Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine

The definition of amino acid requirements for neonates receiving total parenteral nutrition (TPN) is critical for the further improvement of this nutritional regimen. In the present study we investigated the kinetics and requirements of phenylalanine and tyrosine in neonatal piglets receiving TPN. Twenty-four 3-d-old male Yorkshire piglets were fitted with external jugular and femoral catheters and maintained on identical TPN formulations for 5 d. Total amino acid, phenylalanine, tyrosine, and energy intakes were 15, 0.61, and 0.51 g. kg-1 . d-1 and 1.1 MJ . kg-1 . d-1, respectively. On day 5, piglets (three per level) were randomly assigned to receive one of eight phenylalanine intakes: 0.2. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 1.2 g. kg-1 . d-1. On day 6, phenylalanine kinetics were measured during a 4-h primed, continuous infusion of L-[1-14C]phenylalanine. Plasma phenylalanine and phenylalanine oxidation were statistically similar for the three lowest phenylalanine intakes and increased thereafter. Crossover regression analysis yielded estimates for the mean requirement and safe phenylalanine intake of 0.45 and 0.48 g . kg-1 . d-1, respectively (equivalent to 30 and 32 mg/g amino acids, respectively), in the presence of excess tyrosine. An inability of piglets to maintain a linear oxidative response at phenylalanine intakes > 0.8 g . kg-1 . d-1 (equivalent to 53 mg/g amino acids) was found. These data represent the first direct estimates of phenylalanine requirements in neonates receiving TPN and demonstrate the use of oxidation techniques for the estimation of amino acid requirements during parenteral nutrition.     

Threonine requirement of neonatal piglets receiving total parenteral nutrition is considerably lower than that of piglets receiving an identical diet intragastrically

Evidence is accumulating that the amino acid requirements for neonates receiving total parenteral nutrition (TPN) are significantly different than those for oral feeding and need to be determined. The parenteral threonine requirement was determined in 3-d-old male Yorkshire piglets (n = 25) by examining the effect of varying dietary threonine intakes [0.05-0.6 g/(kg.d)] on phenylalanine oxidation. The diet included adequate energy, total amino acids and phenylalanine, with excess tyrosine. Phenylalanine kinetics were determined from a primed, continuous intravenous infusion of L-[1-14C]phenylalanine. Phenylalanine oxidation, estimated from the rate of 14CO2 released in expired air during isotope infusion, decreased (P < 0.05) as threonine intake increased from 0.05 to 0.15 g/(kg.d) and was low and constant for threonine intakes >0.15 g/(kg.d). Using breakpoint analysis with 95% confidence interval (CI), mean requirement and safe level of parenteral threonine intake were estimated to be 0.19 and 0.21 g/(kg. d), respectively (equivalent to 13 and 14 mg/g amino acids, respectively). To compare these data with those of orally fed controls, we then repeated the experiment by infusing identical diets intragastrically to piglets (n = 25); the varying dietary threonine intakes were 0.1-1.2 g/(kg.d). Employing identical kinetics and analyses, the mean requirement and safe level of oral threonine intake were estimated to be 0.42 and 0.51 g/(kg.d), respectively (equivalent to 28 and 34 mg/g amino acids, respectively). These data demonstrate that the threonine requirement of neonates during TPN is approximately 45% of the mean oral requirement.     

Twenty-four-hour L-[1-(13)C]tyrosine and L-[3,3-(2)H2]phenylalanine oral tracer studies at generous, intermediate, and low phenylalanine intakes to estimate aromatic amino acid requirements in adults

