Amino Acid Flux Research Articles

Fluxes of Amino Acid and Hexosamine in Sinking Particles in Subarctic and Arctic Coastal Regions

Spatial and temporal variations in the vertical fluxes of amino acids and hexosamine were studied at coastal sites in the south of the Chukchi Sea, Otsuchi Bay, and the center and the entrance of Funka Bay. Amino acid fluxes were higher in the upper trap than in the lower trap material, but hexosamine fluxes were opposite to amino acid fluxes. The ratios between protein amino acids (Asp, Glu and Arg) and their decomposition products (β-Ala, γ-Aba and Orn) indicate that the labile organic matter was more abundant in the upper traps than in the lower ones and increased during periods of enhanced amino acid fluxes. The ratios of Ser/Gly did not vary temporally and spatially and were nearly constant, indicating that Ser and Gly were preserved in a stable form within sinking particles. Hexosamine/amino acid ratios of these particles were higher in the lower sediment trap than in the upper trap material. This implies that the sinking particles derived from zooplankton are relatively more abundant in deep layers than in the surface layer in the water column. We suggest that the difference in amino acid flux and the composition between the upper and lower trap material is influenced by the processes of microbial decomposition and zooplankton consumption of sinking particulate organic matter.

El Niño/Southern Oscillation (ENSO) Related Variations in Particulate Export Fluxes in the Western and Central Equatorial Pacific

Time-series data from sediment trap moorings intermittently deployed during 1991–1999 show that the fluxes of biogenic material (carbonate, opal and organic matter, including amino acids) and other related parameters are temporally and spatially distinct across the Western Pacific Warm Pool (WPWP). These variations resulted from the El Nino and La Nina conditions, which alternately prevailed over the equatorial Pacific Ocean during the mooring deployments. The westernmost WPWP (a hemipelagic region) recorded relatively high average total mass and amino acid fluxes during the El Nino event. This was in sharp contrast to the eastern part of the WPWP (oligotrophic and weak upwelling regions) which recorded higher flux values during the La Nina event. Settling particulate organic matter was rich in labile components (amino acids) during La Nina throughout the study area. Relative molar ratios of aspartic acid to β-alanine together with relative molar content of non-protein amino acids β-alanine and γ-aminobutyric acid) suggested that organic matter degradation was more intense during La Nina relative to that during El Nino in the WPWP. This study clearly shows that during an El Nino event the well documented decrease in export flux in the easternmost equatorial Pacific is accompanied by a significant increase in export flux in the westernmost equatorial Pacific Ocean.

Vertical and latitudinal variations in amino acid fluxes and compositions of settling particles along 175°E in the North Pacific Ocean

Settling particles collected by sediment traps deployed for about one year (1993–94) at four sites along 175°E longitude in the central North Pacific have been analyzed for amino acids (AA) and amino sugars (or hexosamines, HA). These compounds account for about 12.5–27.9% of particulate organic carbon flux to the deep ocean. Biogeochemical indicator parameters varied in the range comparable with similar studies conducted in other oceanic sites. Correlations between non-protein AA (β-alanine and γ-aminobutyric acid) and flux parameters indicate repackaging of settling particulate organic matter (POM) around intermediate water depth (below 1500 m) at all trap sites. Mean values of indicator parameters in the shallow trap differ significantly (Student’s t-test; p < 0.03) from those in deep traps at the same site. Latitudinal trends in POM flux and its labile composition suggest that relatively fresh POM is being deposited at a higher rate in the subarctic area of the Pacific. However, this trend is not so clear in deep traps, probably due to the influence of bioactive transport and deposition of laterally advected particles in these traps. Total and particulate organic carbon fluxes normalized to primary productivity estimates show a sharp rise in efficiency of the biological pump towards the subarctic area, which is responsible for enhanced fluxes of particles with a high labile content to the deep ocean in this part of the Pacific.

