Incorporation of selenium into egg proteins from dietary selenite

1. The deposition of selenium in egg components has been investigated in two experiments in which sodium selenite was added to a conventional cereal‐based layer diet.

2. Addition of graded amounts of selenite up to 4 mg Se/kg resulted in linear increases in the selenium content of egg white and yolk, and in protein fractions derived from them. The presence of selenium in yolk phosvitin indicates that deposition is not dependent upon the presence of cysteine.

3. Addition of sodium nitroprusside at 0–15 and 0–3 g/kg to diets having an addition of selenite at the highest concentration, 4 mg Se/kg, resulted in substantial reductions in the selenium concentration in egg components.

4. Samples from eggs laid by hens receiving a diet containing an additional 8 mg selenite Se/kg were subjected to dialysis against sodium hydroxide or cysteine, or subjected to reduction with hydrochloric acid and zinc under anaerobic conditions. Comparisons were made with similar samples prepared from eggs laid by hens on the control diet.

5. Both sodium hydroxide and cysteine were more effective at extracting additional diet‐derived selenium from whole white than from whole yolk. The proportion of selenium that could be extracted from the water‐soluble or the high density fractions of yolk by either reagent was similar for both control and high selenium samples. However, neither reagent was effective at removing selenium from the ovalbumin or globin fractions of white from control eggs but substantial amounts were extracted from high selenium samples.

6. Most of the selenium was present in non‐reducible forms in all samples. There was significantly more reducible selenium in ovalbumin from control eggs than from all other samples but even so non‐reducible selenium accounted for two thirds of the selenium present.

7. The differential responses to chemical treatment suggest that selenium can be deposited in eggs in an unspecified number of different forms. These have still to be characterised but site of formation of egg proteins, liver or oviduct, has a bearing on the forms of selenium deposited.     

Regulation of protein degradation pathways by amino acids and insulin in skeletal muscle of neonatal pigs

Background： The rapid gain in lean mass in neonates requires greater rates of protein synthesis than degradation. We previously delineated the molecular mechanisms by which insulin and amino acids, especially leucine, modulate skeletal muscle protein synthesis and how this changes with development. In the current study, we identified mechanisms involved in protein degradation regulation. In experiment 1,6- and 26-d-old pigs were studied during 1） euinsulinemic-euglycemic-euaminoacidemic, 2） euinsulinemic-euglycemiohyperaminoacidemic, and 3） hyperinsulinemic-euglycemic-euaminoacidemic clamps for 2 h. In experiment 2, 5-d-old pigs were studied during 1） euinsulinemic-euglycemic-euaminoacidemic-euleucinemic, 2） euinsulinemic-euglycemic-hypoaminoacidemic- hyperleucinemic, and 3） euinsulinemic-euglycemic-euaminoacidemic-hyperleucinemic clamps for 24 h. We determined in muscle indices of ubiquitin-proteasome, i.e., atrogin-1 （MAFbx） and muscle RING-finger protein-1 （MuRF1） and autophagy-lysosome systems, i.e., unc51-1ike kinase 1 （UKL1）, microtubule-associated protein light chain 3 （LC3）, and lysosomal-associated membrane protein 2 （Lamp-2）. For comparison, we measured ribosomal protein 56 （rpS6） and eukaryotic initiation factor 4E （elF4E） activation, components of translation initiation. Results： Abundance of atrogin-1, but not MuRF1, was greater in 26- than 6-d-old pigs and was not affected by insulin, amino acids, or leucine. Abundance of ULK1 and LC3 was higher in younger pigs and not affected by treatment. The LC3-11/LC3-1 ratio was reduced and ULK1 phosphorylation increased by insulin, amino acids, and leucine. These responses were more profound in younger pigs. Abundance of Lamp-2 was not affected by treatment or development. Abundance of elF4E, but not rpS6, was higher in 6- than 26-d-old-pigs but unaffected by treatment. Phosphorylation of elF4E was not affected by treatment, however, insulin, amino acids, and leucine stimulated rpS6 phosphorylation, and the response     

Regulation of protein degradation pathways by amino acids and insulin in skeletal muscle of neonatal pigs

