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Transamination Governs Nitrogen Isotope Heterogeneity of Amino
Acids in Rats
Alexander Braun,† Armin Vikari,‡ Wilhelm Windisch,‡ and Karl Auerswald*,†
†
Lehrstuhl für Grünlandlehre, Department of Plant Science and ‡Fachgebiet für Tierernährung und Leistungsphysiologie, Technische
Universität München, D-85350 Freising, Germany
ABSTRACT: The nitrogen isotope composition (δ15N) of different amino acids carries different dietary information. We
hypothesized that transamination and de novo synthesis create three groups that largely explain their dietary information. Rats
were fed with 15N-labeled amino acids. The redistribution of the dietary 15N labels among the muscular amino acids was
analyzed. Subsequently, the labeling was changed and the nitrogen isotope turnover was analyzed. The amino acids had a
common nitrogen half-life of ∼20 d, but differed in δ15N. Nontransaminating and essential amino acids largely conserved the
δ15N of the source and, hence, trace the origin in heterogeneous diets. Nonessential and nontransaminating amino acids showed
a nitrogen isotope composition between their dietary composition and that of their de novo synthesis pool, likely indicating their
fraction of de novo synthesis. The bulk of amino acids, which are transaminating, derived their N from a common N pool and
hence their δ15N was similar.
KEYWORDS: amino acid, 15N, transamination, isotopic scrambling
■
INTRODUCTION
regulates nitrogen exchange beyond de novo synthesis and thus
alters the δ15N even of (at least some) essential amino acids.
Transamination by aminotransferase transfers the amino
nitrogen between an amino acid and an α-keto acid; in turn, the
α-keto acid becomes an amino acid and vice versa.6 Since most
transamination reactions have equilibrium constants close to 1,
the direction of a transamination reaction proceeds in large part
as a function of the intracellular concentrations of the
reactants.8 Transaminations can also be chained, providing a
“nitrogen shuttle” even between amino acids of different
functional groups (e.g., essential and nonessential).9,10 For
example, L-serine:glyoxylate aminotransferase (transamination
enzyme, EC 2.6.1.45) catalyzes the reversible amino transfer
from serine to glycine and glycine:2-oxoglutarate aminotransferase (EC 2.6.1.4) catalyzes the reversible amino transfer
from glycine to glutamate (Figure 1A). Glutamate, as the
central amino acid in metabolism,6 is in turn linked to other
transamination chains, like the branched-chain amino acid
metabolism that links the essential amino acids leucine,
isoleucine, and valine.12 Such chains provide continuous
redistribution of amino nitrogen among transaminating amino
acids beyond de novo synthesis.8 Hence, amino acids, which are
subject to transamination and which we term transaminating
amino acids, virtually share one common amino nitrogen
pool.13 This should lead to a reduced heterogeneity in δ15N
among transaminating amino acids when the δ15N of the diet is
heterogeneous. However, since transamination implies an
isotope effect, the δ15N of transaminating amino acids is not
completely equal.14
15
The nitrogen isotope composition (δ N) of organisms
provides information on trophic systems.1,2 While the bulk
δ15N of tissues indicates the trophic position of organisms,3,4
the δ15N of specific compounds has the potential to provide
more detailed insights, because different compounds differ in
biosynthetic pathways and thus may reveal isotope information
from different (dietary) sources.5 This is particularly true for
amino acids, as they provide the main nitrogen reservoir of bulk
materials, but have individual biosynthetic pathways.6 Thus,
alanine, aspartic acid, and glutamic acid (among others) reflect
a similar trophic position as bulk tissues, whereas glycine and
phenylalanine provide an accurate determination of the δ15N at
the base of the food web,2,7 although reasons for this grouping,
as well as the information that can be obtained from other
amino acids, remain unknown. Understanding how the major
biochemical mechanisms that govern nitrogen input and
distribution, namely de novo synthesis and transamination,
affect the δ15N of amino acids would shed light on these
knowledge gaps. Additionally, the time window through which
such isotope information in amino acids is retained in
organisms and differences between amino acids in such time
windows are worthy of investigation.