Daily pattern and rates of whole-body tyrosine oxidation and phenylalanine hydroxylation were determined in young adults (15 men, 1 woman) receiving [13C]tyrosine and [(2)H2]phenylalanine via primed, constant oral infusion and [(2)H4]tyrosine by vein (five subjects also received [(2)H3]leucine simultaneously by vein) continuously for 24 h (12 h fast then 12 h fed). Subjects were given a diet supplying 96.6 (n = 5), 35.6 (the proposed requirement; n = 5), and 18.5 mg phenylalanine x kg(-1) x d(-1) (n = 6) based on an otherwise adequate L-amino acid mixture for 6 d before the 24-h tracer study began. [Each diet was low in tyrosine: 6.79 mg x kg(-1) x d(-1).] Our hypothesis was that subjects would be in tyrosine equilibrium, positive balance, or both, at the 96.6- and 35.6-mg intakes and in distinctly negative balance at the 18.5-mg intake. The diurnal pattern in phenylalanine and tyrosine kinetics was dependent on the intake and, presumably, on the adequacy of dietary phenylalanine. Wholebody tyrosine balances, determined from rates of phenylalanine hydroxylation and tyrosine input and oxidation were negative (0.05 < P < 0.1 from zero balance) with the low (18.5 mg) phenylalanine intake [total aromatic amino acid (AAA) intake: 25.3 mg x kg(-1) x d(-1)] but at equilibrium (P > 0.05 from zero balance) with the two higher phenylalanine intakes. Whole-body AAA balance (AAA intake - tyrosine oxidation) was negative (P < 0.05 from zero balance) with the low intake, at equilibrium with the intermediate intake, and apparently distinctly positive (P < 0.05) with the generous intake. Despite model limitations, as discussed, these findings lend further support for a proposed, tentative value for a total mean requirement of 39 mg AAA x kg(-1) x d(-1).     

Parenteral glycyl-L-tyrosine maintains tyrosine pools and supports growth and nitrogen balance in phenylalanine-deficient rats

Poor solubility hampers the addition of sufficient amounts of free tyrosine to parenteral amino solutions. We investigated the use of a highly soluble synthetic dipeptide, glycyl-L-tyrosine, as a parenteral tyrosine source in 18 male Wistar rats (body weight 180-200 g). The animals were randomized into three equal groups and catheterized to facilitate isoenergetic (1.2 MJ.kg-1.d-1) and isonitrogenous (1.25 g nitrogen.kg-1.d-1) total parenteral nutrition for 7 d. Controls (Group 1) received a complete amino acid solution, Group 2 received the same solution deficient in phenylalanine (nitrogen replaced with glycine), and group 3 received the phenylalanine-deficient solution supplemented with glycyl-L-tyrosine. Between d 4 and 7, weight gain and nitrogen retention were lower in Group 2 and in Group 1 or 3. In plasma and organ samples obtained at the end of the study, amino acids and dipeptides were analyzed by means of reversed phase-HPLC. In Group 2, phenylalanine and tyrosine concentrations were lower than in controls in plasma, muscle and kidney; in liver, only the tyrosine concentration was lower compared with controls. With glycyl-L-tyrosine supplementation, plasma, liver and kidney tyrosine concentrations and the phenylalanine:tyrosine ratio were normal. Intact glycyl-L-tyrosine was not detectable, suggesting a virtually quantitative elimination or utilization of the infused dipeptide. The results indicate that in phenylalanine-deficient rats, parenteral glycyl-L-tyrosine rapidly provides free tyrosine to facilitate normal growth, promote nitrogen metabolism and maintain intra- and extracellular tyrosine pools.     

Continuous 24-h L-[1-13C]phenylalanine and L-[3,3-2H2]tyrosine oral-tracer studies at an intermediate phenylalanine intake to estimate requirements in adults

The daily rates of whole-body phenylalanine oxidation (phe-ox) and hydroxylation (phe-OH) were determined in young men (n = 10) receiving [13C]phenylalanine and [2H2]tyrosine via primed constant oral infusion (four also received simultaneously [2H4]tyrosine and [2H3]leucine via primed constant intravenous infusions) continuously for 24 h (first 12 h fast and then 12 h fed). The subjects were given a diet supplying a proposed requirement phenylalanine intake (six subjects: 39 mg phenylalanine.kg-1.d-1 without tyrosine; four subjects: 36 mg phenylalanine plus 6.8 mg tyrosine), based on an otherwise adequate L-amino acid mixture for 6 d before the tracer study. Our hypothesis was that the subjects would be in approximate body phenylalanine equilibrium at these intakes. Estimates of the daily rate of phe-ox were 26.9 +/- 7.5 mg.kg-1.d-1 (17.2 +/- 5.2 and 9.7 +/- 3.2 mg.kg-1.d-1 during the 12-h fast and fed periods, respectively), and for phe-OH they were 32.1 +/- 11.9 mg.kg-1.d-1 (21.7 +/- 10.5 and 10.4 +/- 2.5 mg.kg-1.d-1 during the 12-h fast and fed periods, respectively). The daily phenylalanine balance was approximately neutral (P > 0.05) when based on phe-ox or phe-OH (+4.73 +/- 7.34 and -0.41 +/- 12.6 mg.kg-1.d-1, respectively). In comparison with recent, comparable 24-h tracer studies at deficient (22 mg.kg-1.d-1) and generous (100 mg.kg-1.d-1) phenylalanine intakes, these results support the hypothesis that a phenylalanine intake of 39 mg.kg-1.d-1 (without significant tyrosine) approximates the mean requirement in healthy adults. This contrasts with the upper requirement value of 14 mg.kg-1.d-1 for the total of the aromatic amino acids proposed in 1985 by FAO/WHO/UNU.     