Background

The rapid gain in lean mass in neonates requires greater rates of protein synthesis than degradation. We previously delineated the molecular mechanisms by which insulin and amino acids, especially leucine, modulate skeletal muscle protein synthesis and how this changes with development. In the current study, we identified mechanisms involved in protein degradation regulation. In experiment 1, 6- and 26-d-old pigs were studied during 1) euinsulinemic-euglycemic-euaminoacidemic, 2) euinsulinemic-euglycemic-hyperaminoacidemic, and 3) hyperinsulinemic-euglycemic-euaminoacidemic clamps for 2 h. In experiment 2, 5-d-old pigs were studied during 1) euinsulinemic-euglycemic-euaminoacidemic-euleucinemic, 2) euinsulinemic-euglycemic-hypoaminoacidemic-hyperleucinemic, and 3) euinsulinemic-euglycemic-euaminoacidemic-hyperleucinemic clamps for 24 h. We determined in muscle indices of ubiquitin-proteasome, i.e., atrogin-1 (MAFbx) and muscle RING-finger protein-1 (MuRF1) and autophagy-lysosome systems, i.e., unc51-like kinase 1 (UKL1), microtubule-associated protein light chain 3 (LC3), and lysosomal-associated membrane protein 2 (Lamp-2). For comparison, we measured ribosomal protein S6 (rpS6) and eukaryotic initiation factor 4E (eIF4E) activation, components of translation initiation.

Results

Abundance of atrogin-1, but not MuRF1, was greater in 26- than 6-d-old pigs and was not affected by insulin, amino acids, or leucine. Abundance of ULK1 and LC3 was higher in younger pigs and not affected by treatment. The LC3-II/LC3-I ratio was reduced and ULK1 phosphorylation increased by insulin, amino acids, and leucine. These responses were more profound in younger pigs. Abundance of Lamp-2 was not affected by treatment or development. Abundance of eIF4E, but not rpS6, was higher in 6- than 26-d-old-pigs but unaffected by treatment. Phosphorylation of eIF4E was not affected by treatment, however, insulin, amino acids, and leucine stimulated rpS6 phosphorylation, and the responses decreased with development.

Conclusions

The rapid growth of neonatal muscle is in part due to the positive balance between the activation of protein synthesis and degradation signaling. Insulin, amino acids, and, particularly, leucine, act as signals to modulate muscle protein synthesis and degradation in neonates.     

Incorporation of intraportally infused [15N]ammonia into urinary uric acid in cockerels pretreated with methionine sulfoximine

1. Measurements were made in situ to determine the incorporation of intraportally infused ammonia-15N into urinary uric acid in cockerels pre-treated with methionine sulfoximine (MSM), a glutamine synthetase inhibitor. 2. The incorporation of 15N into urinary uric acid was 34% of the infused amount in MSM-treated birds. This was not significantly different from the value of 46% for control birds. 3. Pre-treatment with MSM inhibited the activity of liver glutamine synthetase to 7% of the control value and decreased the incorporation of the infused ammonia-15N into plasma glutamine amide-N to 3% of the control. 4. Increases in glutamine concentrations in the blood, liver and kidney caused by the infusion of ammonia were also completely inhibited by the MSM treatment (P less than 0.05). 5. It is concluded that in the cockerel ammonia-N can be incorporated into uric acid other than by glutamine formation.     

Dynamics of amino acid and protein metabolism of laying hens after administration of 15N-labeled wheat protein. 5. Incorporation of 15N into the blood fraction and its amino acids

12 colostomized laying hens which received 15N labelled wheat over 4 days were butchered 12 h, 36 h, 60 h and 108 h (3 animals each) after the last 15N application. The intake of 15N excess (15N') from the wheat amounted to 540 mg 15N' during the application period. The 15N' in the blood plasma decreased after the last 15N' application from 0.76 atom-% to 0.55 atom-% after 108 h, the labelling of the corpuscular components at the same measuring points increased from 0.28 to 0.50 atom-% 15N'. 96.6% of the plasma 15N' and 93.8% of that in the corpuscles is precipitable in trichloric acetic acid. The atom-% 15N' of histidine in the total blood remained unchanged in dependence on the butchering time. The 15N amount in lysine and arginine and that in the non-basic amino acids decreased inconsiderably in the period between 12 h and 108 h after the last 15N' wheat feeding.     

