The Journal of Nutrition
Nutrient Requirements and Optimal Nutrition
A Deficiency or Excess of Dietary Threonine
Reduces Protein Synthesis in Jejunum
and Skeletal Muscle of Young Pigs1,2
Xu Wang,3 Shiyan Qiao,3* Yulong Yin,4 Longyao Yue,3 Zongyi Wang,3 and Guoyao Wu4,5*
3
National Key Laboratory of Animal Nutrition, China Agricultural University, Beijing, China 100094; 4Institute of Subtropical
Agriculture, The Chinese Academy of Sciences, Changsha, China 410125; and 5Faculty of Nutrition and Department of Animal Science
Texas A&M University, College Station, TX 77843
Abstract
Dietary threonine imbalance is known to reduce the growth of the small intestine, liver, and skeletal muscle in young
animals, but the underlying mechanism is largely unknown. Using the pig model, this study was conducted to test the
hypothesis that either a deficiency or an excess of dietary threonine impairs protein synthesis in these tissues. Young pigs
(25 d of age) were fed diets containing 0.37, 0.74 (current NRC requirement) or 1.11% true ileal digestible threonine (TIDT)
(n ¼ 6/diet). Pigs receiving the 0.74 and 1.11% TIDT diets were pair-fed with the same amount of feed as pigs receiving the
0.37% TIDT diet. After a 14-d dietary treatment, the fractional synthesis rate (FSR) of protein in tissues was measured
using a flooding dose of L-phenylalanine plus L-[ring-2H5]phenylalanine. The results indicated that the FSR of protein in liver
was reduced (P , 0.05) in pigs fed the 0.37% TIDT diet compared with pigs fed the 0.74 or 1.11% TIDT diet, and did not
differ between pigs fed the 0.74 and 1.11% TIDT diets. The FSR of protein in longissimus muscle, jejunal mucosa, and
mucins was reduced (P , 0.05) in pigs fed the 0.37 or 1.11% TIDT diet compared with pigs fed the 0.74% TIDT diet. The
absolute synthesis rate of protein in the jejunal mucosa and muscle was also reduced (P , 0.01) in pigs fed the 0.37 and
1.11% TIDT diets compared with the controls. The absolute synthesis rate of hepatic protein was lower (P , 0.01) in pigs
fed the 0.37% TIDT diets when compared with pigs fed the 0.74% TIDT diet. Protein synthesis in skeletal muscle as well
as jejunal mucosa and mucins was reduced to a greater extent than that in liver in response to an imbalance of dietary
threonine. Collectively, these results indicate that either an excess or a deficiency of dietary threonine decreases protein
synthesis in rapidly growing tissues of young pigs. The findings provide a mechanism for the low growth performance of
animals fed a threonine-imbalanced diet. J. Nutr. 137: 1442–1446, 2007.
Introduction
The gastrointestinal tissues have an important impact on the
basal metabolism of animals due to their relatively high protein
turnover rate and high oxygen consumption (1). In neonatal
pigs, the portal-drained viscera (the intestine, pancreas, spleen,
and stomach) accounts for only 4–6% of the total body mass,
but is responsible for 20–50% of the total protein turnover (1–
3). Remarkably, the gastrointestinal tissues of growing pigs
utilize ;50% of amino acids in the diet (4), including 30–50%
of lysine, leucine, and phenylalanine, as well as 60% threonine
(5–7).
Results of a recent study indicate that dietary, rather than
systemic, threonine was preferentially utilized for protein
1
Supported in part by grants from the National Natural Science Foundation of
China (30525029 and 30528006), the National Basic Research Program of China
(2004CB117503), Outstanding Overseas Chinese Scholar Fund of The Chinese
Academy of Sciences (2005–1–4), and Texas Agricultural Experiment Station.
2
Author disclosures: X. Wang, S. Qiao, Y. Yin, L. Yue, Z. Wang, and G. Wu, no
conflicts of interest.
* To whom correspondence should be addressed. E-mail: g-wu@tamu.edu or
qiaoshy@mafic.ac.cn.
