Plasma Concentrations Of Threonine Research Articles

In Vivo Threonine Oxidation Rate Is Dependent on Threonine Dietary Supply in Growing Pigs Fed Low to Adequate Levels

Threonine oxidation was examined in 12 growing pigs fed a well-balanced control diet or a threonine-deficient diet supplemented (Glu) or not (LT) with glutamic acid during constant infusion of L-[1-13C]-threonine, [1-14C]glycine and [1-14C]α-ketobutyrate for 10 h. During these infusions, liver glycine enrichment was significantly lower than plasma enrichment. Moreover, the pancreas to plasma glycine enrichment ratio was higher than the liver to plasma ratio (70–89%), showing that an important part of glycine de novo synthesis in pancreas occurred through the threonine dehydrogenase (TDG) pathway. These results imply that calculation of threonine oxidation into glycine should be made with the assumption of both hepatic and extrahepatic oxidation. Plateau values of plasma threonine, glycine and α-ketobutyrate enrichments and specific radio activities allowed estimations of threonine oxidation through the TDG and threonine dehydratase (TDH) pathways. Threonine oxidation into glycine was 12.16 ± 2.06, 2.89 ± 0.61 and 2.13 ± 0.44 µmol/(kg·h), respectively, in pigs fed the control, LT and Glu diets, and threonine oxidation into α-ketobutyrate was 1.80 ± 0.31, 0.88 ± 0.02 and 0.55 ± 0.06 µmol/(kg·h) for the control, LT and Glu groups, respectively. Total threonine oxidation rates were 75 and 81% lower in the LT and Glu groups, respectively, than in the control group. Liver TDG and TDH activity measured in vitro were not affected by either the level of dietary threonine supply the addition of glutamic acid. On the basis of plasma data, it may be concluded that the addition of glutamic acid to a threonine-deficient diet had no significant effect on threonine oxidation but did reduce the rate of threonine release from protein breakdown. Oxidation appears to be related to plasma threonine concentration.

Evaluation of threonine requirements in mature horses fed 1:1 ratio of forage to concentrate using the indicator amino acid oxidation technique

Threonine has been considered as the second limiting amino acid in equine diets following the first limiting amino acid, lysine, and its digestibility and bioavailability may be affected by dietary fiber content. Therefore, evaluation of threonine requirements in horses fed different diet compositions will improve diet formulation. The objective of this study was to evaluate threonine requirements in mature horses fed a 1:1 ratio of timothy hay and concentrate using the indicator amino acid oxidation (IAAO) technique. Six threonine intake levels [45, 56, 67, 79, 90, and 102 mg/kg body weight (BW)/day] were prepared by mixing two experimental concentrates containing different threonine concentrations (2.9 and 14.8 g of threonine/kg of concentrate). Each of six treatments was administrated to six Thoroughbred mares (4–16 years old; 580.1 ± 47.2 kg) in a randomly determined order. After 5 days of adaptation to each treatment, pre-feeding and 90 min post-feeding blood samples were collected on day 6 to evaluate plasma amino acid concentrations using high performance liquid chromatography. The IAAO procedure was conducted on day 7 of each experimental period. During the first two hour intravenous infusion of [13C]sodium bicarbonate, total CO2 production was evaluated from breath samples. From the subsequent four-hour oral administration of [1-13C]phenylalanine, blood and breath samples were collected to estimate phenylalanine oxidation to CO2. Plasma [13C]phenylalanine concentration and breath 13CO2 enrichment were evaluated using gas chromatography–mass spectrometry and an infrared isotope analyzer, respectively. Experimental data were analyzed using a one-way analysis of variance, including orthogonal linear and quadratic contrasts, in the mixed procedures of SAS version 9.3. Increased threonine intake resulted in higher plasma threonine concentration before and 90 min after feeding (P > 0.05), indicating higher bioavailability of threonine. However, phenylalanine oxidation rate was not affected by threonine consumption as evidenced by the lack of a breakpoint (P > 0.05). The results suggest that threonine requirements might be lower than 45 mg/kg BW/day in mature horses fed a 50% concentrate diet.

Dynamic changes in circulating levels of metabolites in the portal-drained viscera of finishing pigs receiving acute administration of l-arginine.