For carbon isotope composition (δ13C), the amino acids’
ability for de novo synthesis largely explained the information
that can be retrieved from their δ13C.2,7 Essential amino acids
remain biochemically unmodified from diet to consumer and
thus represent the δ13C of the amino acids in the diet. For
example, leucine (essential) was not enriched in 13C between
diet and consumer, whereas glutamic acid (nonessential)
became enriched in 13C. Remarkably, both amino acids were
enriched in 15N,7 indicating that the classification into
essential/nonessential only applies for the carbon backbone
but not for δ15N, because a biochemical process interfered that
© 2014 American Chemical Society
Received:
Revised:
Accepted:
Published:
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May 16, 2014
July 16, 2014
July 18, 2014
July 18, 2014
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Figure 1. (A) Amino acid nitrogen pools and transfers in mammal metabolism. Amino acids (AAs) are given in the standard three-letter code (see
Table 1); essential AAs are in boxes, and AAs analyzed in this study are underlined. A colored background, orange for essential and blue for
nonessential AAs, means no transamination. The reactions are indicated in italics; they are succeeded by a number in parentheses indicating the
reference (1, Berg et al.;6 2, Tomé and Bos;16 3, Hutson et al.;12 4, Matthews;15 5, Nakada;10 6, Voet and Voet11); TA = transamination
(aminotransferation with αKA = α-keto acid) after AA1 + αKA2 ↔ αKA1 + AA2, in most cases dealing with Glu and α-ketoglutaric acid; the double
arrows indicate reversibility exclusively for this reaction. In this case, “equilibration” with the Glu pool is faster than any other reaction of the AAs or
the αKAs. Irreversible TAs, indicated by a one-pointed arrow, may occur, when the αKA is rapidly further converted (Tyr) or when the TA does not
concern the AS/αKA itself but a derivative (Lys and Ser). D = degradation to Glu, S = synthesis from Glu (via different pathways), DH = Glu
dehydrogenase. (B) The atom fraction x(15N) of amino acids in diet and in muscle before diet switch. Amino acids are indicated by their one-letter
code (see Table 1). The 95% confidence intervals had a similar size as the markers. Note the different ranges of x and y axes. (C) Reaction progress
of the atom fraction x(15N) for the amino acids phenylalanine (Phe), serine (Ser), and proline (Pro) after the diet switch. Each of these amino acids
belongs to one of the three hypothesized groups (panel A). Since the y-axis is logarithmic, similar linear slopes indicate similar turnover rates.
hence, amino acids should in the present context be divided
into three groups: (i) transaminating amino acids should
eventually reach a rather homogeneous δ15N irrespective of
their isotope composition in the diet, because their amino
nitrogen is exchanged with that of other transaminating amino
acids (a common nitrogen pool); (ii) nontransaminating and
essential amino acids should retain their δ15N from diet,
because nitrogen is not exchanged; and (iii) nontransaminating
and nonessential amino acids should have a δ15N that ranges
between that of their dietary δ15N and that of the δ15N of the
nitrogen donor pool that supplies de novo synthesis.
The exchange rate by transamination could determine the
nitrogen isotope turnover and thus the time window through
which isotope information can be perceived. If this were the
case, the nitrogen isotope turnover must differ between amino
acids, in particular between transaminating and nontransaminating amino acids. The effect of transamination on nitrogen
isotope turnover could, however, be superimposed by the
continuous release of amino acids from protein turnover, in
particular by the degradation of proteins by proteolysis. In that
Some amino acids differ in their ability to be transaminated.
