scholarly journals The mysteries of nitrogen balance

1999 ◽  
Vol 12 (1) ◽  
pp. 25-54 ◽  
Author(s):  
J. C Waterlow
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Urea Cycle ◽  
N Balance ◽  
Single Amino Acid ◽  
Whole Body ◽  
Acid Oxidation ◽  
Physiological Level ◽  
Body Protein ◽  
Urea Production

AbstractThe first part of this review is concerned with the balance between N input and output as urinary urea. I start with some observations on classical biochemical studies of the operation of the urea cycle. According to Krebs, the cycle is instantaneous and automatic, as a result of the irreversibility of the first enzyme, carbamoyl-phosphate synthetase 1 (EC 6.3.5.5; CPS-I), and it should be able to handle many times the normal input to the cycle. It is now generally agreed that acetyl glutamate is a necessary co-factor for CPS-1, but not a regulator. There is abundant evidence that changes in dietary protein supply induce coordinated changes in the amounts of all five urea-cycle enzymes. How this coordination is achieved, and why it should be necessary in view of the properties of the cycle mentioned above, is unknown. At the physiological level it is not clear how a change in protein intake is translated into a change of urea cycle activity. It is very unlikely that the signal is an alteration in the plasma concentration either of total amino-N or of any single amino acid. The immediate substrates of the urea cycle are NH3 and aspartate, but there have been no measurements of their concentration in the liver in relation to urea production. Measurements of urea kinetics have shown that in many cases urea production exceeds N intake, and it is only through transfer of some of the urea produced to the colon, where it is hydrolysed to NH3, that it is possible to achieve N balance. It is beginning to look as if this process is regulated, possibly through the operation of recently discovered urea transporters in the kidney and colon. The second part of the review deals with the synthesis and breakdown of protein. The evidence on whole-body protein turnover under a variety of conditions strongly suggests that the components of turnover, including amino acid oxidation, are influenced and perhaps regulated by amino acid supply or amino acid concentration, with insulin playing an important but secondary role. Molecular biology has provided a great deal of information about the complex processes of protein synthesis and breakdown, but so far has nothing to say about how they are coordinated so that in the steady state they are equal. A simple hypothesis is proposed to fill this gap, based on the self-evident fact that for two processes to be coordinated they must have some factor in common. This common factor is the amino acid pool, which provides the substrates for synthesis and represents the products of breakdown. The review concludes that although the achievement and maintenance of N balance is a fact of life that we tend to take for granted, there are many features of it that are not understood, principally the control of urea production and excretion to match the intake, and the coordination of protein synthesis and breakdown to maintain a relatively constant lean body mass.

scholarly journals Effect of food intake on hind-limb and whole-body protein metabolism in young growing sheep: chronic studies based on arterio-venous techniques

1992 ◽  
Vol 68 (2) ◽  
pp. 389-407 ◽  
Author(s):  
Patricia M. Harris ◽  
Pat A. Skene ◽  
Vivien Buchan ◽  
E. Milne ◽  
A. G. Calder ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Protein Metabolism ◽  
Hind Limb ◽  
Live Weight ◽  
Whole Body ◽  
Acid Oxidation ◽  
Protein Loss ◽  
Body Protein ◽  
Protein Retention

Whole-body protein synthesis, estimated by the irreversible loss rate procedure, and hind-leg protein metabolism determined by arterio-venous techniques were monitored in response to three nutritional conditions (approximately 0.6, 12 and 1.8 x energy maintenance (M)) in ten wether lambs (33 kg average live weight). In all lambs and treatments measurements were based on radiolabelled phenylalanine, but the terminal procedures (five at 0.6 x M and five at 1.8 x M) also included infusion of [1-13C]leucine; this permitted comparison of amino acids catabolized (leucine) and non-metabolized (phenylalanine) by the hind-limb tissues. Whole-body protein synthesis increased with intake and the relationship with energy expenditure was slightly lower than that reported previously for pigs and cattle. The efficiency of protein retention: protein synthesis did not exceed 0.25 between the two intake extremes. Effects of intake on amino acid oxidation were similar to those observed for cattle. Hind-limb protein synthesis also increased significantly (P < 0.001) in response to intake. Estimates of protein gain, from net uptake values, indicated that the tissues made a greater proportional contribution to total protein retention above M and to protein loss below M, emphasizing the role played by muscle tissue in providing mobile protein stores. The rates of protein synthesis calculated depended on the selection of precursor (blood) metabolite, but rates based on leucine always exceeded those based on phenylalanine when precursor from the same pool was selected. The incremental efficiency of protein retained: protein synthesis was apparently unity between 0.6 and 1.2 x M but 0.3 from 1.2 to 1.8 x M. Blood flow through the iliac artery was also proportional to intake. Leucine and oxo-acid catabolism to carbon dioxide increased with intake such that the metabolic fate of the amino acid was distributed in the proportion 2:1 between protein gain and oxidation. The rates of oxidation were only 1–3% the reported capacity of the rate-limiting dehydrogenase enzyme in muscle, but sufficient enzyme activity resides in the hind-limb adipose tissue to account for such catabolism

scholarly journals Comprehensive assessment of post-prandial protein handling by the application of intrinsically labelled protein in vivo in human subjects