1442
synthesis in the small intestinal mucosa of piglets consuming a
normal protein diet (8). Interestingly, the portal-drained viscera
has a high obligatory requirement for threonine due to its
abundance in mucosal proteins (8). Structurally, the intestinal
mucosa is protected by a complex network of glycoproteins
(mucus), of which mucin is an important component (9). The
protein cores of mucins contain large amounts of threonine (10).
In rats, dietary threonine restriction has been shown to specifically reduce fractional synthesis rate (FSR)6 of intestinal mucins
but had no effect on total mucosal protein (11). However,
although a dietary deficiency of threonine is known to impair the
growth of young animals, little information is available
concerning its effects on protein synthesis in extraintestinal
tissues. Likewise, to our knowledge, there are no reports in the
literature regarding effects of an excess intake of dietary
threonine on protein synthesis in animal tissues. We hypothesized that either a deficiency or an excess of dietary threonine
6
Abbreviations used: ASR, absolute synthesis rate of protein; FSR, fractional
synthesis rate of protein; TIDT, true ileal digestible threonine.
0022-3166/07 $8.00 ª 2007 American Society for Nutrition.
Manuscript received 14 February 2007. Initial review completed 13 March 2007. Revision accepted 4 April 2007.
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may impair tissue protein synthesis. We tested this hypothesis
using the rapidly growing pig (12).
Materials and Methods
The China Agricultural University Animal Care Committee approved
the protocol used in this experiment.
Animals and diets. Three isonitrogenous and isocaloric diets were
formulated to meet the NRC (13) recommended requirements of true
ileal digestible amino acids for pigs weighing 5–10 kg, except for
threonine (13). The ingredients of the diets and the addition of
appropriate amounts of synthetic amino acids are summarized in
Table 1. The 3 experimental diets were supplemented with different
amounts of L-threonine (98.5% purity; Dachen Biochemicals) to provide
true ileal digestible threonine (TIDT) levels of 0.37, 0.74, and 1.11%.
These levels corresponded to 50, 100 (control), and 150% of the
NRC (13) recommended dietary requirement of TIDT for pigs weighing
5–10 kg. Alanine, which is rapidly metabolized by neonatal pigs and
does not have any adverse effects (14), was added to the 0.37 and 0.74%
TIDT diets to obtain the same levels of total nitrogen as the 1.11% TIDT
diet. The TIDT values in corn, peanut meal, and whey were determined
using T-cannula fitted 12–15 cm anterior to the ileocecal valve, as
described (15).
TABLE 1
The composition of diets
TIDT levels, %
Ingredient composition, % as fed
Corn
Peanut meal
Whey
Cornstarch
Sucrose
Soybean oil
Limestone
Dicalcium phosphate
Salt
Vitamin-mineral premix1
Synthetic amino acids2
3
L-Threonine, 98.5%
4
L-Alanine, 98.5%
Chemical analysis, % as fed
Crude protein
Lysine
Methionine
Cystine
Tryptophan
Isoleucine
Valine
Leucine
Histidine
Threonine
Digestible energy, kJ/kg as fed
0.37
0.74
1.11
54.27
20.00
5.00
5.35
5.00
3.00
0.33
2.85
0.20
1.00
2.35
0.00
0.65
54.27
20.00
5.00
5.30
5.00
3.00
0.33
2.85
0.20
1.00
2.35
0.37
0.33
54.27
20.00
5.00
5.25
5.00
3.00
0.33
2.85
0.20
1.00
2.35
0.75
0.00
16.19
1.33
0.53
0.19
0.25
0.73
0.89
1.30
0.40
0.42
14351
16.21
1.35
0.55
0.21
0.28
0.76
0.92
1.29
0.42
0.84
14343
16.17
1.34
0.51
0.20
0.24
0.72
0.90
1.33
0.38
1.24
14339
1
Supplying the following (mg/kg diet): Mn (as MnO), 20; Fe (as FeSO4H2O), 75; Zn (as
ZnO), 100; Cu (as CuSO45H2O), 50; I (as CaI2), 0.48; Se (as Na2SeO3), 0.40; retinyl
acetate, 1.9; cholecalciferol, 0.055; all-rac-a-tocopheryl acetate, 64; menadione
sodium bisulfite (62.5% menadione), 2.2; vitamin B-12, 0.028; riboflavin, 5.5; calcium
pantothenate, 13.8; nicotinic acid, 30.3; and choline chloride, 350.
