Departments of 1 Nutritional Sciences and 2 Paediatrics, University of Toronto, Toronto, Ontario M5S 3E2; 3 The Research Institute, The Hospital for Sick Children, Toronto, Ontario M5G 1X8; and 4 Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2P5
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Tyrosine (Tyr) is an essential amino acid
in phenylketonuria (PKU) because of the limited hydroxylation of
phenylalanine (Phe) to Tyr. The recommended intakes for Tyr in PKU are
at least five times the recommended phenylalanine intakes. This
suggests that Phe and Tyr contribute ~20 and 80%, respectively, of
the aromatic amino acid (AAA) requirement (REQ). In animals and normal
humans, dietary Tyr was shown to spare 40-50% of the Phe
requirement, proportions that reflect dietary and tissue protein
composition. We tested the hypothesis that the Tyr REQ in PKU would
account for 45% of the total AAA REQ by indicator amino acid oxidation (IAAO). Tyr REQ was determined in five children with PKU by examining the effect of varying dietary Tyr intake on lysine oxidation and the
appearance of 13CO2 in breath
(F13CO2) under dietary conditions of adequate
energy, protein (1.5 g · kg1 · day
1),
and phenylalanine (25 mg · kg
1 · day
1).
Lysine oxidation and F13CO2 were determined
using a primed 4-h oral equal-dose infusion of
L-[1-13C]lysine. Lysine oxidation
and F13CO2 decreased linearly as Tyr intake
increased, to a break point that was interpreted as the mean dietary
Tyr requirement (16.3 and 19.2 mg · kg
1 · day
1,
respectively). At Tyr intakes of >16.3 and 19.2 mg · kg
1 · day
1,
lysine oxidation and F13CO2, respectively, were
low and constant. This represents 40.4 and 44.4%, respectively, of the
total AAA intake. The current recommendations for Tyr intake in PKU
patients appear to be overestimated by a factor of ~5. This study is
the first application of the IAAO technique in a pediatric population
and in humans with an inborn error of metabolism.
phenylketonuria; lysine; amino acid requirements
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
PHENYLKETONURIA (PKU) is a disorder of aromatic amino acid metabolism in which phenylalanine cannot be converted to tyrosine, or only to a very limited extent (18, 24, 33). Thus tyrosine is an indispensable amino acid in PKU because it is not supplied endogenously via phenylalanine hydroxylation (18, 27), or only to a very limited degree (33).
The aim of dietary treatment of PKU is to maintain plasma concentrations of phenylalanine, tyrosine, and other amino acids within the normal range, thereby allowing for optimum growth and brain development. Treatment consists of supplying adequate energy, other amino acids and nutrients, while phenylalanine intake is restricted and tyrosine intake is supplemented (8). At present, the dietary management of patients with PKU is empirical because it is based on monitoring plasma amino acid concentrations, blood urea nitrogen, and growth indexes, and not on direct measures of tyrosine and phenylalanine requirements.
The recommended daily aromatic amino acid (phenylalanine plus
tyrosine) intakes in healthy infants and children (9) are shown in Table 1, as are the currently recommended
levels for phenylalanine and tyrosine for children with PKU (8) in
these three age groups. In PKU, the supplemental tyrosine intake, when added to the phenylalanine intake, far exceeds the recommendations for
aromatic amino acid intake in the general, healthy population (8, 9).
In fact, the median tyrosine recommended intake across the different
age groups represents five to seven times the corresponding
phenylalanine intake. This suggests that, of the total aromatic amino
acid requirement, phenylalanine contributes ~20% and tyrosine
~80%, proportions that are significantly different from the relative
contributions of phenylalanine and tyrosine in animals (11, 12, 19, 31,
36) and normal humans (5), in which dietary tyrosine was shown to spare
40-50% of the phenylalanine requirement. This almost equivalent
contribution of phenylalanine and tyrosine to total aromatic amino acid
intake is consistent with the plasma (20, 27) and mixed body protein
(23) ratio of phenylalanine to tyrosine.
|
This study reports the first use of the recently developed technique of indicator amino acid oxidation (4, 41) in children with an inborn error of metabolism. This technique allows a determination of tyrosine requirements by use of lysine as the indicator amino acid. On the basis of ratios of phenylalanine to tyrosine described above, we hypothesized that the tyrosine requirement in PKU would account for ~45% of the total aromatic amino acid requirement.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Study subjects.