15N-labelled lysine in colostomized laying hens. 4. Incorporation of 15N-lysine into various amino acids of egg yolk and white

Each of three colostomized laying hens received per os 0.2% L-Lysine with 48 atom-% 15N-excess (15N') labelled in alpha-position in addition to a pelleted laying hen ration of 120 g over a period of 4 days. On the following 4 days they received equal amounts of unlabelled lysine. The eggs laid during the 8 days of the experiment were separated into the white of egg, the yolk and the eggshell, and the to and heavy nitrogen in the individual fractions were determined. Above that, 17 amino acids and their atom-%15N' were determined in the 19 samples of the white and yolk of egg. Of the total 15N' from the lysine fed in the 4 days, 10.1% were found in the yolk, 10.5% in the white of egg and 1.1% in the eggshells of the eggs laid during the 8 days of the experiment. 85% of the total amino acid-15N' of the yolk and 86% of the white of egg detected to be lysine-15N'. The 15N'-amount of the other 16 amino acids was mainly concentrated in the two acid and basic amino acids. Approximately 50% of the non-lysine 15N' in the egg are contained in aspartic acid, glutamic acid, histidine and arginine. A very low incorporation of the labelled lysine only could be detected in the aromatic and sulphur-containing amino acids from both the yolk and the white of egg. 43% of the 15N' was detected in the 10 essential and semi-essential (except lysine) and 57% in the 6 non-essential amino acids of the yolk and 52% and 48% resp. of the white of egg. One can summarize that the incorporation of 15N' into the egg shows the same development as that of the labelled amino acids of the wheat protein and that 15% of the lysine-15N' could be detected in the 16 other amino acids.     

Studies using labeled urea on lactating ruminants. 4. Incorporation of 15N-urea into the basic amino acids of goat milk]

Goats were used in a trial to investigate the rate of 15N incorporation from labelled urea into the basic amino acids of the milk (excess of 15N = 24.9 atom%). As early as 20 minutes after administration of the first dose of 15N urea nitrogen labelling in arginine and lysine was observed. The highest level of 15N labelling was noticed for lysine and arginine 24 hrs after start of the trial. The peak values of labelling for histidine were essentially lower. Reference is made to peculiarities of the ruminal metabolism of histidine.     

Enzyme studies with the livers of chicks fed semi‐synthetic diets containing crystalline amino acids and diammonium citrate

The levels of glutamate dehydrogenase [NAD(P)], (GDH), aspartate trans‐aminase (AspT) and alanine transaminase (AlT) were measured in livers from chicks fed on a semi‐synthetic diet containing crystalline essential amino acids as the sole nitrogen source (diet A). The effects of a supplement of 12.0% glutamic acid (diet H) or 11.07% diammonium citrate (DAC) (diet B) or 12.0% glutamic acid plus 1.0% proline with an additional 0.6% glycine (diet C) on these enzymes were studied and the results compared with the levels found for control chicks given a typical diet based on cereal protein (diet J). The abilities of livers from chicks given diets A, B and C to synthesise [14C] glutamic acid from [14C]2‐oxoglutaric acid and diammonium citrate (DAC) were assessed.

The levels of GDH, AspT and AlT found in the livers of chicks given the control diet were 54.1, 966 and 123.7 units/mg protein respectively. Non‐essential nitrogen added as glutamic acid or as DAC did not cause induction of the enzymes studied above control levels. Glutamic acid (diets H and B) caused a depression of GDH levels (37.4 and 38.9 units/mg protein respectively) but had no effect on AspT and AlT compared with the controls, whereas DAC caused a decrease in AlT (43.9 units/mg protein) but had no effect on AspT and GDH; diet A depressed AlT and AspT levels (64.4 and 735 units/mg protein respectively).

The livers of chicks given diets A, C and B varied in their ability to synthesise glutamic acid, 39.3%, 31.9% and 24.0% respectively of the radioactivity being recovered as glutamic acid.     

Dynamics of amino acid and protein metabolism of laying hens after the administration of 15N-labeled wheat protein. 8. 15N-labeling of nitrogen and 15N incorporation into the amino acids of the liver

Over 4 days 12 colostomized laying hens received, together with the ration, 36 g wheat with 14.37 atom-% 15N excess (15N'), The basic amino acids were nearly equally labelled. Three animals each were butchered after 12 h, 36 h, 60 h, and 108 h after the last 15N' application. Emission spectrometric determination of 15N' in the liver and in the amino acids was carried out. In addition, atom-% 15N' was determined in the free amino acids and the peptides. The labelling in the liver 12 h after the last 15N' application amounted to 1.75 atom-% 15N' and decreased after 108 h to 0.81 atom-% 15N'. The average TCA precipitable 15N' quota in the total 15N' amounted to 81.4% and was nearly identical at all measuring times. The arginine 15N' amount in the liver was twice as high as that of lysine 15N'. In dependence on the period of time after the last 15N' application the decrease in the labelling of the free arginine is considerable in comparison to free lysine. At the first measuring time (12 h) it was 1.69 atom-% 15N' and at the last one (108 h) 0.57 atom-% 15N'. Based on the results of 15N' labelling of the peptides in the liver further, more detailed series of experiments for studies of the peptide metabolism in the liver should be carried out.     

