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PubMed1998

Amino acid sufficiency and mTOR regulate p70 S6 kinase and eIF-4E BP1 through a common effector mechanism.

The present study identifies the operation of a signal tranduction pathway in mammalian cells that provides a checkpoint control, linking amino acid sufficiency to the control of peptide chain initiation. Withdrawal of amino acids from the nutrient medium of CHO-IR cells results in a rapid deactivation of p70 S6 kinase and dephosphorylation of eIF-4E BP1, which become unresponsive to all agonists. Readdition of the amino acid mixture quickly restores the phosphorylation and responsiveness of p70 and eIF-4E BP1 to insulin. Increasing the ambient amino acids to twice that usually employed increases basal p70 activity to the maximal level otherwise attained in the presence of insulin and abrogates further stimulation by insulin. Withdrawal of most individual amino acids also inhibits p70, although with differing potency. Amino acid withdrawal from CHO-IR cells does not significantly alter insulin stimulation of tyrosine phosphorylation, phosphotyrosine-associated phosphatidylinositol 3-kinase activity, c-Akt/protein kinase B activity, or mitogen-activated protein kinase activity. The selective inhibition of p70 and eIF-4E BP1 phosphorylation by amino acid withdrawal resembles the response to rapamycin, which prevents p70 reactivation by amino acids, indicating that mTOR is required for the response to amino acids. A p70 deletion mutant, p70Delta2-46/DeltaCT104, that is resistant to inhibition by rapamycin (but sensitive to wortmannin) is also resistant to inhibition by amino acid withdrawal, indicating that amino acid sufficiency and mTOR signal to p70 through a common effector, which could be mTOR itself, or an mTOR-controlled downstream element, such as a protein phosphatase.

Adaptor Proteins, Signal Transducing↗

PubMed1977

Dose-response relationships between amino acid intake and blood levels in newborn infants.

The objective of the present investigation was to investigate the nature of the blood amino acid response with amino acid intake in newborn infants. Forty-nine newborn infants of 640 to 4020 g birth weight and 26 to 40 weeks gestational age were placed on one of the following regimes within 48 hr of birth: intravenous dextrose with intravenous feeding of three levels of crystalline amino acid mixture, or of one level of casein hydrolysate, or with oral feeding of two levels of milk formula. Blood amino acid levels at 132 hr after feeding of graded levels of the intravenous amino acid solution were evaluated by regression analyses and compared to blood levels when casein hydrolysate was fed intravenously or milk formula was fed orally. At low levels of amino acid intake, such as were achieved with formula feeding, blood levels were relatively unchanged. However, with higher intakes, as were obtained with intravenous feedings, blood levels increased at rates which could be defined for each amino acid. This observation suggests that if the composition and rate of delivery of an amino acid solution is known, blood levels may be predicted.

Age Factors↗

PubMed1983

Different effect of amino acid deprivation on syntheses of intra- and extracellular proteins in rat hepatocytes in primary culture.

Primary cultured rat hepatocytes synthesize various proteins and secrete half of them into the medium. When the cells were cultured in medium deficient in amino acids for 30 h, they maintained their normal level of intracellular protein synthesis, but their synthesis of extracellular proteins decreased by half. This reduced production of extracellular proteins could be restored by addition of amino acid mixture to the medium. Since secretion of protein was not inhibited and no albumin accumulated in the cells on amino acid deprivation, the reduced secretion of extracellular protein was due to inhibition of its synthesis rather than inhibition of its secretion. Addition of the protease inhibitor leupeptin to cells cultured in medium supplemented with amino acids inhibited protein degradation 60%, but it did not change the rate of protein synthesis. Addition of this inhibitor to cells in amino acid deficient medium strongly inhibited the syntheses of both intra- and extracellular proteins. Electromicroscopic examination of the cells showed that amino acid deprivation markedly decreased the amount of rough endoplasmic reticulum and caused the appearance of autophagic vacuoles. These results show that extracellular amino acids control the syntheses of both intra- and extracellular proteins, but have a more direct effect on the latter, the intracellular proteins being synthesized by reutilization of amino acids liberated on protein degradation. Moreover, protein degradation seems to be accelerated by formation of phagosomes, when the medium is deficient in amino acids. When this protein degradation is inhibited by a protease inhibitor, the synthesis of intracellular proteins decreases. Therefore, there seem to be two compartments of amino acids in hepatocytes, one supplied by lysosomal degradation of proteins and used mainly for synthesis of intracellular protein, and the other supplied from exogenous amino acids and used for syntheses of both intra- and extracellular proteins.