Five children (mean ± SE age, 7.6 ± 0.5 yr) with classical PKU,
treated at the Hospital For Sick Children (HSC), Toronto, participated
on an outpatient basis. Classical PKU was defined as a plasma
phenylalanine concentration 1,200 µM at diagnosis. All subjects
were treated by dietary phenylalanine restriction from early infancy.
None of the subjects had a recent history of weight loss, endocrine
disorders, or medication use. Subject characteristics at the start of
the study are summarized in Table 2. The purpose of the study,
the benefits, and the potential risks involved were explained to the
subjects and their parents. Written consent was obtained from the
subjects' parents, and detailed information sheets about the protocol
were provided. All procedures used in the study were approved by the
University of Toronto Human Experimentation Committee and the Human
Subjects Review Committee of the HSC.
|
Study design.
Each subject was studied for six nonconsecutive days during a 2-mo
period between March and September, 1996. To be able to repeatedly
study 6- to 9-yr-old children by use of amino acid oxidation
techniques, we modified the standard models used in adults (6, 17, 40):
specifically, oral administration of the isotope and ingestion of the
study diet only on the six study days, and sampling of plasma amino
acid isotope enrichment of arterial blood by measuring amino acid
enrichment in urine. We have recently reported our development and
validation of this minimally invasive protocol (3), without which a
study of this type would have been impossible. On the study days, the
subject received one of the seven test intakes of dietary tyrosine, 0, 8, 12, 16, 32, 64, or 130 mg · kg1 · day
1,
assigned in random order. Each subject was studied at six intake levels, which enabled the estimation of individual tyrosine
requirements. Subject MJ was studied at only five test levels
because a minor illness prevented him from completing one study.
Therefore, a total of 29 oxidation studies were performed across the
seven test levels of tyrosine. The distribution of the subjects across these levels can be appreciated from Tables 3 and 4.
Experimental diet.
The experimental diet was based on an amino acid mixture developed for
amino acid kinetic studies (37). A flavored liquid formula
(Protein-Free Powder, Product 80056, Mead Johnson, Evansville, IN) and
protein-free cookies (HSC Research Kitchen) supplied the main source of
energy in the diet. A crystalline amino acid mixture, based on the
amino acid composition of egg protein, was consumed at 1.5 g · kg1 · day
1
and provided the only source of amino nitrogen in the diet. This level
of protein was chosen because it met or exceeded the recommended level
(9) and was of a similar magnitude to the children's habitual protein
intake of 1.9 ± 0.2 g · kg
1 · day
1.
The approximate macronutrient composition of the experimental diet,
expressed as a percentage of dietary energy, was 53% carbohydrate, 38% fat, and 9% protein. The macronutrient composition of the subject's usual diet was 55% carbohydrate, 35% fat, and 10%
protein. The diets were prepared and portioned into eight isoenergetic, isonitrogenous meals. The diet was consumed as hourly meals, and each
meal represented one-eighth of the subject's total daily requirement.