Mutation to phenylalanine of tyrosine 371 in tyrosine hydroxylase increases the affinity for phenylalanine

The aromatic amino acid hydroxylases tyrosine and phenylalanine hydroxylase both contain non-heme iron, utilize oxygen and tetrahydrobiopterin, and are tetramers of identical subunits. The catalytic domains of these enzymes are homologous, and recent X-ray crystallographic analyses show the active sites of the two enzymes are very similar. The hydroxyl oxygens of tyrosine 371 in tyrosine hydroxylase and of tyrosine 325 of phenylalanine hydroxylase are 5 and 4.5 A, respectively, away from the active site iron in the enzymes. To determine whether this residue has a role in the catalytic mechanism as previously suggested [Erlandsen, H., et al. (1997) Nat. Struct. Biol. 4, 995-1000], tyrosine 371 of tyrosine hydroxylase was altered to phenylalanine by site-directed mutagenesis. The Y371F protein was fully active in tyrosine hydroxylation, eliminating an essential mechanistic role for this residue. There was no change in the product distribution seen with phenylalanine or 4-methylphenylalanine as a substrate, suggesting that the reactivity of the hydroxylating intermediate was unaffected. However, the KM value for phenylalanine was decreased 10-fold in the mutant protein. These results are interpreted as an indication of greater conformational flexibility in the active site of the mutant protein.     

Carnitine supplementation accelerates normalization of food intake depressed during TPN

When total parenteral nutrition (TPN; containing glucose, fat, and amino acids; caloric ratio 50:30:20) providing 100% of the rat's daily caloric intake is given for 3-4 days, food intake rapidly decreases by approximately 85%. After stopping TPN, there is a lag period of 3-4 days before food intake returns to previous level, which appears to be related to fatty acid oxidation and fat deposition. Carnitine plays a key role in the oxidation of fatty acids, and was demonstrated to reduce fat deposition in rats receiving TPN, by increasing beta oxidation. We therefore investigated whether rats receiving TPN supplemented with carnitine may prevent either the decrease or speed up the resumption or normalization of food intake, after TPN is stopped. Fourteen adult Fischer-344 rats had a central venous catheter inserted. After 10 recovery days, controls (n = 7) were infused with TPN providing 100% of rat's daily caloric intake for 3 consecutive days, followed by 4 more days of normal saline. The carnitine group (n = 7) received the same solution, but which provided 100 mg/kg/day carnitine. Daily food intake was measured and data were analyzed using ANOVA and Student's t-test. Both parenteral solutions depressed food intake maximally by almost 90% by day 3. Carnitine accelerated the normalization of food intake by decreasing the lag period by 1 day. We conclude that the addition of carnitine enhanced the normalization of post-TPN food intake and argue that this may be on the basis of enhanced fatty acid oxidation, a substrate known to play a significant role in the anorexia induced by TPN.     

Cerebrospinal fluid amino acid levels in newborn infants with intracranial hemorrhage

Cerebrospinal fluid (CSF) amino acid levels including excitatory amino acids (i.e. glutamate and aspartate) in 25 preterm and 18 full-term newborn infants with no serious disease except intracranial hemorrhage (ICH) were measured. ICH was detected in 13 preterm and six full-term infants on the basis of the clinical, lumbar puncture (LP) and cranial ultrasonography (CraUSG) findings. Twelve preterm and 12 full-term infants who were neurologically healthy comprised the control group. The mean concentration of CSF amino acids did not differ between preterm and full-term infants. The CSF concentrations of taurine, threonine, glycine, alanine, valine, isoleucine, leucine, tyrosine and phenylalanine in preterm infants, and threonine, aspartic acid and alanine in full-term infants were significantly elevated in infants with ICH. These abnormalities, especially in preterm infants, are probably related to cerebral hypoxia in CSF amino acid concentrations in newborn infants with ICH.     

