents of the chicken was evaluated using 15N-labeled nitrogenous compounds. 2. About 43% of each of urea nitrogen and glutamine amide nitrogen was converted to ammonia nitrogen, but only 25% of epsilon-nitrogen of the added arginine, a precursor of urea, was found in ammonia. 3. Amino nitrogen of the separately added glutamic acid and glycine to be converted to ammonia was 19-20% of their added amounts, whereas that of alpha-alanine was 11%. 4. It is concluded that dietary and urinary amino acids and urea which find their ways into the caeca are useful nitrogen sources for ammonia production by microflora in the caeca of the chicken.     

The influence of ration size and feeding frequency on ammonia excretion by juvenile Atlantic cod, Gadus morhua L.

Small groups of juvenile Atlantic cod, Gadus morhua L., were kept at 14°C in through-flow tanks and were fed known quantities of a compounded diet of natural food. The cod were fed single and multiple meals with ration size in the range 0.5 to 4.1% of total wet fish body weight. Ammonia production in each feeding experiment was monitored continuously.
For single-meal experiments, significant relationships were derived between ration size and (a) total ammonia excreted, (b) total exogenous ammonia excreted above endogenous excretion levels, (c) duration of the elevated phase of ammonia excretion, (d) maximum rate of ammonia excretion, and (e) time delay after feeding to reach maximum rate of ammonia excretion. Relationships between nitrogen loss as ammonia and nitrogen intake were examined and estimates of endogenous excretion rate and maintenance ration made.
Repetitive feeding resulted in cyclical variation in ammonia excretion. At the lowest ration size, ammonia excretion rates had nearly returned to the pre-feeding level within 24 h. At higher feeding levels, the effect of each successive meal tended to be cumulative, resulting in increasingly higher ammonia excretion rates which only stabilized towards the end of the experiments.     

The bioenergetics of grass carp, Ctenopharyngodon idella (Val.): the influence of body weight, ration and dietary composition on nitrogenous excretion

Nitrogenous excretion by grass carp, Ctenopharyngodon idella (Val.), was measured in the form of ammonia and urea. Endogenous nitrogen excretion (ENE) was estimated as the daily rate of excretion by grass carp which had been starved for 2 days. ENE was scaled allometrically with body weight with weight exponents of 0.75 for ammonia, 0.63 for total nitrogen and 0.63 for the energy lost. The proportion of nitrogen attributable to urea was smaller than that attributable to ammonia and decreased from 25 to 12% as fish weight increased from 2 to over 10 g.
Linear relationships were found between daily rates of ammonia, total nitrogen and energy loss and daily rates of food intake. High carbohydrate and high lipid diets were not shown to have a protein-sparing action compared to a high protein diet. Differences in the amount of nitrogen excreted were explained by the differing nitrogen contents of the diets. Nitrogen budgets were erected and their implications discussed.     

Diel patterns of nitrogen excretion, plasma constituents, and behavior in the gulf toadfish (Opsanus beta) in laboratory versus outdoor mesocosm settings

Nitrogen excretion by the gulf toadfish (Opsanus beta) is of interest because of its high proportion of urea excretion compared with that of other teleosts. To better understand the factors influencing the timing of nitrogen excretion, the ratio of excreted urea∶ammonia, and the effector molecules regulating these processes, gulf toadfish were subjected to a series of experiments that moved them progressively from internal laboratory to outdoor mesocosm settings while assessing their behavior, nitrogen excretion patterns, levels of plasma hormones/effectors, and other parameters. In confined flux chambers in both laboratory and outdoor settings, toadfish nitrogen excretion was largely observed as urea pulses, with no apparent diel patterns to the pulses. Unrestrained toadfish in mesocosms exhibited distinctly nocturnal behavior, remaining exclusively in shelters during the day but taking several forays out into the mesocosm at night. In contrast to nitrogen excretion patterns in chambers, urea and ammonia were coexcreted in mesocosms and ratios for urea∶ammonia were very close to 1∶1 for both fed and fasted toadfish. The majority of measured excretion (and corresponding declines in plasma urea levels) occurred during two distinct periods of pulsing during daylight hours (0600-1000 and 1600-1800 hours). The declines in plasma urea associated with excretion were preceded by/coincided with declines in plasma cortisol. No day/night or hourly patterns in plasma serotonin (5-hydroxytryptamine [5-HT]) were observed, but there was a strong positive correlation among all samples between plasma urea and 5-HT. There was also a negative correlation between plasma cortisol and 5-HT. As expected for a nocturnally active species, plasma melatonin was significantly lower in daylight hours. A variety of enzyme activities (glutamine synthetase, glutaminase) and mRNA levels (glutamine synthetase, urea transporter, and Rhesus proteins) showed no significant variation over a diel cycle. Unlike prior laboratory studies, our results show that gulf toadfish in a natural setting have a distinctly diurnal pattern of nitrogen excretion and that ammonia and urea are coexcreted. The decline in plasma cortisol associated with urea pulses noted in prior laboratory studies was not as evident in the natural setting.     