In this study, we examined the effects of acute intravenous administration of l-arginine on circulating levels of metabolites in the portal-drained viscera (PDV) of 12 barrows surgically fitted with chronic catheters in the portal vein. At day 14 post-surgery, the pigs were fasted for 12hr and then randomly allocated to one of three groups to receive administration of normal saline, l-alanine [103mg/kg body weight (BW), isonitrogenous control] or l-arginine-HCl (61mg/kg BW), via the portal vein. Blood samples were obtained from the carotid artery before and at 30-min intervals for 5hr after the administration of saline or amino acid in order to determine metabolic profiles. The results showed that, compared with the saline treatment, arginine infusion increased plasma concentrations of insulin-like growth factor-I, arginine and cystine in the portal vein plasma, whereas plasma concentrations of threonine, serine, leucine and methionine were reduced. These findings indicate that increasing arginine concentrations in the portal vein alters the metabolic profile in swine, an established animal model for studying human nutrition and metabolism.

Plasma Amino Acid Concentrations of Horses Grazing Alfalfa, Cool-Season Perennial Grasses, and Teff

The impact of forage species on plasma amino acid (AA) concentrations of grazing horses (Equus caballus L.) is unknown. The objectives of this study were to determine the impact of different forage species on plasma AA concentrations and protein synthesis. Research was conducted in July in St. Paul, MN, USA. Alfalfa (Medicago sativa L.), mixed perennial cool-season grasses (CSGs), and teff (Eragrostis tef [Zucc.] Trotter) pastures were grazed by six horses randomly assigned to one of three forage types in a replicated Latin-square design. Horses had access to pasture each day. Jugular venous blood samples were collected from each horse before being turned out (0 hours) and then at 2 and 4 hours after turnout. Corresponding forage samples were taken by hand harvest and analyzed for AA concentrations. Equine muscle satellite cell cultures were treated with sera from grazing horses to assess de novo protein synthesis. Data were analyzed using PROC MIXED in SAS. When evaluating forage, AA concentrations were generally lowest in teff and highest in CSG (P ≤ .05). Significant differences in threonine concentration in the plasma were observed; there was no effect on de novo protein synthesis of cultured equine myotubes treated with plasma obtained from the grazing horses (P ≥ .20). As a result, although there were significant differences in forage AA content, only plasma threonine concentration was different at 4 hours with no effect on protein synthesis of cultured equine satellite cells.

Amino acids levels related to glutamine supplementation for type 2 diabetes patients

Aim: Amino acids levels are changed upon glutamine administration, but the impact of glutamine administration on plasma amino acid levels in type 2 diabetes patients remains unclear. Methods: A placebo controlled, randomized trial was performed with type 2 diabetic patients who consumed 2 supplements containing glutamine (10 g/3 times a day) or placebo (1 g/3 times a day) every day for 6 consecutive weeks. Blood samples were obtained just before initiation of study and again after the patients had received the study supplements. Results: Dietary supplementation with glutamine didn’t affect concentrations of plasma amino acid apart from for threonine. Plasma threonine concentration in intervention group was low at week 6 compared with placebo (glutamine group=111.19 ± 38.60 μmol/l, placebo group=135.30 ± 47.42 μmol/l; P=0.02). Conclusion: In type 2 diabetes patients, oral glutamine supplementation resulted in a decreased level of threonine.

Protein Synthesis in Mucin-Producing Tissues Is Conserved When Dietary Threonine Is Limiting in Piglets