2021 ◽  
pp. 1-9
Author(s):  
Jorn Trommelen ◽  
Andrew M. Holwerda ◽  
Philippe J. M. Pinckaers ◽  
Luc J. C. van Loon
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Stable Isotope ◽  
Dietary Protein ◽  
Protein Metabolism ◽  
Muscle Protein ◽  
Human Subjects ◽  
Whole Body ◽  
Body Protein

All human tissues are in a constant state of remodelling, regulated by the balance between tissue protein synthesis and breakdown rates. It has been well-established that protein ingestion stimulates skeletal muscle and whole-body protein synthesis. Stable isotope-labelled amino acid methodologies are commonly applied to assess the various aspects of protein metabolism in vivo in human subjects. However, to achieve a more comprehensive assessment of post-prandial protein handling in vivo in human subjects, intravenous stable isotope-labelled amino acid infusions can be combined with the ingestion of intrinsically labelled protein and the collection of blood and muscle tissue samples. The combined application of ingesting intrinsically labelled protein with continuous intravenous stable isotope-labelled amino acid infusion allows the simultaneous assessment of protein digestion and amino acid absorption kinetics (e.g. release of dietary protein-derived amino acids into the circulation), whole-body protein metabolism (whole-body protein synthesis, breakdown and oxidation rates and net protein balance) and skeletal muscle metabolism (muscle protein fractional synthesis rates and dietary protein-derived amino acid incorporation into muscle protein). The purpose of this review is to provide an overview of the various aspects of post-prandial protein handling and metabolism with a focus on insights obtained from studies that have applied intrinsically labelled protein under a variety of conditions in different populations.

Leucine metabolism in lactating and dry goats: effect of insulin and substrate availability

1993 ◽  
Vol 265 (3) ◽  
pp. E402-E413 ◽  
Author(s):  
S. Tesseraud ◽  
J. Grizard ◽  
E. Debras ◽  
I. Papet ◽  
Y. Bonnet ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Protein Degradation ◽  
Whole Body ◽  
Initial Slope ◽  
Sensitivity Index ◽  
Early Lactation ◽  
Dry Period ◽  
Body Protein ◽  
Lactating Goats

Early lactating goats show insulin resistance with respect to extramammary glucose utilization. However, much less is known about the two major factors, insulin and plasma amino acid concentration, that regulate protein metabolism in lactating goats. To examine this question, the in vivo effect of acute insulin was studied in goats during early lactation (12-31 days postpartum), midlactation (98-143 days postpartum), and the dry period (approximately 1 yr postpartum). Insulin was infused (at 0.36 or 1.79 nmol/min) under euglycemic and eukaliemic clamps. In addition, appropriate amino acid infusion was used to blunt insulin-induced hypoaminoacidemia or to create hyperaminoacidemia and maintain this condition under insulin treatment. Leucine kinetics were assessed using a primed continuous infusion of L-[1-14C]-leucine, which started 2.5 h before insulin. In all animals the insulin treatments failed to stimulate the nonoxidative leucine disposal (an estimate of whole body protein synthesis) under both euaminoacidemic and hyperaminoacidemic conditions. Thus, in goat as well as humans, infusion of insulin fails to stimulate protein synthesis even when combined with a substantially increased provision of amino acids. In contrast, insulin treatments caused a dose-dependent inhibition of the endogenous leucine appearance (an estimate of whole body protein degradation). Under euaminoacidemia the initial slope from the plot of the endogenous leucine appearance as a function of plasma insulin (an insulin sensitivity index) was steeper during early lactation than when compared with the dry period. A similar trend occurred during midlactation but not to any significant degree. These differences were abolished under hyperaminoacidemia. It was concluded that the ability of physiological insulin to inhibit protein degradation was improved during lactation, demonstrating a clear-cut dissociation between the effects of insulin on protein and glucose metabolism. This adaptation no doubt may provide a mechanism to save body protein.

