Amino Acid Metabolism In Tissues Research Articles

Early changes in tissue amino acid metabolism and nutrient routing in rats fed a high-fat diet: evidence from natural isotope abundances of nitrogen and carbon in tissue proteins

Little is known about how diet-induced obesity and insulin resistance affect protein and amino acid (AA) metabolism in tissues. The natural relative abundances of the heavy stable isotopes of C (δ 13C) and N (δ 15N) in tissue proteins offer novel and promising biomarkers of AA metabolism. They, respectively, reflect the use of dietary macronutrients for tissue AA synthesis and the relative metabolic use of tissue AA for oxidation v. protein synthesis. In this study, δ 13C and δ 15N were measured in the proteins of various tissues in young adult rats exposed perinatally and/or fed after weaning with a normal- or a high-fat (HF) diet, the aim being to characterise HF-induced tissue-specific changes in AA metabolism. HF feeding was shown to increase the routing of dietary fat to all tissue proteins via non-indispensable AA synthesis, but did not affect AA allocation between catabolic and anabolic processes in most tissues. However, the proportion of AA directed towards oxidation rather than protein synthesis was increased in the small intestine and decreased in the tibialis anterior muscle and adipose tissue. In adipose tissue, the AA reallocation was observed in the case of perinatal or post-weaning exposure to HF, whereas in the small intestine and tibialis anterior muscle the AA reallocation was only observed after HF exposure that covered both the perinatal and post-weaning periods. In conclusion, HF exposure induced an early reorganisation of AA metabolism involving tissue-specific effects, and in particular a decrease in the relative allocation of AA to oxidation in several peripheral tissues.

Regional muscle and adipose tissue amino acid metabolism in lean and obese women.

The effect of obesity on regional skeletal muscle and adipose tissue amino acid metabolism is not known. We evaluated systemic and regional (forearm and abdominal subcutaneous adipose tissue) amino acid metabolism, by use of a combination of stable isotope tracer and arteriovenous balance methods, in five lean women [body mass index (BMI) <25 kg/m(2)] and five women with abdominal obesity (BMI 35.0-39.9 kg/m(2); waist circumference >100 cm) who were matched on fat-free mass (FFM). All subjects were studied at 22 h of fasting to ensure that the subjects were in net protein breakdown during this early phase of starvation. Leucine rate of appearance in plasma (an index of whole body proteolysis), expressed per unit of FFM, was not significantly different between lean and obese groups (2.05 +/- 0.18 and 2.34 +/- 0.04 micromol x kg FFM(-1) x min(-1), respectively). However, the rate of leucine release from forearm and adipose tissues in obese women (24.0 +/- 4.8 and 16.6 +/- 6.5 nmol x 100 g(-1) x min(-1), respectively) was lower than in lean women (66.8 +/- 10.6 and 38.6 +/- 7.0 nmol x 100 g(-1) x min(-1), respectively; P < 0.05). Approximately 5-10% of total whole body leucine release into plasma was derived from adipose tissue in lean and obese women. The results of this study demonstrate that the rate of release of amino acids per unit of forearm and adipose tissue at 22 h of fasting is lower in women with abdominal obesity than in lean women, which may help obese women decrease body protein losses during fasting. In addition, adipose tissue is a quantitatively important site for proteolysis in both lean and obese subjects.

Rat adipose tissue amino acid metabolism in vivo as assessed by microdialysis and arteriovenous techniques.

In fed, anesthetized rats, microdialysis demonstrated a net release of glycerol, glutamine, serine, tyrosine, and taurine and a net uptake of glutamate, aspartate, glycine, and arginine across the inguinal adipose depot. However, the results also indicated excessive proteolysis associated with implantation of the microdialysis probe, and a novel arteriovenous difference technique was developed. Arteriovenous difference across the inguinal fat pat demonstrated a net uptake of glucose and a net release of lactate and glycerol. Starvation (48 h) resulted in higher rates of glycerol and lactate release with lower rates of glucose uptake. A net uptake of triacylglycerol was seen in starved-refed animals. Net glutamine, tyrosine, and taurine release was seen in fed and starved animals, but in starved-refed animals taurine and serine were the only amino acids showing significant release. No significant net uptake or release of ammonia, pyruvate, or alanine was observed. These experiments confirm that adipose tissue is a site of glutamine synthesis and suggest that the principal substrates are derived from intracellular proteolysis. The results also demonstrate the viability of an arteriovenous difference technique for the study of adipose tissue in the rat.

Altered tissue amino acid metabolism in acute T-2 toxicosis.

T-2 toxin is a Fusarium trichothecene mycotoxin that has been shown to alter brain neurochemistry and eating behavior in animals eating contaminated diets. Experiments were conducted to determine the role of altered tissue amino acid metabolism in the etiology of acute T-2 toxicosis. Fasted weanling rats were orally dosed with 0 or 2.0 mg T-2 toxin/kg body weight. Blood, brain, liver, and muscle tissue were excised 4 and 8 hr after dosing, and amino acid concentrations were determined. Hepatic enlargement coupled with reduced liver concentrations of free small neutral, large neutral, and basic amino acids were seen 4 hr after dosing. Brain and muscle amino acid concentrations were largely refractory to treatment, while the plasma concentrations of tyrosine and lysine, and the sum of the basic amino acids fell. Hepatic amino acid concentrations returned to control levels 8 hr after dosing at which time aminoacidemia was seen. This was due partially to an increase in plasma concentrations of large neutral amino acids including particularly the branched-chain amino acids. A subsequent experiment was conducted to determine the effect of T-2 toxin on 14C-leucine uptake and incorporation into protein in liver slices 4 hr after dosing. Exposure to T-2 toxin reduced total (free + protein-bound) uptake of leucine due primarily to reduced incorporation of leucine into newly-synthesized hepatic protein. It was concluded that reduced amino acid uptake by liver preceded aminoacidemia in acute T-2 toxicosis, although it is not clear how this might influence subsequent changes in brain neurochemistry and behavior.