BackgroundThe neonatal gastrointestinal tract extracts the majority of dietary threonine on the first pass to maintain synthesis of threonine-rich mucins in mucus. As dietary threonine becomes limiting, this extraction must limit protein synthesis in extraintestinal tissues at the expense of maintaining protein synthesis in mucin-producing tissues. ObjectiveThe objective was to determine the dietary threonine concentration at which protein synthesis is reduced in various tissues. MethodsTwenty Yucatan miniature piglets (10 females; mean ± SD age, 15 ± 1 d; mean ± SD weight, 3.14 ± 0.30 kg) were fed 20 test diets with different threonine concentrations, from 0.5 to 6.0 g/100 g total amino acids (AAs; i.e., 20–220% of requirement), and various tissues were analyzed for protein synthesis by administering a flooding dose of [3H]phenylalanine. The whole-body requirement was determined by [1-14C]phenylalanine oxidation and plasma threonine concentrations. ResultsBreakpoint analysis indicated a whole-body requirement of 2.8–3.0 g threonine/100 g total AAs. For all of the non–mucin-producing tissues as well as lung and colon, breakpoint analyses indicated decreasing protein synthesis rates below the following concentrations (expressed in g threonine/100 g total AAs; mean ± SE): gastrocnemius muscle, 1.76 ± 0.23; longissimus dorsi muscle, 2.99 ± 0.50; liver, 2.45 ± 0.60; kidney, 3.81 ± 0.97; lung, 1.95 ± 0.14; and colon, 1.36 ± 0.29. Protein synthesis in the other mucin-producing tissues (i.e., stomach, proximal jejunum, midjejunum, and ileum) did not change with decreasing threonine concentrations, but mucin synthesis in the ileum and colon decreased over threonine concentrations <4.54 ± 1.50 and <3.20 ± 4.70 g/100 g total AAs, respectively. ConclusionsThe results of this study illustrate that dietary threonine is preferentially used for protein synthesis in gastrointestinal tissues in piglets. If dietary threonine intake is deficient, then muscle growth and the functions of other tissues are likely compromised at the expense of maintenance of the mucus layer in mucin-producing tissues.

Effects of dietary lysine levels on plasma free amino acid profile in late-stage finishing pigs.

Muscle growth requires a constant supply of amino acids (AAs) from the blood. Therefore, plasma AA profile is a critical factor for maximizing the growth performance of animals, including pigs. This research was conducted to study how dietary lysine intake affects plasma AA profile in pigs at the late production stage. Eighteen crossbred (Large White × Landrace) finishing pigs (nine barrows and nine gilts; initial BW 92.3 ± 6.9 kg) were individually penned in an environment controlled barn. Pigs were assigned randomly to one of the three dietary treatments according to a randomized complete block design with sex as block and pig as experiment unit (6 pigs/treatment). Three corn- and soybean meal-based diets contained 0.43 % (lysine-deficient, Diet I), 0.71 % (lysine-adequate, Diet II), and 0.98 % (lysine-excess, Diet III) l-lysine, respectively. After a 4-week period of feeding, jugular vein blood samples were collected from the pigs and plasma was obtained for AA analysis using established HPLC methods. The change of plasma lysine concentration followed the same pattern as that of dietary lysine supply. The plasma concentrations of threonine, histidine, phenylalanine, isoleucine, valine, arginine, and citrulline of pigs fed Diet II or III were lower (P < 0.05) than that of pigs fed Diet I. The plasma concentrations of alanine, glutamate, and glycine of pigs fed Diet II or III were higher (P < 0.05) than that of pigs fed Diet I. The change of plasma leucine and asparagine concentrations followed the patterns similar to that of plasma lysine. Among those affected AAs, arginine was decreased (P < 0.05) in the greatest proportion with the lysine-excess diet. We suggest that the skeletal muscle growth of finishing pigs may be further increased with a lysine-excess diet if the plasma concentration of arginine can be increased through dietary supplementation or other practical nutritional management strategies.

Influence of a flooding dose of valine on key indicators of metabolic status in the growing pig.

A key concern with the flooding dose technique for measuring protein synthesis is that a large dose of amino acid (AA) can potentially change the animals' hormonal and nutritional status, which in turn can influence protein synthesis. Among stable isotope tracers, 1-[(13)C]-valine is the preferred AA for measuring protein synthesis in gut tissue and mucins. A study was conducted to determine the impact of a flooding dose of valine on the metabolic status of pigs. Six barrows [16.5 kg body weight (BW)] were randomly assigned to intravenous infusions of either 150 mM valine (1.5 mmol/kg BW) or physiological saline, following a crossover design. Blood samples were taken 10 min prior to infusion, at the end of infusion, at 10-min intervals for 60 min post-infusion, and at 90 and 120 min post-infusion. Plasma concentrations of insulin, glucose, AA, urea nitrogen and packed cell volume (PCV) were measured. Infusion of valine increased plasma valine concentrations (4129 vs. 582 μM; P < 0.05) but had no influence on PCV (26.4% vs. 27.2%) and plasma concentrations of glucose (6.0 vs. 5.8 mM) and insulin (8.2 vs. 8.5 μU/ml; P > 0.10). Plasma urea nitrogen concentration was reduced with valine infusion (8.5 vs. 7.8 mg/dl; P < 0.05). A flooding dose of valine had no impact on plasma concentrations of AA, and specifically branched-chain AA such as leucine (240 vs. 231 μM) and isoleucine (310 vs. 331 μM; P > 0.10). There was, however, a slight increase in the plasma concentrations of threonine (224 vs. 263 μM; P < 0.05) and a tendency towards reduced glycine (1387 vs. 1313 μM; P < 0.10). The results indicate that a flooding dose of valine does not cause a substantial change in the metabolic status of growing pigs and is therefore suitable for measuring protein synthesis rates.

