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Nutrient Metabolism

Aromatic Amino Acids Are Utilized and Protein Synthesis Is Stimulated during Amino Acid Infusion in the Ovine Fetus¹

Herman B. Wells Center for Pediatric Research, Department of Pediatrics, Indiana University School of Medicine, Indianapolis, IN 46202

²To whom correspondence should be addressed.

	ABSTRACT

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The purpose of this study was to determine whether the ovine fetus is capable of increased disposal of an amino acid load; if so, would it respond by increased protein synthesis, amino acid catabolism or both? A further purpose of the study was to determine whether the pathways of aromatic amino acid catabolism are functional in the fetus. Late gestation ovine fetuses of well-nourished ewes received an infusion of Aminosyn PF alone (APF), and Aminosyn PF + glycyl-L-tyrosine (APF+GT) at rates estimated to double the intake of these amino acids. The initial study, using APF, was performed at 126 ± 1.4 d; the APF+GT study was performed at 132 ± 1.7 d (term = 150 d). Phenylalanine and tyrosine kinetics were determined using both stable and radioactive isotopes. Plasma concentrations of most amino acids, but not tyrosine, increased during both studies; tyrosine concentration increased only during the APF+GT study. Phenylalanine rate of appearance and phenylalanine hydroxylation increased during both studies. Tyrosine rate of appearance increased only during the APF+GT study; tyrosine oxidation did not increase during either study. Fetal protein synthesis increased significantly during both studies, producing a significant increase in fetal protein accretion. Fetal proteolysis was unchanged in response to either amino acid infusion. These results indicate that the fetus responds to an acute increase in amino acid supply primarily by increasing protein synthesis and accretion, with a smaller but significant increase in amino acid catabolism also. Both phenylalanine hydroxylation and tyrosine oxidation are active in the fetus, and the fetus is able to increase phenylalanine hydroxylation rapidly in response to increased supply.

KEY WORDS: • phenylalanine hydroxylation • stable isotopes • amino acid metabolism • mass spectrometry • sheep

	INTRODUCTION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Infusion of amino acids leads to increases in disposal by increased protein synthesis and amino acid catabolism. This has been shown in animals (Watt et al. 1992 ), adult humans Giordano and Castellino 1997 , Jurasinski et al. 1995 , Volpi et al. 1998 ), and premature infants (Denne et al. 1996 ). Most studies, however, have taken place during the postabsorptive state, when the only source of circulating amino acids is from proteolysis. The mammalian fetus, however, receives a continuous supply of amino acids via the umbilical circulation during gestation. The uptake of most amino acids exceeds accretion requirements by a large degree (Lemons et al. 1976 ). It is unknown whether protein synthesis and accretion can be further augmented by the provision of excess amino acids, or whether the fetus would be able to increase catabolic disposal of such amino acids.

Phenylalanine metabolism is poorly understood in the mammalian fetus and newborn. These infants have been thought to have limited capacity for phenylalanine hydroxylation and limited capacity for tyrosine oxidative disposal. Premature human newborns receiving parenteral nutrition often exhibit elevated phenylalanine plasma concentrations; tyrosine concentrations are variable but often are elevated also (Polberger et al. 1990 , Walker and Mills 1990 ). On the basis of both plasma concentration data and in vitro tissue analysis (Delvalle and Greengard 1977 , McLean et al. 1973 , Raiha and Schwartz 1973 ), it has been suggested that the specific enzymes responsible for phenylalanine conversion to tyrosine and for subsequent tyrosine oxidation may be developmentally regulated, with activity gradually increasing during the final trimester of gestation and early neonatal period. Adult activities are reached in the postnatal period. These findings have led to the speculation that tyrosine may be conditionally essential in the newborn, especially if born prematurely (Snyderman 1971 ).

