Tyrosine Flux Research Articles

Tyrosine Amino Acid as a Foulant for the Heterogeneous Anion Exchange Membrane.

The features of organic fouling have been revealed for highly basic anion exchange membranes during prolonged electrodialysis in solutions containing the aromatic amino acid tyrosine. With increased operation time when using MA-41 heterogeneous membranes in tyrosine solution, an increase in hydrophobicity and roughness characteristics of the material surface is detected. A reduction in tyrosine flux through the membrane occurs which is caused by its pores plugging and deposition of the amino acid at the membrane surface induced by tyrosine adsorption and local supersaturation of the solution in the membrane phase. The long-term contact of the anion exchange membrane with a solution of tyrosine leads to some structural changes in the anion exchange material. An accumulation of the studied amino acid with phenolic fragment and tyrosine oxidation products (DOPA, DOPA-quinone) is found and confirmed by IR- and UV-spectroscopy techniques. The organic fouling is accompanied by an increase in density and a decrease in moisture content of the studied membrane. A comparative analysis of the chemical and electrochemical cleaning results for fouled samples of the MA-41 membrane demonstrates a partial restoration of the material transport characteristics using electrochemical cleaning in the intensive current mode of electrodialysis. The best efficiency of regeneration is reached when carrying out chemical cleaning with a solution of hydrochloric acid, providing almost complete restoration of the membrane characteristics.

Systemic arginine depletion: effects on protein synthesis and blood pressure in mice (258.4)

Arginine (Arg) is a semi‐essential amino acid involved in protein synthesis (PS) and blood pressure (BP) regulation. Arg deiminase (ADI) is a bacterial enzyme that hydrolyses Arg imino group to yield citrulline (Cit) and NH3. Pegylated ADI (ADI‐PEG) administration in animals and humans results in the depletion of circulating Arg. To investigate the effect of ADI‐PEG on PS and BP, ADI‐PEG or Saline (Sal) were administered i.m. to male mice. Plasma Arg concentration was negligible (<1; Sal 80 µmol/L), but Cit increased several fold (1100; Sal 80 µmol/L) in ADI‐PEG treated mice. Arg and Cit fluxes were greater in ADI‐PEG mice (Arg 543 vs 910, Cit 126 vs 402 µmol•kg‐1•h‐1, Sal and ADI‐PEG, resp.). The conversion of Arg to Cit, usually used as a proxy for NO production, increased from 4.1 to 496 µmol•kg‐1•h‐1, demonstrating the activity of the deiminase. Phenylalanine and tyrosine fluxes and phenylalanine hydroxylation were not different between the two treatments, indicating no changes in whole body protein kinetics. Protein fractional synthesis rate was not different for most tissues, but it was lower in the brain, thymus and testicles of ADI‐PEG treated mice. 14C Cit incorporation showed an identical pattern. Mean BP was reduced in ADI‐PEG treated mice (105 vs Sal 118 mm Hg, P< 0.008), despite the virtual absence of Arg in blood. These results demonstrate the capacity of many cell types to derive ARG from Cit to meet their needs.Grant Funding Source: Supported by USDA/ARS Cooperative Agreement No 58‐6250‐6‐001

Anabolic signaling and protein deposition are enhanced by intermittent compared with continuous feeding in skeletal muscle of neonates

Orogastric tube feeding is indicated for neonates with impaired ability to ingest and can be administered by intermittent bolus or continuous schedule. Our aim was to determine whether feeding modalities affect muscle protein deposition and to identify mechanisms involved. Neonatal pigs were overnight fasted (FAS) or fed the same amount of food continuously (CON) or intermittently (INT; 7 × 4 h meals) for 29 h. For 8 h, between hours 20 and 28, pigs were infused with [(2)H(5)]phenylalanine and [(2)H(2)]tyrosine, and amino acid (AA) net balances were measured across the hindquarters. Insulin, branched-chain AA, phenylalanine, and tyrosine arterial concentrations and whole body phenylalanine and tyrosine fluxes were greater for INT after the meal than for CON or FAS. The activation of signaling proteins leading to initiation of mRNA translation, including eukaryotic initiation factor (eIF)4E·eIF4G complex formation in muscle, was enhanced by INT compared with CON feeding or FAS. Signaling proteins of protein degradation were not affected by feeding modalities except for microtubule-associated protein light chain 3-II, which was highest in the FAS. Across the hindquarters, AA net removal increased for INT but not for CON or FAS, with protein deposition greater for INT. This was because protein synthesis increased following feeding for INT but remained unchanged for CON and FAS, whereas there was no change in protein degradation across any dietary treatment. These results suggest that muscle protein accretion in neonates is enhanced with intermittent bolus to a greater extent than continuous feeding, mainly by increased protein synthesis.