In mammals, tyrosine can be degraded but not synthesized by
transamination, because the equivalent α-keto acid that would
accept nitrogen and turn into tyrosine is unavailable.15 Hence,
tyrosine can only be a nitrogen donor to the common nitrogen
pool. The amino acids lysine,16 threonine,17 proline,18 and
phenylalanine (except for the dysfunction phenylkentonuria6)
do not transaminate at all. Due to their disability of
bidirectional transamination, these amino acids are termed
“non-transaminating” in this study.
Since many diet reconstruction studies based on isotopes
focus on feeding generalists (e.g., refs 2, 13) and feeding
generalists likely have an isotopically heterogeneous diet, it
should be investigated how the δ 15 N of isotopically
heterogeneous amino acids changes from diet to consumer,
after nitrogen redistribution by de novo synthesis and
transamination. We hypothesize that the joint consideration
of transamination and de novo synthesis can largely explain the
variation in δ15N among amino acids in consumers. Since
transamination is the final step in de novo synthesis, its effect
on δ15N of amino acids should be of a major priority, and
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acids were supplied by Chemotrade Chemiehandelsgesellschaft mbH,
Leipzig, Germany, except for threonine, which was supplied by
Campro Scientific, Veenendaal, The Netherlands. However, in the
mixture, aspartic acid and serine were substituted isonitrogenously by
glycine, cysteine was substituted by methionine, and tyrosine was
substituted by phenylalanine. Proline was not added as a 15N-enriched
amino acid. These modifications led to a large variation in labeling of
the individual amino acids in the diet, with the atom fractions of 15N
ranging from 0.004 to 0.033 (Table 1). This pronounced labeling was
chosen because any fractionation by the metabolism will be much
smaller and can thus be neglected. For example, biosynthesis of
glutamic acid by N transfer from aspartic acid to α-keto glutaric acid is
accompanied by N isotope fractionation,14 but this will lower the
atomic fraction only by ∼0.004 and thus does not interfere with the
labeling, which caused an atom fraction in the labeled diet of 0.022.
At day 50, the six-week “chase period” started and supplementation
of the 15N-enriched amino acids was stopped in order to induce an
exponential reaction progress in nitrogen isotope composition in the
animals. To achieve reaction progress in less time, the protein content
was increased to 2.4 g d−1 (while starch was accordingly decreased to
2.1 g d−1 20).
All procedures and protocols were approved by the Animal Care
and Use Committee of Technische Universität München.
Sampling. Rats were sacrificed by decapitation under anesthesia
(diethyl ether) in groups of four rats at day 39 of the equilibration
period and on days 2, 7, 13, 20, 34, and 41 during the chase period to
investigate the reaction progress of the isotope composition following
the diet switch (Figure 1C). The animals were dissected to provide
muscle tissue (musculus femoris and musculus quadriceps). The
muscles were combined, homogenized with an Ultra-Turrax (IKALabortechnik Jahnke & Kunkel GmbH & Co KG, Staufen i. Br.,
Germany), rinsed with a solution of Hexane−2-propanol (3:2) to
eliminate lipids, and filtrated. The fat-free muscle material was dried
for 12 h at 70 °C and stored at −80 °C. The diet was collected daily,
starting 5 days before the chase period to day 7 of the chase period and
additionally on days 10, 13, 17, 22, 27, 34, and 41 of the chase period.