The role of biominerals in the sinking flux and preservation of amino acids in the Southern Ocean along 170°W

Abstract In a study of the transport and transformation of particulate amino acids in the Southern Ocean, we found that silicate and calcium carbonate biominerals play an important role in the preservation of amino acids throughout the water column and surface sediments. Plankton, sinking particle and sediment samples were collected during the USJGOFS AESOPS transect across the Antarctic Polar Front (APF) along 170°W. Total hydrolyzable amino acids (THAA) made up of 17–27% of total organic carbon (Corg) in sinking particles and 6–23% of Corg in surface sediments. In addition to THAA, we measured amino acids bound in silicate (SiTHAA) and calcium carbonate (CaTHAA) biominerals. Although the fraction of biomineral bound to total amino acids in plankton was small, Amino acid composition has been used to develop a Degradation Index (sensu Dauwe and Middelburg, 1998). The unusual amino acid composition of Southern Ocean plankton, i.e., dominated by diatom cell walls, resulted in an apparent mismatch between the absolute value of the Degradation Index and the presumed extent of degradation. However, changes in amino acid composition that accompanied degradation were similar to those found in previous studies. Principal components analysis suggests that the greatest change in THAA composition occurred between the sediment surface floc layer and deeper sediments where particles had the longest residence time. Compositional changes observed in the water column suggested that degradation processes resulted in complete removal of amino acids, whereas changes in sediments were consistent both with selective degradation of plankton amino acids with depth and with the conversion of primarily phytoplankton biomass to that of bacterial biomass.

Protein turnover, amino acid profile and amino acid flux in juvenile shrimp Litopenaeus vannamei: effects of dietary protein source.

The effect of dietary protein on protein synthesis and growth of juvenile shrimps Litopenaeus vannamei was investigated using three different diets with equivalent protein content. Protein synthesis was investigated by a flooding dose of tritiated phenylalanine. Survival, specific growth and protein synthesis rates were higher, and protein degradation was lower, in shrimps fed a fish/squid/shrimp meal diet, or a 50% laboratory diet/50% soybean meal variant diet, than in those fed a casein-based diet. The efficiency of retention of synthesized protein as growth was 94% for shrimps fed the fish meal diet, suggesting a very low protein turnover rate; by contrast, the retention of synthesized protein was only 80% for shrimps fed the casein diet. The amino acid profile of the casein diet was poorly correlated with that of the shrimps. 4 h after a single meal the protein synthesis rates increased following an increase in RNA activity. A model was developed for amino acid flux, suggesting that high growth rates involve a reduction in the turnover of proteins, while amino acid loss appears to be high.

Development, amino acid utilization and cell allocation in bovine embryos after in vitro production in contrasting culture systems

The effects of protein-supplemented and protein-free media on amino acid uptake, protein synthesis and cell differentiation in bovine blastocysts were investigated. Four formulations of synthetic oviduct fluid were used. Each formulation was identified by the principal supplement: bovine serum albumin (0.4%, w/v); polyvinyl alcohol (0.3%, w/v); or either of two steer sera (10%, v/v). After zygote culture, blastocyst yields (day 7.5) were lowest in protein-free medium and highest in albumin-supplemented medium. Subsequent 12 h incubation in the presence of both essential and non-essential amino acids was used for the measurement of amino acid flux. All blastocysts released alanine but consumed aspartate (P < 0.001) and the extent was influenced by prior culture conditions. Aspartate uptake was lower in blastocysts produced in protein-free conditions (P < 0.05) than in blastocysts produced in albumin-supplemented conditions. Consumption indices for 16 other amino acids were not influenced by blastocyst source. Cell counts and hatching incidences were highest for albumin-supplemented blastocysts, but were similar among blastocysts from the protein-free and serum-dependent treatments. Crucially, the use of protein-free medium for zygote culture did not compromise resultant blastocysts in terms of either de novo protein synthesis ([3H]phenylalanine incorporation) or trophectoderm function (phenotype based on interferon-tau detection). Thus, although blastocyst yields were compromised after zygote culture in a protein-free (vis-à-vis albumin-supplemented) medium, amino acid flux was qualitatively conserved, and only quantitatively modified in the case of alanine and aspartate. Moreover, vital properties of blastocysts that were produced, including de novo protein synthesis and trophectodermal cell function, apparently were not adversely affected by protein deprivation.