Amino Acids↗

PubMed1981

[Effect of nitrogens parenteral nutrition on the amino acid spectrum of the blood and liver in thyroxine poisoning].

The effect of nitrogen parenteral nutrition on the blood and liver amino acid spectrum was studied in white rats with experimental thyrotoxicosis. The content of free amino acids in the blood plasma and liver was shown to be considerably increased in thyrotoxicosis in the presence of protein starvation. Injection of an amino acid mixture containing polyamin and improved casein hydrolysate for 7 days favoured the normalization of the content of most free amino acids in the blood and liver but did not completely eliminate their imbalance.

Amino Acids↗

PubMed1994

[Cholesterol levels in lipoproteins and various intake of amino acids in an experiment].

In the conditions of physiological nutrient content and variable amino-acid mixtures made of casein K, gluten G and KG combination (1:1, standard diet), the 10-week effect of amino acids (until the age of 105 days, end of maturation stage) was investigated in growing rats with respect to protein utilization (net protein ratio) and to the values of lipid parameters at the end of the experiment (cholesterol, triacylglycerols, atherogenic indexes, cholesterol distribution into lipoprotein classes). A significant decrease in NPR for gluten, an insignificant decrease for casein in the growth period are related to the amino acid score of limiting amino acids--they determine rate of proteosynthesis and potential biological value--amount percentage of amino acids of the given diet for plastic processes of proteins (AS for KG 69, for K 67, for G 28; BHp for KG 83.1, for K 70.0, for G 54.1). Since day 105 amino acid utilization is balanced in all groups, hence it does not depend upon the quality of aminograms (lower rate of proteosynthesis in comparison with the growth period, for which the values of limiting amino acids are sufficiently high). Significantly decreased values of cholesterolemia and triacylglycerolemia for gluten food, insignificantly higher values for casein food, are in direct correlations with the values of Lys/Arg, Met/Arg, Met, Lys, and in indirect correlations with Arg and Arg+Glu+Gly. Distribution of HDL-cholesterol as a cholesterol percentage in HD lipoproteins is higher by 8.7% for gluten and all atherogenic indexes are significantly decreased in contrast to casein food, in which a slight decrease in HDL-cholesterol and insignificantly higher atherogenic indexes were recorded.

Amino Acids↗

PubMed1993

Down regulation of epidermal growth factor receptors in liver proliferation induced by a mixture of triiodothyronine, amino acids, glucagon, and heparin (TAGH).

This study investigated the mechanisms by which TAGH solution (a mixture of triiodothyronine, amino acids, glucagon, and heparin) induces DNA synthesis in hepatocytes in the liver of intact rats, with particular reference to events at the epidermal growth factor (EGF) receptor. Both partial hepatectomy and infusion of TAGH stimulated DNA synthesis at 24 hours and both procedures resulted in a reduction of EGF receptors assessed in plasma membranes isolated from rat liver at this time. In cell cultures, while EGF strongly stimulated DNA synthesis and started EGF receptor down regulation, TAGH had only a minor effect (1.5 x basal) on DNA synthesis and did not interact with or down regulate the EGF receptor. Membrane phosphorylation studies, however, showed that TAGH induced phosphorylation of tyrosine residues in the EGF receptor. The in vivo action of TAGH seems to entail recruitment of similar changes in the EGF receptor to those that occur after partial hepatectomy.

Amino Acids↗

PubMed1996

Increased hepatic urea synthesis in patients with active inflammatory bowel disease.

BACKGROUND/METHODS: Patients with active inflammatory bowel disease are often reported to be in negative nitrogen balance. Therefore, we examined basal and amino acid stimulated urea synthesis in 11 patients with active inflammatory bowel disease (six with Crohn's disease and five with ulcerative colitis) and in 10 patients with non-active disease (six with Crohn's disease and four with ulcerative colitis). A primed continuous infusion of an amino acid mixture was given from t = 1 h to t = 5 h; during the first and the last 2 h no amino acid infusion was given. Urea nitrogen synthesis rate was calculated in hourly intervals for 7 consecutive hours. Urea nitrogen synthesis rate was quantified independent of changes in blood amino acid concentration by means of the functional hepatic nitrogen clearance, i.e. the linear slope of the regression of urea nitrogen synthesis rate of blood amino acid concentration. RESULTS: Basal urea nitrogen synthesis rate was 24.5 +/- 2.9 mmol/h in the patients with no disease activity and 43.8 +/- 2.2 mmol/h in patients with active disease (p < 0.01). During amino acid infusion urea nitrogen synthesis rate was elevated two-fold in the patients with active disease. Functional hepatic nitrogen clearance was 28.2 +/- 1.5 1/h in patients with no disease activity and 56.1 +/- 4.1 1/h in patients with active disease (p < 0.01). No differences between the two groups were observed as regards basal or stimulated plasma glucagon and cortisol and serum levels of interleukin-1 alpha, interleukin-1 beta, tumor necrosis factor alpha and interleukin-6. CONCLUSIONS: The results show that the liver function related to conversion of amino-nitrogen to urea is increased in patients with active inflammatory bowel disease. No differences among known and possible regulators of urea synthesis were found between the two groups. The accelerated hepatic amino-nitrogen conversion contributes to the less efficient nitrogen economy in patients with active inflammatory bowel disease.