Our data, as well as data from other investigators (Kilani et al. 1995 ) from premature human infants suggest that adequate conversion of phenylalanine to tyrosine occurs in these infants (Denne et al. 1994 ). There is some evidence, however, that the birth process may result in enzyme induction (Delvalle and Greengard 1977 ). It is important therefore to understand the metabolism of phenylalanine and tyrosine in the late gestation fetus, which has not experienced the stress of premature birth. We hypothesized that the late gestation sheep fetus would exhibit substantial phenylalanine hydroxylation.

In addition, immaturity of tyrosine transaminase may lead to hypertyrosinemia when phenylalanine is supplied to the newborn in excessive amounts. We hypothesized that the fetus would be able to utilize the glycyl-L-tyrosine and that this would result in the sparing of phenylalanine catabolism, thereby increasing protein synthesis.

This study had three aims as follows: 1) to determine whether acute provision of excess amino acids would result in increases in protein synthesis, reduction in proteolysis and amino acid accretion; 2) to determine whether significant rates of phenylalanine hydroxylation occur in the fetus, and if such hydroxylation increases with excess phenylalanine supply; 3) to determine whether the fetus is able to oxidize tyrosine and whether such oxidation increases during infusion of excess tyrosine.

To accomplish these aims, the late gestation ovine fetus was infused with Aminosyn PF a commercially available source of amino acids commonly used in neonatal intensive care units. Fetal and maternal phenylalanine kinetics were determined before and during the amino acid infusion. A second study, identical except for the addition of glycyl-L-tyrosine, was performed several days later.

	MATERIALS AND METHODS

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Animals and surgical procedures.

Nine ewes (Purdue University Sheep Production Facility, Lafayette, IN) of 115–120 d gestation were used for this study. Animal care was in strict compliance with National Institutes of Health guidelines, and the study protocols were approved by the Animal Care Review Committee.

Surgical procedures were performed aseptically under general anesthesia. Anesthesia was accomplished by intravenous ketamine for induction and isoflurane inhalation for maintenance. Catheters were placed in the common umbilical vein, fetal descending aorta and inferior vena cava, and maternal descending aorta and inferior vena cava. Catheters were exteriorized and irrigated daily with 9 g/L saline, containing 3000 units heparin/L. All ewes consumed ad libitum a diet consisting of hay and pelletized alfalfa and had constant access to water and salt lick.

Study design.

The ewes were allowed to recover from surgery for a minimum of 5 d and were feeding ad libitum before the initial studies were performed. Access to feed was continued throughout the performance of the fed study. On the day of the study, preinfusion samples were obtained for isotopic enrichment of phenylalanine and tyrosine. An infusion of L-[1-¹³C]phenylalanine and L-[²H₂]tyrosine (Tracer Technologies, Sommerville, MA) was begun into the fetal inferior vena cava. A priming dose consisting of 60 min infusate was given as a bolus. The rate of tracer administration was 0.8 and 0.15 µmol/min for Phe and Tyr, respectively. An infusion of L-[¹⁴C]-tyrosine (10.833 kBq) was also begun, specifically to determine the rate of tyrosine oxidation. L- [ring d₅]-Phenylalanine was infused into the maternal inferior vena cava at 4.2 µmol/min. Ethanol was simultaneously infused at 420 µg/min for determination of umbilical blood flow. After a 120-min equilibration period, four sets of samples were obtained at 20-min intervals. An infusion of Aminosyn PF (APF³ ; Abbott Laboratories, Abbott Park, IL) was then begun into the fetal inferior vena cava (APF study). The rate of infusion of each amino acid is provided in Table 1. After 120 min, an additional set of four samples was obtained. The ewe was then returned to housing and continued ad libitum food intake. A second study was performed 5–7 d later. This study was identical to the initial study with the exception that glycyl-L-tyrosine was infused at a rate of 85.7 mg/h (APF+GT study).

Whole-blood amino acid and tracer infusate phenylalanine and tyrosine concentrations were determined by standard ion exchange chromatography methodology, utilizing a Beckman 6300 automated Amino Acid Analyzer (Beckman Coulter, Fullerton, CA) The intra-assay coefficient of variation was <3%.

