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Laboratory of Human Nutrition, Massachusetts Institute of Technology, Cambridge, MA 02139 and * MassTrace Incorporated, Woburn, MA 01801
2To whom correspondence should be addressed.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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KEY WORDS: • glutamate • reactivity • evolution • properties • kinetics • physiology • uptake
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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Albert Einstein (1879–1955)
For purposes of this review, it was suggested by the
symposium organizers that we address two issues; first, we would try to
answer the following question: why is L-glutamate such an
abundant biomolecule? Second, we would provide an update of Munro’s
1979 review on the regulation of glutamate metabolism (Munro 1979
). To cover each remit in detail is not possible within the
space permitted. In any event, considering the many functions of
glutamate (Table 1
) and that a substantial update of our understanding of glutamate
metabolism/function will be covered elsewhere in these proceedings, it
is perhaps appropriate for us to focus more, but not exclusively, on
the first of the foregoing issues. We use, therefore, as a springboard,
the quote given above that is attributed to Albert Einstein.
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Well, perhaps both Einstein and Haldane missed the point; they might have approached their speculations from the perspectives of nutritional biochemists and organic chemists; upon examination of the details, the characteristics and the workings of living things, we are led inescapably to the conclusion that when God created the world, it was done so with glutamic acid in mind!
Our goal, therefore, at the beginning of these proceedings, will
be to summarize the lines of evidence for such a conclusion. Leaving
issues of creationism aside, at the very least, we will reinforce the
notion that glutamate plays a pivotal role in the design of life
processes, and especially those of importance in human metabolism.
Further, we will take a somewhat unorthodox approach and let glutamic
acid, the molecule itself, set much of the agenda for our coverage.
That is, the physical and chemical properties of this five-carbon
amino acid will provide the framework for a discussion about its unique
reactivity in a biological setting and, for purposes of further
illustration, this will be contrasted with that of its lower and higher
homologs, aspartic acid and 2-aminoadipic acid. We begin with a brief,
rather selective statement about the prebiotic dawn of events and some
evolutionary features of glutamic acid in the biological and metabolic
world. The properties of glutamate will then be examined in four
contexts that have proven to be consequential for its mediation in
biological processes. In doing so, we will also include reference to
and finish with some contemporary findings that expand slightly upon
some topics included in Munro’s earlier review (Munro 1979
).
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Prebiotic and evolutionary considerations of glutamic acid |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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], the appearance of this and other amino
acids on Earth arose in the far geological past, as many studies
suggest, via abiotic synthesis (Harada 1974
). Indeed, it
is now quite apparent that the formation of amino acids occurs readily
under inferred primitive earth conditions. Amino acids can be formed
when various types of energies are applied to gas mixtures or from
chemically reactive precursors; glutamic acid and its homolog, aspartic
acid, have been detected when various sources of energy including
electrical discharge, UV rays, ionizing radiation and thermal energy
are introduced into such systems. Furthermore, organic compounds,
including amino acids, have been thought to have appeared >3.5 x 109 y ago as a consequence of
atmospheric shock waves within the early, strongly reducing atmosphere
(Chyba and Sagan 1992
). In a recent paper, Huber
and Wächtershauser (1998)
reported that they produced
peptides from amino acids under geochemically relevant conditions.
These investigators concluded that their findings supported a
thermophilic organ of life and an early appearance of peptides in the
evolution of a primordial metabolism. In addition, others have
demonstrated the formation of biopolymers, via, for example, the
thermal polycondensation of amino acids on the surface of minerals,
including formation of copolymers of glutamic acid and glycine and of
glutamic acid and aspartic acid [see Harada (1974)
for
review] by processes akin to modern-day solid phase synthesis in
which the negatively charged residues, principally of glutamate, form a
template lattice upon anion exchange minerals, such as hydroxyapatite
or illite (Hill et al. 1998
, Orgel 1998
).