Increased cationic amino acid flux through a newly expressed transporter in cells overproducing nitric oxide from patients with septic shock

Increased production of nitric oxide (NO) is thought to be a factor in the pathogenesis of many human diseases - among them the hypotension that often accompanies sepsis. The supply of the cationic amino acid arginine is known to be rate-limiting for NO production. We hypothesized that cationic amino acid transport might be increased in cells producing excess NO from patients with septic shock. Peripheral blood mononuclear cells were isolated from patients with sepsis and from healthy control subjects. The rates of both NO production and cationic amino acid uptake were increased in cells from patients with septic shock. The increased transport was due almost entirely to an increase in the activity of one transporter, subtype y+. The activity of the other major cationic amino acid transporter (y+L) was unchanged. The expression of CAT2 mRNA, which encodes a y+ transporter protein, was also increased in these cells. We suggest that CAT2 might be a therapeutic target to prevent excess NO production in sepsis and possibly other human disease states, while leaving basal production unchanged.

Increased cationic amino acid flux through a newly expressed transporter in cells overproducing nitric oxide from patients with septic shock

Increased production of nitric oxide (NO) is thought to be a factor in the pathogenesis of many human diseases - among them the hypotension that often accompanies sepsis. The supply of the cationic amino acid arginine is known to be rate-limiting for NO production. We hypothesized that cationic amino acid transport might be increased in cells producing excess NO from patients with septic shock. Peripheral blood mononuclear cells were isolated from patients with sepsis and from healthy control subjects. The rates of both NO production and cationic amino acid uptake were increased in cells from patients with septic shock. The increased transport was due almost entirely to an increase in the activity of one transporter, subtype y+. The activity of the other major cationic amino acid transporter (y+L) was unchanged. The expression of CAT2 mRNA, which encodes a y+ transporter protein, was also increased in these cells. We suggest that CAT2 might be a therapeutic target to prevent excess NO production in sepsis and possibly other human disease states, while leaving basal production unchanged.

Non-invasive amino acid turnover predicts human embryo developmental capacity.

IVF is limited by low success rates and a confounding high multiple birth rate contributing to prematurity, increased neonatal mortality and child handicap. These problems could be overcome if single embryos of known developmental competence could be selected for transfer on day 2/3 of development, but current methods, which rely on morphological appearance, are poor predictors of viability. We have measured non-invasively the depletion/appearance (i.e. turnover) of a physiological mixture of 18 amino acids by single human embryos during in-vitro culture using high performance liquid chromatography. From the time of transfer (day 2/3), embryos with future competence to develop to the blastocyst stage (day 5/6) exhibit amino acid flux patterns distinct from those of embryos with similar morphological appearance which arrest. Significantly, the profiles of Ala, Arg, Gln, Met and Asn flux predict blastocyst potentiality at >95%. The amino acid most consistently depleted throughout development by those embryos which form blastocysts was leucine. Of the amino acids which were produced, the most striking was alanine, which appeared in increasing amounts throughout development. Non-invasive amino acid profiling has the potential to select developmentally competent single embryos for transfer, thereby increasing the success rate and eliminating multiple births in IVF.

Nutritional and metabolic consequences of the early Maillard reaction of heat treated milk in the pig. Significance for man.

During the processing of foods, the Maillard reaction may occur contributing to altering the nutritional value of proteins. In dairy products the formation of lactuloselysine reduces the availability of lysine but the effects on the other nutrients are not very well known. Determination of the consequences of a high level of lactuloselysine in milk on the bioavailability of skim milk nutrients and the kinetics of their appearance in the portal blood and of their urinary and faecal excretions and extrapolation to lower heat treatments and to man, using the pig model. Sub-adult pigs were fitted, under anaesthesia, with permanent catheters in the portal vein, carotid artery and urethra, and with an electromagnetic flow probe around the portal vein. Each animal was successively fed with two experimental meals containing an equal amount of dried skim milk (SM), either lyophilised or heat treated to obtain an intense Maillard reaction, (M-SM) resulting in a 50% lysine blockage. Portal and arterial concentrations and flux of individual amino acids (AA), glucose, galactose and fructoselysine were measured for a period of 12h after the meals. Lysine, fructoselysine and AA excreted in the urine and faeces within 72h were also determined. In M-SM containing 50% blocked lysine, no other AA was chemically modified. Fructoselysine appeared in the portal blood very late compared to amino acids resulting from a very slow release and corresponded to 8.2 and 18.6% of the ingested amount after 12 and 72h, respectively. Significant changes of the appearance in the portal blood were observed only for lysine (-60%), alanine (-17%) and cystine (+37%). A small decrease in the digestibility of most AA during the same period was observed, which was significant after 48h for lysine, phenylalanine, cystine, aspartic acid, glycine and total AA (-6%). It was confirmed that lactuloselysine was not bioavailable. The loss in protein nutritive value was mainly due and proportional to the deterioration of lysine and, to a lesser extent, to the decrease in the digestibility of some essential AA. Taking into account the very high level of lactuloselysine in the M-SM sample studied, it may be concluded that in common foods such as milk, infant formulas, biscuits, bread, pasta, containing lower levels of blocked lysine, the nutritional loss is primarily due to the loss of lysine and to a less extent to the decrease in the digestibility of other essential AA.

