Department of Clinical and Experimental Medicine, University of Padova, 35128 Padua, Italy
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ABSTRACT |
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Whether phenylalanine-tyrosine (Phe-Tyr)
tracers yield estimates of postprandial protein synthesis comparable to
those of the widely used leucine (Leu) tracer is unclear. We measured
Leu oxidation (Ox), Phe hydroxylation (Hy), and their disposal into whole body protein synthesis before and after the administration of a
mixed meal (62 kJ/kg body wt, 22% of energy as protein), over 4 h
in healthy subjects. Both plasma and intracellular precursor pools were
used. The amino acid data were extrapolated to body protein by assuming
a fixed ratio of Leu to Phe in the proteins. In the postabsorptive
state, whole body protein synthesis (expressed as
mg · kg1 · min
1)
was similar between Leu and Phe-Tyr tracers irrespective of the
precursor pool used. After the meal, Leu Ox, Phe Hy, and body protein
synthesis increased (P
0.01 vs. basal). With the use of
intracellular precursor pools, the increase of protein synthesis with
Phe-Tyr (+0.51 ±0.21
mg · kg
1 · min
1)
and Leu tracers (+0.57 ± 0.14) were similar (P = not significant). In contrast, with plasma pools the increase of
protein synthesis was more than twofold greater with Phe-Tyr
(+1.17 ± 0.19 mg · kg
1 · min
1)
than that with Leu (0.50 ± 0.13 mg · kg
1 · min
1,
P < 0.01). Direct correlations were found between Leu
and Ox [using both plasma and intracellular pools (r
0.65, P
0.01)] but not between Phe and either plasma or
intracellular Hy. In conclusion, 1) Phe-Tyr and Leu tracers
yield comparable estimates of body protein synthesis postprandially,
provided that intracellular precursor pools are used; 2)
both Leu Ox and Phe Hy are stimulated by a mixed meal; 3)
Phe does not correlate with Hy, which might be better related to the
(unknown) portal Phe.
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INTRODUCTION |
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A PRECISE ESTIMATE of postprandial whole body protein synthesis is essential to determine the anabolic effects of meal ingestion, i.e., the key physiological stimulus of net protein deposition in body tissues (13, 14, 20, 27). By use of amino acid tracer techniques, whole body protein synthesis is commonly calculated from the portion of amino acid flux (e.g., disposal) that is not irreversibly catabolized and therefore must be incorporated into the proteins (3, 27, 31). Protein synthesis is stimulated by a meal of adequate energy and protein content (13, 18, 27). An index of such an adequacy is the stimulation of postprandial amino acid catabolism (33). Therefore, the accurate measurement of both the flux and the catabolism of indispensable amino acid(s) is essential to estimate whole body protein synthesis precisely.
Leucine and phenylalanine tracers are extensively used for this purpose (3, 13, 14, 20, 27, 31). The first step of leucine irreversible catabolism is oxidation (3, 24, 31); that of phenylalanine is hydroxylation to tyrosine (7, 21). The in vivo measurement of leucine oxidation (Leu Ox) requires analyses in both plasma and expired air, whereas phenylalanine hydroxylation (Phe Hy) is measured after the combined infusion of phenylalanine and tyrosine tracers, with analyses only in plasma (7). Therefore, the latter method is attractive because it requires a simpler sample collection.
Whether these two isotopic amino acid methods are equivalent as regards the measurements of changes in protein synthesis and amino acid catabolism after a mixed meal is unclear. In previous studies (9, 18), Leu Ox was stimulated postprandially, whereas Phe Hy was not. However, neither plasma amino acid concentrations nor meal composition was provided; therefore, the tested meal might not have been a sufficient stimulus for hydroxylation. In addition, although protein synthesis was enhanced to a similar extent with both tracers by use of adjusted intracellular precursor pools (18), the impact of the choice of the precursor pools on amino acid kinetics was not thoroughly evaluated.
Therefore, the aims of the present study were the following: 1) to measure whole body protein synthesis and amino acid catabolism in healthy volunteers by use of leucine and phenylalanine-1-tyrosine tracers simultaneously both before and after the ingestion of a mixed meal of generous energy and protein content; 2) to evaluate the impact on the kinetic measurements of the choice of the precursor pool(s); and 3) to look for the relationships between leucine and phenylalanine concentrations, catabolism, and protein synthesis.