Total energy intakes were based on each subject's calculated resting
metabolic rate (9), multiplied by an activity factor of 1.5. Subjects
were instructed to maintain their usual level of physical activity and
to fast for 10-12 h overnight before the study. Within 1 wk of the
first study day and on the morning of each subsequent study day, each
subject had his height and weight measured. Standing height was
measured without shoes, to the nearest 0.1 cm, by use of a wall-mounted
stadiometer. Body weight was measured to the nearest 0.1 kg on a
balance scale (Toledo Scale, model 2020, Windsor, ON, Canada) while
subjects wore light clothing and were without shoes, after an overnight
fast, and after voiding. Body weight and height from one study day were used to calculate the total energy and protein (amino acid) content of
the experimental diet for the next study day. Subjects were not adapted
to the experimental diet, because it was possible to match the protein
and phenylalanine intakes of the experimental diet to the usual intakes
of the subjects who followed a highly regulated diet (see Table 2). In
addition, the protein source for these subjects with PKU was composed
of crystalline amino acids, similar to the amino acid mix used in the
experimental diet. The phenylalanine intake provided by the
experimental diet on the study day was 24 mg/kg. The lysine intake on
the study day was 64 mg/kg. This value reflected the upper limit of the recommended lysine intake for children aged 2-12 (9).
Oral isotope infusion studies.
Each isotope study took 8 h (480 min) and was divided into a 4-h period
to allow background isotope equilibration to the experimental diet (3)
and a 4-h period after the isotope administration was started. Time
0 was defined as when isotope administration was started, with 240 min before and after time 0. Hourly meals were consumed
beginning at time 240 min. The level of lysine in each meal was
the same. This was achieved by reducing the dietary lysine content of
the last four meals by an amount that corresponded to the amount of
[13C]lysine administered. The level of amino
nitrogen in the diet was kept constant despite the different tyrosine
test levels by the addition of molar equivalents of
L-alanine.
|
Analytical procedures. Amino acids in 500 µl of urine were derivatized by the method described by Patterson et al. (25) to their N-heptafluorobutyryl-n-propyl esters. Isotopic enrichment for urinary free [13C]lysine was measured by gas chromatography-[selected ion monitoring-negative chemical ionization]-mass spectrometry [Hewlett-Packard model 5890 Series II GC (Mississauga, ON) VG Trio-2 quadrupole mass spectrometer system]. Selected ion chromatographs were obtained by monitoring mass-to-charge ratios of 560 and 561 for [13C]lysine, corresponding to the unenriched (m) and enriched (m+1) peaks, respectively. The areas under the peaks were integrated by a Digital DECp 450D2LP computer by use of a Lab-Base program (VG Biotech, Altringham, UK).
The trapped 13CO2 was released from the NaOH by addition of an equal volume (0.25 ml) of 85% phosphoric acid in a twin-limbed preevacuated Rittenberg Tube (13). The percentage enrichment of the expired 13CO2 was measured on a dual-inlet magnetic sector isotope ratio mass spectrometer (VG Micromass 602D, Cheshire, UK) by use of techniques described in earlier work (13). Breath enrichments from baseline samples and from those taken during the isotope infusion were expressed as atoms percent excess (APE) 13CO2 over a reference standard of compressed CO2 gas.Data analysis. Results are presented as means ± SE. A stochastic model was used to evaluate lysine kinetics (35). Isotopic steady state in the metabolic pool was represented by plateaus in 13CO2 enrichment in breath and in [13C]lysine enrichment in urine. This state was achieved in breath and urine by 120 min from the start of the isotope infusion and was maintained to the end of the study at 240 min. Plateaus in urine and breath isotope enrichments were defined first by visual inspection (Fig. 1) followed by regression analysis demonstrating that the slope was not different from zero. The mean breath isotope enrichment values of the three baseline samples and the four plateau samples were used to determine APE above baseline at isotopic steady state. The mean ratio of the enriched peak (m+1) to the unenriched peak (m) in urine for both baseline and plateau samples was used to calculate molecules percent excess (MPE) for [13C]lysine.
Lysine kinetics were estimated from breath and urine enrichment data with standard equations (17). Examples of the use of these equations in indicator amino acid oxidation are described in detail elsewhere (40). Briefly, lysine flux (Q) was calculated using the following equation
![]() |
(1) |
![]() |
(2) |
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The mean lysine flux was 114.0 ± 7.9 mg · kg1 · day
1,
data not shown. Lysine flux was not affected by the level of test
tyrosine intake (P = 0.89). However, there were differences
between individual subjects in their lysine fluxes (P = 0.0001); conversely, the sex of the subject (male vs. female) had no
effect (P = 0.14) on the estimate of lysine flux.