Amino acid metabolism in the piglet. 2. Influence of fasting on plasma free amino acid concentration and in vivo oxidation of methionine, isoleucine and threonine

1. The influence of a 24 h fast on the concentrations of free amino acids in the plasma, and upon the oxidation rates of methionine, isoleucine and threonine was studied (using early weaned, 4-week-old piglets which were receiving a semi-purified diet. 2. There was no change in the total concentration of the essential amino acids as a result of the 24 h fast: the concentration of the branched-chain amino acids increased, but the effect of this was offset by decreases in the concentrations of arginine, histidine, lysine, methionine and phenylalanine. There was a reduction in the concentration of the non-essential amino acids. 3. The piglets received infusions of L-[I-14C]methionine, L-[U-14C]isoleucine and L-[U-14C]-threonine, and the recovery of the label in carbon dioxide was determined. Less than 5% of the activity from methionine was recovered in the CO2 from the fed piglets, whereas 12% was recovered from the fasted piglets. The corresponding values with threonine were 11 and 19% but there was no effect of fasting on the recovery of the label from isoleucine in CO2. 4. The initial dilution of a single dose of a labelled amino acid infused into the bloodstream depends on the plasma concentration of the amino acid. Nutritional regimens may effect the free amino acid concentration in the plasma. Thus comparisons based upon direct determination of activity recovered in CO2 from the labelled dose of an amino acid with animals on different nutritional regimens could not misleading, unless the differences in the concentrations of the amino acid in the plasma are considered.     

Phenylalanine hydroxylase from Chromobacterium violaceum. Uncoupled oxidation of tetrahydropterin and the role of iron in hyroxylation

A gene encoding phenylalanine hydroxylase has been cloned from Chromobacterium violaceum and expressed in Escherichia coli. The purified phenylalanine hydroxylase contains copper, which does not support enzymatic activity. Upon removal of copper by dithiothreitol (DTT), the enzyme contains substoichiometric amounts of calcium and zinc but little or no redox-active metal ions. The copper-depleted hydroxylase catalyzes the phenylalanine-dependent oxidation of 6, 7-dimethyltetrahydropterin (DMPH4) by O2 in a reaction in which phenylalanine is not hydroxylated and does not appear to undergo a chemical change, and hydrogen peroxide is produced. Analogs of phenylalanine also activate the oxidation of DMPH4. Both the copper-phenylalanine hydroxylase and the copper-depleted hydroxylase catalyze the hydroxylation of phenylalanine in the presence of DTT and FeSO4 in a reaction in which hydrogen peroxide is not produced. The apparent values of Km for Fe2+ and DTT are 0.28 microM and 1.1 mM, respectively, at 1.0 mM phenylalanine, 120 microM DMPH4 and pH 7. 4 and 23 degreesC. The apparent value of kcat is 14.3 s-1 under these conditions. Glutathione, mercaptoethanol, and dihydrolipoate support the hydroxylation of phenylalanine essentially as well as DTT. Incubation of copper-depleted hydroxylase with FeSO4, phenylalanine, and DTT followed by gel permeation chromatography leads to an iron-hydroxylase containing approximately 1 molecule of iron per molecule of enzyme. The iron-hydroxylase displays an optical absorption band extending from 300 to 600 nm, and it catalyzes the hydroxylation of phenylalanine at the same maximum rate as the iron-activated hydroxylase but does not require added Fe2+. We conclude that iron participates in the hydroxylation of phenylalanine. Iron is not required for the oxidation of DMPH4, although it may exert a modest acceleration effect. A hypothetical mechanism is presented wherein the reaction of iron with the putative 4a-hydroperoxy-DMPH4 leads to 4a-hydroxy-DMPH4 and a high valent iron-oxy species. The iron-oxy species is postulated to react with phenylalanine in the hydroxylation process.     

L-[1-(13)C] Phenylalanine oxidation as a measure of hepatocyte functional capacity in end-stage liver disease

