1. Measurements have been made of whole-body and skeletal muscle protein synthesis in fed and fasted adults with L-[1-13C]leucine. 2. The marked increase in whole-body synthesis on feeding largely reflects the changes in protein synthesis in muscle, which doubles on feeding, compared with a 40% increase in that of the rest of the body. 3. Skeletal muscle in fed man contributes more than half to total protein synthesis occurring in the whole body.

Muscle protein synthesis measured by stable isotope techniques in man: the effects of feeding and fasting.

The keto analog of leucine, alpha-ketoisocaproate (KIC), is formed intracellularly from leucine and is released, in part, into the systemic circulation. Therefore. KIC can be used to estimate intracellular leucine tracer enrichment in man during labeled-leucine tracer experiments without requiring tissue biopsy samples. This approach was studied in young, healthy, male adults maintained on different dietary protein intakes from generous (1.5 g kg-1d-1) to deficient (0.0 g kg-1d-1) for 5-7 day periods. At the end of each dietary period, the volunteers were given a primed, continuous infusion of L-[1-13C]leucine either after an overnight fast (postabsorptive state) or while being fed hourly aliquots of the same diet. The plasma concentrations of all 3 branched-chain amino and keto acid pairs were measured from early morning blood samples taken from 4 subjects at 4 different levels of protein intake. Leucine concentration showed a weak correlation, and valine concentration showed a strong correlation with protein intake; isoleucine and the 3 keto acids did not. However, each branched-chain amino acid concentration was strongly correlated with its corresponding keto acid concentration. In plasma samples obtained during the L-[1-13C]leucine infusions, the ratio of [1-13C]KIC to [1-13C]leucine enrichment ratio remained relatively constant (77 +/- 1% over the wide range of dietary protein intakes and for both the fed and postabsorptive states. For the tissues from which the plasma KIC originates, the rate of plasma leucine into cells will account for approximately 77% of the intracellular leucine flux with the remaining 23% coming primarily from leucine release via protein breakdown. The constant nature of the plasma KIC to leucine 13C enrichment ratio implies that relative changes in leucine kinetics will appear the same under many dietary circumstances regardless of whether plasma leucine or KIC enrichments are used for the calculations.

Relationship of plasma leucine and α-ketoisocaproate during a L-[1-13C]leucine infusion in man: A method for measuring human intracellular leucine tracer enrichment

Among the reasons suggested for the discrepancy between N balance and tracer-derived indispensable amino acid (IAA) requirement estimates is the possibility that the metabolic requirement is met not only by the diet but also by IAA synthesized de novo by the gastrointestinal microflora, which are then absorbed. It is therefore crucial to better understand and quantify the microbial biosynthesis of amino acids in the human gastrointestinal tract and its potential role in providing IAA to meet human amino acid requirement. Here, the available evidence on the contribution of microbial amino acids to the host's amino acid homeostasis, applying the (15)N labeling paradigm, is summarized. Between 1 and 20% of circulating plasma lysine, urinary lysine and body protein lysine of the host, respectively, is derived from intestinal microbial sources and corresponds to a gross microbial lysine contribution of 11-68 mg. kg(-1). d(-1) in adult humans with an adequate protein intake when fecal or ileal microbial lysine enrichment is used as precursor. Factors affecting estimates of net microbial IAA contribution are discussed. It appears that the small intestine is responsible for a large part of microbial lysine uptake, although some absorption from the large intestine cannot be excluded. Nonoxidative lysine losses from the human gastrointestinal tract, which were found to be between 3.9 to 8.5 mg. kg(-1). d(-1), are necessary to estimate the net contribution of microbial IAA. It is reasonable to assume that microbial amino acid synthesis in the human gastrointestinal tract utilizes a mixture of various nitrogen sources, i.e., endogenous amino acids, urea and ammonia. Microbes in the small intestine may rely more on endogenous amino acids. Deprivation of nutrients, the intake of certain dietary nonstarch oligosaccharides, lipids, as well as protein intake level and source and level of consumption of certain amino acids can affect the composition and metabolic activity of the intestinal microflora and thus its fermentation products potentially available to the host. In conclusion, with the use of the (15)N labeling paradigm, a significant contribution of microbial lysine to the host lysine homeostasis is found. However, to assess the net contribution of microbial IAA and its importance in defining the adult IAA requirement, this is not the ultimately successful experimental strategy because the interpretation of results is complicated by the nitrogen recycling in the gut, the uncertainty of the precursor pool of absorption and the limited data on nonoxidative IAA losses from the human gastrointestinal tract.

Contribution of Microbial Amino Acids to Amino Acid Homeostasis of the Host

Numerous studies have been reported that relate to the influence of cattle type on production efficiency; however, most reported studies have been concerned with output characteristics. This review concentrates on the influence of cattle type on the input requirements and more specifically, energy requirements. Data are presented that show that energy is required for maintenance, growth, gestation and lactation and requirements for each of these functions vary among cattle types. Data are presented that suggest 70 to 75% of total annual energy requirements are required for maintenance functions and variation in requirements for maintenance appear to be greater than variation in requirements for growth, gestation or lactation. In general, variation in maintenance requirements appears to be positively associated with genetic potential for measures of production, e.g., rate of growth or milk production. Reported results suggest that animals having genetic potential for high productivity may have less advantage or be at a disadvantage in a more restrictive environment, suggesting the need for synchronization of production environment and germ plasm resources. Some of the sources of variation in maintenance requirements have been discussed. Data are cited that suggest little of the variation in maintenance energy expenditures is attributable to variation in total body composition per se. However, considerable evidence has accumulated to show that a relatively large proportion of maintenance energy expenditures can be attributed to energy expenditures of visceral organs, especially the liver and gastrointestinal tract. High rates of energy expenditures of these tissues appear to be directly or indirectly associated with the high rates of protein synthesis in these tissues. Greater use of differing research approaches and assimilation of the results are needed to develop an understanding of the reasons for variation among animals in maintenance energy expenditures or energetic efficiency.