The rate of 13CO2 released by lysine tracer
oxidation is shown in Table 3. There was a
significant decrease in the mean rate of 13CO2
released by lysine tracer oxidation between tyrosine intakes of 0 and
12 mg · kg1 · day
1
and no change in the mean rate of 13CO2
released between 12 and 130 mg · kg
1 · day
1.
The individual subject (P = 0.21) and the sex of the subjects (P = 0.67) did not have significant effects on
F13CO2. The mean break point in the
F13CO2 data, as analyzed by two-phase linear
regression crossover design (not shown), occurred at a dietary intake
of 19.2 mg · kg
1 · day
1
of tyrosine (95% confidence limits of 13.3-25.2).
|
Similar results were apparent from lysine oxidation rates (Table
4). There was a significant decrease in the
mean rate of lysine oxidation between tyrosine intakes of 0 and 12 mg · kg1 · day
1
and no change between 12 and 130 mg · kg
1 · day
1.
The individual subject had a significant effect on lysine oxidation (P = 0.0001), whereas the sex of the subjects did not
(P = 0.20). Figure 2 shows the
effect of tyrosine intake on the mean rates of lysine oxidation. A
break point in the lysine oxidation curve occurred at a dietary intake
of 16.3 mg · kg
1 · day
1
(95% confidence limits of 5.8-26.8).
|
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Our study was prompted by the lack of direct measurement of aromatic amino acid requirements in individuals with PKU. Up until the present, tyrosine requirement estimates were based on plasma tyrosine levels (the timing of which in relationship to meals was not standardized) as well on growth measurements and blood urea nitrogen levels (1, 8). It was further prompted by the observation that amino acid homeostasis, although disturbed in untreated PKU, is not fully normalized in treated PKU (10). If untreated, patients with PKU show low-to-normal plasma tyrosine concentrations (14). Other authors have reported low plasma tyrosine concentrations in treated PKU patients after an overnight fast (29). Treated patients with PKU have both lower than healthy control and higher than healthy control postprandial plasma tyrosine concentrations while consuming the current recommended tyrosine enriched amino acid mixtures (34). The clinical implications of low fasting plasma tyrosine concentrations are not known; however, low plasma tyrosine levels have been the basis for tyrosine supplementation in PKU. As shown in Table 1, the current recommendations for aromatic amino acids in PKU far exceed the recommended intake for the general population (9). The source of this difference is the supplemental tyrosine provided to PKU patients. We regarded the current recommendation for tyrosine, which is 80% of total aromatics, as being too high, and we hypothesized it would be ~45% (of total aromatic amino acid needs), as in the healthy population.
Indicator amino acid oxidation is based on the principle that when one
amino acid is in limited supply for protein synthesis, other essential
amino acids will be in relative excess and hence will be oxidized (4,
41). It overcomes most of the limitations of direct amino oxidation, in
that 1) the precursor pool for oxidation does not change as the
level of the test amino acid is varied; and 2) all amino acids
can be studied, not only those whose carboxyl group is directly
released to the bicarbonate pools (41). Indicator oxidation was
developed and validated in growing piglets, and the short-term
indicator oxidation estimates of requirement were shown to correspond
to classical longer-term measurements such as nitrogen balance, growth,
and changes in body composition (2, 4). When we first used the
indicator oxidation technique to determine amino acid requirements of
humans, we used the classical amino acid metabolism model (17) with
intravenous isotope administration, sampling of tracer enrichment in
plasma, and adaptation to an amino acid-based diet for a period of 8
days (38). With the objective of adapting the indicator amino acid
oxidation technique to be acceptable in children, we developed a
minimally invasive model (3), which we then applied, for the first time
in children, in the present study. The estimate of tyrosine requirement
that was derived in these children with PKU is one for optimal protein synthesis and, hence, growth. Tyrosine's other role is as a precursor for the adrenergic neurotransmitters. A recent study has investigated whether tyrosine supplementation of 100 mg · kg
1 · day
1,
in addition to their dietary tyrosine intake (total tyrosine intakes
were ~130
mg · kg
1 · day
1),
had any effect on neuropsychological performance, and none was found
(30). Because this was a randomized controlled trial with adequate
numbers, the possibility that higher levels of tyrosine are needed to
enhance brain function is currently unproven.