Phenylalanine and tyrosine isotopic enrichments were determined in plasma after derivatization to their tertiary butyl dimethyl silyl derivatives (Schwenk et al. 1984 ). These derivatives were injected into a Hewlett-Packard 5970 GC/MS (Hewlett-Packard, Palo Alto, CA) with electron impact ionization and selected ion monitoring of ions 234 and 239 (d₅-phenylalanine), 366 and 367 (¹³C-Phe), 466, 467 (¹³C-tyrosine) and 468 (²H-tyrosine). Tracer:tracee ratios were calculated by the method of Rosenblatt and Wolfe (1988) .

To determine ¹⁴CO₂ content, ~0.3 mL whole blood was injected into the center well of a previously weighed scintillation vial (BioRad, Melville, NY) containing 1.0 mL Solvable (New England Nuclear, Boston, MA). The actual amount of blood injected was then determined gravimetrically; 0.5 mL of 1.0 mol/L HCl was added to the center well to liberate the CO₂, which was trapped in the Solvable. After overnight incubation, the center well was removed, 15 mL Econofluor was added to the vial and the sample radioactivity was counted. Counts per sample were converted to becquerels, and the sample becquerels per gram whole blood were converted to becquerels per liter whole blood by using the density of whole blood. The intraassay coefficient of variation was <5%.

Calculations.

Umbilical blood flow was calculated by the steady-state ethanol diffusion method (Meschia et al. 1966 ). The net fluxes of phenylalanine and tyrosine from placenta to the fetus were calculated as the product of the umbilical arteriovenous concentration difference and the umbilical blood flow.

Phenylalanine and tyrosine kinetics, including phenylalanine conversion to tyrosine, were estimated by the method of Clark and Bier (1982) , modified for the fetus as we have previously described (Liechty et al. 1996 ).

The flux of maternal phenylalanine from the maternal plasma to fetal plasma was estimated by dilution of the maternal tracer in fetal plasma, according to the following equation:

where IE_f^m is the isotopic enrichment of maternal tracer (d₅-Phe) in fetal plasma; IE_f^f is the isotopic enrichment of fetal tracer (1-¹³C- Phe) in fetal plasma; Q^f is the rate of appearance of phenylalanine in fetal plasma; Q^m is the rate of appearance of phenylalanine in maternal plasma; and IR^m is the rate of infusion of maternal tracer.

This estimate assumes that there is no utilization of maternally derived phenylalanine by the placenta. Therefore it will overestimate F_fp^Phe to the extent that maternal tracer is utilized by the placenta for protein synthesis.

The total rate of disappearance is partitioned into flux rates from the fetus to the placenta, protein synthesis and irreversible fetal disposal. For phenylalanine, this latter flux represents the conversion of phenylalanine to tyrosine via irreversible hydroxylation.

Endogenous phenylalanine appearance (appearance from protein breakdown) was calculated as follows:

Phenylalanine for protein synthesis was calculated as follows:

These rates were converted to daily rates of protein synthesis and breakdown using a conversion factor of 211 µmol Phe/g protein (Meier et al. 1981b ). Protein accretion was estimated as the difference between PS and PB.

Tyrosine oxidation was calculated as follows:

where IR ¹⁴C-Tyr is the infusion rate of ¹⁴C-Tyr (Bq/min) (Van Veen et al. 1984 ). The maternal phenylalanine rate of appearance was used only to estimate F_fp^Phe. It was not a primary outcome parameter; therefore the data are not reported in the results.

Statistics.

Data are presented as means ± SEM. Three-way ANOVA was used for data analysis. The experimental factors were the amino acid mixture (APF vs. APF+GT) and basal vs. infused; interanimal variation was controlled for as a random factor. A priori hypotheses were tested by three-way ANOVA and Student-Neuman-Keuls multiple comparison technique, with P < 0.05 as the level of significant difference (JMP, SAS Institute, Cary, NC).Comparisons of two outcome variables were made by paired t test.