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METHODS |
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Subjects.
Eight healthy male subjects (age 34 ± 6 yr; body mass index
23.7 ± 1.2 kg/m2), metabolically stable and adapted
to a standard weight-maintaining diet, were recruited. They were
informed about the aims of the study and signed their consent to it.
The protocol was approved by the Ethics Committee of the Medical
Faculty at the University of Padova, Italy, and it was performed
according to the Helsinki Declaration (as revised in 1983)
(32) as well as following the recommendations of the local
Radiation Safety Officer. Whole body radiation exposure due to the
[14C]leucine isotope was estimated to be 1 mGy
(8).
Isotopes.
L-[1-14C]leucine {[14C]Leu;
specific activity (SA) 2 GBq/mmol} and sodium
[14C]bicarbonate (SA
2 GBq/mmol) were purchased from
Amersham (Buckinghamshire, UK).
L-[ring-2H5]phenylalanine
([2H5]Phe) and
L-[2H2]tyrosine
([2H2]Tyr) were purchased from
MassTrace (Woburn, MA).
L-[ring-2H4]tyrosine
([2H4]Tyr) was obtained from Eurisotop
(Gif-Sur-Yvette, France). The stable isotopes were >99% mole percent
enriched. All tracers were dissolved in sterile saline and proven to be
sterile and pyrogen free before use.
Experimental design.
The study was conducted as previously described in detail
(27). Briefly, a polyethylene catheter was placed
percutaneously in retrograde fashion into a superficial vein of one
arm, which was kept at +60°C in a heated box for arterialized venous
blood sampling. Another catheter was placed into an antecubital vein of
the opposite arm for isotope infusions. At 240 min, continuous infusions of [14C]Leu (5,841 ± 407 dpm · kg
1 · min
1),
[2H5]Phe (0.0503 ± 0.0016 mg · kg
1 · min
1),
and [2H2]Tyr (0.0214 ± 0.0009 mg · kg
1 · min
1)
were started by means of calibrated pumps. Priming doses of each
isotope (equivalent to 60× continuous infusion rate/min), as well as
priming doses of [2H4]Tyr (0.08 mg/kg) and of
[14C]bicarbonate (3 µCi), were administered at
240
min. Samples were frequently taken over 3 h to allow the
achievement of steady state in plasma amino acid concentrations,
[14C]leu and
-[14C]ketoisocaproate (KIC)
SAs, and phenylalanine and tyrosine enrichments. Steady state was
defined as absence of a slope significantly different from zero as well
as of changes in concentrations, SAs, and enrichments >5%, and it was
usually achieved after
2.5 h (data not shown). Between
60 and 0 min, four 10-ml blood samples were collected at 20-min intervals into
EDTA tubes and rapidly centrifuged at +4°C. The plasma was then
stored at
20°C before assay. Samples of expired air (2-min
collections) for 14CO2 measurements were taken
at the same time points.
Analytical measurements. Plasma leucine, phenylalanine, and KIC concentrations, and plasma leucine and KIC 14C SAs were determined by high-pressure liquid chromatography (HPLC) (17, 26). Plasma [Tyr] was determined by ion exchange chromatography using a Beckman amino acid analyzer. Plasma [2H5]Phe, [2H2]Tyr, and [2H4]Tyr mole percent enrichments were determined by gas chromatography-mass spectrometry (GC-MS) as tert-butyldimethylsilyl derivatives and electron impact ionization (23). The monitored fragments were mass-to-charge ratios (m/z) 239/234 for [2H5]Phe, 468/466 for [2H2]Tyr, and 470/466 for [2H4]Tyr, respectively. Enrichments were expressed as tracer-to-tracee ratios (TTR) (31). The 14CO2 in the expired air was determined as described previously (4, 27). Insulin and glucagon concentrations were measured by radioimmunoassay, as referenced elsewhere (27). Plasma glucose was determined using a Beckman Glucose Analyzer 2.
Calculations. The values of plasma leucine and KIC SA, and of phenylalanine and tyrosine enrichments in the two steady-state periods, i.e., in the last 60 min of the basal, postabsorptive state as well as in the last 60 min of meal administration (i.e., between 190 and 250 min), were averaged. All calculations were performed using these mean values.