This study demonstrates the first use of lysine as an indicator for amino acid requirements in a human population. Both phenylalanine and lysine were used as indicators in a study of tryptophan requirements in piglets (2). Because of the enzyme defect in PKU, lysine was used as the indicator amino acid in this study. The pattern of F13CO2, or lysine oxidation, was similar to those observed in previous human studies in which phenylalanine was used as the indicator (7, 15, 40). Both F13CO2 and lysine oxidation decreased and then reached a plateau as tyrosine intake increased from deficient levels to levels above requirement. The results from this study demonstrate that [13C]lysine can be used as an indicator amino acid in the estimation of amino acid requirement in humans.
Lysine flux was not affected by changes in tyrosine intakes. The lysine
flux observed in the current PKU group (81.7-128.2 µmol · kg1 · h
1)
was in an expected range given the levels reported from studies in
adults (3, 6, 21, 22, 39). An important assumption of the indicator
oxidation method is that changes in intake of the test amino acid do
not alter the flux of the indicator (4, 38).
The mean tyrosine requirement in children with classical PKU was
estimated at 19.2 and 16.3 mg · kg1 · day
1
by the F13CO2 and oxidation response curves,
respectively. From the F13CO2 and oxidation
data, the upper 95% confidence interval of the break-point estimate
was calculated as 25.2 and 26.8 mg · kg
1 · day
1,
respectively. The mean requirement estimates determined by
F13CO2 released and by lysine oxidation
represent 44.4 and 40.4%, respectively, of the total aromatic amino
acid intake and not 80%, as is suggested by the current
recommendations used in clinical practice. The requirement estimates
derived from these two data sets are similar and support the hypothesis
that tyrosine contributes ~45% of the total aromatic amino acid
requirement in PKU, if we assume that the mean habitual phenylalanine
intake of 24 mg · kg
1 · day
1
determined by clinical monitoring is a reasonable estimate of the true
phenylalanine requirement of this population.
The proportion of tyrosine to phenylalanine determined in the present
experiment in PKU children is consistent with the animal literature, in
which tyrosine was shown to spare 40-46% of phenylalanine requirement by both nitrogen balance (19, 31, 36) and tracer oxidation
methods (11, 12). This proportion is also consistent with the
requirements estimated in normal humans by nitrogen balance (5, 16, 32)
and is also in keeping with human plasma phenylalanine-to-tyrosine ratios (27) and with the ratio in mixed piglet body protein (23).
Finally, these results are consistent with an estimate of the tyrosine
requirement of 25 mg · kg1 · day
1
in children with hypertyrosinemia, aged 9-10 yr (1). Given the
agreement between our data and the literature on animals, healthy
humans, and children with hypertyrosinemia, we believe that our data
are a reasonable estimate of the true requirement and that current
clinical practice is in error.
Each subject was studied at six test tyrosine levels in the present
study, thus uniquely enabling an estimate of his individual requirement. Significant differences among individuals were observed with respect to the lysine oxidation response, but not with respect to
the F13CO2 response to changes in tyrosine
intake. The individual patterns of lysine oxidation were, however,
consistent with the group oxidation data and with the
F13CO2 data. The oxidation pattern observed in
these individual measurements suggests an inflection in oxidation
occurring between 15.0 and 19.0 mg · kg1 · day
1
of tyrosine intake. Individual tyrosine requirements based on inflections in the F13CO2 curves ranged from
16.0 to 25.0 mg · kg
1 · day
1.