DIETARY EFFECTS OF MAGNESIUM ON HISTAMINE METABOLISM AND URINE ACIDITY IN DOMESTIC FELINES

Magnesium deficiency has been associated with increased histamine production in rats. Limitation of Mg with acidifying foods is common practice for management of urinary tract health in domestic cats. Nine healthy adult female shorthair cats were used in a 3 period random crossover experiment with fixed treatment sequences to test the effects of dietary Mg (0.06, 0.12 and 0.18% DM) on histamine in blood and urine. The dry-extruded test foods were fed in sufficient amounts to maintain ideal body weight and obtain a target urine pH of 6.3. Each experimental period was preceded by a 7d wash out period, in which the 0.06% Mg food was fed, followed by a 14d feeding period of the appropriate food. Two 24 h total urine collections were performed (d13: Acidified, d14: Un-acidified; immediately iced) and blood was collected on d14. Dry matter intake (p≥0.13) and BW (p≥0.13) were not affected by treatment. Plasma Mg concentration increased linearly with increasing dietary Mg (0.54, 0.56, 0.58 mM; p = 0.001). In contrast, plasma concentrations of threonine, histidine and tryptophan were lower in cats fed 0.12% Mg compare with 0.06 or 0.18% Mg (quadratic, p≤0.03). Urine output (p≥0.17), pH (p≥0.55), NH3 (p≥0.21) and titratable acidity of urine (p≥0.14) were similar across treatments. Urinary histamine excretion responded quadratically (p = 0.02) to treatment (3483, 3369, 3986 ng/d), whereas urinary histamine: Creatinine (p≥0.43) and plasma histamine concentration (p≥0.55) were unaffected. Differences were not detected among treatments in total histamine, cellular + noncellular histamine, (p≥0.11) or antigen-induced (p≥0.21) histamine release in whole blood. These data indicate that dietary Mg concentration, from 0.06-0.18%, does not affect urinary acidity or circulating histamine concentrations, however, supplying Mg at 0.18% may increase urinary histamine excretion.

Safety and adequacy of a semi-elemental formula for children with gastro-intestinal disease

A prospective, open trial was conducted to evaluate the nutritional adequacy of a semi-elemental diet in 47 children with functional gastro-intestinal disorders. Nutritional adequacy was assessed based on growth relative to Euro-growth standards for body mass index (BMI)-for-age z-scores and evaluations of blood parameters. Twenty-five patients completed the study. In total, 533 l of “New-Alfare” was consumed during 775 trial-days. The mean intake per infant was 85.8 ± 26.8 kcal/kg/day or 122.5 ± 38.3 ml/kg/day. Weight and length evolution during the 4 weeks trial were within normal range. The mean BMI-for-age z-score (P < 0.05) and albumin concentration (P < 0.01) increased significantly after 4 weeks. Plasma threonine concentration decreased significantly (P = 0.01) and the tryptophan concentration increased (P = 0.06). No adverse events related to the study formula were reported. These results show that “New Alfaré” is safe and nutritionally adequate for pediatric patients with gastro-intestinal disease.

Adequate oral threonine is critical for mucin production and gut function in neonatal piglets