Protein turnover in the human fetus studied at term using stable isotope tracer amino acids

1993 ◽  
Vol 265 (1) ◽  
pp. E31-E35 ◽  
Author(s):  
P. F. Chien ◽  
K. Smith ◽  
P. W. Watt ◽  
C. M. Scrimgeour ◽  
D. J. Taylor ◽  
...  
Keyword(s):  
Amino Acids ◽  
Protein Synthesis ◽  
Placental Transfer ◽  
Safety Margin ◽  
Whole Body ◽  
Acid Oxidation ◽  
Maternal Circulation ◽  
Body Protein ◽  
Isotope Tracer ◽  
Elective Cesarean

Before elective cesarean delivery (4 h), we infused L-[1-13C]leucine and L-[15N]phenylalanine into the maternal circulation and measured enrichment and concentration of amino acids and carbon dioxide in cord blood of six normal human fetuses at delivery. There were net fetal uptakes of leucine (2.22 +/- 0.29 mumol.kg-1.min-1) and phenylalanine (0.80 +/- 0.11 mumol.kg-1.min-1) with net outputs of CO2 (6.11 +/- 1.12 ml.kg-1.min-1) and the transamination product of leucine, alpha-ketoisocaproate (1.04 +/- 0.32 mumol.kg-1.min-1). Fetal amino acid oxidation accounted for a substantial proportion of the flux from the mother (leucine, 0.36 +/- 0.09 mumol.kg-1.min-1 and phenylalanine, 0.18 +/- 0.04 mumol.kg-1.min-1). Fetal whole body accretion of leucine carbon (0.82 +/- 0.21 mumol.kg-1.min-1) was 69% of the umbilical uptake, and that of phenylalanine (0.62 +/- 0.08 mumol.kg-1.min-1) was 78%. Fetal whole body protein synthesis was approximately 13 g.kg-1.day-1, i.e., much faster than in adults but similar to that in the newborn. Net protein accretion was 2-4 g.kg-1.day-1. The placental supply of leucine and phenylalanine exceeds the fetal demand for protein synthesis by only a small amount, suggesting that the safety margin of placental transfer may be small for these amino acids. The results suggest that the method could be applied safely to studies of fetal growth retardation.

scholarly journals The effect of a high dose of 3-hydroxy-3-methylbutyrate on protein metabolism in growing lambs

1997 ◽  
Vol 77 (6) ◽  
pp. 885-896 ◽  
Author(s):  
Isabelle Papet ◽  
Piotr Ostaszewski ◽  
Francoise Glomot ◽  
Christiane Obled ◽  
Magali Faure ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Free Amino Acid ◽  
Protein Metabolism ◽  
Free Amino ◽  
Skeletal Muscles ◽  
Whole Body ◽  
Control Group ◽  
High Dose ◽  
Body Protein

AbstractThe effect of a high dose of 3-hydroxy-3-methylbutyrate (HMB, a leucine catabolite) on protein metabolism was investigated in growing male lambs fed on hay and concentrate. Concentrate was supplemented with either Ca(HMB)2 (4g/kg) or Ca(C03)2 in experimental (HMB) and control groups respectively. Both groups consisted of six 2-month old lambs. Three complementary methods to study protein metabolism were carried out consecutively 2·5 months after beginning the dietary treatment: whole body phenylalanine fluxes, postprandial plasma free amino acid time course and fractional rates of protein synthesis in skeletal muscles. Feeding a high dose of HMB led to a significant increase in some plasma free amino acids compared with controls. Total, oxidative and non-oxidative phenylalanine fluxes were not modified by dietary HMB supplementation. Similarly, an acute infusion of HMB, in the control group, did not change these fluxes. In skeletal muscles, fractional rates of protein synthesis were not affected by long-term dietary supplementation with HMB. Taken together our results showed that administration of a high dose of HMB to lambs was able to modify plasma free amino acid pattern without any effect on whole-body protein turnover and skeletal muscle protein synthesis

Meal stimulation of albumin synthesis: a significant contributor to whole body protein synthesis in humans

1992 ◽  
Vol 263 (4) ◽  
pp. E794-E799 ◽  
Author(s):  
P. De Feo ◽  
F. F. Horber ◽  
M. W. Haymond
Keyword(s):  
Amino Acids ◽  
Protein Synthesis ◽  
Amino Acid ◽  
Combined Treatment ◽  
Essential Amino Acids ◽  
Whole Body ◽  
Albumin Synthesis ◽  
Body Protein ◽  
Peripheral Tissues ◽  
Stimulation Of