Data were not normalized to fetal weight because the fetal weight at the time of the APF studies was unknown.

	RESULTS

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The initial study, using Aminosyn PF was performed at 126 ± 1.4 d; the second study was performed at 132 ± 1.7 d, and 4.9 ± 0.5 d elapsed between the two studies. Fetal weight at the completion of the second study was 3780 ± 330 g.

Concentrations of phenylalanine, leucine, isoleucine and valine increased significantly during the infusion in both studies (Table 2). Methionine, threonine, glycine, alanine and glutamine increased during the APF study only. Tyrosine concentration increased during the APF+GT study only. In general, the concentrations achieved were less during the APF+GT study than during the APF study.

Table 3

Table 4

View larger version (36K): [in this window] [in a new window]   Figure 1. The rates of protein synthesis, protein breakdown, and accretion estimated from phenylalanine kinetics, in late gestation ovine fetuses infused with Aminosyn PF alone (APF) and Aminosyn PF + glycyl-L-tyrosine (APF + GT). The phenylalanine content of fetal protein was assumed to be 211 µmol/g of protein (Meier et al. 1981a ). Data are means ± SEM, n = 9. *P < 0.05, basal vs. infused.

View larger version (15K): [in this window] [in a new window]   Figure 2. The correlation between tyrosine rate of appearance and tyrosine oxidation (upper panel) and phenylalanine rate of appearance and phenylalanine hydroxylation (lower panel) in late gestation ovine fetuses infused with Aminosyn PF alone (APF) and Aminosyn PF + glycyl-L-tyrosine (APF + GT). Data were analyzed by linear regression.

	DISCUSSION

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

This study indicates that the fetus is capable of increasing amino acid disposal significantly in response to an acute increase in amino acid supply. It does so by an increase in amino acid catabolism and a large increase in protein synthesis. However, there is no change in proteolysis. Thus, both the ovine fetus and premature infants are apparently resistant to the antiproteolytic effects of exogenous amino acids (Denne et al. 1996 ). In contrast, full-term human infants reduce proteolysis in a dose-responsive fashion in response to amino acid infusion (Poindexter et al. 1997 ).

The metabolisms of phenylalanine and tyrosine are particularly important to study in this model and in neonates because the enzymes that catalyze the conversions may be developmentally regulated. In addition, congenital absence of the genes encoding these enzymes is the basis for several important disorders of amino acid metabolism, such as phenylketonuria. We carried out these experiments to verify that phenylalanine and tyrosine disposal, by hydroxylation and oxidation, respectively, occurs at physiologically important rates in response to an excess supply of either amino acid. This was done by first infusing Aminosyn PF to the fetus. This is an amino acid mixture commonly used in neonatal intensive care units. At the rate used, it supplied phenylalanine at 3.88 µmol/min, thereby approximately doubling the fetal intake of phenylalanine. However, it supplied only a negligible amount of tyrosine. During the second study, the Aminosyn PF was supplemented with glycyl-L-tyrosine, supplying 3.9 µmol Tyr/min. This also approximately doubled the fetal tyrosine intake.

Our data support the hypothesis that the fetus possesses adequate enzymatic capacity to dispose of both phenylalanine and tyrosine. During the infusion of Aminosyn PF, which has little tyrosine, phenylalanine Ra and hydroxylation increased significantly. The relative increase was similar for Ra and hydroxylation, and therefore the change in the percentage of total phenylalanine that underwent hydroxylation was not significantly altered. The percentage of total tyrosine Ra that was derived from phenylalanine increased significantly. Neither tyrosine concentration nor tyrosine Ra was significantly altered during Aminosyn PF infusion, in spite of the significant increase in phenylalanine hydroxylation. However, this study had only 20% power to detect a change as small as 0.5 µmol/min in Tyr Ra, which was the degree to which phenylalanine hydroxylation increased.