Effect of liver cirrhosis on phenylalanine and tyrosine metabolism

Phenylalanine conversion to tyrosine (i.e., 'hydroxylation') is the first irreversible step in phenylalanine catabolism and a source of circulating tyrosine. The purpose of the present review is both to examine hydroxylation from a biochemical standpoint and to report data measured in vivo under physiological conditions, as well as in liver and kidney disease. The simultaneous infusion of phenylalanine and tyrosine tracers in humans allows us to determine the hydroxylation rate in vivo. Hydroxylation accounts for a minor ( approximately 10-20%) although significant portion of tyrosine flux. The liver and the kidney are the key organs accounting for virtually the whole-body hydroxylation rates. It is regulated by substrate availability, being acutely stimulated by mixed meal ingestion and by dietary adaptation to high phenylalanine intakes. Theoretically, it may be impaired in advanced liver and kidney disease. Nevertheless, in compensated liver cirrhosis, hydroxylation as well as tyrosine flux are not decreased but rather increased. Only in end stage liver disease hydroxylation may be impaired and is corrected by transplantation. Hydroxylation is also reduced in end stage renal disease. Phenylalanine hydroxylation in vivo appears to represent a regulatory step of phenylalanine disposal and tyrosine production under acute and/or extreme conditions.

Increased Protein Turnover and Proteolysis Is an Early and Primary Feature of Short-Term Experimental Hyperthyroidism in Healthy Women

Hyperthyroidism increases energy expenditure, glucose turnover, lipolysis, and protein breakdown. Our objective was to test whether increased protein breakdown occurs independently of other catabolic effects in mild experimental hyperthyroidism. We conducted a single-blind, randomized, placebo-controlled, crossover study. Protein dynamics of the whole body and of the forearm muscles were measured by amino acid tracer dilution technique ([(15)N]phenylalanine and [(2)H(4)]tyrosine). All subjects underwent a 3-h study in the basal state followed by a 3-h euglycemic clamp study. The study took place at a university clinical research unit. Eight healthy women (24-46 yr old) participated. Intervention included 6 d thyroid hormone (T(4) 50 microg and T(3) 0.67 microg/kg.d) or placebo administration. Thyroid hormone administration led to mild T(3) hyperthyroidism with more than a doubling of T(3) levels and suppression of TSH. Energy expenditure and body composition was unchanged. Glucose infusion rates, forearm glucose uptake, and levels of lipid intermediates were also alike. Basal whole-body phenylalanine flux and tyrosine flux (reflecting whole-body protein breakdown) were increased (P < 0.05) as were whole-body protein synthesis rate (P = 0.05). Basal forearm rate of appearance and disappearance for phenylalanine (reflecting muscle protein breakdown and synthesis) were similar. Mild short-term experimental hyperthyroidism increases whole-body protein turnover and breakdown before any measurable changes in energy expenditure or glucose and fat metabolism, suggesting that amino acid and protein metabolism is an early and primary target for thyroid hormone action in humans.

The Muscle Protein Synthetic Response to Carbohydrate and Protein Ingestion Is Not Impaired in Men with Longstanding Type 2 Diabetes 3