Whole body leucine rate of appearance (Leu Ra) was calculated using both [14C]Leu SA (plasma data) and [14C]KIC SA (intracellular data) as precursor pools (22), i.e., by dividing the [14C]Leu infusion rate (in dpm · kg
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(1) |
Statistical analysis.
All data are expressed as means ± SE. The comparison between the
postprandial and the basal amino acid kinetic data (i.e., Leu, Phe, and
Tyr Ra and Leu Ox, Phe Hy, NOLD, and NHPD) within each
group was performed using the two-tailed Student's t-test for paired data. The same test was used to compare two sets of pair-related data (such as the relative differences vs. baseline, expressed as either % or , of Ox vs. Hy, NOLD vs. NHPD, Leu
Ra vs. Phe Ra, etc.). The one-way analysis of
variance (ANOVA) was used to compare more than two sets of data (such
as the relative changes vs. basal of Leu, Phe, and Tyr Ra).
The two-way ANOVA for repeated measurements was used to analyze whole
body protein synthesis, determined with the two tracers, in the fasting
and postprandial states simultaneously. A P value
<0.05 was considered statistically significant.
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RESULTS |
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Substrate concentrations, SAs, and enrichments.
Meal ingestion increased (P < 0.01) plasma glucose
(from 84 ± 6 to 101 ± 8 mg/dl), insulin (from 13 ± 2 to 77 ± 12 mU/l), leucine, phenylalanine, and tyrosine
concentrations (Table 1). The percent
postprandial increments of leucine and phenylalanine concentrations
were similar (+63 ± 10% and +61 ± 3%, respectively). Glucagon concentrations did not change (from 106 ± 14 to 115 ± 16 ng/l). Plasma SAs and enrichments decreased postprandially, whereas the expired 14CO2 increased (Table 1).
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Amino acid kinetics.
Leu Ra, Phe Ra, and Tyr Ra
increased postprandially (Table 2). The
percent increments of the flux of each amino acid were substantially
similar (50-70%) using either plasma or intracellular precursor pools, with the exception of the increase of phenylalanine flux using the calculated intracellular precursor pool, which was lower
(+23 ± 9%, P < 0.005 by ANOVA vs. the other
increments; Table 2).
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DISCUSSION |
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The aim of this study was to test whether the phenylalanine-tyrosine tracer method is suitable to measure whole body protein synthesis in human subjects postprandially. The use of phenylalanine-tyrosine tracers is attractive, because the measurement of phenylalanine catabolism (hydroxylation), which is required for the calculation of whole body protein synthesis (3, 18), does not require expired-air collection. As a reference, we used the widely employed leucine tracer method. Both plasma and intracellular precursor pools were used.
The data show that, after the acute ingestion of a mixed meal of abundant energy and nitrogen content, whole body protein synthesis increased with both amino acid tracers when either plasma or intracellular calculations were used. When expressed as milligrams of newly synthesized protein, the use of intracellular precursor pool(s) resulted in similar rates of protein synthesis with the leucine and phenylalanine-tyrosine tracers, in both the postabsorptive and the postprandial states. In contrast, by use of plasma precursor pools, the phenylalanine-tyrosine approach led to an apparent overestimation (twofold; Fig. 2) of the increment of postprandial protein synthesis with respect to the leucine data.
Amino acid kinetics are calculated using SA and/or enrichments either measured in accessible pools (i.e., plasma), or estimated intracellularly, i.e., at the sites where the metabolic reactions take place. As regards leucine, either the SA or the enrichment of its deamination product KIC, measured in plasma, is commonly used as an indicator of intracellular leucine SA/enrichment (1, 22). However, no similar compounds are available for phenylalanine and tyrosine (21). Therefore, the intracellular phenylalanine enrichment in the fasting state as well as the intracellular tyrosine enrichments in both the fasted and fed states have been extrapolated by assuming a ratio between intracellular and extracellular enrichments similar to that of KIC to leucine SA measured in each subject (18). In the postprandial state, the intracellular/plasma phenylalanine enrichment ratio has been set to 1 (i.e., plasma phenylalanine enrichment has been used throughout) on the basis of the phenylalanine enrichment attained in a liver-synthesized protein, i.e., apoB-100, measured at steady state following meal ingestion (20). We acknowledge that some of these assumptions are unproven. However, the observation that by use of these adjustments protein synthesis was similar between the two tracer models in both the fasting and the fed states indirectly supports their validity.