Absorption and utilization of amino acids infused into the cecum of growing pigs. 2. 15N-labeled lysine

Over a period of 4 days 15N-labelled lysine was infused into two growing female pigs (live weight approximately 50 kg) through a caecal cannula. The feeding was restrictive (1,400 g dry matter/day) and, with regard to lysine, it didn't meet the requirement. In a 7-day experiment the N- and 15N-content was measured periodically in the excretions (feces and urine), in various fractions of the blood and in selected slaughtering samples. From the infused 15N 3-5% are excreted as lysine in feces, another 5% are in other amino acids of the bacteria protein. The disappearance rate of 15N' from the large intestine makes greater than or equal to 90%. The biggest part of this 15N (78-88%) is excreted with the urine in form of 15N-urea. Obviously the infused amino acid is decomposed to NH3 in the large intestine and then absorbed. The absorbed ammonia is changed into urea in the ornithine cycle and excreted in urine. The recovery rate of the 15N infused as 15N-lysine is 93 and 84% resp. Incorporation of 15N in to serum protein or other body protein could not be detected so that the remaining difference of 7-16% cannot necessarily be interpreted as incorporation rate of 15N into the body protein. Under practical conditions the maximal utilisation of lysine from the feed in the large intestine is 1.6% and should thus be without importance.     

Utilization of 15N-labeled urea in laying hens. 7. 15N-incorporation into the amino acids of different muscle types

3 colostomized laying hybrids received 1% 15N labelled urea with 96.06 atom-% 15N excess (15N') with a commercial ration over a period of 6 days. After the application of the same ration with unlabelled urea on the following 2 days the animals were butchered. In the muscles of the breast, the leg and the heart, the labelling of total nitrogen and the incorporation of urea 15N' into 15 amino acids of the 3 different kinds of muscles were ascertained. On average, significant differences could be ascertained between the atom-% 15N of the muscles of the skeleton and those of the heart. The 15N' of the breast and leg muscles was 0.25 and 0.34 atom-% resp.; that of the cardial proteins 0.71 atom-% 15N'. The incorporation of urea 15N into the basic amino acids is low and varies both between the kinds of muscles and between the amino acids. On average the highest level of labelling was found among the essential amino acids valine, isoleucine and leucine; the average atom-% 15N' for the muscles of the breast is 0.13, of the leg 0.17, and of the heart 0.27; the 15N' quota of branched chain amino acids in the total 15N' of the respective muscle is accordingly 6.0%, 5.0% and 4.5%. The non-essential amino acids, particularly glutamic acid, are more highly labelled in the muscles than the essential ones. A 15N' for glutamic acid of 0.24 atom-% in the breast muscles, of 0.27 atom-% in those of the legs and of 0.64 atom-% in the heart muscle could be detected. The average quota of the 15N' of these acid amino acids in the 15N' for breast, leg and heart muscles is 7.4, 6.2 and 6.7 resp. The quota of the 15N' in the 6 non-essential amino acids in the total 15N' in all 3 kinds of muscles is approximately two thirds and in the 9 essential ones one third of the total 15N'. Although the results show that there is a certain incorporation of 15N' from urea into the amino acids of the muscle proteins, their contribution to meeting the demands is to be considered irrelevant.     

Effect of dietary excess of branched‐chain amino acids on performance and serum concentrations of amino acids in growing pigs

Depressed performance and availability of some amino acids (AA) in pigs fed excess Leu diets appear to be related to lower feed intake. Surplus Ile and Val may help to overcome this effect. An experiment was conducted with 24 pigs (31.8 ± 1.2 kg initial BW) to evaluate the effect of dietary excess of either Leu alone or with surplus Ile and Val on performance and serum concentration (SC) of essential AA. Treatments were as follows: T1, basal diet; T2, basal plus 0.43% L‐Leu (excess Leu); T3, basal added with 0.43% L‐Leu, plus 0.20% L‐Ile and 0.25% L‐Val (excess LIV). The basal diet was formulated to contain 0.90% standardized ileal digestible Lys and added with crystalline L‐Lys, L‐Thr, DL‐Met, L‐Trp, L‐Leu, L‐Ile, L‐His and L‐Val to create essential AA:Lys ratios close to an ideal protein for growing pigs. All pigs were fed the same amount of feed twice a day (average, 3.42× the requirement of NEm). Blood samples were collected at 2.5 (absorptive) and 11.0 h (post‐absorptive) post‐prandial to analyse SC of AA. Excess of either Leu or LIV did not affect growth rate nor feed conversion. Excess Leu increased Leu SC and decreased Ile and Val SC (p < 0.05) at both absorptive and post‐absorptive phases, but excess LIV restored the SC of Ile and Val. The SC of other essential AA was not affected by excess of either Leu or LIV. The SC of all AA during absorptive, on average, was about two times higher than that of post‐absorptive phase. These results suggest that the reduced availability (SC) of Ile and Val in pigs consuming excess Leu diets is attributed to a reduced absorption and increased cellular degradation rates of them.     