BACKGROUND: Liver disease is associated with impaired metabolism of these amino acids phenylalanine and tyrosine. Decreased metabolism of these amino acids leads to abnormal plasma elevations and impaired clearance rates. We have developed a noninvasive breath test that measures hepatic cytosolic enzyme activity. METHODS: The rate of hepatic phenylalanine metabolism was quantitatively calculated from the appearance of 13CO2 in the breath using the nonradioactive tracer L-[1-(13)C]phenylalanine. RESULTS: Normal controls (n = 47) oxidized phenylalanine more than twice that of end-stage liver disease patients (n = 117). Significant differences in the percent of phenylalanine oxidized per hour (mean +/- SEM) were found between controls (7.08% +/- 0.33%, 95% CI: 6.42%-7.74%) and Child Pugh classification patients, class A (4.96% +/- 0.69%, 95% CI: 3.50%-6.42%), class B (2.88% +/- 0.13, 95% CI: 2.39%-3.38%) and class C (1.75% +/- 0.13, 95% CI: 1.50%-2.01%). The phenylalanine breath test score significantly correlated with albumin levels, prothrombin time and total bilirubin. CONCLUSION: We have demonstrated that phenylalanine oxidation is significantly decreased with end-stage liver disease and is correlated with the best clinical measures of liver disease.     

Protein balance in the first week of life in ventilated neonates receiving parenteral nutrition

BACKGROUND: Protein intake is frequently delayed in ill neonates because of concerns about their ability to metabolize substrates. OBJECTIVE: We aimed to determine the factors affecting protein balance in ventilated, parenterally fed newborns during the first week of life. DESIGN: Leucine kinetic studies were performed in 19 neonates by using the [1-(13)C]leucine tracer technique after 24 h of a stable total parenteral nutrition (TPN) regimen. TPN intakes were prescribed by rotating attending physicians, enabling assessment of protein metabolism over a range of clinically used nutrient intakes. RESULTS: Mean (+/-SD) birth weight was 1.497 +/- 0.779 kg, gestational age at birth was 30.3 +/- 4.0 wk, and age at study was 3.9 +/- 1.4 d. Amino acid intakes (AAIs) ranged from 0.0 to 2.9 g x kg(-1) x d(-1). Based on leucine kinetic data, protein balance was calculated as the difference between protein synthesis and catabolism. By multiple regression analysis, AAI was the only predictor associated independently with protein balance (P < 0.01); energy intake, lipid intake, glucose intake, birth weight, and gestational age were not. Both leucine oxidation and nonoxidative leucine disposal rates were significantly correlated with leucine intake (P < 0.0005 and P < 0.01, respectively). Of the 12 infants with AAIs > 1 g x kg(-1) x d(-1), only 1 infant was significantly catabolic (protein balance <-1 g x kg(-1) x d(-1)). There was no evidence of protein intolerance as determined by elevated creatinine (69 +/- 31 micromol/L), plasma urea nitrogen (6.7 +/- 2.53 mmol/L), or metabolic acidosis (pH: 7.36 +/- 0.05). CONCLUSIONS: Ill neonates can achieve a positive protein balance in the first days of life without laboratory evidence of protein toxicity.     

Diurnal changes in plasma amino acids in maple syrup urine disease

Protein turnover is a cyclic process with a net loss of protein in the (catabolic) fasted state and a net gain in the (anabolic) fed state. In maple syrup urine disease (MSUD) the early block of degradation of the branched-chain amino acids (BCAA) brings about the opportunity for evaluation of the diurnal variation in net protein anabolism and catabolism by studying cyclic changes in the plasma concentrations of BCAA. The alterations in plasma BCAA in a 3-y-old boy with classical MSUD were studied in the fed and fasted state over a period of 19 months. For each amino acid a total of 34 data pairs was calculated. The plasma concentrations of the BCAA leucine, valine and isoleucine were constantly higher in the fasted than in the fed state. Plasma concentrations of alloisoleucine, being a non-protein amino acid, did not participate in cyclic changes. In contrast, the essential amino acid pair tyrosine and phenylalanine increased after meals. The fasting concentration of alanine increased after feeding, while glycine did not change significantly. Healthy subjects show a decrease in all amino acids in the fasted (mild catabolic) state and an increase in the fed state. These findings in MSUD suggest a net decrease in non-BCAA as result of a greater rate of amino acid oxidation rate than of protein breakdown and a net entry of BCAA into plasma in the fasted state due to the specific metabolic block. Such changes in amino acid plasma pools have to be taken into account during monitoring of treatment and especially when in vivo leucine oxidation is assessed.     