Adolescent↗

PubMed1979

Determination of erythrocyte amino acids by gas chromatography.

Erythrocyte amino acid levels were determined, by gas chromatography, in a group of 34 normal human adults. No significant sex or age correlations were noted. A method for the quantitative gas chromatographic analysis of free amino acids in erythrocytes is described. Following hemolysis and deproteinization the amino acids were isolated on a cation-exchange resin. Glutathione was removed from the amino acid mixture by adsorption on an anion-exchange resin. Following conversion to their N-acetyl-n-propyl esters, 19 amino acids were separated and quantitated by gas chromatography on a single column in 18 min. Typical reproducibility data indicate that a coefficient of variation of 2-5% is attainable.

Adult↗

PubMed1972

Protein synthesis in synaptosomal fractions. Ultrastructural radioautographic study.

A quantitative ultrastructural radioautographic study of in vitro protein synthesis has been carried out in rat synaptosomal fractions incubated with tritiated leucine or a tritiated amino acid mixture. Analysis of grain density distribution demonstrated that presynaptic endings are labeled. 30-50% of the developed grains, representing tritiated amino acids incorporated into proteins, were related to presynaptic endings which accounted for 75-77% of the total processes. 34-45% of the grains were related to processes containing ribosomes which accounted for only 4-7% of the total processes. The relative specific activity of these ribosome-containing processes, some of which could be identified as postsynaptic elements, was up to ten times higher than that of the presynaptic ending. These findings indicate that protein synthesis takes place in vitro in presynaptic terminals although to a significantly lesser degree than that occurring in ribosome-containing processes, which, with other nonpresynaptic processes, are at the present time unavoidable contaminants of synaptosomal fractions. Presynaptic endings that in radioautographs contained no mitochondria were labeled. Also, presynaptic endings were labeled after incubation in the presence of chloramphenical which inhibited 20% of the protein synthesis of the synaptosomal fraction. It is concluded that besides mitochondrial protein synthesis, another protein synthesizing system operates in presynaptic endings in vitro.

Amino Acids↗

PubMed1976

Glycine-serine interrelations in the chick.

Glycine and serine are involved in numerous important functions in the body in addition to protein synthesis. Glycine is synthesized by higher animals; however, the rate is not adequate to support maximal growth of the chick. Studies indicate that chicks fed a glycine-serine free crystalline amino acid diet grow at approximately 80% the rate of chicks fed the same diet supplemented with glycine. An equimolar quantity of L-serine has been shown to support equal chick performance as glycine, thus indicating that serine conversion is adequate to meet the dietary need for glycine. The serine-glycine interconversion is catalyzed by the folic acid containing enzyme sering hydroxymethyltransferase, and a deficiency of this vitamin decreases the effectiveness of serine in meeting the chick's dietary need for glycine. Studies with chicks fed normal and high levels of a crystalline amino acid mixture devoid of glycine and serine indicate that the chick has the metabolic potential to synthesize more of these two amino acids than should be required for normal growth. The observed dietary response to glycine or serine or both indicates, however, that this synthetic potential is not being utilized.

Animals↗

PubMed1997

[Increased urea synthesis in patients with active inflammatory bowel disease].