When glycyl-L-tyrosine was added to the amino acid infusion, the changes in phenylalanine kinetics were similar to those during the APF study. Contrary to findings in adult humans, there was no correlation between the rate of phenylalanine hydroxylation and phenylalanine concentration. Rather, it was correlated with phenylalanine supply, estimated as Ra. However, supplementation with either Aminosyn PF or Aminosyn PF + glycyl-L-tyrosine did result in a two- to threefold increase in estimated phenylalanine accretion.

Conflicting evidence exists regarding the fetal capacity for phenylalanine hydroxylation. Studies of enzyme activity in rat and human fetuses have suggested a developmental regulation of phenylalanine hydroxylase activity, with late fetal activities <50% of adult activities (Delvalle and Greengard 1977 ). However, other investigators, studying human fetal liver, have determined that the hydroxylase activity is present at adult activity levels by 14–20 wk (Bessman et al. 1977 , Räihä 1973 ). Although the evidence is mixed, the preponderance of studies suggests that hydroxylation likely occurs in the mammalian fetus.

It is clear from our data that substantial conversion of phenylalanine to tyrosine occurs in the ovine fetus. Under normal conditions, ~10% of total phenylalanine turnover is accounted for by conversion to tyrosine; 10–15% of total tyrosine appearance results from phenylalanine hydroxylation. These data are consistent with conversion percentages found in postnatal animals and humans. Nearly 50% of the flux of phenylalanine from the ewe to the fetus is converted to tyrosine. Absolute rates of phenylalanine conversion ranged from 0.61 to 1.31 µmol/min, with a mean of 0.5 µmol/min. These conversion rates are much greater than the 0.1 µmol/(kg · min) reported by Clark and Bier (1982) for adult humans. Thus, our data are consistent with the suggestion that phenylalanine hydroxylase is fully active by late gestation. This is not surprising in that net phenylalanine uptake has been determined to be significantly greater than phenylalanine accretion in several previous studies (Lemons et al. 1976 , Meier et al. 1981b ). On the basis of Meier's data, phenylalanine accretion accounts for 0.54 µmol/min compared with a net umbilical uptake of 2.8 µmol/min. Excess phenylalanine would by necessity be converted to tyrosine because the predominant pathway for phenylalanine degradation involves hydroxylation as a first step.

Tyrosine metabolism was altered by the addition of glycyl-L-tyrosine. A significant increase in tyrosine concentration was observed, as well as an increase in tyrosine Ra. Neither of these parameters was altered by infusion of Aminosyn PF alone. We did not directly determine the contribution of the tyrosine supplied as dipeptide to overall tyrosine turnover. However, taken together, these findings suggest that the tyrosine in the dipeptide is bioavailable to the fetus.

The fetus also is also able to dispose of excess tyrosine adequately. Tyrosine concentrations were unchanged during the Aminosyn PF study and increased by only 30%, to 119 ± 10 µmol/L, during the APF+GT study. This is within the range of tyrosine concentrations found in other investigations in unstressed, unperturbed fetal lambs (Lemons et al. 1976 , Lemons and Schreiner 1983 ). This is a smaller increase in concentration than was observed for several other amino acids. Tyrosine oxidation accounted for 10–20% of total tyrosine Ra and was correlated with Tyr Ra, but not tyrosine concentration.

It is interesting that the amino acid concentrations were in general greater during the APF study compared with during the APF+GT study. There are two possibilities that might explain this difference. The first possibility is that tyrosine is, in fact, conditionally essential in the fetus. Under this hypothesis, tyrosine is limiting for maximal rates of protein synthesis if it is not supplied as part of an amino acid infusion. If tyrosine is supplied, all amino acids are used more efficiently, and hence the plasma concentrations do not rise to the same degree as when tyrosine is not supplied. This may also explain why tyrosine oxidation did not increase significantly during tyrosine supplementation. However, there were no differences in rates of protein synthesis during the two study periods; therefore, this hypothesis remains speculative. An alternative possibility is that fetal growth took place between the two studies. Therefore, a lesser amount of amino acids per kilogram was infused during the APF+GT study, accounting for the difference in plasma concentrations.