In the present study, the mean tyrosine requirement was estimated to be
16.3 to 19.2 mg · kg1 · day
1,
which represents 40.4-44.4% of the total aromatic amino acid intake in children with PKU. However, direct measurement of the phenylalanine requirement in PKU may be required to refine the relative
contribution of tyrosine and phenylalanine to the total aromatic amino
acid requirement in PKU. Therefore, the current recommendations for
tyrosine intake in PKU patients are overestimated by a factor of ~5.
The findings of this study have significant implications with respect
to the dietary treatment of individuals with PKU.
![]() |
ACKNOWLEDGEMENTS |
---|
This research was supported by Grant MT 10321 from the Medical Research Council of Canada. Mead Johnson Canada generously provided the protein-free powder. R. Bross received partial support from the Fonds four la Formation de Chercheurs a l'Aide a la Recherche, Quebec QC, Canada.
![]() |
FOOTNOTES |
---|
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. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: P. B. Pencharz, Division of Gastroenterology and Nutrition, The Hospital for Sick Children, 555 Univ. Ave., Toronto, Ontario, Canada M5G 1X8 (E-mail: paul.pencharz{at}sickkids.on.ca).
Received 13 May 1999; accepted in final form 11 October 1999.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Acosta, P. B.,
and
L. J. Elsas II.
Dietary Management of Inherited Metabolic Disease. Atlanta, GA: ACELMU Publishers, 1970, p. 40.
2.
Ball, R. O.,
and
H. S. Bayley.
Tryptophan requirement of the 2.5-kg piglet determined by the oxidation of an indicator amino acid.
J. Nutr.
114:
1741-1746,
1984[ISI][Medline].
3.
Bross, R.,
R. O. Ball,
and
P. B. Pencharz.
Development of a minimally invasive protocol for determination of phenylalanine and lysine kinetics in humans during the fed state.
J. Nutr.
128:
1913-1919,
1998
4.
Brunton, J. A.,
R. O. Ball,
and
P. B. Pencharz.
Determination of amino acid requirements by indicator amino acid oxidation: applications in health and disease.
Curr. Opin. Clin. Nutr. Metabolic Care
1:
449-453,
1998[Medline].
5.
Burrill, L. M.,
and
C. Schuck.
Phenylalanine requirements with different levels of tyrosine.
J. Nutr.
83:
202-208,
1964[ISI].
6.
Conway, J. M.,
D. M. Bier,
K. J. Motil,
J. F. Burke,
and
V. R. Young.
Whole-body lysine flux in young adult men: effects of reduced total protein and of lysine intake.
Am. J. Physiol. Endocrinol. Metab.
239:
E192-E200,
1980
7.
Duncan, A. M.,
R. O. Ball,
and
P. B. Pencharz.
Lysine requirement of adult males is not affected by decreasing dietary protein.
Am. J. Clin. Nutr.
64:
718-725,
1996[Abstract].
8.
Elsas, L. J.,
and
P. B. Acosta.
Nutritional support of inherited metabolic diseases.
In: Modern Nutrition in Health and Disease, edited by M. E. Shils,
J. A. Olson,
and M. Shike. Malvern, PA: Lea & Febiger, 1994, p. 1147-1206.
9.
FAO/WHO/UNU..
Energy and Protein Requirements. Geneva: World Health Organization, 1985.
10.
Hjelm, M.,
J. Seakins,
and
A. Antoshecheckin.
Indications of changed amino acid homeostasis in untreated and treated PKU.
Acta Paediatr. Suppl.
407:
57-59,
1994[Medline].
11.
House, J. D.,
P. B. Pencharz,
and
R. O. Ball.
Tyrosine kinetics and requirements during total parenteral nutrition in the neonatal piglet: the effect of glycyl-L-tyrosine supplementation.
Pediatr. Res.
41:
575-583,
1997[Abstract].
12.
House, J. D.,
P. B. Pencharz,
and
R. O. Ball.
Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine.
Am. J. Clin. Nutr.
65:
984-993,
1997[Abstract].
13.