Both phenylalanine and leucine catabolism increased using either the plasma or the intracellular precursor pool, indicating the nutritional adequacy (i.e., beyond requirement) of the meal tested. This finding is new, because in previous studies Phe Hy did not increase after a meal, at variance with the reported increase of Leu Ox (9, 18). These discrepancies between previous and present data might be due to the lower energy and protein content of the meals previously tested, to differences in the amino acid composition of the meals, to the limited number of subjects studied previously (9), or to other, unknown factors.
In our study, the percent increase of Leu Ox (100% using both
plasma and intracellular pools), as well as that of Phe Hy using the
plasma pool (
95%), was greater than that of Phe Hy using the
intracellular pool (
30%), despite similar percent increments in the
amino acid concentrations. In addition, plasma leucine concentration
was tightly correlated with Leu Ox calculated using either precursor
pool (Fig. 3, A and B), whereas phenylalanine concentration did not correlate with Phe Hy when either plasma or
intracellular precursor pools were used.
These findings are somewhat unexpected, because amino acid oxidation/catabolism has been shown to be tightly related to their concentrations (2, 33). A possible explanation is that Leu Ox is almost ubiquitous (24, 30); therefore a peripherally infused tracer might provide a balanced measurement of individual tissue oxidation. In contrast, Phe Hy occurs to a significant extent within the splanchnic area (15, 25); therefore, it may not be precisely estimated by peripherally infused tracers, despite the above-described adjustments of the precursor pools. In other words, Phe Hy may be more related to the (unknown) portal than to the peripheral concentrations, a relationship that may be even stronger in the postprandial period, because of the direct influx of dietary amino acid into the portal circulation as well as of the high splanchnic extraction of the ingested aromatic amino acids (4).
The postprandial leucine and phenylalanine concentrations achieved in
this study were well below the Km values of the
enzymes regulating the catabolic steps of these two amino acids.
Indeed, the Km of the branched-chain
dehydrogenase complex is >1 mM (11), whereas that of
phenylalanine hydroxylase is between 200 and 300 µM (6,
29). In dose-response studies in vivo, Leu Ox increased linearly
up to 600 µM of leucine concentration (10).
Similarly, at increasing phenylalanine intakes, both phenylalanine
oxidation and its conversion to tyrosine were dose dependent and did
not reach a plateau (34). Thus it is unlikely that the
differences between the increments of Leu Ox and Phe Hy observed
postprandially were due to a saturation of the phenylalanine-catabolic
step; rather, it is possible that other factors, either not considered or unmeasured in this study, played a role in the different response of
the catabolism of these two essential amino acids to the meal.
The source of the meal protein used in this study was a casein hydrolysate. In casein, the leucine-to-phenylalanine ratio is 3.03 (5), i.e., greater than the commonly assumed ratio between leucine and phenylalanine in average body proteins (2.14; see Calculations) (30). This may imply that more leucine had to be oxidized (with respect to the amount of phenylalanine to be hydroxylated) to meet the body requirements for these essential amino acids. This may provide another possible explanation for the larger increase of leucine than phenylalanine catabolism observed postprandially.
No correlation was found between protein synthesis and any of the measured variables (in particular, amino acid and insulin concentrations). Thus the factors regulating the postprandial increase of whole body protein synthesis are probably more complex than is currently believed, and only partially known.
In conclusion, this study demonstrates that the leucine and phenylalanine tracer models yield similar estimates of postprandial protein synthesis in human subjects provided that adequate adjustments for intracellular specific activities or enrichments of the precursor pool are adopted. The observed differences in the relationships between leucine and phenylalanine concentrations and their catabolism might be related to the specific site of each amino acid catabolism. These limitations should be kept in mind in the choice of amino acid tracer(s) to measure amino acid catabolism and whole body protein synthesis in humans postprandially.
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ACKNOWLEDGEMENTS |
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We thank M. Vettore for excellent technical assistance.
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FOOTNOTES |
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This study was supported by a grant from The National Research Council (CNR) of Italy (Grant no. 9704295CT04) and by the Joint Project between CNR and Regione Veneto: Study of Energy Metabolism in the Elderly.