2
Contained L-lysine (78%), 1.0%; DL-methionine (99%), 0.30%; L-tryptophan (99%),
0.11%; L-Isoleucine(99.9%), 0.29%; L-Valine (99.5%), 0.27%; L-Leucine (98.5%),
0.29%; L-Histidine (99%), 0.09%.
3
Feed grade obtained from Dachen Biochemicals.
4
Food grade obtained from Sigma.
Eighteen crossbred (Large White 3 Landrace) barrows, weaned at
21 d of age were obtained from the China-Holland Pig Breeding Farm.
During a 4-d adaptation period, the pigs were fed a mixture of the 0.74%
TIDT diet and a commercial diet (50:50, w:w). The composition of
nutrients in the commercial diet was: 14.226 MJ digestible energy/kg;
crude protein, 19.5%; lactose, 5.6%; ether extract, 5.2%; crude fiber,
2.8%; ash, 2.8%; L-lysine, 1.3%; L-methionine 1 L-cystine, 0.85%;
L-threonine, 0.89%; Ca, 0.82; and P, 0.5% (Provimi). After the
adaptation period, all pigs were weighed (7.34 6 0.55 kg) and assigned
randomly into 1 of 3 groups (n ¼ 6/group) on the basis of the origin of
litters and body weights. The experimental pigs were housed individually
in 1.20 3 0.45 m2 pens over plastic-coated, expanded-metal floors, in an
environmentally controlled nursery (25–27C) with a 12-h light and12-h
dark cycle.
Equally sized meals (15 g feed/kg body wt) were provided to pigs
3 times daily at 0800, 1600, and 2400 h. Pigs assigned to the 0.37%
TIDT treatment group had free access to feed, whereas pigs receiving the
0.74 and 1.11% TIDT diets were individually pair-fed with the same
feed intake as pigs receiving the 0.37% TIDT diet. Pair-feeding was
necessary to ensure similar intakes of all dietary nutrients except for
threonine. Between meals, pigs had free access to drinking water. The
pigs were weighed on d 0 and 14. These values were then used to
calculate daily weight gain, daily feed consumption, and feed conversion.
Infusion protocol and sample collection. At ;0800 h on the morning
of d 14, the pigs received their normal morning allotment of feed. Blood
samples (;7 mL) were collected 1 h after feeding by jugular vein
puncture using vacutainer tubes coated with EDTA (Greiner Bio-one).
Plasma was separated from whole blood by centrifugation at 2,000 3 g
for 20 min at 4C and then stored at 220C until needed for plasma
amino acid analysis.
Immediately after blood sampling, the pigs received i.p. administration of a flooding dose of L-phenylalanine (1.5 mmol/kg body wt) plus
2
L-[ring- H5]phenylalanine (Cambridge Isotopes Laboratories; 0.6 mmol
per kg body wt), as previously described (16). The injection was
completed in 5–10 s. Thirty minutes after the isotope administration,
pigs were killed with an intracardial injection of sodium pentobarbital
(50 mg/kg body wt) and jugular exsanguination. After the abdomen was
exposed, liver and longissimus muscle were quickly isolated. The whole
small intestine was removed and flushed with ice-cold saline to remove
the digesta. The jejunum was obtained, and mucosal samples were then
collected by scrapping. All the samples were immediately frozen in liquid
nitrogen and stored at 270C until analysis.