Phenylketonuria. The in vivo hydroxylation rate of phenylalanine into tyrosine is decreased

In phenylketonuria (PKU), the enzyme phenylalanine hydroxylase is deficient, resulting in a decreased conversion of phenylalanine (Phe) into tyrosine (Tyr). The severity of the disease is expressed as the tolerance for Phe at 5 yr of age. In PKU patients it is assumed that the decreased conversion of Phe into Tyr is directly correlated with the tolerance for Phe. We investigated this correlation by an in vivo stable isotope study. The in vivo residual hydroxylation was quantitated using a primed continuous infusion of L-[ring- 2H5]Phe and L-[1-13C]Tyr and the determination of the isotopic enrichments of L-[ring-2H5]Phe, L-[ring-2H4]Tyr, and L-[1-13C]Tyr in plasma. Previous reports by Thompson and coworkers (Thompson, G.N., and D. Halliday. 1990. J. Clin. Invest. 86:317-322; Thompson, G.N., J.H. Walter, J.V. Leonard, and D. Halliday. 1990. Metabolism. 39:799-807; Treacy, E., J.J. Pitt, K. Seller, G.N. Thompson, S. Ramus, and R.G.H. Cotton. 1996. J. Inherited Metab. Dis. 19:595- 602), applying the same technique, showed normal in vivo hydroxylation rates of Phe in almost all PKU patients. Therefore, our study was divided up in two parts. First, the method was re-evaluated. Second, the correlation between the in vivo hydroxylation of Phe and the tolerance for Phe was tested in seven classical PKU patients. Very low (0.13- 0.95 micromol/kg per hour) and normal (4.11 and 6.33 micromol/kg per hour) conversion rates were found in patients and controls, respectively. Performing the infusion study twice in the same patient and wash-out studies of the labels at the end of the experiment in a patient and control showed that the method is applicable in PKU patients and gives consistent data. No significant correlation was observed between the in vivo hydroxylation rates and the tolerances. The results of this study, therefore, showed that within the group of patients with classical PKU, the tolerance does not depend on the in vivo hydroxylation.     

Formation and fate of tyrosine. Intracellular partitioning of newly synthesized tyrosine in mammalian liver

Tyrosine in an hepatocyte is transported from the plasma, synthesized from phenylalanine, or released during protein turnover. Effects of phenylalanine and tyrosine on the formation and fate (partitioning) of tyrosine from the different sources were examined in primary rat hepatocyte cultures. Rates of tyrosine degradation, transport, incorporation into and release from protein, and synthesis from phenylalanine were measured as well as the intracellular dilution of labeled tyrosine and phenylalanine incorporated into protein. We found tyrosine had little effect on phenylalanine hydroxylation over a wide range of conditions, that transported tyrosine and tyrosine from phenylalanine are in different metabolic pools, and that there appears to be channeling of newly synthesized tyrosine during degradation. In addition, under some conditions, intracellular partitioning of tyrosine is determined by tyrosine concentration. Specifically, if extracellular tyrosine is low and phenylalanine is at a normal plasma level, tyrosine use in protein synthesis takes precedence over tyrosine degradation or export. It is proposed that the mechanism controlling this is kinetic, based on relative rates of tyrosyl-tRNA formation and tyrosine degradation and export. A quantitative model of tyrosine and phenylalanine in-flow and out-flow in hepatocytes is given, incorporating tyrosine synthesis, degradation, plasma membrane transport, and tyrosine and phenylalanine use and release during protein turnover.     

Second derivative spectrophotometry as an effective tool for examining phenylalanine residues in proteins

The second derivative absorption spectra of N-acetyl ethyl esters of phenylalanine, tyrosine and tryptophan, as models of the aromatic amino acid residues in proteins, were measured. The second derivative spectra of tyrosine and tryptophan were found to have no influence on the spectrum of phenylalanine over the range of 245 to 270 nm, where characteristic absorbance bands of phenylalanine were observed. Thus the second derivative spectrum is a good tool for examining the optical properties of phenylalanine residues in proteins.     

Nitrogen balance, 3-methylhistidine excretion, and plasma amino acid profile in infants after cardiac operations for congenital heart defects: the effect of early nutritional support