Address for reprint requests and other correspondence: P. Tessari, Dept. of Clinical and Experimental Medicine, Chair of Metabolism, Policlinico Universitario, via Giustiniani 2, 35128 Padua, Italy (E-mail: paolo.tessari{at}unipd.it).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
10.1152/ajpendo.00416.2002
Received 26 September 2002; accepted in final form 14 January 2003.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ahlman, B,
Charlton M,
Fu A,
Berg C,
O'Brien P,
and
Nair KS.
Insulin's effect on synthesis rates of liver proteins. A swine model comparing various precursors of protein synthesis.
Diabetes
50:
947-954,
2001
2.
Barazzoni, R,
Zanetti M,
Vettore M,
and
Tessari P.
Relationship between phenylalanine hydroxylation and plasma aromatic amino acid concentrations in humans.
Metabolism
47:
669-674,
1998[ISI][Medline].
3.
Bier, DM.
Intrinsically difficult problems: the kinetics of body proteins and amino acids in man.
Diabetes Metab Rev
5:
111-152,
1989[ISI][Medline].
4.
Biolo, G,
Tessari P,
Inchiostro S,
Bruttomesso D,
Fongher C,
Sabadin L,
Fratton MG,
Valerio A,
and
Tiengo A.
Leucine and phenylalanine kinetics during mixed meal ingestion: a multiple tracer approach.
Am J Physiol Endocrinol Metab
262:
E455-E463,
1992
5.
Boirie, Y,
Dangin M,
Gachon P,
Vasson MP,
Maubois JL,
and
Beaufrère B.
Slow and fast dietary proteins differently modulate postprandial protein accretion.
Proc Natl Acad Sci USA
94:
14930-14935,
1997
6.
Carr, FPA,
and
Pogson CI.
Phenylalanine metabolism in isolated liver cells.
Biochem J
198:
655-660,
1981[ISI][Medline].
7.
Clarke, JTR,
and
Bier DM.
The conversion of phenylalanine to tyrosine in man. Direct measurement by continuous intravenous tracer infusion of L-[ring-2H5]phenylalanine and L-[1-13C]tyrosine in the postabsorptive state.
Metabolism
31:
999-1005,
1982[ISI][Medline].
8.
Classic KL, Schwenk WF, and Haymond MW. Radiobiological half-lives
for carbon-14 and hydrogen-3 leucine in man. In: Proc
International Radiopharmaceutical Dosimetry Symp 4th, Oak Ridge TN
1985, p. 681-687.
9.
Cortiella, J,
Marchini JS,
Branch S,
Chapman TE,
and
Young VR.
Phenylalanine and tyrosine kinetics in relation to altered protein and phenylalanine and tyrosine intakes in healthy young men.
Am J Clin Nutr
56:
517-525,
1992[Abstract].
10.
Giordano, M,
Castellino P,
and
DeFronzo RA.
Differential responsiveness of protein synthesis and degradation to amino acid availability in humans.
Diabetes
45:
393-399,
1996[Abstract].
11.
Goldberg, AL,
and
Odessey R.
Oxidation of amino acids by diaphragms from fed and fasted rats.
Am J Physiol
223:
1384-1391,
1972
12.
Hoerr, RA,
Yong-Ming Y,
Wagner DA,
Burke JF,
and
Young VR.
Recovery of 13C in breath from NaH13CO3 infused by gut and vein: effect of feeding.
Am J Physiol Endocrinol Metab
257:
E426-E438,
1989
13.
McNurlan, MA,
and
Garlick PJ.
Influence of nutrient intake on protein turnover.
Diabetes Metab Rev
2:
165-189,
1989.
14.
Millward, DJ,
and
Rivers JPW
The need for indispensible amino acids: the concept of the anabolic drive.
Diabetes Metab Rev
5:
191-212,
1989[ISI][Medline].
15.
Möller, N,
Meek S,
Bigelow M,
Andrews J,
and
Nair KS.
The kidney is an important site for in vivo phenylalanine-to-tyrosine conversion in adult humans: a metabolic role of the kidney.
Proc Natl Acad Sci USA
97:
1242-1246,
2000
16.
Munro, HN,
and
Crim MC.
The proteins and amino acids.
In: Modern Nutrition in Health and Disease, edited by Shils ME,
and Young VR.. Philadelphia, PA: Lea & Febiger, 1988, p. 1-37.
17.
Nissen, SL,
Van Huysen C,
and
Haymond MW.
Measurement of branched-chain amino acids and branched-chain -ketoacids in plasma by high-performance liquid chromatography.