Protein fractional synthesis rates in tissues and mucins. Sample
preparation in duplicate and the isotopic enrichment of L-[2H5]phenylalanine in the free and protein-bound pools were measured as
described by Bregendahl et al. (16). The preparation and purification of
jejunal mucins in duplicate was carried out according to the procedures
of Faure et al. (17). The isotopic enrichment of L-[2H5] phenylalanine in
the mucosal free pool, mucins, and mucosal protein-bound pool were
measured according to the procedures of Faure et al. (17), except that the
n-propyl hepta-fluorobutyrate derivative of phenylalanine was measured
using a model 6890 GC linked to a 5973N quadruple MS set on Electron
Ionization mode (Agilent Technologies) (18,19). Ions with mass-tocharge ratios of 91 and 96 were monitored and converted to percentage
of molar enrichment (mol %) using calibration curves.
FSR in tissues and mucins was calculated following the procedures
of Frank et al. (20). FSR in tissues and mucins was calculated as: FSR
(%/d) ¼ (EBound 3 1440 3 100%) / (EFT 3 t). FSR is the fractional
protein synthesis rate; EBound is the isotopic enrichment (%) of the tracer
phenylalanine in the protein pool; 1440 is the number of min/d; EFT is the
enrichment of the tracer phenylalanine in the free pool at time t; and t is
the exact time (min) of protein labeling between the end of i.p. tracer
injection and the time the tissue sample was placed in liquid nitrogen (16).
Absolute protein synthesis rates (ASR) were calculated by fractional
synthesis rates (FSR) time tissue protein mass (TPM) as previously
described (20,21): ASR (g/d) ¼ FSR (%/d) 3 TPM (g). Tissue protein
mass was obtained as dissected tissue mass (DTM) 3 total protein
concentration (TPC) of the tissue: TPM (g) ¼ DTM (g) 3 TPC (%).
Threonine and tissue protein synthesis
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1443
Concentrations of free amino acids in plasma were determined
simultaneously with the measurement of isotopic enrichment using
L-norleucine (200 mL of 2.5 mmol/L L-norleucine solution; Sigma) as an
internal standard (18).
TABLE 2
Growth performance and relative tissue weights
of weaned pigs fed diets containing 0.37, 0.74,
and 1.11% TIDT between 25 and 39 d of age
TIDT levels, %
Statistical analysis. All data are presented as means 6 SEM. The effects
of dietary TIDT on the measured variables were analyzed by 1-way
ANOVA using the General Linear Model procedures of SAS statistical
software (22). Duncan’s multiple range test was performed to identify
differences among groups. Significance was set at P , 0.05.
Results
Feed intake, growth performance, and tissue weights. Feed
intake [g/(d kg body wt)] by pigs fed the 0.37% TIDT diet during the experimental period was reduced by 6.0% compared
with the value for the basal period. Intakes by pigs fed the 0.37,
0.74, and 1.11% TIDI diets during the experimental period were
470, 477, and 474 g/d, respectively. Dietary intakes of digestible
energy by the corresponding 3 groups of pigs during the
experimental period were 6745, 6841, and 6795 kJ/d, respectively. Pigs fed the 0.37% TIDT diet had lower daily weight gain
and feed conversion efficiency (P , 0.05) than pigs fed the
0.74% (control) and 1.11% TIDT diets (Table 2). However,
there were no differences in growth performance between pigs
fed the 0.74 and 1.11% TIDT diets. Feed intakes did not differ
among the 3 groups, because pigs fed the 0.74 and 1.11% TIDT
diets were pair-fed with the same amount of feed consumed by
pigs fed the 0.37% TIDT diet (Table 2). The relative tissue
weights (g/kg body wt) for liver, longissimus muscle, and
jejunum did not differ among the 3 groups of pigs.
Plasma amino acid concentrations. Increasing dietary levels
of threonine dose-dependently increased (P , 0.001) its
concentration in plasma (Table 3). Concentrations of glycine
in plasma also increased (P , 0.010) with increasing dietary
levels of TIDT from 0.37 to 0.74% and leveled off thereafter.
Dietary levels of threonine did not affect concentrations of other
amino acids, including alanine, arginine, glutamine, and
branched-chain amino acids (data not shown).