Aminotransfer from Alanine and Glutamate to Glycine and Serine during Photorespiration in Oat Leaves

¹⁵N-Labeled glutamate and alanine were used to examine the photorespiratory nitrogen metabolism in oat (Avena sativa L.) leaf slices. Glutamate and alanine supply amino groups for glycine formation during photorespiration. The nitrogen flux from alanine to glycine was estimated to be 3 times higher than that from glutamate. It is concluded from these results that alanine is a direct and important amino donor for photorespiratory glycine formation in oat leaves. The ¹⁵N labeling of serine was almost as high as that of glycine during the initial period of the labeling experiments. Thereafter, the ratio of ¹⁵N label in serine to ¹⁵N label in glycine declined substantially.     

Nitrogen metabolism and excretion in the aquatic chinese soft-shelled turtle, Pelodiscus sinensis, exposed to a progressive increase in ambient salinity

This study aimed to determine effects of 6-day progressive increase in salinity from 1 per thousand to 15 per thousand on nitrogen metabolism and excretion in the soft-shelled turtle, Pelodiscus sinensis. For turtles exposed to 15 per thousand water on day 6, the plasma osmolality and concentrations of Na+, Cl- and urea increased significantly, which presumably decreased the osmotic loss of water. Simultaneously, there were significant increases in contents of urea, certain free amino acids (FAAs) and water-soluble proteins that were involved in cell volume regulation in various tissues. There was an apparent increase in proteolysis, releasing FAAs as osmolytes. In addition, there might be an increase in catabolism of certain amino acids, producing more ammonia. The excess ammonia was retained as indicated by a significant decrease in the rate of ammonia excretion on day 4 in 15 per thousand water, and a major portion of it was converted to urea. The rate of urea synthesis increased 1.4-fold during the 6-day period, although the capacity of the hepatic ornithine urea cycle remained unchanged. Urea was retained for osmoregulation because there was a significant decrease in urea excretion on day 4. Increased protein degradation and urea synthesis implies greater metabolic demands, and indeed turtles exposed to 15 per thousand water had significantly higher O2 consumption rate than the freshwater (FW) control. When turtles were returned from 15 per thousand water to FW on day 7, there were significant increases in ammonia (probably released through increased amino acid catabolism) and urea excretion, confirming that FAAs and urea were retained for osmoregulatory purposes in brackish water.     

The role of energy,serine, glycine,and 1-carbon units in the cost of nitrogen excretion in mammals and birds

The efficiency with which dietary protein is used affects the nitrogen excretion by the animal and the environmental impact of animal production. Urea and uric acid are the main nitrogen excretion products resulting from amino acid catabolism in mammals and birds, respectively. Nitrogen excretion can be reduced by using low-protein diets supplemented with free amino acids to ensure that essential amino acids are not limiting performance. However, there are questions whether the capacity to synthesize certain nonessential amino acids is sufficient when low-protein diets are used. This includes glycine, which is used for uric acid synthesis. Nitrogen excretion not only implies a nitrogen and energy loss in the urine, but energy is also required to synthesize the excretion products. The objective of this study was to quantify the energy and metabolic requirements for nitrogen excretion products in the urine. The stoichiometry of reactions to synthesize urea, uric acid, allantoin, and creatinine was established using information from a publicly available database. The energy cost was at least 40.3, 60.7, 64.7, and 65.4 kJ/g excreted N for urea, uric acid, allantoin, and creatinine, respectively, of which 56, 56, 47, and 85% were retained in the excretion product. Data from a broiler study were used to carry out a flux balance analysis for nitrogen, serine, glycine, and so-called 1-carbon units. The flux balance indicated that the glycine intake was insufficient to cover the requirements for growth and uric acid excretion. The serine intake was also insufficient to cover the glycine deficiency, underlining the importance of the de novo synthesis of serine and glycine. One-carbon units are also a component of uric acid and can be synthesized from serine and glycine. There are indications that the de novo synthesis of 1-carbon units may be a “weak link” in metabolism, because of the stoichiometric dependency between the synthesized 1-carbon units and glycine. The capacity to catabolize excess 1-carbon units may be limited, especially in birds fed low-protein diets. Therefore, there may be an upper limit to the 1-carbon-to-glycine requirement ratio in relation to nutrients that supply 1-carbon units and glycine. The ratio can be reduced by increasing uric acid excretion (i.e., reducing protein deposition) or by dietary supplementation with glycine. The hypothesis that the 1-carbon-to-glycine requirement ratio should be lower than the supply ratio provides a plausible explanation for the growth reduction in low-protein diets and the positive response to the dietary glycine supply.     