In previous experiments, we found that the threonine requirement of neonatal piglets fed parenterally was 40% of that when fed intragastrically; we hypothesized that much of the oral supply of threonine is being used for mucin production. To investigate this hypothesis, intragastrically fed 2-day-old piglets were fed one of three treatments for 8 days: 1) a threonine-adequate diet (IG-A; 0.6 g threonine.kg(-1).day(-1) fed intragastrically); 2) a threonine-deficient diet (IG-D; 0.1 g threonine.kg(-1).day(-1) fed intragastrically); or 3) a threonine-deficient diet with adequate threonine delivered parenterally (IV-A; 0.5 g threonine.kg(-1).day(-1) fed parenterally plus 0.1 g threonine.kg(-1).day(-1) fed intragastrically). IG-D piglets experienced higher nitrogen excretion, higher plasma urea, and lower plasma threonine concentrations versus both of the other groups (P < 0.05), indicating profound threonine deficiency. Mucosal mass and total crude mucin content were lower in the colons of IG-D pigs (P < 0.05). Histopathological analysis showed lower numbers of acidic mucin-producing goblet cells in the duodenum and ileum of IG-D pigs. In IG-D pigs, acidic mucin subtypes were lower in the small intestine but higher in the colon, which corresponded with persistent diarrhea. The parenteral supply of threonine was adequate to maintain most outcome parameters, although IV-A pigs did have smaller colonic goblet cells with more acidic mucins compared with IG-A pigs. Overall, our results suggest that adequate dietary threonine was critical in the production of mucus and that a parenteral threonine supply can ameliorate most of the symptoms of oral threonine deficiency.

Metabolism of threonine in newborn infants

Threonine kinetics, threonine oxidative pathway, and the relationship between threonine and whole body protein turnover were quantified in 10 healthy term infants during the first 48 h after birth. The kinetic data were obtained 6 h after the last feed (fasting) and in response to formula feeding, using [U-(13)C(4),(15)N]threonine, [(2)H(5)]phenylalanine, and [(15)N]glycine tracers. The rate of carbon dioxide production (Vco(2)) and (13)C enrichment of the expired CO(2) were measured to quantify the rate of oxidation of threonine. The rate of appearance (R(a)) of threonine (136 +/- 37 micromol.kg(-1).h(-1)) was higher in newborn infants than that reported in adults. Formula feeding resulted in a significant decrease in threonine R(a) (P < 0.05). A significant positive correlation was seen between phenylalanine R(a) and threonine R(a), both during fasting and after formula feeding (r(2) = 0.65). In contrast to a 1:1 ratio of threonine and phenylalanine in mixed muscle protein, threonine R(a) relative to phenylalanine R(a) was 2.2 +/- 0.4. The fractional rate of threonine flux oxidized was 20% during fasting and 26% (P < 0.05) in response to nutrient administration. There was a significant correlation between plasma threonine concentration and threonine oxidation (r(2) = 0.75). No measurable incorporation of threonine in plasma glycine was seen. These data suggest that threonine is exclusively degraded by the glycine-independent serine/threonine dehydratase pathway. A higher flux of threonine relative to phenylalanine indicates higher turnover of threonine enriched proteins.

Phenylalanine Kinetics Differ between Formula-Fed and Human Milk–Fed Preterm Infants

Infants fed casein-dominant formulas have higher plasma phenylalanine and tyrosine concentrations than those fed mother's milk. Conversely, elevated plasma threonine concentrations are observed in infants fed whey-dominant formulas. We recently showed that formula-fed preterm infants have a lower capacity to degrade threonine than do preterm infants fed mother's milk. We hypothesized that these same infants (n = 18) would differ in their catabolism of phenylalanine in response to phenylalanine loads provided by formulas with increasing casein content of formulas (whey:casein 60:40, 40:60, and 20:80) compared with preterm infants fed mother's milk. Plasma phenylalanine concentrations significantly rose (49, 46, 79 micromol . L(-1) for whey:casein 60:40, 40:60, and 20:80, respectively, pooled SD 8, P < 0.05); and plasma phenylalanine concentrations in infants fed mother's milk were low (40 +/- 4 micromol . L(-1)). Using [1-(13)C]phenylalanine tracer and (13)CO(2) production in breath we found that although there was a significant positive relation between phenylalanine oxidation and phenylalanine intake in formula-fed infants (r(2) = 0.43, P = 0.03), these infants were not able to increase their oxidation of phenylalanine enough to prevent a significant rise in plasma phenylalanine when fed the 20:80 formula. Compared to infants fed mother's milk, formula-fed infants had significantly lower phenylalanine oxidation (39.1 vs. 30.7% of phenylalanine intake, respectively, P < 0.05). We conclude that one of the mechanisms for the differences in plasma amino acid concentration between formula-fed and mother's milk-fed preterm infants may be in vivo down-regulated catabolism of 2 important essential amino acids (phenylalanine in addition to threonine) in formula-fed preterm infants.