Protein-like material (proteinoid) has been produced via
copolymerization of glutamic acid and aspartic acid with 16 of the
other natural amino acids common to proteins, and these products show
weak catalytic activity, including that of hydrolyses, decarboxylation
and amination. This suggests, therefore, the possible origin of the
transamination activity that is so fundamental to amino acid and
nitrogen metabolism, as we know it today.
To continue briefly on our evolutionary journey and as
Piccirilli (1995)
has pointed out, there is ample
evidence that catalytic RNA rose in the early stages of evolution of
life on Earth and that before RNA, with its component of
ß-ribofuranoside-5'-phosphates, it may be that peptide nucleic acid
(PNA) molecules were a prebiotic form (Böhler et al. 1995
). These molecules appear to be capable of facilitating the
synthesis of complementary PNA strands, and it is further speculated
that these early informational molecules possibly underwent
metamorphosis to a ribose-phosphate backbone-based RNA.
Parenthetically, it is not clear why D-nucleotides were
chosen during evolution of the first organized biological world (the
RNA world) but Kozlov et al. (1998)
have observed, at
least, that oligomerization of activated guanosine mononucleotides
proceeded past one or two L-dC residues when introduced
into D-oligo (dC)s as templates. They suggested that once a
"predominantly D-metabolism" existed, occasional
L-residues in a template would not have led to the
termination of self-replication.
The link between these observations on the appearance of RNA and amino
acids together with the mammalian ribosome, with its capacity to serve
as a site of peptide bond synthesis, is of course hard to establish.
However, it might be of interest that recently, Nitta et al. (1998)
made naked transcripts corresponding to each domain of
23S ribosomal RNA and found that a mixture of all six domains was
capable of stimulating peptide bond formation. In addition,
dePouplana et al. (1998)
proposed that tRNAs preceded
their synthetases, which may have been the first protein enzymes to
emerge from the RNA world. Further, Bailey (1998)
demonstrated that when RNA is retained on a surface, mimicking
prebiotic surface monolayers, it becomes automatically selective for
the L-enantiomer of the amino acid. He concluded that
L-amino acids dominate in metabolism as a consequence of
the selection of D-ribose, and not L-ribose, in
the early RNA molecules. Finally, in this context, the origin of and
amplification of biomolecular chirality and of the almost exclusive use
and presence in nature of L-amino acids, and of
L-glutamate in particular, require mention. This, again, is
a puzzle (Bonner 1991
) but it has been suggested that
there was an extraterrestrial origin of amino acids with an
L-enantiomer excess, which predated the origin of life on
Earth. Support for this comes from the discovery of an excess of
L-amino acids in the Murchinson meteorite (Engel and Macko 1997
), and astronomical sources of circular polarization
might account for substantial levels of enantiomeric excess in
interstellar chiral molecules (Bailey et al. 1998
).
Thus, there is some reason for us to understand how and why
L-glutamate is in higher abundance than its
D-enantiomer, although a fully sequential account of
L-glutamate’s appearance and further enrichment in Nature
remains to be established. It is worth noting, however, that
D-amino acids are present in food sources in which they
arise as a consequence of food processing or are present in bacterial
walls, marine bivalves and in plant material (Man and Bada 1987
). They occur in tissues and are removed by the kidney or
metabolized via D-amino acid oxidases, which are present in
liver and kidney. There appears to be some oxidation of
D-aspartic acid but not of D-glutamate, which
competes with L-glutamate for uptake by the kidney
(Carter and Welbourne 1998
); it appears in urine as
D-5-oxoproline (Sekura et al. 1976
).
Finally, the central position of L-glutamate in mammalian
metabolism (Fig. 1
) and, in particular, its presence in metabolic cycles, including the
citric acid cycle, is noteworthy and should be brought into the
prebiotic and evolutionary schema. Indeed, some further comment in the
context of its appearance at the beginnings of evolution of metabolism
might be useful. Thus, in brief, Waddell and Miller (1992)
demonstrated that the major product from sunlight
photolysis of glutamate was succinate and, analogously, aspartate is
photolyzed to malonic acid. These investigators suggest that the
photochemical, oxidative decarboxylation of glutamate parallels its
metabolism in modern cells, and that this may provide an evolutionary
link between simple amino acids and reactions of the citric acid cycle.