Protein ingestion stimulates muscle protein synthesis and improves net muscle protein balance. Insulin resistance has been suggested to result in a reduced muscle protein synthetic response to food intake. As such, we hypothesized that type 2 diabetes patients have a impaired muscle protein synthetic response to food ingestion. To test this hypothesis, 10 male type 2 diabetes patients using their normal oral glucose-lowering medication (68 ± 2 y) and 10 matched, normoglycemic men (65 ± 2 y) were randomly assigned to 2 crossover treatments in which whole body and muscle protein synthesis were measured following the consumption of either carbohydrate (CHO) or carbohydrate with a protein hydrolysate (CHO+PRO). Primed, continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied and blood and muscle samples were collected to assess whole-body protein balance and mixed muscle protein fractional synthetic rate over a 6-h period. Whole-body phenylalanine and tyrosine flux were higher after the CHO+PRO treatment compared with the CHO treatment in the diabetes and control group (P < 0.01). Protein balance was negative following CHO but positive following CHO+PRO treatment in both groups. Muscle protein synthesis rates were higher in both groups following the CHO+PRO (0.086 ± 0.014%/h) treatment than in the CHO treatment (0.040 ± 0.003%/h; P < 0.01) with no difference between the diabetes patients and normoglycemic controls. We conclude that the muscle protein synthetic response to CHO or CHO+PRO ingestion is not substantially impaired in longstanding, type 2 diabetes patients treated with oral blood glucose-lowering medication.

Protein metabolism in Turner syndrome and the impact of hormone replacement therapy

Studies have documented an altered body composition in Turner syndrome (TS). Body fat is increased and muscle mass is decreased. Ovarian failure necessitates substitution with female hormone replacement therapy (HRT), and HRT induces favourable changes in body composition. It is unknown how HRT affects protein metabolism. To test whether alterations in body composition before and after HRT in TS are a result of altered protein metabolism. We performed a randomized crossover study with active treatment (HRT in TS and oral contraceptives in controls) or no treatment. We studied eight women (age 29.7 +/- 5.6 (mean +/- SD) years) with TS, verified by karyotype, and eight age-matched controls (age 27.3 +/- 4.9 years). All subjects underwent a 3-h study in the postabsorptive state. Protein dynamics of the whole body and of the forearm muscles were measured by an amino acid tracer dilution technique using [(15)N]phenylalanine and [(2)H(4)]tyrosine. Substrate metabolism was examined by indirect calorimetry. Energy expenditure was comparable among TS and controls, and did not change during active treatment. Whole-body phenylalanine and tyrosine fluxes were similar in the untreated situations, and did not change during active treatment. Amino acid degradation and protein synthesis were similar in all situations. Muscle protein breakdown was similar among groups, and was not affected by treatment. Muscle protein synthesis rate and forearm blood flow did not differ among groups or due to treatment. Protein metabolism in TS is comparable to controls, and is not affected by HRT.

Phenylalanine and Tyrosine Metabolism in Chronic Kidney Failure

In chronic kidney failure, there is impairment in the conversion of phenylalanine to tyrosine. As a result, tyrosine and the tyrosine/phenylalanine ratio are reduced in plasma and many tissues, and phenylalanine concentrations tend to be normal or slightly increased. Although animal studies indicate that the kidney is not a major contributor to the conversion of phenylalanine to tyrosine, human studies conducted in the postabsorptive state suggest that the kidney plays a major role in the uptake of phenylalanine and its hydroxylation and release as tyrosine. The human splanchnic bed in the postabsorptive state also displays net uptake of both phenylalanine and tyrosine and hydroxylation of substantial amounts of phenylalanine to form tyrosine. In chronic renal failure (CRF) patients, splanchnic uptake of tyrosine appears to be reduced in the postabsorptive state. After an amino acid meal, there is net release of phenylalanine from the splanchnic bed in normal subjects and to an even greater degree in CRF patients; tyrosine is released postprandially in both normal subjects and CRF patients. In the postabsorptive state, tyrosine release from the kidney is largely derived from the hydroxylation of phenylalanine. In CRF, the release of tyrosine from the kidney is reduced and this reduction may be marked with advanced CRF. These observations, as well as isotope studies indicating normal phenylalanine flux, reduced tyrosine flux and impaired conversion of phenylalanine to tyrosine in CRF patients, raise the possibility that tyrosine may be an essential amino acid in this condition. Further research will be necessary to answer this question. Oxidative stress, which often increases in CRF patients, may lead to increased formation of chlorotyrosine and nitrotyrosine in plasma proteins and of nitrotyrosine in the brain. Increased nitrotyrosine is also found in kidneys of patients with diabetic nephropathy or allograft nephropathy. Increased serum concentrations of oxidation products of phenylalanine have also been observed in patients with CRF. Impaired urinary excretion also may lead to accumulation of metabolic products of both phenylalanine and tyrosine in CRF. It is not known whether the elevated protein chlorotyrosine or nitrotyrosine or increased oxidative products of phenylalanine cause adverse metabolic or toxic effects in patients with CRF.