Patients with active inflammatory bowel disease are often reported to be in negative nitrogen balance. Therefore, we examined basal and amino acid stimulated urea synthesis in 11 patients with active inflammatory bowel disease and in 10 patients with non-active disease. A primed continuous infusion of an amino acid mixture was given from t = 1 h to t = 5 h; during the first and the last two hours no amino acid infusion was given. Urea nitrogen synthesis rate was quantified independently of changes in blood amino acid concentration by means of the functional hepatic nitrogen clearance, i.e. the linear slope of the regression of urea nitrogen synthesis rate on blood amino acid concentration. Basal and amino acid stimulated urea nitrogen synthesis rate as well as functional hepatic nitrogen clearance were elevated twofold in the patients with active disease. No differences between the two groups were observed as regards basal or stimulated plasma glucagon, cortisol, catecholamines and serum levels of interleukin-1 alpha, interleukin-1 beta, tumor necrosis factor-alpha and interleukin-6. The results show that liver function related to conversion of amino-nitrogen to urea is increased and may contribute to the less efficient nitrogen economy in patients with active inflammatory bowel disease.

Adult↗

PubMed1989

Identification of phenylthiocarbamyl amino acids for compositional analysis by thermospray liquid chromatography/mass spectrometry.

The chromatographic separation of amino acids for compositional analysis of peptides and proteins is commonly performed by reverse-phase high-performance liquid chromatography of amino acid residues that have been derivatized with phenylisothiocyanate. The present report describes an extension of this method, which employs thermospray liquid chromatography/mass spectrometry to confirm the identification of the resulting phenylthiocarbamyl (PTC) amino acids. A standard HPLC separation method has been adapted for use with the thermospray technique, and on-column mass spectra of standard synthetic PTC-amino acids have been acquired. These spectra show characteristic fragmentation patterns not seen in the corresponding cyclic phenylthiohydantoin amino acid derivatives. The LC/MS method has been tested on hydrolysates of bovine serum albumin, porcine insulin, and human placental collagen. In each case, the mass spectra of components eluting with the same retention times as the standard PTC-amino acids are similar to those observed in the standard amino acid mixture. Other components display mass spectra that can be interpreted in terms of known in vivo or in vitro modifications to amino acid side chains in these proteins. The LC/MS method has assisted in the identification of by-products of the derivatization reaction. It has also been applied to a study in which an enzyme, citrate synthase, isolated from porcine heart, was compared to the protein expressed by a recombinant porcine citrate synthase gene in Escherichia coli. The data showed that the recombinant protein lacks a modified residue, trimethyllysine, which is present in the enzyme expressed in mammalian tissues.

Amino Acids↗

PubMed1990

Amino acid tolerance in cirrhotic patients following oral protein and amino acid loads.

Plasma amino acid and venous blood ammonia concentrations were measured in six patients with well-compensated cirrhosis and in six healthy volunteers, both in the fasting state and serially for 5 h following ingestion of 30 g mixed protein and 30 g amino acid mixture, administered on separate occasions. Mean fasting plasma concentrations of threonine, serine, proline, glycine, and of the three branched-chain amino acids, valine, isoleucine and leucine, were significantly reduced in the cirrhotic patients compared with the control subjects, while mean (+/- 1 s.d.) fasting venous blood ammonia concentrations were comparable 71.2 +/- 31.4 cf. 56.0 +/- 25.4 mumol/L. Following the oral protein and amino acid loads, increases were observed in plasma amino acid concentrations in the majority of subjects with a return to baseline values by the end of the study. Changes in the circulating concentrations of most amino acids were independent of their concentration in the oral protein and amino acid loads, and their relative distribution in the circulation varied over time. The increases in the concentrations of the three branched-chain amino acids did, however, reflect their concentrations in the two nitrogen loads and did remain constant, relative to one another, over time. There were wide intra- and inter-individual variations in plasma amino acid concentrations following protein and amino acid ingestion in both study groups, and in general no significant differences in responses were observed between them. Similarly, no significant inter-group differences were observed in the ammonia response to the two nitrogen loads. No fundamental differences exist in the ways in which patients with well-compensated cirrhosis handle oral protein or amino acid loads of the magnitude employed in the present study.

Adult↗

PubMed1976

Intestinal absorption of amino acids and peptides in Hartnup disorder.

Absorption of free and peptide-bound amino acids was investigated in a girl with Hartnup disorder aged 26 months. Plasma levels of amino acids were followed after oral administration of (1) an amino acid mixture simulating casein and (2) an equivalent dose of a partial enzymic hydrolysate of casein containing oligopeptides in addition to free amino acids. The results suggested that many neutral amino acids were poorly absorbed when given in the free form, but much more readily absorbed when given as peptides. Unexpectedly, the results also suggested that glutamic acid was poorly absorbed when given in the free form. The results obtained with threonine could not be interpreted. There was an increased renal clearance of many neutral amino acids, including glycine, but clearance of proline was not increased. Most amino acids with an increased renal clearance also appeared to be poorly absorbed when given by mouth in the free form.