Growth performance
Initial body weight, kg
Final body weight, kg
Body weight gain, g/d
Daily feed intake, g/(d kg
body wt)
Feed conversion, g weight
gain/g feed
Tissue weights, g/kg body wt
Liver
Longissimus muscle
Jejunum
1444
Wang et al.
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0.74
1.11
SEM1
P-Value
7.36
10.92
254b
43.0
7.32
11.77
318a
40.5
7.34
11.85
322a
40.0
0.55
0.58
17
1.2
0.987
0.180
0.021
0.920
0.03
0.001
1.42
0.62
1.61
0.734
0.476
0.990
0.540b
26.1
20.0
28.0
0.667a
0.679a
24.5
19.0
28.0
25.4
19.8
28.2
1
Values are means and pooled SEM, n ¼ 6/group. Means in a row with superscripts
without a common letter differ, P , 0.05.
Discussion
The results of growth performance indicated that pigs fed the
0.37% TIDT diet had a 20% lower weight gain and 19% lower
feed efficiency compared with pigs fed the control (0.74%
TIDT) diet. Similar reductions in the efficiency of feed utilization
have been observed for pigs fed threonine-deficient diets (23–
25). Interestingly, in contrast to rats (11), dietary levels of
threonine did not affect the relative weights of any tissues in
young pigs (Table 2). This discrepancy can be explained likely by
a more severe restriction of dietary threonine in the rodent study
(11). Nonetheless, the finding that pig growth performance did
not differ between the 0.74 and 1.11% TIDT diets (Table 2)
suggests that feeding 150% of NRC (13) TIDT is not beneficial
for pigs weighing 5–10 kg.
TABLE 3
Fractional and absolute protein synthesis rates in the liver,
muscle, and jejunum. The FSR of protein was reduced (P ,
0.05) in the liver of pigs fed the 0.37% TIDT diet, compared
with pigs fed the 0.74 and 1.11% TIDT diets, and did not differ
between pigs fed the 0.74 and 1.11% TIDT diets (Table 3).
However, in both longissimus muscle and jejunal mucins, the
FSR of protein was reduced (P , 0.05) in pigs fed the 0.37 or
1.11% TIDT diets compared with pigs fed the 0.74% TIDT diet,
and did not differ between pigs fed the 0.37% and 1.11% TIDT
diets. Notably, the FSR of protein in the jejunal mucosa of pigs
fed the 1.11% TIDT diet was lower (P , 0.05) than that in pigs
fed the 0.74% TIDT diet but was greater (P , 0.05) than the
value for pigs fed the 0.37% TIDT diet.
The ASR of hepatic protein was lower (P , 0.05) in pigs fed
the 0.37% TIDT diet than in pigs fed the 0.74 and 1.11% TIDT
diets (Table 3). In contrast, the ASR of protein in longissimus
muscle was lower (P , 0.05) in pigs fed the 0.37 and 1.11%
TIDT diets than in pigs fed the 0.74% TIDT diet, and did not
differ between pigs fed the 0.37 and 1.11% TIDT diets.
Interestingly, the ASR of protein synthesis was highest and
lowest (P , 0.05) in pigs fed the 0.74 and 0.37% TIDT diets,
respectively, with the intermediate value for pigs fed the 1.11%
TIDT diet (P , 0.05).
0.37
Plasma concentrations of threonine and glycine as
well as fractional and absolute synthesis rates of
protein in jejunum, longissimus muscle, and liver of
weaned pigs fed diets containing 0.37, 0.74,
and 1.11% TIDT
TIDT levels, %
Tissue or variable
Plasma amino acid, mmol/L
Threonine
Glycine
FSR,2 %/d
Jejunal mucosa
Jejunal mucins
Longissimus muscle
Liver
ASR,3 g/d
Jejunual mucosa
Longissimus muscle
Liver
1
0.37
0.74
1.11
SEM1
P-Value
34c
485b
294b
862a
1230a
877a
67
79
0.001
0.010
75.5c
52.8b
5.0b
99.9b
115.8a
96.7a
9.8a
136.8a
88.8b
52.6b
5.2b
144.0a
3.12
3.06
1.18
5.20
0.001
0.001
0.017
0.001
0.87
0.04
1.39
0.001
0.001
0.001
17.8c
0.60b
46.3b
29.6a
1.95a
69.4a
22.8b
0.68b
70.3a
Values are means and pooled SEM, n ¼ 6/group. Means in a row with superscripts
without a common letter differ, P , 0.05.