OBJECTIVE: The objective of this study was to evaluate the effect of nutritional support on proteolysis and plasma amino acid profile in infants early after cardiac operations for congenital heart defects. METHODS: Thirty-seven patients, 2 to 12 months old, were randomized on postoperative day 1 for 24-hour isocaloric metabolic study. Group STANDARD (18 patients) received glucose as the maintenance fluid, and group PN (19 patients) received glucose and crystalloid amino acid solution at a dosage of 0.8 +/- 0.1 gm/kg per day. The nonprotein caloric intake in the two groups was 25 +/- 15 and 33 +/- 9 kcal/kg, respectively (p = not significant). RESULTS: The nitrogen balance was markedly less negative in group PN than in group STANDARD (-114 +/- 81 vs -244 +/- 86 mg/kg, respectively, p = 0.001). There was a highly significant inverse correlation between the nitrogen balance and urinary 3-methylhistidine excretion in both groups, but the muscle proteolysis was blunted more effectively in patients receiving amino acids. Concentrations of the plasmatic branched-chain amino acids, alanine, glycine, and proline, decreased significantly in group STANDARD but not in group PN on postoperative day 2. Glutamine and threonine levels declined significantly on postoperative day 2 in both groups. Low levels of arginine were observed in our patients before operation and in the early postoperative period. The amino acid concentrations normalized on postoperative day 7 in all patients. CONCLUSION: Significant proteolysis and hypoaminoacidemia were observed in infants early after cardiac operations. This hypercatabolic response was blunted by parenteral nutritional support.     

Membrane transport of aromatic amino acids by Hymenolepis diminuta (Cestoda)

Aromatic amino acids (phenylalanine, tryptophan and tyrosine) are absorbed by Hymenolepis diminuta through a combination of mediated (non-Na+-sensitive) transport and diffusion. All 3 amino acids are accumulated against an apparent concentration difference during a 30-min incubation of tapeworms in 0.1 mM 3H-labelled amino acid. Inhibitor studies demonstrate that phenylalanine, tryptophan and tyrosine are mutually competitive inhibitors of the uptake of each other, and the uptake of these amino acids is inhibited by aliphatic amino acids but not by basic or dicarboxylic amino acids. The D- and L-isomers of aromatic amino acids are equally effective in inhibiting aromatic amino acid uptake. The data indicate that at least 3 amino acid transport loci are involved in aromatic amino acid transport by H. diminuta.     

Plasma insulin and growth hormone concentrations in pregnant sheep II: post-absorptive levels in mid- and late pregnancy

1. The incorporation of L-[U-14C]leucine, L[U-14C]histidine and L-[U-14C]phenylalanine into casein secreted during perfusion of isolated guinea-pig mammary glands was demonstrated. 2. The extent of incorporation of label into casein residues was consistent with their being derived from free amino acids of the perfusate plasma. 3. The mean transit time of the amino acids from perfusate into secreted casein was approx. 100 min. 4. Whereas radioactive histidine and phenylalanine were incorporated solely into milk protein, radioactivity from [U-14C]valine was also transferred to CO2 and to an unidentified plasma component, and from [U-14C]leucine to plasma glutamic acid. 5. Evidence from experiments with [U-14C]phenylalanine suggests that, as in rats, but in contrast with ruminant species, guinea-pig mammary tissue does not possess phenyl alanine hydroxylase activity. 6. The results are discussed in relation to the possible role of essential amino acid catabolism in the control of milk-protein synthesis.     

Protection against influenza after annually repeated vaccination: a meta-analysis of serologic and field studies

We report the effects of a tyrosine (and phenylalanine)-free amino acid mixture on tyrosine levels, ex vivo catecholamine synthesis and in vivo catecholamine release in brain regions of the rat. Administration of a tyrosine-free amino acid load reduced tissue levels of tyrosine (-50% after 2 h) in all brain regions examined (frontal cortex, hippocampus, striatum). The tyrosine-free amino acid mixture also reduced DOPA accumulation: this effect was most marked in striatum (-44%) and nucleus accumbens (-34%), areas with a predominantly dopaminergic innervation. Smaller decreases (-20-24%) were detected in other areas (cortex, hippocampus and hypothalamus). The effect on DOPA accumulation was prevented by supplementing the mixture with tyrosine/phenylalanine. The tyrosine-free amino acid mixture did not alter 5-HTP accumulation in any region. In microdialysis experiments, the tyrosine-free amino acid mixture did not consistently alter striatal extracellular dopamine under basal conditions but markedly, and dose-dependently, reduced the release of dopamine induced by amphetamine. In contrast, the tyrosine-free amino acid mixture did not alter either basal or amphetamine-evoked release of noradrenaline in hippocampus. Overall, these studies indicate that administration of a tyrosine-free amino acid mixture to rats depletes brain tyrosine to cause a decrease in regional brain catecholamine synthesis and release. Dopaminergic neurones appear to be more vulnerable to tyrosine depletion than noradrenergic neurones.