Co-ingestion of protein and leucine stimulates muscle protein synthesis rates to the same extent in young and elderly lean men

The progressive loss of skeletal muscle mass with aging is attributed to a disruption in the regulation of skeletal muscle protein turnover. We investigated the effects on whole-body protein balance and mixed-muscle protein synthesis rates of the ingestion of carbohydrate with or without protein and free leucine after simulated activities of daily living. Eight elderly (75 +/- 1 y) and 8 young (20 +/- 1 y) lean men were randomly assigned to 2 crossover experiments in which they consumed either carbohydrate (CHO) or carbohydrate plus protein and free leucine (CHO+Pro+Leu) after performing 30 min of standardized activities of daily living. Primed, continuous infusions with L-[ring-13C6]phenylalanine and L-[ring-2H2]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover and the protein fractional synthetic rate in the vastus lateralis muscle over a 6-h period. Whole-body phenylalanine and tyrosine flux were significantly higher in the young than in the elderly men (P < 0.01). Protein balance was negative in the CHO experiment but positive in the CHO+Pro+Leu experiment in both groups. Mixed-muscle protein synthesis rates were significantly greater in the CHO+Pro+Leu than in the CHO experiment in both the young (0.082 +/- 0.005%/h and 0.060 +/- 0.005%/h, respectively; P < 0.01) and the elderly (0.072 +/- 0.006%/h and 0.043 +/- 0.003%/h, respectively; P < 0.01) subjects, with no significant differences between groups. Co-ingestion of protein and leucine with carbohydrate after activities of daily living improves whole-body protein balance, and the increase in muscle protein synthesis rates is not significantly different between lean young and elderly men.

Co-ingestion of Protein and Leucine Stimulates Muscle Protein Synthesis in Young and Elderly Men

Aging is associated with a progressive loss of skeletal muscle mass, which is attributed to a disruption in the regulation of skeletal muscle protein turnover, which results in a chronic imbalance between muscle protein synthesis and breakdown rate. Knowledge of differences in responses of young and elderly subjects under normal daily living conditions are of crucial importance to our understanding of the etiology of sarcopenia and are currently lacking in the literature. PURPOSE: We investigated the response to the ingestion of carbohydrate with or without protein and free leucine following simulated activities of daily living (ADL) on whole-body protein balance and mixed muscle protein synthesis rates. METHODS: Eight elderly (75±1 y) and eight young (20±1 y) male subjects were randomly assigned to 2 trials in which they consumed either carbohydrate (CHO) or carbohydrate, protein and free leucine (CHO+PRO+leu) after performing 30 min of standardized ADL activities. Primed, continuous infusions with L-[ring−13C6]phenylalanine and L-[ring−2H2]tyrosine were applied, and blood and muscle samples were collected to assess whole-body protein turnover as well as fractional protein synthetic rate (FSR) in the vastus lateralis muscle over a 6 h period. RESULTS: Plasma insulin responses were significantly higher in the CHO+PRO+leu trial than in the CHO trial in both the young and elderly subjects (p<0.05). Whole-body phenylalanine and tyrosine fluxes were significantly higher in the young versus the elderly (p<0.01). Protein balance was negative in the CHO trial, but positive in the CHO+PRO+leu trial in both groups. In the CHO trial, muscle FSR was significantly lower in the elderly compared with the young subjects (p<0.05). Mixed muscle protein synthesis rates were significantly greater in the CHO+PRO+leu compared with the CHO trial in both the young (0.082±0.005%.h−1 vs. 0.060±0.005%h−1, respectively; p<0.01), and elderly subjects (0.072±0.006%h−1 vs. 0.043±0.003 %h−1 respectively; p<0.01), with no differences between groups. CONCLUSIONS: Co-ingestion of protein hydrolysate and leucine with carbohydrate following ADL activities improves whole-body protein balance and increases muscle protein synthesis rates to the same extent in young and elderly men.