2
FSR, fractional synthesis rate.
3
ASR, absolute synthesis rate.
Plasma threonine concentrations increased markedly as the
dietary TIDT level increased (Table 3), as previously reported
(26,27). An exceedingly low concentration of threonine in
plasma of pigs fed the 0.37% TIDT diet indicates its gross
inadequacy for supporting tissue protein synthesis. A high
concentration of threonine in plasma of pigs fed the 1.11%
TIDT diet may result from an increase in exogenous provision
and a reduced rate of tissue protein synthesis. Because glycine is
a product of threonine catabolism in mammals, including pigs
(28), increasing dietary levels of TIDT from 0.37 to 0.74% increased plasma concentrations of glycine (Table 3). This result
aids in explaining previous observations that glycine is the most
abundant amino acid in pigs fed a diet that met NRC nutrient
requirement (29,30). However, a further increase in dietary
threonine provision beyond 0.74% TIDT did not result in an
additional increase in plasma concentrations of glycine, suggesting a limit in threonine degradation via the hepatic threonine
dehydrogenase pathway.
The mucus layer forms a gel adherent to the mucosal surface
to resist exogenous and endogenous luminal irritants and,
therefore, plays an important protective function in the intestine
(31–33). The mucin is an important component of the mucus
(34) and contributes to the lubrication of the gut epithelium, the
protection of the intestinal lumen from an acidic environment
and bacterial protease, colonization resistance, and the repair of
the epithelium (35). Malnutrition decreases the absolute amount
of intestinal mucin, resulting in impaired resistance to enteric
infection (36). In turn, this affects nutrient uptake by the
intestine (37). Although one study with rats reports that a dietary deficiency of threonine reduced intestinal mucin synthesis
(11), it is not known whether an excess dietary threonine can
affect this biochemical event. Also, little information is available
regarding the effects of dietary threonine levels on protein
synthesis in tissues of young pigs. A novel and important finding
from the present study is that the FSR of small intestinal mucosal
protein and mucins was reduced by an imbalanced intake of
dietary threonine (both a threonine deficiency and an excess). It
appears that, in young pigs, the small intestine is more sensitive
to dietary threonine levels than the liver with regard to tissue
protein synthesis (both fractional and absolute protein synthesis
rates), particularly in response to an excess of dietary threonine.
A reduced availability of threonine in the intestinal lumen is
expected to impair intestinal protein synthesis (1). However, it
is not clear how an excess of threonine in the intestinal lumen
also results in the same outcome. A possible explanation may
be a reduction in the uptake of neutral amino acids (including
branched-chain amino acids) by the intestinal mucosa due to
their sharing the same transport systems (4), thereby limiting the
local synthesis of both global and specific proteins. Additionally,
an imbalance of threonine intake may affect the secretion of
local and systemic hormones that regulate intestinal protein
metabolism. Further studies are required to test this hypothesis.
Another new and important finding from the present study is
that a dietary deficiency of threonine also reduces protein
synthesis in skeletal muscle and liver of young pigs (Table 3).
Particularly, the ASR of protein in longissimus muscle and liver
of threonine-deficient piglets decreased by 70 and 33%, respectively, compared with the control group. This result is consistent
with the previous report that rates of protein synthesis in skeletal
muscle of young pigs are more responsive to changes in dietary
intake of nutrients (including amino acids) than those in the liver
(38–40). Furthermore, it is noteworthy that FSR of protein in
longissimus muscle (;7%/d) was much lower than that in
intestinal mucosal protein (;80%/d) and mucin protein
(;65%/d). On the basis of a percentage of decline in FSR of
protein, longissimus muscle or the intestinal mucosa responded
similarly to both a deficiency and an excess of dietary threonine.