J Chromatogr
232:
170-175,
1982[Medline].
18.
Pacy, PJ,
Price GM,
Halliday D,
Quevedo MR,
and
Millward DJ.
Nitrogen homeostasis in man: the diurnal responses of protein synthesis and degradation and amino acid oxidation to diets with increasing protein intakes.
Clin Sci (Colch)
86:
103-118,
1994[ISI][Medline].
19.
Reeds, PJ,
Hachey DL,
Patterson BW,
Motil KJ,
and
Klein PD.
VLDL apolipoprotein B-100, a potential indicator of the isotopic labeling of the hepatic protein synthetic precursor pool in humans: studies with multiple stable isotopically labeled amino acids.
J Nutr
122:
457-466,
1992[ISI][Medline].
20.
Rennie, MJ,
Edwards RHT,
Halliday D,
Matthews DE,
Wolan SL,
and
Millward DJ.
Muscle protein synthesis measured by stable isotope techniques in man: the effects of feeding and fasting.
Clin Sci (Colch)
59:
519-523,
1992.
21.
Rosenberg, LE,
and
Scriver CR.
Disorders of amino acid metabolism.
In: Metabolic Control and Disease, edited by Bondy PK,
and Rosenberg LE.. Philadelphia, PA: Saunders, 1980, p. 707-710.
22.
Schwenk, WF,
Beaufrère B,
and
Haymond MW.
Use of reciprocal pool specific activities to model leucine metabolism in humans.
Am J Physiol Endocrinol Metab
249:
E646-E650,
1985
23.
Schwenk, WF,
Berg PJ,
Beaufrère B,
Miles JM,
and
Haymond MW.
Use of t-butyl-dimethylsilylation in the GC/MS analysis of physiologic compounds found in plasma using electron impact ionization.
Anal Biochem
141:
101-109,
1984[ISI][Medline].
24.
Shinnick, FL,
and
Harper AE.
Branched-chain amino acid oxidation by isolated rat tissue preparations.
Biochim Biophys Acta
437:
477-486,
1976[ISI][Medline].
25.
Tessari, P,
Deferrari G,
Robaudo C,
Vettore M,
Pastorino N,
De Biasi L,
and
Garibotto G.
Phenylalanine hydroxylation across the kidney in humans.
Kidney Int
56:
2168-2172,
1999[ISI][Medline].
26.
Tessari, P,
Inchiostro S,
Vettore M,
Marescotti MC,
and
Biolo G.
A fast HPLC method for the measurement of plasma concentration and specific activity of phenylalanine.
Clin Biochem
24:
425-428,
1991[ISI][Medline].
27.
Tessari, P,
Zanetti M,
Barazzoni R,
Vettore M,
and
Michielan F.
Mechanisms of post-prandial protein accretion in human skeletal muscle: insight from leucine and phenylalanine forearm kinetics.
J Clin Invest
98:
1361-1372,
1996
28.
Thompson, GN,
Pacy PJ,
Merritt H,
Ford GC,
Read MA,
Cheng KN,
and
Halliday D.
Rapid measurement of whole body and forearm protein turnover using a [2H5]phenylalanine model.
Am J Physiol Endocrinol Metab
256:
E631-E639,
1989
29.
Tourian, A,
Goddard J,
and
Puck TT.
Phenylalanine hydroxylase activity in mammalian cells.
J Cell Physiol
73:
159-170,
1969[ISI][Medline].
30.
Waterlow, JC,
Garlick PJ,
and
Millward DJ.
Protein Turnover in Mammalian Tissues and in the Whole Body. Amsterdam: North Holland Publishing, 1978.
31.
Wolfe, RR.
Radioactive and Stable Isotope Tracers in Biomedicine. Principles and Practice of Kinetics Analysis. New York: Wiley-Liss, 1992, p. 62-70.
32.
World Medical Association.
Declaration of Helsinki.
Law Med Health Care
19:
264-265,
1991[Medline].
33.
Young, VR,
and
Marchini JS.
Mechanisms and nutritional significance of metabolic responses to altered protein and amino acids, with reference to nutritional adaptation in humans.
Am J Clin Nutr
51:
270-289,
1990[Abstract].
34.
Zello, GA,
Pencharz PB,
and
Ball RO.
Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C]phenylalanine.
Am J Physiol Endocrinol Metab
259:
E835-E843,
1990
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