Impact of suboxia on sinking particulate organic carbon: Enhanced carbon flux and preferential degradation of amino acids via denitrification

Fluxes of particulate organic carbon (POC) through the oxygen deficient waters in the eastern tropical North Pacific were found to be relatively less attenuated with depth than elsewhere in the eastern North Pacific. The attenuation coefficient (b) for the flux was found to be 0.40 versus the composite value of 0.86 determined by Martin et al. (1987). To examine this further, sinking POC was collected using sediment traps and allowed to degrade in oxic and suboxic experiments. Using a kinetic model, it was found that degradation proceeded at similar rates (roughly 0.8 day−1) under oxic and suboxic conditions, but a greater fraction of bulk POC was resistant to degradation in the suboxic experiments (61% vs. 23%). Amino acids accounted for 37% of POC collected at 75m, but following degradation the value dropped to 17% and 16% in the oxic and suboxic experiments respectively. POC collected from 500m was 10% amino acids. The non-AA component of POC collected at 75m was not degraded under suboxic conditions, while under oxic conditions it was. These results suggest that microbes degrading OC under suboxic conditions via denitrification preferentially utilize nitrogen-rich amino acids. This preferential degradation of amino acids suggests that 9% more nitrogen may be lost via water column denitrification than is accounted for when a more “Redfieldian” stoichiometry for POC is assumed.

Impact of ENSO variability on the flux and composition of sinking POM in the western equatorial Pacific Ocean: Amino acids and hexosamines

Abstract Sediment trap moorings deployed along the equator at three distinct oceanographic zones within the western Pacific Ocean during 1991–93, 1994–95, and 1999 provided data on total mass and amino acid fluxes and composition of particulate organic matter (POM) sinking in this part of the Pacific. An intercomparison of annual averages of various parameters revealed discrete patterns in flux and composition of POM under the El Nino (1991–93, 1994–95) and La Nina (1999) conditions that prevailed over the equatorial Pacific during the sampling periods. In the hemipelagic zone of the far western equatorial Pacific, where terrestrial deposition of particles larger than 5 μm accounts for >25% of the total sediment, average total mass and amino acid fluxes were relatively higher during El Nino than during La Nina. However, at the oligotrophic warm pool and upwelling sites, total mass and amino acid fluxes were higher during La Nina. Influence of ENSO-related changes in the sinking particle flux was much clearer in the hemipelagic zone compared to that in the warm pool. Average values of biogeochemical parameters such as glucosamine/galactosamine and amino acids/hexosamines, and bulk parameters like amino acid carbon and nitrogen contents relative to organic carbon and total nitrogen (THAA-C% and THAA-N%, respectively), and organic carbon normalised amino acid concentrations indicated that sinking POM was more labile during La Nina at all the sites. The ratios Aspartic acid/β-alanine, Glutamic acid/γ-aminobutyric acid, and relative concentration of non-protein amino acids (β-alanine+γ-aminobutyric acid) suggested that POM degradation was more intense during La Nina than during El Nino conditions at all sites. >

The effect of jugular or abomasal infusion of amino acids on milk yield in lactating cows fed a protein deficient diet.

The influence of first-pass splanchnic metabolism was investigated by comparing the response of 5 lactating cows to an infusion of an amino acid mixture into the abomasum or a jugular vein over 5 d according to a complete block design. The basal diet and the amino acid infusion provided 71% and 14% of crude protein requirements, respectively. The jugular infusion increased (P = 0.01) milk yield by 0.80 kg in comparison to the abomasal infusion, but milk protein yield was not altered. The jugular infusion tended to increase (P = 0.06) the arterial concentration of total essential amino acids by 11% relatively to the abomasal infusion. Mammary plasma flow and net fluxes of amino acids and glucose were not affected by the infusion sites. Variations in essential amino acid concentrations suggest that splanchnic metabolism alters peripheral delivery of amino acids but the recirculation of amino acids within the animal decreased the impact of the first-pass splanchnic metabolism on lactational performances.