Amino Acids↗

PubMed1988

O6-methylguanine-DNA methyltransferase levels in tissues of methionine-cysteine deficient subadult and adult mice.

1. Subadult and adult mice were fed during 6 days a diet containing a complete mixture of amino acids or a mixture low in methionine-cysteine. 2. O6-methylguanine-DNA methyltransferase (MT) is the acceptor protein for methyl groups present in DNA at the O-6 position of a guanine that has been methylated by alkylating carcinogenic agents. 3. Upon methionine-cysteine deficiency O6-methylguanine-DNA methyltransferase levels decreased significantly in liver, but seemed unaffected in kidney, lung, testis and brain. 4. Age associated differences were found in liver, with lower values in the subadult than the adult mice leaving the young animals more vulnerable to exposure to alkylating agents. 5. To ensure an efficient repair of DNA lesions both age groups depended on a complete amino acid mixture in the diet.

Aging↗

PubMed1983

o-Phthaldialdehyde precolumn derivatization and reversed-phase high-performance liquid chromatography of polypeptide hydrolysates and physiological fluids.

A rapid and ultrasensitive fluorescence amino acid analysis system has been developed which utilizes omicron-phthaldialdehyde as a precolumn derivatizing agent. omicron-Phthaldialdehyde in the presence of mercaptan reacts rapidly with primary amino acids to form intensely fluorescent derivatives. These derivatives are analyzed with good selectivity by high-performance liquid chromatography employing 3-microns particle size reversed-phase columns. Resolution of the amino acid derivatives is accomplished with a methanol gradient in 0.1 M aqueous sodium acetate, pH 7.2. The quantitation of the individual amino acid derivatives is reproducible within an average relative deviation of +/- 1.5% and has a detection limit of less than 100 fmoles. Amino acid mixtures obtained by either enzymatic or acid hydrolysis of polypeptides are efficiently resolved with an analysis time of less than 18 min. Methods for the amino acid analysis of physiological fluids such as serum, urine and cerebrospinal fluid were also developed which employ the above separation procedure for the identification and quantitation of amino acids and other biological amines. Mixtures which contained as many as 48 components were resolved with an analysis time of less than 50 min.

Aldehydes↗

PubMed1985

Nutritional treatment of liver cirrhosis by branched-chain amino acid-enriched nutrient mixture.

The effects of branched-chain amino acid (BCAA)-enriched nutrient mixture (nutrient-mixture) on the nitrogen metabolism and nutritional state were clinically investigated in 10 patients with liver cirrhosis. Nutrient-mixture-supplemented diet was prepared by adding 150 g nutrient-mixture daily to low-protein diet, and comparisons were made with a regular diet (control diet). Each diet supplied 2,100 kcal energy and 80 g protein per day. Patients were given control diet for 2 weeks and thereafter treated successively with nutrient-mixture-supplemented and control diet each for 2 weeks. Nitrogen balance improvement and positive balance were observed during the feeding of nutrient-mixture-supplemented diet. The composition of nitrogen compounds in urine and the fecal nitrogen excretion did not alter during the test period. Plasma aromatic amino acid (AAA) concentrations decreased and BCAA/AAA molar ratios increased significantly during the 1st and 2nd week of nutrient-mixture-supplemented diet administration. Plasma methionine concentration also decreased in the 1st week. Plasma pre-albumin levels rose significantly during the 1st and 2nd week of nutrient-mixture-supplemented diet administration, and the number connection test improved significantly following the supplemented diet. These results suggest that the use of nutrient-mixture in the nutritional treatment of liver cirrhosis had no deleterious effects on nitrogen metabolism and is useful for the improvement of plasma amino acid imbalance and protein-energy malnutrition.

Adult↗

PubMed1999

Nutritional modulation of liver regeneration by carbohydrates, lipids, and amino acids: a review.

The survival of patients after a life-threatening hepatic injury of varying etiology depends on the ability of the remaining hepatocytes to regenerate. Thus, the stimulation of hepatic regeneration can have tremendous therapeutic relevance. Experimental studies--performed mostly on a model of regenerating rat liver after partial hepatectomy--indicate that glucose administration inhibits, whereas infusion of a lipid emulsion can enhance, the rate of liver regeneration. However, the inhibitory effect of glucose on liver regeneration is not observed when glucose is administered together with other nutrients. The results further indicate that administration of a standard amino acid mixture without energy substrate has an inhibitory effect and that development of liver regeneration can be favorably influenced by branched-chain amino acids (valine, leucine, and isoleucine) and glutamine.

Amino Acids↗