Cow Type and the Nutritional Environment: Nutritional Aspects

Whole-body leucine and lysine metabolism was explored in young adult men by a primed constant intravenous infusion of a mixture of L-[1–13C]leucine and L-[alpha-15N]lysine over a 4-h period. Subjec...

Whole-body leucine and lysine metabolism: response to dietary protein intake in young men

1. The fractional synthetic rates of tissue proteins were studied in growing pigs using the constant-infusion technique of tracer-labelled amino acids ([14C]leucine and [14C]lysine) and the mathetmatical model for calculation, employed in rats by Garlick, Millward & James (1973). 2. During a 6 h infusion, samples were taken from blood and muscle and at the end of the infusion from liver, muscle, pancreas, heart, duodenum, jejunum, ileum, colon, and skin. The specific radioactivity of free and protein-bound leucine and lysine was estimated. 3. A quasi-steady-state in the specific radioactivity of free plasma leucine and lysine was reached within approximately 2 h, the rate-constants being 35 and 48/d respectively. 4. The specific radioactivity of free leucine and lysine in plasma was used to calculate the flux of these amino acids. It was found to be higher than the daily intake. 5. The average fractional rate of protein synthesis in muscle and heart 8.1% d, in small and large intestine the values were 50 and 33% d respectively and in liver and pancreas more than 100% d. 6. The calculation of protein synthetic rate in pig tissue using the constant-infusion method of labelled amino acids seems to be a suitable toof for study of this species.

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Estimation of rate of protein synthesis by constant infusion of labelled amino acids in pigs

(1983) Investigations on the pancreatic and stomach secretion in pigs by means of continuous infusion of 14C-amino acids Archiv fur Tierernaehrung: Vol 33, No 1, pp 9-22

Investigations on the pancreatic and stomach secretion in pigs by means of continuous infusion of 14C-amino acids

The effects of thyroid hormones on the range of tissue protein synthesis in growing pigs using the constant infusion technique with [14C]lysine were studied. 2. During a 6 h infusion, samples were taken from blood and, at the end of the infusion, from liver, pancreas, stomach, small and large intestines, kidney cortex, kidney medulla, muscle and skin. 3. Lower relative specific radioactivities of free leucine and lysine in several tissues were observed in the hormone-treated group than in the untreated one. 4. The range of protein synthesis rate and the daily amount of protein synthesized in tissues was higher in all tissues after application of thyroid hormones. 5. Assuming that the organs analysed represented 70% of the total trichloroacetic acid-precipitable protein of the pig, the estimated range of daily protein synthesis was 251-490 and 312-880 g in untreated and hormone-treated pigs respectively.

/pdf/studies-on-the-range-of-tissue-protein-synthesis-in-pigs-the-qskoxeoiac.pdf

Studies on the range of tissue protein synthesis in pigs: the effect of thyroid hormones.

A trial was performed with 2 fistula pigs (each with 2 fistulas, one located about 30 cm below the pyloric orifice and the other at the end of the small intestine). Animal A received a casein diet containing 14% crude protein for a period of 2 weeks before the tracer amino acid was administered. Animal B received the same diet for a period of 10 days and was then fed a diet (at the same protein level) containing gluten as sole protein source. The two tracer amino acids, 14C-U-L-leucine and 3H-4,5-(N)-L-lysine, were injected intravenously. The passage rates for dry matter, organic matter and N measured at the beginning of the small intestine were higher than the rate of intake. The rate of passage of amino acids was also found to be increased relative to the rate of intake. In general, this increase involved the non-essential amino acids to a much larger extent. A considerable proportion of the amino acids passing into the large intestine is not excreted with the faeces but is probably converted in catabolic processes. It is for this reason that any values for the efficiency of amino acid absorption calculated on the basis of data on the faecal excretion of amino acids will not provide conclusive evidence for the availability of dietary amino acids in processes of the intermediate metabolism. The rate of secretion of 3H and 14C radioactivity into the digesta of the small intestine was found to increase rapidly within 1-2 hrs after administration of the tracer amino acids. The 14C radioactivity detected was found to be almost exclusively derived from 14C leucine while only about 60% of the 3H activity found in the digesta of fistula I were shown to be bound to lysine. Labelled lysine and leucine (of endogenic origin) are absorbed into the small intestine at a slower rate (i.e. endogenic proteins are less efficiently digested) than the non-radioactive amino acids (of exogenic origin) so that a process of concentration of endogenic amino acids is observed towards the end of the small intestine.

Secretion of endogenous amino acids in the gastrointestinal tract and amino acid resorption in the swine

(1982). Investigations on the amino acid secretion and absorption in the stomach of the growing pig. Archiv fur Tierernaehrung: Vol. 32, No. 10-11, pp. 703-710.

Münchmeyer R

Papers

Estimation of rate of protein synthesis by constant infusion of labelled amino acids in pigs

Investigations on the pancreatic and stomach secretion in pigs by means of continuous infusion of 14C-amino acids

Studies on the range of tissue protein synthesis in pigs: the effect of thyroid hormones.

Secretion of endogenous amino acids in the gastrointestinal tract and amino acid resorption in the swine

Investigations on the amino acid secretion and absorption in the stomach of the growing pig