母猪日粮中支链氨基酸水平对仔猪血液生化指标和部分免疫指标的影响

试验探讨了母猪日粮中添加支链氨基酸对哺乳仔猪血清生化指标和部分免疫指标的影响。选取胎次、体况、分娩期相近的法系(JALAX)母猪144头,随机均分成3个处理(对照组、处理A和处理B),每个处理6个重复,每个重复8头母猪。对照组饲喂基础日粮(总支链氨基酸为3.24%,缬氨酸:赖氨酸=0.86:1),处理A日粮添加低水平支链氨基酸(总支链氨基酸为3.56%,缬氨酸:赖氨酸=1.10:1),处理B日粮添加高水平支链氨基酸(总支链氨基酸为3.74%,缬氨酸:赖氨酸=1.20:1)。试验结果表明:①支链氨基酸添加组仔猪10、21日龄血清尿素氮水平有所降低,但10日龄血糖的浓度显著提高(P<0.05);②低水平支链氨基酸显著提高了10日龄仔猪血清白蛋白浓度(P<0.05)以及10、21日龄血清中乳酸脱氢酶活性(P<0.01);③与对照组相比,支链氨基酸的添加极显著提高了10、21日龄仔猪血清中IgG的浓度(P<0.01)。这说明母猪日粮中添加支链氨基酸能通过母体效应进而影响仔猪生长发育与免疫功能,但作用效果与添加水平有关。     

Impact of dietary tryptophan and behavioral type on growth performance and plasma amino acids of young pigs

Tryptophan (TRP) content in the protein of the weaning diet was varied from deficient (.70 g/16 g N) to adequate (1.15 g/16 g N) and excess (1.60 g/16 g N) in diets fed to 108 pigs from d 5 to d 26 after weaning (W) and in 72 pigs from d 26 after weaning to slaughter (100 kg live weight) to assess immediate and long-term effects of TRP on performance. Daily weight gain and feed efficiency were improved when dietary TRP was increased from deficient to adequate (+60 and +40%, respectively). Concurrently, daily feed intake was elevated moderately (+15%). No further improvement was observed with excess TRP. In the low TRP group, gain/feed was significantly poorer up to 25 kg live weight, but this effect did not continue later. Although no compensatory growth could be shown in the group fed the deficient diet, growth retardation was very small (1.5%; P greater than .10) at slaughter. Early changes in TRP supply did not affect either carcass or meat quality. Behavioral reactivity, as determined on day W + 5 in an "open-field" test, did not affect early performance, but growth rate during the growing-finishing stage (3.2%) or the whole period (2.5%) was greater by nonemotional than by emotional pigs. Plasma amino acid contents in blood samples, withdrawn on day W + 15 (fed state) and W + 17 (fasted state), were consistent with the effect of TRP on growth rates. However, in the fasted state, a diet x reactivity interaction suggested that TRP removal from the plasma was less rapid in nonemotional than in emotional pigs. Furthermore, increased plasma concentrations of essential amino acids and urea in the latter group suggested that protein and amino acid catabolism was more rapid in emotional than in nonemotional pigs. These data are discussed relative to the effect of the behavioral type of pig on its TRP requirement.     

Utilization of N15-labelled urea in the laying hen. 6. Incorporation in the liver and kidneys

In an experiment 3 colostomized laying hybrids received a normal ration containing 1% 15N labelled urea with 96.06% atom-% 15N excess (15N') over six days. Subsequently the same ration with unlabelled urea was given over 2 days, after which the animals were butchered. In the kidneys the 15N' amounted to 1.1 atom-% and 1.8 atom-% in the liver. The TCA soluble N fraction and the ammonia were more highly labelled than the total N. Lysine, histidine and arginine were lowly labelled in the kidneys. This also applies to the liver with the exception of histidine. In the branch-chained and aromatic amino acids of the liver the 15N' was between 0.2 and 0.3 atom-%. The highest labelling of non-essential amino acids was found in glutamic acid with 0.9 atom-% 15N' and aspartic acid with 1.1 atom-% 15N'. The evaluation of the amino acid in the liver showed that the 6 non-essential amino acids account for two thirds of the total amino acid 15N' whereas the 9 essential ones account for one third of the amino acid 15N' only.     