For more details on the keeping and sampling, see the work of Braun
et al.20
Isotope Analyses of Amino Acids. Isotope analysis was carried
out at the Leibniz-Institut für Nutztierbiologie, Dummerstorf,
Germany, following the method of Metges and Petzke.21 In brief,
samples of 2.5−3.5 mg of muscle lyophilizate were double-washed
with 10% trichloroacetic acid, acetone, and ether. Washed samples
were hydrolyzed for 24 h with 2 mL of 6 N HCl at 110 °C and
derivatized to their N-pivaloyl-isopropyl esters. To this end, 2 mL of
CH2Cl2 was added to 3−8 μmol of amino acids, and the solution was
dried and solved in 100 μL of pyridine. An aliquot of 100 μL of
pivalochloride was added, the solution was acetylated (30 min, 60 °C)
and cooled, and 2 mL of CH2Cl2 was added. The mixture was then
passed through a 4 cm silica gel (silica gel 60, 200−400 mesh) column
(4 mm i.d.) and the filtrate was dried under a stream of N2 before
adding 100 μL of ethyl acetate. The samples were then analyzed for
nitrogen isotope composition on a gas-chromatograph (HP 5890,
Waldbronn, Deutschland), connected to a gas-isotope-ratio-mass
spectrometer (Finnigan MAT, Bremen, Deutschland), via a GC
combustion interface. The interface comprised a combustion furnace
reactor filled with copper and nickel oxide and platinum (980 °C) and
a reduction furnace filled with elemental copper (600 °C).
Calculation of x(15N) of Dietary Amino Acids. In the following,
we denote isotope composition as the atom fraction [x(15N)], because
its use is preferred over the use of δ values for tracer and mixing
calculations:22
case the nitrogen isotope turnover should be similar among all
amino acids.
In order to examine the effect of dietary heterogeneity in
δ15N of amino acids on the δ15N of amino acids in the
consumer involving the nitrogen isotope turnover of amino
acids, we fed rats a diet with 15N-labeled amino acids and
analyzed the δ15N of amino acids in muscle protein.
Subsequently, we switched the δ15N of the amino acids in
the diet to quantify their nitrogen isotope turnover rate. This
was necessary to evaluate potential differences in half-lives
among amino acids that could induce differences in δ15N due to
different levels of labeling.
■
MATERIALS AND METHODS
Animals and 15N Labeling. Eighteen juvenile female Sprague−
Dawley rats (Rattus norvegicus, Berkenhout) were assigned to cages
and maintained at 25 °C ambient temperature, 60% relative humidity,
and a 12 h day−12 h night cycle for 13 weeks. All rats had ad libitum
access to deionized water supplemented with 0.014% NaCl and
received a diet of 8 g d−1 (dry mass), consisting of 2.2 g of sucrose, 0.6
g of coconut fat, 0.3 g of minerals, 0.2 g of cellulose, 0.1 g of sunflower
oil, 0.02 g of vitamins, and 0.02 g of DL-methionine. Diets were
supplemented with starch and casein depending on the nutritional
requirements of the juvenile rats. During the first 7 weeks, termed the
“isotope equilibration period”, the supplement was 3.2 g d−1 (days 1−
17), 3.5 g d−1 (days 18−34), and 3.7 g d−1 (days 35−49) starch and
1.3, 1.0, and 0.8 g d−1 casein, respectively. Digestibility of casein in
young rats is close to 100%.19 A mixture of 12 L-amino acids that was
similar to the amino acid profile of casein but highly enriched in 15N
(atom fraction 0.95 or 0.99) was added, approximately at a ratio 1:100
to casein, which remained constant over time (Table 1). All amino
Table 1. Amino Acid Profile in Dietary Casein and in the LAmino Acid Labeling Mixture and Resulting Atom Fraction
of 15N [x(15N)] in the Bulk Dieta
content in bulk diet (g/100 g)
amino acid
alanine
arginine
asparagine
aspartic acid
cysteine
glutamine
glutamic acid
glycine
histidineb
isoleucineb
leucineb
lysineb
methionineb
phenylalanineb
proline
serine
threonineb
tryptophanb
tyrosine
valineb
Ala
Arg
Asn
Asp
Cys
Gln
Glu
Gly
His
Ile
Leu
Lys
Met
Phe
Pro
Ser
Thr
Trp
Tyr
Val
A
R
N
D
C
Q
E
G
H
I
L
K
M
F
P
S
T
W
Y
V
from casein
from label mixture
x(15N)
5
2.5
4.1
2.2
0.8
7.7
9.3
2.2
1.9
5.7
9.1
7.2
3.8
3.9
9.8
7.4
5.7
0.7
4.2
7.3
0.02
0.01
0
0
0
0
0.18
0.07
0.03
0.05
0.08
0.04
0.05
0.05
0
0
0.05
0.01
0
0.06
0.008
0.007
0.004
0.004
0.004
0.004
0.022
0.033
0.016
0.012
0.012
0.009
0.020
0.016
0.004
0.004
0.011
0.017
0.004
0.011
x(15N) = n(15N)/(n(14 N) + n(15N))
a
The x(15N) of amino acids in the bulk diet is calculated from mass
balance and a x(15N) of 0.004 for casein and 0.950 for the enriched
amino acids except for Tyr and Try, for which x(15N) was 0.990. The
name of the amino acids is followed by their three and one letter
codes, respectively. bThis amino acid is essential for rats and most
other mammals.