Umbilical Threonine Uptake during Maternal Threonine Infusion in Sheep

Objective: The infusion into the maternal circulation of amino acid solutions failed to increase umbilical threonine (THR) uptake above normal even when THR was present in the infusate at a relatively high concentration. The purpose of the present study was to determine whether umbilical THR uptake can be increased by infusing a THR solution that does not contain any other amino acids.Study design: Five pregnant sheep (130±1.0 days after conception) were infused for 2h with a threonine solution (4.4±0.2μmol·kg−1·min−1). Plasma amino acids, glucose and lactate, hematocrit, blood O2 content in maternal arterial, uterine venous, umbilical arterial and venous blood were measured. Uterine and umbilical blood flows were measured before and during the infusion and were used to calculate uterine and umbilical uptakes. Maternal and foetal plasma insulin and glucagon concentrations were also measured.Results: The THR infusion increased maternal plasma THR (904 vs 236μm , P< 0.001), foetal plasma THR (539 vs 334μm , P< 0.01), and both uterine (20.4 vs 4.7μmol·min−1·kg−1fetalweight, P< 0.05) and umbilical (8.6 vs 3.8μmol·min−1·kg−1fetalweight, P< 0.001) THR uptakes. The uterine-umbilical THR uptake difference increased significantly (11.8 vs 0.9μmol·min−1·kg−1fetalweight, P< 0.05). There were significant (P< 0.001) decreases in the foetal arterial plasma concentrations of tyrosine and the branched chain amino acids, as well as in isoleucine umbilical uptake (P< 0.05). There was a significant increase in maternal plasma glucagon (P< 0.01).Conclusion: A maternal THR infusion that causes a 3.8-fold increase in maternal plasma THR concentration above normal, with no significant increase in the concentration of other amino acids, leads to a 2.3-fold increase in umbilical THR uptake. This contrasts with the absence of a significant increase in umbilical THR uptake when THR was infused as part of an amino acid mixture in previous studies. The evidence supports the hypothesis that, in vivo, THR flux from placenta to foetus is mediated by a saturable, rate limiting transport system which is subject to inhibition by other neutral amino acids.

The optimal true ileal digestible lysine and threonine requirement for nursery pigs between 25 and 55 lb (2003)

A total of 360 pigs (initially 22.2 lb and 31 d of age) was used in a 21-d growth assay. This trial was conducted as a combination of two separate trials in order to simultaneously examine both the true ileal digestible lysine and true ileal digestible threonine requirement and determine the appropriate threonine:lysine ratio. The first part of the trial consisted of five treatments with increasing dietary lysine (1.0. 1.1, 1.2, 1.3 and 1.4% true digestible lysine). The second part consisted of five treatments with increasing dietary threonine (0.66, 0.72, 0.78, 0.84 and 0.91% true ileal digestible threonine). The highest level of both lysine and threonine (1.4% and 0.91% respectively) served as a positive control, and this diet was combined as one treatment to give a total of nine treatments. Average daily gain increased to 1.3% true ileal digestible lysine, and then plateaued, while ADG increased to 0.78% true ileal digestible threonine, suggesting a threonine:lysine ratio of 60% for ADG. Increasing dietary lysine improved F/G linearly through 1.4% true ileal digestible lysine, while F/G improved up to a level of 0.84% true ileal digestible threonine. Using a level of 1.4% true ileal digestible lysine, a threonine:lysine ratio of approximately 60% is implicated for F/G. Amino acid and plasma urea N values were measured on d 10 of the trial. Plasma lysine concentrations were maintained steadily as the true ileal digestible lysine level increased, with a slight increase in plasma lysine concentration observed as the true ileal digestible lysine level increased from 1.3% to 1.4%. A linear increase (P<.0001) in plasma threonine concentration was observed as true ileal digestible threonine increased from 0.66% to 0.91%. Plasma urea N decreased linearly (P<0.0003) with increasing true ileal digestible lysine. As true ileal digestible threonine increased, there was no difference seen in plasma urea N concentration. Following analysis of the data, a true ileal digestible threonine to lysine ratio of 60% is suggested. A second study is in progress where the higher true digestible lysine level of 1.5% is used to verify trial results.; Swine Day, 2003, Kansas State University, Manhattan, KS, 2003

Temporal Response of Hepatic Threonine Dehydrogenase in Chickens to the Initial Consumption of a Threonine-Imbalanced Diet