The present studies were performed to test the hypothesis that the liver, by increasing the synthesis of specific plasma proteins during the absorption of an amino acid meal, may play an important role in the temporary "storage" of ingested essential amino acids and to explore the effects of glucocorticosteroids and recombinant human growth hormone (rhGH) on these processes. The fractional synthetic rates of albumin and fibrinogen were determined using simultaneous infusions of intravenous [1-14C]leucine and intraduodenal [4,5-3H]leucine after 22 h fasting and during absorption of glucose and amino acids in four groups of normal subjects treated for 1 wk with placebo, prednisone (0.8 mg.kg-1.day-1), rhGH (0.1 mg.kg-1.day-1), or combined treatment. When compared with the fasted state and independent of the route of tracer delivery and hormonal treatment, albumin, but not fibrinogen, synthesis increased (P < 0.0001) during absorption of a mixed glucose amino acid meal in all groups. This increase in albumin synthesis accounted for 28% of the increase in whole body protein synthesis associated with feeding and for 24, 22, and 14% in the prednisone, rhGH, and combined treatment groups, respectively. These data suggest that the stimulation of albumin synthesis observed during feeding prevents irreversible oxidative losses of a significant fraction of ingested essential amino acids and may serve as a vehicle to capture excess dietary amino acids and transport them to peripheral tissues to sustain local protein synthesis.

Whole body protein synthesis: studies with different amino acids in the rat

1989 ◽  
Vol 257 (5) ◽  
pp. E639-E646 ◽  
Author(s):  
C. Obled ◽  
F. Barre ◽  
D. J. Millward ◽  
M. Arnal
Keyword(s):  
Amino Acids ◽  
Protein Synthesis ◽  
Amino Acid ◽  
Protein Turnover ◽  
Specific Radioactivity ◽  
Whole Body ◽  
Body Protein ◽  
Tissue Specific ◽  
Dose Method ◽  
Infusion Method

These studies were undertaken to determine to what extent constant infusion measurements and plasma sampling could provide sensible answers for rates of whole body protein turnover and also which amino acid would be the most representative probe of whole body protein turnover. Whole body protein synthesis rates were estimated in 70-g rats with L-[U-14C]threonine, L-[U-14C]lysine, L-[U-14C]tyrosine, L-[U-14C]phenylalanine, and L-[1-14C]leucine by either simultaneous tracer infusion of four amino acids or by injections of large quantities of 14C-labeled amino acids. In the infusion experiment, indirect estimates of whole body protein turnover based on free amino acid specific radioactivity and stochastic modeling were compared with direct measurement of the incorporation of the tracer into proteins. These two methods of analysis provided similar results for each amino acid, although in each case fractional synthesis rates were lower (by between 26 and 63%) when calculations were based on plasma rather than tissue specific radioactivity. With the flooding-dose method, whole body fractional protein synthesis rates were 41.4, 25.6, 31.1, and 31.4% with threonine, lysine, phenylalanine, and leucine, respectively. These values were similar to those obtained by the continuous infusion method using tissue specific radioactivity for threonine and lysine. For leucine, however, the flooding-dose method provided an intermediate value between the two estimates derived either from the plasma or the tissue specific radioactivity in the infusion method.(ABSTRACT TRUNCATED AT 250 WORDS)

scholarly journals Basal and Postprandial Myofibrillar Protein Synthesis Rates Do Not Differ between Lean and Obese Middle-Aged Men

2019 ◽  
Vol 149 (9) ◽  
pp. 1533-1542 ◽  
Author(s):  
Imre W K Kouw ◽  
Jan Willem van Dijk ◽  
Astrid M H Horstman ◽  
Irene Fleur Kramer ◽  
Joy P B Goessens ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Amino Acid ◽  
Muscle Protein ◽  
Myofibrillar Protein ◽  
Whole Body ◽  
Protein Digestion ◽  
Body Protein ◽  
Postprandial Period ◽  
Protein Ingestion ◽  
Myofibrillar Protein Synthesis