Branched chain amino acids as the protein component of parenteral nutrition in cancer cachexia

A prospective randomized trial was conducted to determine the effects of branched chain amino acids (BCAA) as the protein component of total parenteral nutrition (TPN) on protein kinetics in patients with intraabdominal adenocarcinoma. Nine malnourished patients were given both conventional TPN containing 19 per cent BCAA (AA) and isocaloric, isonitrogenous TPN containing 50 per cent BCAA (BCAA-TPN), in random order. Both [13C]leucine and [14C]tyrosine were employed as tracers to avoid the potential bias due to the different amino acid composition of the two TPN solutions. With BCAA-TPN, leucine and tyrosine flux increased significantly from (mean +/- s.d.) 158.0 +/- 37.2 to 243.5 +/- 75.8 mumol kg-1 h-1 (P less than 0.025) and from 35.0 +/- 8.4 to 42.6 +/- 11.0 mumol kg-1 h-1 (P less than 0.05) respectively. Leucine oxidation was significantly higher on BCAA-TPN (24.1 +/- 6.3 on AA versus 68.3 +/- 37.1 mumol kg-1 h-1, P less than 0.025) while tyrosine oxidation was significantly lower (3.7 +/- 1.8 mumol kg-1 h-1 on AA versus 2.5 +/- 2.0 mumol kg-1 h-1 on BCAA-TPN, P less than 0.05). Whole body protein synthesis and breakdown was significantly higher on BCAA-TPN by the tyrosine tracer (31.3 +/- 7.3 on AA versus 40.1 +/- 9.3 mumol kg-1 h-1, P less than 0.025 and 33.0 +/- 8.4 on AA versus 41.3 +/- 11.1 mumol kg-1 h-1, P less than 0.05) respectively. Using the leucine tracers both synthesis and breakdown were increased, but not significantly, from 133.8 +/- 40.0 to 175.3 +/- 65.1 mumol kg-1 h-1 and from 127.9 +/- 33.6 to 167.7 +/- 71.2 mumol kg-1 h-1 respectively. The fractional albumin synthetic rate increased significantly on BCAA-TPN from 4.3 +/- 2.9 on AA to 8.0 +/- 5.1 per cent per day (P less than 0.05). The reduction in tyrosine oxidation, suggesting improved protein utilization, coupled with an increase in protein and albumin synthesis, strongly support a positive benefit from BCAA-TPN in cancer cachexia.

Supplementation with Aromatic Amino Acids Improves Leucine Kinetics but Not Aromatic Amino Acid Kinetics in Infants with Infection, Severe Malnutrition, and Edema

We investigated whether supplementation with an aromatic amino acid (AAA) cocktail consisting of 0.5 mmol each of phenylalanine, tryptophan, and tyrosine compared with isonitrogenous amounts of alanine (Ala) would improve measures of protein kinetics in 14 (8 with AAA, 6 Ala) children with edematous malnutrition (aged 6-24 mo) during the infected acute malnourished state. Supplementation started immediately after the baseline experiment, 2 d postadmission and continued to the end of the acute phase of treatment. The second (postsupplementation) experiment was done approximately 12 d postadmission. We measured leucine kinetics, phenylalanine and tyrosine fluxes, using an i.g. 8-h prime continuous infusion of (2)H(3)-leucine, and an i.v. 6-h prime continuous infusion of (13)C-leucine, (2)H(2)-tyrosine, and (2)H(5)-phenylalanine in the fed state. Leucine flux tended to be faster (P = 0.06) in the AAA group compared with Ala group after supplementation (mean difference +/- SEM): 22.6 +/- 10.9 micromol/(kg . h). The rate of leucine appearance from protein breakdown [28.1 +/- 9.4 micromol/(kg . h)] and the nonoxidative disposal of leucine [i.e., leucine to protein synthesis; 35.4 +/- 12.9 micromol/(kg . h)] were faster (P < 0.02) in the AAA group than in the Ala group. There was no significant effect of supplementation on leucine splanchnic metabolism, phenylalanine, and tyrosine fluxes. These findings are consistent with the hypothesis that the blunting of the protein catabolic response to infection in children with edematous malnutrition syndrome is due to limited availability of aromatic amino acids.