Amino acid imbalances contribute to higher requirements of amino acids than would occur if the dietary profile of amino acids perfectly matched the requirements. The mechanisms of imbalances have not been fully elucidated. Because threonine dehydrogenase (TDH) activity in liver mitochondria increases in chicks and rats subjected to threonine imbalance, the current study was carried out to determine whether the change in TDH activity occurs rapidly enough after the consumption of an imbalanced diet to be considered a possible primary metabolic response. In a series of experiments, Leghorn chicks were allowed free access to a semipurified basal diet marginally limited in threonine or the same diet containing a mixture of indispensable amino acids (IAA) lacking threonine to cause a threonine imbalance. In the first experiment, dietary supplements of 5.5 and 11.1% IAA were used to determine a level of supplement that would cause a robust response in the specific activity of TDH. Feed intake, body weight gains and efficiency of feed utilization were lower and specific activities of TDH were higher in chicks fed 11.1% IAA than in those fed 5.5% IAA. In subsequent experiments, hepatic TDH activities and plasma amino acid profiles of the control and experimental groups were determined at 1. 5, 3, 6, 12 and 24 h after the first offering of the diet containing 11.1% IAA. The specific activities of TDH in chicks fed the IAA supplement were 40-150% higher (P < 0.05) and plasma threonine concentrations were 42-53% lower (P < 0.05) than in chicks fed the basal diet at all times except 1.5 h. These results indicate that changes in the capacity for threonine degradation via TDH may occur in the liver within a few hours after the consumption of a threonine-imbalanced diet and suggest the possibility that altered TDH activity may contribute to the increased threonine requirement associated with threonine imbalance.

Chronic alcohol feeding impairs hepatic translation initiation by modulating eIF2 and eIF4E.

The present study examined potential cellular mechanisms responsible for the inhibition of protein synthesis in liver after chronic alcohol consumption. Rats were maintained on an alcohol-containing diet for 14 wk; control animals were fed isocalorically. Hepatic ATP content was not different in alcohol-fed and control animals. No alcohol-induced reduction in total hepatic RNA content (an estimate of ribosomal RNA) was detected, suggesting that alcohol decreased translational efficiency. Alcohol feeding increased the proportion of 40S and 60S ribosomal subunits in the nonpolysome-associated fraction by 30%. To identify mechanisms responsible for the impairment in initiation, several eukaryotic initiation factors (eIF) were analyzed. Alcohol feeding decreased hepatic eIF2B activity by 36%. This reduction was associated with a 20% decrease in eIF2Bepsilon content and a 90% increase in eIF2alpha phosphorylation. Alcohol also dramatically influenced the distribution of eIF4E. Compared with pair-fed control values, alcohol feeding increased the amount of eIF4E present in the inactive 4E-binding protein 1 (4E-BP1). eIF4E complex by 80% and decreased binding of eIF4G to eIF4E by 70%. However, the phosphorylation status of 4E-BP1 and eIF4E was not altered by alcohol. Although the plasma concentrations of threonine, proline, and citrulline were mildly decreased, the circulating amount of total amino acids was not altered by alcohol feeding. In summary, these data suggest that chronic alcohol consumption impairs translation initiation in liver by altering eIF2B activity as well as eIF4F function via changes in eIF4E availability.

Threonine kinetics in preterm infants fed their mothers' milk or formula with various ratios of whey to casein

Plasma threonine concentrations are elevated in infants fed formula containing a whey-to-casein protein ratio of 60:40 compared with concentrations in infants fed formula containing a ratio of 20:80 or human milk (60:40). We studied whether degradation of excess threonine was lower in formula-fed infants than in infants fed their mothers' milk. Threonine kinetics were examined in 17 preterm infants (gestational age: 31+/-2 wk: birth weight: 1720+/-330 g) by using an 18-h oral infusion of [1-13C]threonine at a postnatal age of 21+/-11 d and weight of 1971+/-270 g. Five infants received breast milk. Formula-fed infants (n = 12) were randomly assigned to receive 1 of 3 formulas (5.3 g protein/MJ) that differed only in the whey-to-casein ratio (20:80, 40:60, and 60:40). Threonine intake increased significantly in formula-fed infants with increasing whey content of the formula (48.5, 56.4, and 63.2 micromol.kg(-1).h(-1), respectively; pooled SD: 2.2; P = 0.0001), as did plasma threonine concentrations (228, 344, and 419 micromol/L, respectively; pooled SD: 75; P = 0.03). Despite a generous threonine intake by infants fed breast milk (58.0+/-16.0 micromol.kg(-1).h(-1), plasma threonine concentrations remained low (208+/-41 micromol/L). Fecal threonine excretion and net threonine tissue gain, estimated by nitrogen balance, did not differ significantly among groups. Threonine oxidation did not differ significantly among formula-fed infants but was significantly lower in formula-fed infants fed than in infants fed breast milk (17.1% compared with 24.3% of threonine intake, respectively). Formula-fed infants have a lower capacity to oxidize threonine than do infants fed breast milk.