In summary, this study demonstrates that phenylalanine conversion to tyrosine represents a large proportion of phenylalanine utilization and tyrosine production in the late gestation ovine fetus. The fetus also is able to dispose of an increase in tyrosine load with only minimal elevation of arterial tyrosine concentration. Supplementation of the fetus with amino acids also increases protein synthesis and protein accretion and may be a feasible method for nutritional supplementation of the growth-retarded fetus.

	FOOTNOTES

 
¹ Supported by PHS grants RO1-HD19089, PH60-DK-20542, and the James Whitcomb Riley Memorial Association.

³ Abbreviations used: APF, Aminosyn PF; APF+GT, Aminosyn PF + L-glycyl-tyrosine; Ra, rate of appearance; F_fp^Phe, flux of phenylalanine to the fetus from the placenta; IE_f^m, isotopic enrichment of maternal tracer (d₅ Phe) in fetal plasma; IE_f^f, isotopic enrichment of fetal tracer (1-¹³C- Phe) in fetal plasma; Q^f, rate of appearance of phenylalanine in fetal plasma Q^m, rate of appearance of phenylalanine in maternal plasma; IR^m, rate of infusion of maternal tracer; PB _phe, rate of phenylalanine appearance from protein breakdown; PS _phe, rate of phenylalanine use for protein synthesis.

Manuscript received October 5, 1998. Initial review completed November 27, 1998. Revision accepted February 18, 1999.

	REFERENCES

TOP ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 

1. Bessman S. P., Wapnir R. A., Towell M. E. Development of liver phenylalanine hydroxylase and brain aromatic hydroxylases in human fetuses. Biochem. Med. 1977;17:1-7[Medline]

2. Clarke J. T., Bier D. M. The conversion of phenylalanine to tyrosine in man. Direct measurement by continuous intravenous tracer infusions of L-[ring-²H₅]phenylalanine and L-[1-¹³C] tyrosine in the postabsorptive state. Metabolism 1982;31:999-1005[Medline]

3. Delvalle J. A., Greengard O. Phenylalanine hydroxylase and tyrosine aminotransferase in human fetal and adult liver. Pediatr. Res. 1977;11:2-5[Medline]

4. Denne S. C., Karn C. A., Ahlrichs J., Dorotheo A. R., Wang J., Liechty E. A. Leucine and phenylalanine kinetics in response to parenteral nutrition in extremely premature infants. Pediatr. Res. 1994;35:310A(abs.)

5. Denne S. C., Karn C. A., Ahlrichs J. A., Dorotheo A. R., Wang J. Y., Liechty E. A. Proteolysis and phenylalanine hydroxylation in response to parenteral nutrition in extremely premature and normal newborns. J. Clin. Investig. 1996;97:746-754[Medline]

6. Giordano M., Castellino P. Correlation between amino acid induced changes in energy expenditure and protein metabolism in humans. Nutrition 1997;13:309-312[Medline]

7. Jurasinski C., Gray K., Vary T. C. Modulation of skeletal muscle protein synthesis by amino acids and insulin during sepsis. Metabolism 1995;44:1130-1138[Medline]

8. Kilani R. A., Cole F. S., Bier D. M. Phenylalanine hydroxylase activity in preterm infants: is tyrosine a conditionally essential amino acid?. Am. J. Clin. Nutr. 1995;61:1218-1223[Abstract/Free Full Text]

9. Lemons J. A., Adcock E. W., Jones M., Naughton M., Meschia G., Battaglia F. C. Umbilical uptake of amino acids in the unstressed fetal lamb. J. Clin. Investig. 1976;58:1428-1434

10. Lemons J. A., Schreiner R. L. Amino acid metabolism in the ovine fetus. Am. J. Physiol. 1983;244:E459-E466[Abstract/Free Full Text]