Changes in Creatine and Urea Formation in Rats Fasted and Fed Low Protein Diets

Changes in the time course of the urinary excretion of creatinine, creatine and urea, and the activities of kidney transamidinase and liver urea-cycle enzymes were investigated in rats fasted and fed on a 10% casein diet and 10% casein diets supplemented with 10% glycine and/or 1.4% arginine.

The urinary total-creatinine of the fasted rats increased extremely during fasting for 7 days, while that of the animals given the 10% casein diet supplemented with glycine and arginine rose exceedingly on the 3rd day and thereafter no significant change was observed. Most of the increase of total-creatinine could be accounted for by the increase of creatine. The activity of kidney transamidinase in the fasted rats decreased in the 3rd day and thereafter kept nearly constant. The transamidinase activity of rats fed on the 10% casein diet after giving a protein-free diet for 5 days increased in the 3rd day. An inverse relation was observed between the urinary creatine and the transamidinase activity. The urinary urea increased in the rats fasted or fed on the 10% casein diets with the supplement of glycine and/or arginine. In fasting, the activities of liver urea-cycle enzymes, except arginase, had a tendency of increasing with the lapse of time. The arginase activity remained more or less constant. The reason of the extreme increase of urinary creatine during starvation was discussed.     

Postprandial exchange of urea and ammonia nitrogen between the digestive tract and portal blood in the conscious pig after ingestion of a diet with or without urea

The concentrations of urea and ammonia were measured in the portal and arterial blood simultaneously to the blood flow rate in the portal vein during the postprandial period (8 hrs.) after ingestion of a normal protein diet with 3% urea (10 meals) or without urea (12 meals) in conscious pigs (mean body weight: 55.5 +/- 2.3 kg). When no urea was present in the diet, there was a slight and permanent uptake of blood urea by the gut (570 mg/h, i.e. 9,5 mmoles/h) as well as a permanent appearance of ammonia in the portal vein (258 mg/h i.e. 15.2 mmoles/h), increasing with time (P less than 0.05). The absorbed ammonia nitrogen represented a maximum of 70% of urea nitrogen taken up. 2. Addition of urea to the diet brought about a large absorption of that substance (73% of the ingested amount) followed by a rather large excretion (960 mg/h, i.e. 16 mmoles/h), 5-6 hrs. after the meal and led to an increase (P less than 0.05) in the absorption of ammonia.     

Dietary protein source affects lipid metabolism in the European seabass (Dicentrarchus labrax)

The study was undertaken to evaluate the effects of dietary protein sources on lipogenesis and fat deposition in a marine teleost, the European seabass (Dicentrarchus labrax). Four isonitrogenous (crude protein (CP, Nx6.25), 44% DM) and isoenergetic (22-23 kJ/g DM) diets were formulated to contain one of the following as the major protein source: fish meal (FM), one of two soy protein concentrates (SPC) and corn gluten meal (CGM). Apparent digestibility coefficients of the diets and raw ingredients, as well as soluble nitrogen (ammonia and urea) and phosphorus excretion were measured. Growth rates of seabass fed plant protein-based diets were significantly lower than those fed fish meal based diet. The protein utilisation was strongly correlated to the dietary essential amino acids index. Measurements of N excretion (ammonia and urea nitrogen) confirmed these data. Daily fat gain at the whole body level ranged between 1.1 to 1.7 g/kg BW, with the highest values being recorded in fish fed the fish meal based diet. Levels of plasma triglycerides and cholesterol were lower in fish fed soy protein diets than in those fed the diet solely based on fish meal. Soy protein rich diets decreased the activities of selected hepatic lipogenic enzymes (glucose 6-phosphate dehydrogenase, malic enzyme, ATP-citrate lysase, acetylcoenzyme A carboxylase and fatty acid synthetase). Highest lipogenic enzyme activities where found in fish fed the fish meal diet, except for fatty acid synthetase which was increased in seabass fed the corn-gluten meal based diets. Overall data suggest that dietary protein sources affects fat deposition and the lipogenic potential in European seabass.