Production of glutamine and utilization of glutamate by rat subcutaneous adipose tissue in vivo.

Information about adipose tissue amino acid metabolism is limited, with most data derived from studies in vitro. The purpose of this study was to further characterize the role of adipose tissue in glutamine metabolism in the rat in vivo. The extracellular concentrations of glutamine, glutamate, alanine, and ammonia were measured in the rat inguinal fat pad using a microdialysis sampling technique. A calibration method was used to accurately assess the extracellular levels of metabolites, and a comparison of these concentrations with those in arterial blood allowed determination of the net flux of each compound. The adipose tissue-arterial blood concentration differences were 122 +/- 19, 54 +/- 37, -61 +/- 21, and -28 +/- 13 microM for glutamine, alanine, glutamate, and ammonia, respectively, indicating a production of glutamine and an uptake of glutamate by subcutaneous adipose tissue. The magnitude of glutamine production suggests that adipose tissue may play a significant role in whole body glutamine homeostasis.

Exercise-Mediated Peripheral Tissue and Whole-Body Amino Acid Metabolism during Intravenous Feeding in Normal Man

1. The effect of a daily submaximal exercise regimen on whole-body and peripheral tissue amino acid metabolism during weight-stable intravenous feeding (IVF) was evaluated in 11 normal volunteers. Five of the subjects performed 1 h of daily bicycle exercise at 75 W during IVF, while the remaining six subjects received IVF without daily exercise. Body nitrogen balance, leg and forearm plasma amino acid flux and whole-body kinetics were measured before and on day 10 of IVF using a [1-13C]leucine and [15N]glycine tracer. 2. At the end of the IVF period, exercised subjects demonstrated leg uptake of total amino acids (237 +/- 103 nmol min-1 100 ml-1 of tissue, mean +/- SEM) which was significantly (P less than 0.05) different than in non-exercised subjects (-1101 +/- 253 nmol min-1 100 ml-1 of tissue). 3. In the non-exercised forearm, a significant (P less than 0.05) decrease in total amino acid flux was observed in exercised subjects (-162 +/- 88 nmol min-1 100 ml-1 of tissue) compared with non-exercised subjects (-460 +/- 105 nmol min-1 100 ml-1 of tissue) on day 10 of IVF. 4. Efflux of 3-methylhistidine significantly (P less than 0.05) decreased from the leg in those subjects who performed daily exercise (-0.29 +/- 0.12 nmol min-1 100 ml-1 of tissue) compared with those subjects receiving IVF without daily exercise (-1.46 +/- 0.35 nmol min-1 100 ml-1 of tissue).(ABSTRACT TRUNCATED AT 250 WORDS)

The role of endotoxin and nutritional support on peripheral tissue amino acid metabolism during critical illness : [formula omitted], K.J. Tracey, Y. Fong, J.D. Albert, A. Legaspi, P.S. Barie, S.F. Lowry, Department of Surgery, The New York Hospital-Cornell Medical Center, New York City, USA

The role of endotoxin and nutritional support on peripheral tissue amino acid metabolism during critical illness : [formula omitted], K.J. Tracey, Y. Fong, J.D. Albert, A. Legaspi, P.S. Barie, S.F. Lowry, Department of Surgery, The New York Hospital-Cornell Medical Center, New York City, USA

The regulation of uptake and output of amino acids by rat tissues

1. 1)|Uptake and output of amino acids by individual tissues of rats were assessed by measuring differences between concentrations of amino acids in the plasma of various blood vessels. In most tissues of fasted rats, uptake or output of glutamine and alanine remarkably exceed those of other amino acids. So, these amino acids are considered to be “end products” of amino acid metabolism in tissues and to serve as “nitrogen carriers” which convey nitrogen from tissue to tissue. (Glycine and serine also play similar, although minor, roles.) This concept is consonant with the finding that uptake of glutamine and alanine by fetal and tumor tissues exceeds those of other amino acids. In meal-fed rats, movements of most amino acids between the plasma and tissues are very different from those observed in fasted rats, but some features are common to fasted and meal-fed rats. Under both conditions, the nitrogen of amino acids metabolized in tissues is released into the blood plasma in the form of glutamine, alanine, glycine or serine, and glutamine is utilized by the kidney and portal-drained viscera. 2. 2)|In fasted rats, the uptake of alanine by the liver exceeds that of other amino acids and the carbon of alanine is rapidly incorporated into blood glucose. However, the uptake of serine by the liver is relatively small and glutamine is even released from the liver. The carbon of serine and glutamine is only slowly incorporated into blood glucose. So, alanine is an effective precursor for hepatic gluconeogenesis, but glutamine and serine are not. 3. 3)|Portal-drained viscera is a major site for metabolism of glutamine in the plasma. Glutamine is metabolized in these viscera and its nitrogen is released into the plasma in the form of alanine and ammonia. 4. 4)|The concentration of glycine in the arterial plasma is significantly lowered by feeding protein, while concentrations of other amino acids are raised remarkably. In addition, the content of glycine in the liver is also reduced by feeding protein, whereas the uptake of glycine by the liver is increased. It is suggested that the uptake and metabolism of glycine by the liver is stimulated by feeding protein, resulting in the reduction of glycine content in the blood plasma and liver.