Effect of increasing dietary threonine intakes on amino acid metabolism of the central nervous system and peripheral tissues in growing rats.

The threonine content of most of the infant formulas currently on the market is approximately 20% higher than the threonine concentration in human milk. Due to this high threonine content the plasma threonine concentrations are up to twice as high in premature infants fed these formulas than in infants fed human milk. To study the effect of different threonine intakes on plasma and tissue amino acid concentrations, 24 young male Wistar rats were fed three experimental diets based on a mixture of bovine proteins with a whey protein/casein ratio of 60/40 with different threonine contents [group A, 0.86 g of threonine/100 g (n = 8); group B, 1.03 g of threonine/100 g (n = 8); group C, 2.21 g of threonine/100 g (n = 8)]. Eight animals were fed a typical rat diet based on bovine casein as controls. After a feeding period of 15 d, amino acids were measured in plasma and in homogenates of the cerebral cortex, brain stem, liver, and muscle. There was a significant correlation between threonine intake and plasma threonine levels (r = 0.687, p < 0.001). The plasma threonine concentration correlated significantly with the threonine concentration in the cortex (r = 0.821, p < 0.01) and the brain stem (r = 0.882, p < 0.01). There was a positive significant correlation between threonine and glycine concentrations in the cortex (r = 0.673, p < 0.01), and the brain stem (r = 0.575, p < 0.01), whereas the glycine concentration decreased with increasing threonine intakes in the liver and muscle. The presented data indicate that increasing the threonine in plasma leads to increasing brain glycine and thereby affects the neurotransmitter balance in the brain. This may have consequences for brain development during early postnatal life. Therefore, excessive threonine intake during infant feeding should be avoided.

Effects of Dietary Long-Chain Polyunsaturated Fatty Acids on Plasma Amino Acids and Indices of Protein Metabolism in Infants: Results from a Randomized Clinical Trial

Background/Aim: Previous studies in vitro and in animals in vivo found that α-linolenic acid (C18:3ω3) may enhance oxidative damage of essential amino acids. We investigated whether the addition of the long-chain polyunsaturated fatty acids (LCPUFA) arachidonate (C20:4ω-6; AA) and docosahexaenoate (C22:6-ω3; DHA) in the form of egg phospholipids to infant formula affects plasma amino acid concentrations and indices of protein metabolism in term infants. Methods: In a double-blind, randomized clinical trial, healthy infants were fed from day 5 of life formula with or without preformed LCPUFA (n = 10 and 12, respectively). At the age of 5 days and 1, 2, 3 and 4 months, blood samples were obtained and analyzed for plasma amino acids by high-performance liquid chromatography and for plasma phospholipid fatty acid composition by gas chromatography. Results: At the age of 3 months, plasma threonine concentrations were significantly lower in infants receiving dietary LCPUFA than in controls (124 ± 16 vs. 216 ± 28 µmol/l, p < 0.05). Values of other plasma essential amino acids, total protein, albumin, creatinine and urea nitrogen did not differ between the two feeding groups throughout the study. At the age of 5 days, plasma phospholipid AA and DHA concentrations were inversely correlated with histidine concentrations (AA: r = –0.60, p = 0.01; DHA: r = –0.53, p < 0.05). At the age of 3 months, DHA concentrations were inversely related to plasma histidine, methionine and threonine concentrations (r = –0.66, –0.62, and –0.64, respectively, p < 0.05). Conclusions: The dietary LCPUFA supplementation of infant formula used in this study has no adverse effects on infant plasma amino acid concentrations and indicators of protein metabolism. Nonetheless, the apparent interaction of LCPUFA with some amino acids in formula-fed infants warrants further investigation.