Jones, P. J. H.,
P. B. Pencharz,
L. Bell,
and
M. T. Clandinin.
Model for determination of 13C substrate oxidation rates in humans in the fed state.
Am. J. Clin. Nutr.
41:
1277-1282,
1985[Abstract].
14.
Koepp, P.,
and
K. R. Held.
Serum-tyrosine in patients with hyperphenylalaninaemia.
Lancet
2:
92-93,
1977[ISI][Medline].
15.
Lazaris-Brunner, G.,
M. Rafii,
R. O. Ball,
and
P. Pencharz.
Tryptophan requirement in young adult women determined by indicator amino acid oxidation using L-[13C]phenylalanine.
Am. J. Clin. Nutr.
68:
303-310,
1998[Abstract].
16.
Leverton, R. M.,
N. Johnson,
J. Ellison,
D. Geschwender,
and
F. Schmidt.
The quantitative amino acid requirements of young women. IV. Phenylalanine, with and without tyrosine.
J. Nutr.
58:
341-353,
1956[ISI].
17.
Matthews, D. E.,
K. J. Motil,
D. K. Rohrbaugh,
J. F. Burke,
V. R. Young,
and
D. M. Bier.
Measurement of leucine metabolism in man from a primed, continuous infusion of L-[1-13C]leucine.
Am. J. Physiol. Endocrinol. Metab.
238:
E473-E479,
1980
18.
Medical Research Council Working Party on Phenylketonuria.
Phenylketonuria due to phenylalanine hydroxylase deficiency: an unfolding story.
Br. Med. J.
306:
115-119,
1993[ISI][Medline].
19.
Milner, J. A.,
R. L. Garton,
and
R. A. Burns.
Phenylalanine and tyrosine requirements of immature beagle dogs.
J. Nutr.
114:
2212-2216,
1984[ISI][Medline].
20.
Mitchell, G. A.,
G. Lambert,
and
R. M Tanguay.
Hypertyrosinemia.
In: The Metabolic Basis of Inherited Disease, edited by C. R. Scriver,
A. L. Beaudet,
W. S. Sly,
and D. Valle. New York: McGraw-Hill, 1995, p. 1077-1106.
21.
Motil, K. J.,
D. M. Bier,
D. E. Matthews,
J. F. Burke,
and
V. R. Young.
Whole body leucine and lysine metabolism studied with [1-13C]leucine and [alpha-15N]lysine: response in healthy young men given excess energy intake.
Metabolism
30:
783-791,
1981[ISI][Medline].
22.
Motil, K. J.,
A. R. Opekun,
C. M. Montandon,
H. K. Berthold,
T. A. Davis,
P. D. Klein,
and
P. J. Reeds.
Leucine oxidation changes rapidly after dietary protein intake is altered in adult women but lysine flux is unchanged as is lysine incorporation into VLDL-apolipoprotein B-100.
J. Nutr.
124:
41-51,
1994[ISI][Medline].
23.
Munro, H. N.,
and
A. Fleck.
Analysis of tissues and body fluids for nitrogenous constituents.
In: Mamamalian Protein Metabolism, edited by H. N. Munro. New York: Academic, 1969, vol. III, chapt. 30, p. 423-525.
24.
Okano, Y.,
R. C. Eisensmith,
F. Guttler,
U. Lichter-Konecki,
D. S. Konecki,
F. K. Trefz,
M. Dasovich,
T. Wang,
K. Henriksen,
H. Lou,
and
S. L. Woo.
Molecular basis of phenotypic heterogeneity in phenylketonuria.
N. Engl. J. Med.
324:
1232-1238,
1991[Abstract].
25.
Patterson, B. W.,
D. L. Hachey,
G. L. Cook,
J. M. Amann,
and
P. D. Klein.
Incorporation of a stable isotopically labeled amino acid into multiple human apolipoproteins.
J. Lipid Res.
32:
1063-1072,
1991[Abstract].
26.
SAS Institute.