Effect of Tyrosine Intake on the Rate of Phenylalanine Hydroxylation in Adult Males

This study evaluated the effect of varying levels of tyrosine intake on the estimation of phenylalanine hydroxylation. Healthy men were fed 1 g protein kg(-1) x d(-1) for a 2-day period. On the third day, subjects consumed a formula diet containing 1 g protein kg(-1) x d(-1) hourly over 10 hours, and primed hourly oral doses of L-[15N]phenylalanine and L-[3,3-2H2]tyrosine for the last 6 hours. Each subject was studied at 7 levels of tyrosine intake (3.0, 4.5, 6.0, 7.5, 9.0, 10.5, and 12.0 mg x kg(-1) x d(-1)) at a constant intake of phenylalanine (9 mg x kg(-1) x d(-1), 4.55 micromol x kg(-1) x h(-1)). Phenylalanine hydroxylation was estimated from the ratio of plasma amino acid isotope enrichment of [15N]phenylalanine and [15N]tyrosine and the tyrosine flux estimated from [2H2]tyrosine enrichment. Phenylalanine and tyrosine fluxes showed no significant response to alterations in the intake of tyrosine. Linear regression analysis showed a significant response such that the rate of phenylalanine hydroxylation decreased as tyrosine intake increased (R2 = .21; P = .003). The mean rates of phenylalanine hydroxylation were 3.89 to 8.06 micromol x kg(-1) x h(-1). Given model uncertainties, the apparent protein breakdown observed at tyrosine intake levels less than 10.5 mg x kg(-1) x d(-1), and the significant differences observed between the present data and our prior data, we cannot estimate the tyrosine requirement with any degree of certainty with the present hydroxylation results.

Evaluation of phenylalanine and tyrosine metabolism in late human pregnancy

The [ 2H 5]-phenylalanine method for measurement of protein metabolism requires the phenylalanine hydroxylation to tyrosine to be calculated from the tyrosine flux. Although this can be estimated, for pregnancy, we made a direct measurement of the molar ratio of the fluxes of tyrosine and phenylalanine from protein breakdown ( Pt Pp ) using [ 2H 2]-tyrosine infusion. Six normal pregnant women were studied at 37 weeks' gestation. While fasting, they were administered a 3-hour primed-constant infusion with [ 13C]-leucine, [ 2H 5]-phenylalanine, and [ 2H 2]-tyrosine. Leucine (α-ketoisocaproic acid [KIC]) flux was 136.2 ± 15.1 μmol/kg/h (mean ± SD], phenylalanine flux 41.2 ± 5.6, and tyrosine flux 25.0 ± 6.0, and phenylalanine hydroxylation was 3.3 ± 2.1 μmol/kg/h. The mean tyrosine to phenylalanine molar flux ratio ( Pt Pp ) was 0.52 ± 0.10, lower than the ratio of 0.65 to 0.85 reported in normal nonpregnant subjects and 0.73 estimated from animal studies. We studied protein metabolism in six additional pregnant women and six nonpregnant women using [ 13C]-leucine and [ 2H 5]-phenylalanine infusions only and applied the lower Pt Pp ratio to the former group. Tyrosine flux (42.0 ± 7.2 μmol/kg/h) and phenylalanine hydroxylation (9.2 ± 4.2 μmol/kg/h) were significantly higher in nonpregnant subjects than in both groups of pregnant subjects. The percent contribution of phenylalanine hydroxylation to total tyrosine flux was reduced from 20% to 14%. When using [ 2H 5]-phenylalanine to study whole-body protein metabolism in pregnancy and tyrosine flux is not measured directly by infusion of [ 2H 2]-tyrosine, the lower Pt Pp ratio is required. The phenylalanine model shows that tyrosine flux derived from protein breakdown and phenylalanine hydroxylation are both reduced in pregnancy.

Protein dynamics in whole body and in splanchnic and leg tissues in type I diabetic patients.