Glutamine Uptake by Neurons: Interaction of Protons with System A Transporters

Astrocytes provide the glutamine required by neurons to synthesize glutamate and GABA. However, the mechanisms involved in glutamine transfer from glia to neurons have remained poorly understood. Recent work has implicated the System N transporter SN1 in the efflux of glutamine from astrocytes and the very closely related System A transporters SA1 and SA2 in glutamine uptake by neurons. To understand how these closely related proteins mediate flux in different directions, we have examined their ionic coupling. In contrast to the electroneutral exchange of H+ for Na+ and neutral amino acid catalyzed by SN1, we now show that SA1 and SA2 do not couple H+ movement to amino acid flux. As a result, SA1 and SA2 are electrogenic and do not mediate flux reversal as readily as SN1. Differences between System N and A transporters in coupling to H+ thus contribute to the delivery of glutamine from glia to neurons. Nonetheless, although they are not transported, H+ inhibit SA1 and SA2 by competing with Na+.

Coupled and uncoupled proton movement by amino acid transport system N.

The system N transporter SN1 has been proposed to mediate the efflux of glutamine from cells required to sustain the urea cycle and the glutamine-glutamate cycle that regenerates glutamate and gamma-aminobutyric acid (GABA) for synaptic release. We now show that SN1 also mediates an ionic conductance activated by glutamine, and this conductance is selective for H(+). Although SN1 couples amino acid uptake to H(+) exchange, the glutamine-gated H(+) conductance is not stoichiometrically coupled to transport. Protons thus permeate SN1 both coupled to and uncoupled from amino acid flux, providing novel mechanisms to regulate the transfer of glutamine between cells.

Effect of dietary protein degradability on net fluxes of free and peptide amino acids across the portal-drained viscera of steers

Four steers, average body weight 260±15 kg, fitted with portal catheters were used in a 4×4 Latin Square design to evaluate the influence of dietary protein degradability in the rumen on peptide and amino acid fluxes across the gastrointestinal tract. Dietary protein degradability was regulated by using different protein sources and the diets were calculated to contain 130 g CP/kg and 9·62 MJ ME per kg DM. Plasma concentrations of amino acids were analysed before and after acid hydrolysis of samples first subjected to chemical deproteinization and physical ultrafiltration, and peptide amino acids (PAA) were calculated as the difference between total and free amino acids (FAA). Portal blood flow and arterial concentrations of FAA and PAA were not affected by protein degradability or by diet. Venoarterial concentration difference and net portal flux of FAA tended to increase (P &lt; 0·10) with increase of degradable protein intake. Portal-arterial concentration difference (P &lt; 0·05) and net portal flux (P &lt; 0·10) of PAA increased linearly as dietary protein degradability increased. The proportion of PAA in total amino acid (FAA+PAA) net flux was not modified by dietary protein degradability or by diet, and the mean value as a proportion was 0·32. The major PAA absorbed were glutamate, leucine, aspartate and lysine for all diets, accounting in total for 0·50 of PAA flux. The results demonstrate that PAA may contribute significantly to AA flux across the portal-drained viscera (PDV) of steers, and both FAA and PAA net fluxes can be affected by degradable protein intake.

Effects of the feed solution concentration on the separation degree in Donnan dialysis for binary systems of amino acids

The separations of amino acids by Donnan dialysis using an ion-exchange membrane were studied. Donnan dialytic experiments were carried out using an anion-exchange membrane, glutamic acid–phenylalanine or glutamic acid–alanine mixed solutions as the feeds, and sodium hydroxide solutions as the stripping ones. The initial concentrations of the two kinds of amino acids in the feed solutions were equal and in the range of 0.5–50 mol m −3. The amino acid fluxes were measured for each feed solution. Above the feed concentration of 10 mol m −3, the glutamic acid flux was over 100 times greater than that of the other amino acid, and it was found that the Donnan dialysis was applicable to the separation of the amino acids. On the other hand, below 10 mol m −3, the amino acid fluxes varied in a complicated manner with the concentration, and below 1 mol m −3 there was little difference between the fluxes of the two amino acids. Furthermore, after soaking the membrane in solutions having the same concentrations as the feed in the Donnan dialysis, uptake of the amino acids into the membrane was also measured. By comparing the experimental results of both the flux and uptake of the amino acids, the reason why the flux varied in a complicated manner with the concentration was discussed.