(1)
where n(14N) and n(15N) are the amounts of isotope 14N and 15N of
element N, respectively.
The x(15N) of the amino acids in the bulk diet was calculated
according to the mass balance of an amino acid provided by casein and
provided by the enriched amino acids of the labeling mixture as
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x(15N) = (mcasx(15N)cas + mlabel x(15N)label )/(mcas + mlabel )
variation was small among replicates for all amino acids (∼2%
of the respective mean), indicating equal labeling among rats.
Turnover of Amino Acid 15N. After the diet switch the
15
x( N) of all amino acids exponentially approached the x(15N)
of the unlabeled diet (Figure 2). The mean half-life of the
(2)
where mcas and mlabel are the masses of the specific amino acid provided
by casein and by the labeled amino acid mixture, respectively, while
x(15N)cas and x(15N)label are the corresponding x(15N) values.
Turnover Estimation and Statistical Evaluation. Turnover as
quantified herein only applies for nitrogen but not for the carbon
skeletons of the amino acids, which follow different pathways of
recycling, degradation, and synthesis. A previous study that used the
same animals had shown that the reaction progress of all organs
including the muscle exhibited a common delay of 0.5 d following a
diet switch, which likely was caused by the transit time in the digestive
tract.20 Since neglecting a delay would lead to an underestimate in
isotope turnover, the delay (0.5 d) was subtracted from the sampling
times to provide a delay-corrected reaction progress.
Since growth affects the apparent reaction progress20,23 and the
muscle from which the amino acids were sampled had a growth rate of
∼0.7 g d−1,20 the effects of growth and turnover rate were disentangled
from the delay-corrected reaction progress by fitting the function
x(15 N)t = x(15 N)∞ − (x(15 N)∞ − x(15 N)0 )(1 − Δw/wt )(1 + r / g )
(3)
Figure 2. Half-lives of nitrogen in amino acids (quantified herein) and
in muscle tissue (quantified in Braun et al.20), obtained from the same
animals. The error bars indicate the 95% confidence intervals.
where x(15N)t is the atom fraction at time t, x(15N)∞ is the atom
fraction of the asymptote at the end of the reaction progress, and
x(15N)0 is the atom fraction at time zero (time of isotope switch plus
delay). The parameter wt is the dry mass of an amino acid at time t and
Δw is wt − w0, where the subscript 0 denotes time zero. The growth
rate constant is denoted g and estimated for each amino acid
specifically as 0.7/γ g d−1, where γ was the ratio of the mass of a
specific amino acid to the mass of all amino acids in the muscle (0.7 g
d−1 was the growth rate of the entire muscle; see above and in Braun et
al.20). The ratio r/g gives the relative contribution of isotope turnover
(r) and growth (g) to the apparent reaction progress. The half-life
(t1/2) is then given by ln(2)/r. The parameter x(15N)0 was obtained by
measuring the isotope composition at the end of the “equilibration
period” (as described above), as this value characterizes the constant
isotope composition during the delay and simultaneously the intercept
of the following reaction progress model when delay time is
subtracted. The standard errors of the fitting function for x(15N)0
and t1/2 were then converted to confidence intervals for p = 0.05 and a
two-sided error. All data analyses were done in R 2.15.1.
nitrogen in amino acids was 20 d (13−27 d), and the nitrogen
half-lives were not significantly different among amino acids,
and they were not statistically different from the nitrogen halflife of the entire muscle. In particular, turnover was not
different for essential/nonessential and transaminating/nontransaminating amino acids (Figure 1C).