To elucidate the mechanism of insulin's anticatabolic effect in humans, protein dynamics were evaluated in the whole-body, splanchnic, and leg tissues in six C-peptide-negative type I diabetic male patients in the insulin-deprived and insulin-treated states using two separate amino acid models (leucine and phenylalanine). L-(1-13C,15N)leucine, L-(ring-2H5)phenylalanine, and L-(ring-2H2) tyrosine were infused intravenously, and isotopic enrichments of [1-13C,15N]-leucine, (13C)leucine, (13C)ketoisocaproate, (2H5)phenylalanine, [2H4]tyrosine, (2H2)tyrosine, and 13CO2 were measured in arterial, hepatic vein, and femoral vein samples. Whole-body leucine flux, phenylalanine flux, and tyrosine flux were decreased (< 0.01) by insulin treatment, indicating an inhibition of protein breakdown. Moreover, insulin decreased (< 0.05) the rates of leucine oxidation and leucine transamination (P < 0.01), but the percent rate of ketoisocaproate oxidation was increased by insulin (P < 0.01). Insulin also reduced (< 0.01) whole-body protein synthesis estimated from both the leucine model (nonoxidative leucine disposal) and the phenylalanine model (disposal of phenylalanine not accounted by its conversion to tyrosine). Regional studies demonstrated that changes in whole body protein breakdown are accounted for by changes in both splanchnic and leg tissues. The changes in whole-body protein synthesis were not associated with changes in skeletal muscle (leg) protein synthesis but could be accounted for by the splanchnic region. We conclude that though insulin decreases whole-body protein breakdown in patients with type I diabetes by inhibition of protein breakdown in splanchnic and leg tissues, it selectively decreases protein synthesis in splanchnic tissues, which accounted for the observed decrease in whole-body protein synthesis. Insulin also augmented anabolism by decreasing leucine transamination.

Phenylalanine Kinetics in Sick Preterm Neonates with Respiratory Distress Syndrome

The cause of hyperphenylalaninemia in sick preterm infants has yet to be determined; one reason may be reduced tolerance to phenylalanine as a consequence of immaturity of phenylalanine hydroxylase. Phenylalanine metabolism was studied in vivo in 23 ventilated preterm infants of gestational age 23 to 36 wk within the first 6 d of life using a continuous i.v. infusion of the stable isotope-labeled amino acids [2H5]phenylalanine, [2H4]tyrosine, and [2H2]tyrosine. Phenylalanine hydroxylation was calculated from two different methods. In the first method, used in all 23 infants receiving glucose and in seven of these infants who subsequently received parenteral nutrition, phenylalanine hydroxylation was calculated from the plasma enrichments of [2H5]phenylalanine and [2H4]tyrosine and from the molar ratio of tyrosine to phenylalanine in mammalian tissue protein. In this instance, the mean hydroxylation was 16.0 (SD 10.9) and 48.4 (SD 14.9) mumol/kg/h, which was 17.3% (SD 8.4%) and 33.2% (SD 9.8% of the total phenylalanine flux for infants receiving glucose and parenteral nutrition, respectively. Additionally, in six infants receiving glucose, hydroxylation was calculated from the measured phenylalanine (2H5), independent tyrosine (2H2) fluxes, and the plasma enrichments of (2H5) phenylalanine and its hydroxylation product [2H4]tyrosine. In this case, hydroxylation was 20.5 (SD 13.0) mumol/kg/h, which represented 22.3% (SD 9.8%) of the phenylalanine flux. In the same six infants, phenylalanine hydroxylation derived using the first method was 22.2 (SD 13.1) mumol/kg/h, 23.6% (SD 9.9%) of the total phenylalanine flux.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylalanine and Tyrosine Kinetics in Critically Ill Children with Sepsis

ABSTRACT: To better understand the impact of severe illness on the amino acid economy and nutritional needs of pediatric patients, we studied plasma phenylalanine and tyrosine kinetics in eleven critically ill patients (six full-term newborns and five young infants). Within 48 h of the diagnosis of sepsis they were given primed constant i.v. infusions of L-[1-13C]phenylalanine and L-[3,3,2H2]tyrosine for 4 h. Routine nutritional support continued during this period by parentcral administration of dextrose, lipid emulsion, and an amino acid mixture low in tyrosine. Phenylalanine and tyrosine fluxes and rate of phenylalanine hydroxylation did not differ significantly between the two age groups, and so the data were combined for evaluation. For the entire group, values (μmol-kg−1-h−1; mean ± SD) for phenylalanine and tyrosine fluxes and rate of phenylalanine hydroxylation were 132 ± 24, 66 ± 16, and 29 ± 12, respectively. Plasma phenylalanine to tyrosine concentration ratio was 1.67 ± 0.6. From a comparison of the rate of phenylalanine hydroxylation with measured phenylalanine intakes, it was concluded that their routine, clinical nutritional support was inadequate to achieve body phenylalanine balance. In comparison with published data, the relative rate of phenylalanine hydroxylation appears to be high. We speculate that tyrosine is a conditionally indispensable amino acid under these conditions; it would be desirable to establish the intake levels and ratio of phenylalanine to tyrosine that effectively support aromatic amino acid balance in these critically ill patients.