SAS/STAT Guide For Personal Computers. Cary, NC: SAS Institute, 1991.
27.
Scriver, C. R.,
S. Kaufman,
and
S. L. Woo.
The hyperphenylalanemias.
In: The Metabolic Basis of Inherited Disease, edited by C. R. Scriver,
A. L. Beaudet,
W. S. Sly,
and D. Valle. New York: McGraw-Hill Information Services, 1989, p. 495-546.
28.
Seber, G. A. F.
Linear Regression Analysis. New York: Wiley, 1977.
29.
Smith, I.,
M. G. Beasley,
and
A. E. Ades.
Intelligence and the quality of dietary treatment in phenylketonuria.
Arch. Dis. Child.
65:
472-478,
1990[Abstract].
30.
Smith, M. L.,
W. B. Hanley,
J. T. Clarke,
P. Klim,
W. Schoonheyt,
V. Austin,
and
D. C. Lehotay.
Randomized controlled trial of tyrosine supplementation on neuropsychological performance in phenylketonuria.
Arch. Dis. Child.
78:
116-121,
1998
31.
Stockland, W. L.,
Y. F. Lai,
R. J. Meade,
J. E. Sowers,
and
G. Oestemer.
L-Phenylalanine and L-tyrosine requirements of the growing rat.
J. Nutr.
101:
177-184,
1971[ISI][Medline].
32.
Tolbert, B.,
and
J. H. Watts.
Phenylalanine requirement of women consuming a minimal tyrosine diet and the sparing effect of tyrosine on the phenylalanine requirement.
J. Nutr.
80:
111-114,
1963[ISI].
33.
Van Spronsen, F. J.,
D. J. Reijngoud,
G. P. Smit,
G. T. Nagel,
F. Stellaard,
R. Berger,
and
H. S. Heymans.
Phenylketonuria. The in vivo hydroxylation rate of phenylalanine into tyrosine is decreased.
J. Clin. Invest.
101:
2875-2880,
1998
34.
Van Spronsen, F. J.,
T. van Dijk,
G. P. Smit,
M. van Rijn,
D. J. Reijngoud,
R. Berger,
and
H. S. Heymans.
Large daily fluctations in plasma tyrosine in treated patients with phenylketonuria.
Am. J. Clin. Nutr.
64:
916-921,
1996[Abstract].
35.
Waterlow, J. C.,
P. J. Garlick,
and
D. J. Millward.
Protein Turnover in Mammalian Tissues and in the Whole Body. Amsterdam: North Holland, 1978.
36.
Williams, J. M.,
J. G. Morris,
and
Q. R. Rogers.
Phenylalanine requirement of kittens and the sparing effect of tyrosine.
J. Nutr.
117:
1102-1107,
1987[ISI][Medline].
37.
Zello, G. A.,
P. B. Pencharz,
and
R. O. Ball.
The design and validation of a diet for studies of amino acid metabolism in adult humans.
Nutr. Res.
10:
1353-1365,
1990[ISI].
38.
Zello, G. A.,
P. B. Pencharz,
and
R. O. Ball.
Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C]phenylalanine.
Am. J. Physiol. Endocrinol. Metab.
259:
E835-E843,
1990
39.
Zello, G. A.,
P. B. Pencharz,
and
R. O. Ball.
Dietary lysine requirement of young adult males determined by oxidation of L-[1-13C]phenylalanine.
Am. J. Physiol. Endocrinol. Metab.
264:
E677-E685,
1993
40.
Zello, G. A.,
J. Telch,
R. Clarke,
R. O. Ball,
and
P. B. Pencharz.
Reexamination of protein requirements in adult male humans by end-product measurements of leucine and lysine metabolism.
J. Nutr.
122:
1000-1008,
1992[ISI][Medline].
41.
Zello, G. A.,
L. J. Wykes,
R. O. Ball,
and
P. B. Pencharz.
Recent advances in methods of assessing amino acid requirements for adult humans.
J. Nutr.
125:
2907-2915,
1995[ISI][Medline].