■
DISCUSSION
We hypothesized that the joint consideration of transamination
and de novo synthesis divides amino acids into three groups
that largely explain the variation in x(15N) among amino acids
in animals: (i) transaminating, (ii) nontransaminating and
essential, and (iii) and nontransaminating and nonessential
amino acids.
Transaminating amino acids had a homogeneous x(15N)
value in consumer tissues, despite their x(15N) value in the diet
varying by an order of magnitude (Figure 1B). For example,
serine was not provided by the labeled amino acid mixture but
had the same x(15N) value in the muscle tissue as the labeled
amino acids (Table 1). The x(15N) value of transaminating
amino acids was similar to the x(15N) of the bulk diet (plus an
expectable isotope enrichment24), and it was similar to the
mean of x(15N) of transaminating amino acids in the diet. This
indicated that complete redistribution of amino nitrogen
among transaminating amino acids in consumer tissues can
be expected, because transamination is the most common
reaction involving free amino acids.17 However, the transaminations cause isotope fractionation25,26 that appears herein
only as minor variation around the homogenized x(15N),
because of the very high labeling. Future research should gain
detailed knowledge on the isotope fractionations of the specific
transamination enzymes.
The harmonization of nitrogen isotope composition of
transaminating amino acids seems to be consistent across
trophic levels, environments, and diets, as similar nitrogen
isotope compositions can be found for transaminating amino
acids within herbivores, e.g., pig, and terrestrial and marine
carnivores, e.g., lion and whale (see results of Hare et al.13).
Thus, our results would be applicable to the many studies using
■
RESULTS
x(15N) of Amino Acids. At the end of the equilibration
period, the x(15N) of amino acids varied by about a factor of 2
in muscle tissue, ranging from 0.0067 in tyrosine to 0.0158 in
phenylalanine. The x(15N) of dietary amino acids did not
correlate significantly with the x(15N) of muscular amino acids
(p > 0.05, r2 = 0.29), but the following pattern was apparent
(Figure 1B):
(i) The transaminating amino acids alanine, glutamic acid,
glycine, isoleucine, leucine, serine, and valine had a common
x(15N) of 0.0108 (SD 0.0007), which was not statistically
different from the mean x(15N) of the diet (0.0101 ± 0.0001).
Hence, their x(15N) had changed substantially, including
depletion and enrichment in 15N. The change ranged from
−0.0211 in glycine to 0.0071 in serine.
(ii) The x(15N) of nontransaminating and essential amino
acids lysine, threonine, and phenylalanine was quite similar in
diet and muscle; hence, they appeared on the 1:1 line (Figure 1
B).
(iii) The nontransaminating and nonessential amino acids
proline and tyrosine had a x(15N) between their initial (dietary)
x(15N) and the x(15N) of their specific nitrogen donor pools,
glutamine and phenylanaline, respectively. The coefficient of
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N in mammals.27−30 This finding is, however, in contrast to
x(15N) values in a zooplankter,7 indicating that the ability of
transamination may differ between organisms that are more
remote in their evolution and which do not share conserved
biochemical pathways. This calls for the need to repeat such
investigations in other animal groups that are in the focus of
isotope animal ecology, e.g., birds or insects.