Nutritional and metabolic variables correlate with amino acid forearm flux in patients with severe head injury

To measure the arterial-venous amino acid flux across the forearm muscle in patients with severe head injury. Prospective, interventional study. Level I trauma hospital in the neurosurgery intensive care unit (ICU) at a university medical center. Eight nonsteroid-treated patients with severe head injury. Patients were prospectively randomized to receive either standard or supplemental intravenous zinc therapy. Net forearm alanine, glutamine, tyrosine, phenylalanine, and branch-chain amino acid forearm flux were measured and compared with metabolic markers of energy expenditure and nitrogen excretion. There was a significant inverse relationship between the measured energy expenditure/predicted energy expenditure ratio and glutamine flux (r2 = .62; p < .05). The patients with the highest measured energy expenditure/predicted energy expenditure ratio had the greatest release of glutamine from forearm muscle. Nitrogen balance was significantly correlated with leucine flux (r2 = .53; p < .05) and with isoleucine flux (r2 = .67; p < .05). The patients with the most positive nitrogen balance had the least release of branch-chain amino acids from skeletal muscle. Tyrosine flux was highly correlated with net amino acid flux (r2 = .76; p < .01). Tyrosine flux was therefore indicative of overall muscle catabolism. Four patients had an overall negative flux of amino acids from skeletal muscle. Three patients had an overall negative flux of branch-chain amino acids. This preliminary descriptive report suggests that increased skeletal muscle efflux of amino acids correlates significantly with metabolic variables of hypermetabolism and hypercatabolism in nonsteroid-treated, head-injured patients.

Effect on whole-body protein synthesis after institution of intravenous nutrition in cancer and non-cancer patients who lose weight

Cancer and non-cancer patients received total parenteral nutrition (TPN) corresponding to either 120% or 200% non-protein energy resting energy expenditure. Whole-body tyrosine flux and leg exchange of various metabolites were measured in the fasted and fed state. Feeding with the moderate TPN rate did not stimulate whole-body protein synthesis in either group, but the high rate did. Both TPN rates switched an efflux of branched-chain aminoacids from the leg to an uptake in both groups, but this did not apply to tyrosine or phenylalanine. Only the high TPN rate stimulated glucose uptake across the leg in both groups. The leg exchanges of lactate, glycerol and free fatty acids were not significantly influenced by moderate or high TPN rates in either group, although changes in arterial concentrations indicated significant exchanges in compartments other than leg tissues. Thus standard TPN is insufficient to stimulate overall protein synthesis in both malnourished cancer and non-cancer patients, which may explain why previous studies have demonstrated insignificant functional effects with nutritional support to cancer patients.

Effect of systemic hyperinsulinemia on amino acid flux across human legs in postabsorptive state

The effect of physiological hyperinsulinemia (approximately 110 mU/l) on leg tissue protein balance was investigated in eight weight-stable healthy individuals. A primed constant infusion of L-[U-14C]tyrosine was used to measure the disposal and release of tyrosine across the leg before and during 2 h of euglycemic clamp studies. The leg exchange of 3-methyl-L-histidine (3-MH) and all amino acids in blood were measured before and during insulinization, including the muscle tissue content of amino acids. Hyperinsulinemia decreased whole body tyrosine flux from 52 +/- 2 to 35 +/- 1 mumol/min (P less than 0.0001), whereas neither disposal (53 +/- 9 vs. 45 +/- 9 nmol.min-1.100 g-1) nor release of tyrosine across the leg (76 +/- 11 vs. 66 +/- 10 nmol X min-1 X 100 g-1) was significantly influenced. The arterial concentration and the leg exchange of 3-MH were not significantly affected by 2 h of hyperinsulinemia, but the sum of all amino acids declined significantly. The net leg balance of tyrosine was not affected at all by hyperinsulinemia, whereas the balance of the branched-chain amino acids and methionine were switched from efflux toward influx. Phenylalanine efflux from the leg only showed a trend to a significant effect by insulin. The muscle tissue concentration of six individual amino acids decreased significantly during hyperinsulinemia, particularly the branched-chain amino acids. The leg exchange of glucose, free fatty acids, and glycerol immediately changed significantly, as expected in response to insulinization.(ABSTRACT TRUNCATED AT 250 WORDS)