Our results are in contrast to the previous report that a dietary
deficiency of threonine did not reduce muscle protein synthesis
in rats (11). This discrepancy may be explained by a difference in
either species or the relative age of the animals used in that the
previous study involved rats at several weeks postweaning (11).
Additionally, a deficiency of dietary threonine (30% of the
control intake) in rats reduced plasma levels of threonine by only
48%, whereas a more severe reduction (by 88%) was observed
in pigs fed the 0.37% TIDT diet (Table 3).
Although the Faure et al. method (11) may not be the best for
purifying mucins from the large intestine that contains a large
amount of proteoglycans, it was the only technique that was
published for isolating mucins from the small intestine at the
time of our study. Thus, the values for jejunal mucin synthesis
in the present study represent all mucins combined. Also, our
determination of muscle FRS deserves comment. There were no
differences in plasma concentrations of adrenocorticotropic
hormone or cortisol in pigs between the time of the morning
blood sampling and 30 min after isotope administration (data
not shown), suggesting a lack of stress from the procedures. In
the previous study that measured protein synthesis in tissues of
pigs using the i.p. administration of [2H]phenylalanine (16), its
isotopic enrichment in the free pool of longissimus doris muscle
and visceral tissues did not differ between 15 and 75 min post
tracer injection. The isotopic enrichment of the protein pool
increased at 30 min post i.p. administration of the tracer in
skeletal muscle as in visceral tissues when compared with the
values at 15 min, but did not further increase in the muscle at
45–75 min (16). Thus, muscle FSR values were substantially
lower at 45–75 min than at 15 min, but differed by ;20%
between 15 and 30 min (16). Therefore, although FRS values of
longissimus doris muscle measured at 30 min post i.p. administration of [2H]phenylalanine may be modestly underestimated
in the present study, it is valid to assess their relative changes in
pigs fed the 0.37 and 1.11% TIDT diets, compared with pigs fed
the 0.74% TIDT diet (control, 100% NRC requirement for
threonine). In support of this view, we found that a marked
decline in plasma concentrations of threonine in pigs fed the
0.37% TIDT diet, compared with pigs fed the 0.74% TIDT diet
was associated with a substantial reduction in muscle protein
synthesis (Table 3).
Feed intake by the 3 groups of pigs (Table 2) was 96% of that
recommended by NRC for pigs weighing 5–10 kg (13). Note
that dietary intakes of energy and essential amino acids (EAA)
were ;96% of the values recommended by NRC (13). On the
basis of a previous study with young pigs (41), such a small
reduction in energy and EAA intake may not result in a
substantial decrease in tissue protein synthesis. Because all
groups of pigs had a similar intake of nutrients except for
threonine, we believe that the changes in tissue FSR values in
pigs fed the 0.37 and 1.11% TIDT diets, in comparison with pigs
fed the 0.74% TIDT diet (100% NRC requirement), could be
interpreted to be brought about by a deficiency or excess of
dietary threonine.
In conclusion, a dietary deficiency or excess of threonine
reduced the synthesis of intestinal mucosal protein and mucins
as well as muscle protein in weaned pigs. An inadequate intake
of threonine also impairs hepatic protein synthesis. The syntheses of global proteins and mucins in the small intestine and of
skeletal-muscle protein were reduced to a greater extent than
those in the liver. These results indicate that a sufficient intake of
Threonine and tissue protein synthesis
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1445
dietary threonine plays an important role in maintaining an
adequate amount of mucosal mucins and protein synthesis in
major tissues of rapidly growing pigs but an excess level of
dietary threonine is detrimental for protein synthesis in extrahepatic tissues and, thus, their growth and development.
Acknowledgments
The authors thank Dr. Defa Li for helpful discussion and Ms.
Frances Mutscher for office support.
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