Homogeneous x(15N) in consumer but varying x(15N) in
diet implies variation in trophic shifts among transaminating
amino acids, including enrichment and depletion in 15N, as
found herein (Figure 1B) and in other studies.5,31 Amino acids
become enriched in 15N when their dietary x(15N) is below the
homogenized x(15N) and vice versa; the height of the trophic
shift will be defined by the difference between isotope
composition of an amino acid and the homogenized x(15N).
Hence, the trophic shift of a transaminating amino acid cannot
indicate the trophic position of the consumer.
Nontransaminating and nonessential amino acids changed
their x(15N) from diet to consumer to less extent. Considering
their biochemical pathways, each amino acid of this group in
consumer is either derived from its dietary amino acid pool
(without nitrogen exchange by transamination) or synthesized
de novo by the organism. Hence, the change in x(15N) should
be a function of their dietary x(15N), the x(15N) of the donor
pool supplying de novo synthesis, the fraction of de novo
synthesized amino acids, and isotope fractionation during de
novo synthesis:14 Principally, the x(15N) of these amino acids
may retain their dietary x(15N) in the case of no de novo
synthesis (e.g., when dietary supply exceeds the need); the
x(15N) of these amino acids should be equal to the x(15N) of
the precursor pool (±isotope fractionation) in the case of the
fraction of de novo synthesis being 100%; in cases where the
fraction of de novo synthesis is between 0 and 100%, the
x(15N) of the amino acid should be between the dietary x(15N)
and x(15N) of the de novo synthesis pool (±isotope
fractionation). When neglecting the relatively small isotope
fractionation compared to the high labeling herein, the de novo
synthesis fraction as calculated by mass balance was 25% for
tyrosine and 40% for proline, despite their near identical x(15N)
in consumer. This contrast resulted from differing nitrogen
donor pools (tyrosine is supplied by phenylalanine, whereas
proline is supplied by the common nitrogen pool, as glutamic
acid is the precursor32) that also differed in x(15N) from the
common nitrogen pool.
Nontransaminating and essential amino acids almost retained
their dietary x(15N) value and hence provide highly conserved
isotope information from the base of the food web. However,
minor changes in isotope compositions might occur (e.g., small
deviations from the 1:1 line in Figure 1B) due to isotope
fractionation, e.g., caused by the deamination of threonine.13
The half-lives were rather constant among amino acids
(Figure 2), indicating that the differences in x(15N) values
among amino acids were not influenced by differences in
isotope turnover rates that determine the kinetics of label
incorporation. This also indicated that the common turnover
was not an effect of transamination that differed between amino
acids, but rather an effect of proteolysis that governs the
degradation of whole proteins and thus affects nearly all amino
acids at once. This is not surprising, as transamination is a
kinetically perfect reaction, which is so efficient that it is only
controlled by the provision of the substrate.
Amino acids had shorter half-lives than their bulk muscle
tissue,20 although statistically indistinguishable. This might
15
result from the slower turnover of other nitrogen-containing
components in bulk muscle, like nucleic acids.33
■
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Phone: +49 (0)8161
713965. Fax: +49 (0)8161 713243.
Author Contributions
W.W. designed the feeding experiment, A.V. carried out the
feeding experiment, K.A. designed data analysis, A.B. analyzed
the data, and A.B and K.A. wrote the manuscript. All authors
read and approved the final manuscript.
Funding
The “Deutsche Forschungsgemeinschaft” (grant number AU
183/3-1) is thanked for supporting this project.
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
The authors acknowledge the valuable discussions with Hans
Schnyder on experimental design and with Hanns-Ludwig
Schmidt on biochemical pathways. Alan Hopkins provided
linguistic support.
■
ABBREVIATIONS USED
C, stable isotope of carbon with isotopic mass 13 u; δ13C,
carbon isotope composition in permil; 15N, stable isotope of
nitrogen with isotopic mass 15 u; δ15N, nitrogen isotope
composition in permil; x(15N) nitrogen isotope composition in
atom fraction; EC, enzyme commission number; SD, standard
deviation.
13
■
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