Flux of amino acids and energy substrates across the leg in weight-stable HIV-infected patients with acute opportunistic infections: indication of a slow protein wasting process.

Increased whole-body proteolysis with muscle protein net degradation has been suggested as one of the causes of weight loss in patients infected with human immunodeficiency virus (HIV). We studied the exchange rates of amino acids and energy substrates across the lower extremity in 16 HIV patients and 16 age-matched controls with similar body cell mass. The patients had either opportunistic infections or chronic diarrhea but no signs of clinical malnutrition. The following findings were obtained in the HIV patients: an augmented peripheral net release of arginine and lysine; an increase in both the negative arterial-venous difference and the efflux of the nitrogen contained in nonmetabolized amino acids; diminished export of 3-methylhistidine; lowered plasma and erythrocyte amino acid concentrations; reduced output of glycerol and furthermore; and neither a net release nor a net uptake of free fatty acids. The findings concerning nitrogen metabolism support the hypothesis that, in the presence of a reduction in protein breakdown, peripheral protein synthesis is severely depressed, making a slow protein wasting process likely to occur. The balances of glycerol and free fatty acids are due not only to the leg tissues but perhaps also in part to increased net retention of these substrates by skeletal muscle.

The unexpected versatility of plants: organic nitrogen use and availability in terrestrial ecosystems

The recently recognized importance of organic nitrogen (ON), particularly amino acids, to plant nutrition in many types of agricultural and natural ecosystems has raised questions about plant-microbe interactions, N availability in soils, and the ecological implications of ON use by plants in the light of climate change and N pollution. In this review we synthesize the recent work on availability and plant uptake of amino acids with classic work on ON in soils. We also discuss recent work on the use of natural abundance levels of (15)N to infer N sources for plants. Reliance on ON is widespread among plants from many ecosystems. Authors have reached this conclusion based on laboratory studies of amino acid uptake by plants in pure culture, amino acid concentrations in soils, plant uptake of isotopically labeled amino acids in the field and in plant-soil microcosms, and from plant natural abundance values of (15)N. The supply of amino acids to plants is determined mainly by the action of soil proteolytic enzymes, interactions between amino acids and the soil matrix, and competition between plants and microbes. Plants generally compete for a minor fraction of the total amino acid flux, but in some cases this forms a significant N resource, especially in ecosystems where microbial biomass undergoes large seasonal fluctuations and contributes labile ON to the soil. A quantitative understanding of ON use by plants is confounded by incomplete data on partitioning of ON between plants, mycorrhizal fungi, and competing soil microbes. Further research is needed to predict the ecological implications of ON use by plants given the influence of climatic change and N pollution.

The effect of glutamine on protein balance and amino acid flux across arm and leg tissues in healthy volunteers.

Glutamine is important in nitrogen transportation and the physiological control of acid-base regulation. In addition, it has been assumed that glutamine regulates protein balance in skeletal muscles based on findings in both experimental and clinical studies. However, little information on glutamine and its effect on protein dynamics in normal individuals is available. Therefore, the aim of this study was to evaluate whether glutamine improves protein balance and uptake of various indispensable amino acids across peripheral tissue in healthy individuals. Standard primed constant infusions of L-[ring-2H5]phenylalanine and [ring 3,3-2H2]tyrosine (2 micromol kg(-1) h(-1)) were performed after overnight fast in five healthy male volunteers before and during infusions of a standard and a glutamine/tyrosine enriched amino acid solution. Flux measurements of amino acids (AA) including 3-methylhistidine, glucose, lactate and free fatty acids (FFA) were performed across arm and leg tissues. Infusion of the standard AA solution (0.2 g N kg(-1) day(-1)) increased the net uptake of individual amino acids, but provision of the enriched solution (0.4 g N kg(-1) day(-1)) with increased amounts of glutamine and tyrosine seemed to compete unfavourably with the net uptake of other key amino acids as methionine and phenylalanine, which are indispensable in muscles for protein synthesis. Increased flux of amino acids across peripheral tissues did not influence on flux of glucose, free fatty acid and lactate. Glutamine provision did neither stimulate protein synthesis nor attenuate breakdown of either globular or myofibrillar proteins in skeletal muscles of healthy volunteers.