Genotype and dietary lysine deficiency affect carcass and muscle amino acid composition of pigs growing from 10 to 25 kg body weight

Amino acid (AA) composition of body protein is considered constant although there are evidences that AA pattern in pigs may be altered by different factors. Pigs with different body composition and protein deposition rates—like fatty and lean pigs—may differ in AA composition, with possible consequences on their AA requirements. This work investigates effects of genotype and dietary lysine deficiency on AA composition of carcass and muscles of Iberian and Landrace × Large White pigs. Twenty‐eight barrows (10 kg body weight [BW]), 14 from each breed, were used. They were randomly assigned to two experimental diets according to a factorial arrangement (two breeds × two diets). Diets were isonitrogenous and isoenergetic (200 ± 1 g CP/kg dry matter (DM); 14.7 ± 0.1 MJ ME/kg DM) and with identical chemical composition except for lysine concentration (10.9 and 5.20 g lysine/kg DM, for lysine‐adequate (AL) diet and lysine‐deficient (DL) diet respectively). Pigs were individually housed, and daily feed allowance was adjusted on a weekly basis according to BW. Pigs were slaughtered at 25 kg BW. Isoleucine, valine and phenylalanine concentration were higher in carcass protein of Iberian pigs (p < .01). In longissimus muscle, higher concentration of arginine, isoleucine, phenylalanine, lysine and valine (p < .001–p < .05), and lower of methionine (p < .001) were detected in Iberian pigs, whereas phenylalanine, leucine, lysine, threonine and methionine concentration decreased and arginine increased (p < .001–p < .05) when pigs were fed DL diet. Genotype and lysine deficiency effects were moderate in the AA composition of protein of biceps femoris muscle. The results show that AA proportions in protein of carcass and longissimus muscle can be influenced by pig genotype and conditions of lysine shortage. The biceps femoris muscle, with different functional and metabolic properties, shows more constant AA composition than longissimus, which seem to prevail independent from genotype or nutritional challenges.     

Studies on the protein and amino acid metabolism of laying hens using 15N labeled casein. 4. Fecal excretion of nitrogen and amino acids, digestibility of crude protein and absorption of basic amino acids]

Generally, the faeces of laying hens fed 15N casein rations were found to contain equal proportions of TCE-precipitable and TCE-soluble nitrogen. Considerable variations were observed to occur between the 64 samples investigated (27%-75%) and no explanation was found to account for this fact. The content of basic amino acids in faecal proteins was found to differ considerably from that of the proteins in the intestinal contents. A high lysine content was found after the feeding of wheat. The present trial substantiated this result, provided the casein contained a certain proportion of non-available lysine. The apparent and true digestibility of dietary N was 88% or 91%, that of 15N (2nd and 6th day of experiment) and 92%. During the feeding of labelled casein a higher level of N labelling was found in the TCE-soluble portion of the faeces, whereas on the 8th to 12th day a higher level of labelling was observed in the TCE-precipitable portion of the faeces. The peak of 15N excretion occurred on the 3rd day of experiment. When 15N administration terminated the atom% 15N in the faeces and in urine was found to decrease rapidly approximating the initial level of labelling asymptotically.     

The utilization of 15N-labeled urea by the laying hen. 3. Incorporation of labeled nitrogen into the amino acids of the white and egg yolk

In an experiment with 3 colostomized laying hens the incorporation of heavy nitrogen from urea into the amino acids of the 21 eggs laid during the 8-day experiment was ascertained. In these eggs the content of 15 amino acids was ascertained separately in the whites and yolks of the eggs and their atom-% 15N-excess (15N') was determined. The heavy nitrogen could be detected in all amino acids investigated. The incorporation of 15N' into the essential amino acids of the white and yolk of eggs is very low. Of the applied 15N'-amount of the urea 0.18% could be detected in the 9 essential amino acids of the white of egg and 0.12% in those of the yolk. For the 6 analysed nonessential amino acids the rediscovery quota of 15N' in the white of egg was 0.50% and in the yolk 0.81%. The conclusion from these results is that the NPN-source urea is insignificant for egg protein synthesis.