11. Liechty E. A., Boyle D. W., Lee W.-H., Bowsher R. R., Denne S. C. Effects of circulating IGF-I on glucose and amino acid kinetics in the ovine fetus. Am. J. Physiol. (Endocrinol. Metab.) 1996;271:E177-E185[Abstract/Free Full Text]

12. McLean A., Marwick M. J., Clayton B. E. Enzymes involved in phenylalanine metabolism in the human foetus and child. J. Clin. Pathol. 1973;26:678-683[Abstract/Free Full Text]

13. Meier P., Peterson R., Bonds D., Meschia G., Battaglia F. C. Rates of protein synthesis and turnover in fetal life. Am. J. Physiol. (Endocrinol. Metab.) 1981;240:E320-E324[Abstract/Free Full Text]

14. Meier P., Teng C., Battaglia F. C., Meschia G. The rate of amino acid nitrogen and total nitrogen accumulation in the fetal lamb. Proc. Soc. Exp. Biol. Med. 1981;167:463-468[Medline]

15. Meschia G., Cotter J. R., Makowski E. L., Barron D. H. Simultaneous measurement of uterine and umbilical blood flows and oxygen uptakes. Q. J. Exp. Physiol. 1966;52:1-18

16. Poindexter B. B., Karn C. A., Ahlrichs J. A., Wang J., Leitch C. A., Liechty E. A., Denne S. C. Amino acids suppress proteolysis independent of insulin throughout the neonatal period. Am. J. Physiol. (Endocrinol. Metab.) 1997;272:E592-E599[Abstract/Free Full Text]

17. Polberger S.K.T, Axelsson I. E., Räihä N.C.R. Amino acid concentrations in plasma and urine in very low birth weight infants fed protein-unenriched or human mild protein enriched human milk. Pediatrics 1990;86:909-915[Abstract/Free Full Text]

18. Räihä N. C. Phenylalanine hydoxylase in human liver during development. Pediatr. Res. 1973;7:1-4

19. Räihä N.C.R., Schwartz A. L. Enzyme induction in human fetal liver in organ culture. Enzyme 1973;15:330-339[Medline]

20. Rosenblatt J., Wolfe R. R. Calculation of substrate flux using stable isotopes. Am. J. Physiol. (Endocrinol. Metab.) 1988;254:E526-E531[Abstract/Free Full Text]

21. Schwenk W. F., Berg P. J., Beaufrère B., Miles J. M., Haymond M. W. Use of t-butyldimethylsilylation in the gas chromatographic/mass spectrometric analysis of physiologic compounds found in plasma using electron-impact ionization. Anal. Biochem. 1984;141:101-109[Medline]

22. Snyderman S. E. The protein and amino acid requirements of the premature infant. Jonxis J.H.P. Visser H.K.A. Troelstra J. A. eds. Metabolic Processes in the Fetus and Newborn Infant 1971:128-141 Stenfert Kroese Leiden, The Netherlands.

23. Van Veen L. C., Hay W. W., Jr, Battaglia F. C., Meschia G. Fetal CO₂ kinetics. J. Dev. Physiol. 1984;6:359-365[Medline]

24. Volpi E., Ferrando A. A., Yeckel C. W., Tipton K. D., Wolfe R. R. Exogenous amino acids stimulate net muscle protein synthesis in the elderly. J. Clin. Investig. 1998;101:2000-2007[Medline]

25. Walker V., Mills G. A. Metabolism of intravenous phenylalanine by babies born before 33 weeks of gestation. Biol. Neonate 1990;57:155-166[Medline]

26. Watt P. W., Corbett M. E., Rennie M. J. Stimulation of protein synthesis in pig skeletal muscle by infusion of amino acids during constant insulin availability. Am. J. Physiol. (Endocrinol. Metab.) 1992;263:E453-E460[Abstract/Free Full Text]

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