ABSTRACT Background Excess lipid availability has been associated with the development of anabolic resistance. As such, obesity may be accompanied by impairments in muscle protein metabolism. Objective We hypothesized that basal and postprandial muscle protein synthesis rates are lower in obese than in lean men. Methods Twelve obese men [mean ± SEM age: 48 ± 2 y; BMI (in kg/m2): 37.0 ± 1.5; body fat: 32 ± 2%] and 12 age-matched lean controls (age: 43 ± 3 y; BMI: 23.4 ± 0.4; body fat: 21 ± 1%) received primed continuous L-[ring-2H5]-phenylalanine and L-[ring-3,5-2H2]-tyrosine infusions and ingested 25 g intrinsically L-[1-13C]-phenylalanine labeled whey protein. Repeated blood and muscle samples were obtained to assess protein digestion and amino acid absorption kinetics, and basal and postprandial myofibrillar protein synthesis rates. Results Exogenous phenylalanine appearance rates increased after protein ingestion in both groups (P < 0.001), with a total of 53 ± 1% and 53 ± 2% of dietary protein–derived phenylalanine appearing in the circulation over the 5-h postprandial period in lean and obese men, respectively (P = 0.82). After protein ingestion, whole-body protein synthesis and oxidation rates increased to a greater extent in lean men than in the obese (P-interaction < 0.05), resulting in a higher whole-body protein net balance in the lean than in the obese (7.1 ± 0.2 and 4.6 ± 0.4 µmol phenylalanine · h−1 · kg−1, respectively; P-interaction < 0.001). Myofibrillar protein synthesis rates increased from 0.030 ± 0.002 and 0.028 ± 0.003%/h in the postabsorptive period to 0.034 ± 0.002 and 0.035 ± 0.003%.h−1 in the 5-h postprandial period (P = 0.03) in lean and obese men, respectively, with no differences between groups (P-interaction = 0.58). Conclusions Basal, postabsorptive myofibrillar protein synthesis rates do not differ between lean and obese middle-aged men. Postprandial protein handling, including protein digestion and amino acid absorption, and the postprandial muscle protein synthetic response after the ingestion of 25 g whey protein are not impaired in obese men. This trial was registered at www.trialregister.nl as NTR4060.

scholarly journals Protein Intake to Maximize Whole-Body Anabolism during Postexercise Recovery in Resistance-Trained Men with High Habitual Intakes is Severalfold Greater than the Current Recommended Dietary Allowance

2019 ◽  
Author(s):  
Michael Mazzulla ◽  
Sidney Abou Sawan ◽  
Eric Williamson ◽  
Sarkis J Hannaian ◽  
Kimberly A Volterman ◽  
...  
Keyword(s):  
Protein Synthesis ◽  
Steady State ◽  
Amino Acid ◽  
Resistance Exercise ◽  
Protein Intake ◽  
Whole Body ◽  
Total Amino Acid ◽  
Acid Oxidation ◽  
Amino Acid Oxidation ◽  
Habitual Protein Intake

ABSTRACT Background Dietary protein supports resistance exercise–induced anabolism primarily via the stimulation of protein synthesis rates. The indicator amino acid oxidation (IAAO) technique provides a noninvasive estimate of the protein intake that maximizes whole-body protein synthesis rates and net protein balance. Objective We utilized IAAO to determine the maximal anabolic response to postexercise protein ingestion in resistance-trained men. Methods Seven resistance-trained men (mean ± SD age 24 ± 3 y; weight 80 ± 9 kg; 11 ± 5% body fat; habitual protein intake 2.3 ± 0.6 g·kg−1·d−1) performed a bout of whole-body resistance exercise prior to ingesting hourly mixed meals, which provided a variable amount of protein (0.20–3.00 g·kg−1·d−1) as crystalline amino acids modeled after egg protein. Steady-state protein kinetics were modeled with oral l-[1-13C]-phenylalanine. Breath and urine samples were taken at isotopic steady state to determine phenylalanine flux (PheRa), phenylalanine excretion (F13CO2; reciprocal of protein synthesis), and net balance (protein synthesis − PheRa). Total amino acid oxidation was estimated from the ratio of urinary urea and creatinine. Results Mixed model biphasic linear regression revealed a plateau in F13CO2 (mean: 2.00; 95% CI: 1.62, 2.38 g protein·kg−1·d−1) (r2 = 0.64; P ˂ 0.01) and in net balance (mean: 2.01; 95% CI: 1.44, 2.57 g protein·kg−1·d−1) (r2 = 0.63; P ˂ 0.01). Ratios of urinary urea and creatinine concentrations increased linearly (r = 0.84; P ˂ 0.01) across the range of protein intakes. Conclusions A breakpoint protein intake of ∼2.0 g·kg−1·d−1, which maximized whole-body anabolism in resistance-trained men after exercise, is greater than previous IAAO-derived estimates for nonexercising men and is at the upper range of current general protein recommendations for athletes. The capacity to enhance whole-body net balance may be greater than previously suggested to maximize muscle protein synthesis in resistance-trained athletes accustomed to a high habitual protein intake. This trial was registered at clinicaltrials.gov as NCT03696264.

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