Subsequently, Melendez-Hevia et al. (1996)
attempted to
demonstrate that the evolution of the Krebs cycle emerged as a
consequence of the opportunistic reorganizing and assembling of
preexisting organic chemical reactions. They concluded that the modern
design of the Krebs cycle is a unique solution arrived at by
the assembly of pieces of chemical steps previously functioning for
amino acid biosynthesis; significantly, they emphasized that this
conclusion was strongly dependent on the biosynthesis of amino acids
and of glutamate and aspartate, in particular.
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). It
is also present in free form in relatively higher concentrations in
some foods, such as tomatoes and a variety of fruits (Giacometti 1979
) and it is the most abundant free anion in
Escherichia coli, where it serves both as an important
osmolyte (Cayley et al. 1991
) and as a nutrient.
Parenthetically, it might be mentioned here that the ligand-binding
domain in glutamate metabotropic receptors [see Meldrum (2000)
] is related to bacterial periplasmic binding proteins
(O’Hara et al. 1993
, Stern-Boch et al. 1994
), which are involved in nutrient uptake. Hence, during its
early evolution in metabolism, L-glutamate appears to have
taken on the multiple roles of a nutrient, a catalytic intermediate and
also of an excitatory molecule that is involved in cell-cell
communication. In the following section, therefore, we will attempt to
highlight the properties of glutamate that contribute to its function
and presumably to its relatively high abundance in nature. |
Properties of glutamate |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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). We begin with some comments on the overall
structural motif in which the "entire molecule," so to speak,
participates in its unique function, and then we will consider the
biochemical/chemical reactivity of specific positions within the
glutamate molecule. |
Overall structural motif and contributions to protein structure/function |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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, the odd carbon number in chain length clearly differentiates glutamate
from its homologs, 2-aminoadipic acid and aspartic acid. It might be
seen that this fact orients the 2-amino and 1-carboxylic oxygen atoms
on different planes of symmetry; in the case of glutamate, the oxygen
atoms form a "pocket" that is not present in its even-carbon
chain homologs. Further, from the linear presentation shown in
Figure 3
, it should also be clear that when the three homologs are aligned
according to their molecular centers, the axes of symmetry have
different angles and an opposite declination in the odd vs. even chain
length molecule; the interatomic distance between the oxygens in the
glutamate pocket is the smallest, thus contributing to tighter
hydration and cation retention as a coordinating structure. In
addition, the five-carbon skeleton of glutamate permits the
keto-enol tautomers and transient, shared-electron activation
states at the 2,3- and 4,5-positions to be in conjugation during active
site binding under enzyme catalysis. This is not so for its homologs,
which will be commented upon below.
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-helices (Zubay 1996
) (aspartic acid is in the bottom third with respect to
frequency and amino adipic acid is not a player because it is a
"post-translational" amino acid). Glutamate is a principal
contributor to the hydrogen bonding, hydration and
coordination-chelation phenomena leading to the characteristic
tighter packing of chain folds that permits -helices to function as
molecular springs. Not only does glutamic acid induce helicity to a
greater extent than its homologs (Kohn et al. 1995a
and 1995b
), but also, in its protonated form, the acid residue then
stabilizes the resulting coiled-coils by modulating their hydration
state in response to pH changes, in step with the physiologic processes
that require proteins to do "work" by folding and unfolding
repeatedly (Kohn et al. 1995a
and 1995b
,
Urry 1998
).
In contrast, ß-sheets have opposite requirements; glutamate
turns out to be the least abundant of the amino acids, although it is
present in polyglutamate runs in ß-sheets that must have special
rigidity; such is the case in proteoglycans, in which motifs such as
EENEEEEEE (where E = glutamate and N = asparagine) impart
rod-like structural regions. Examples of such
polyglutamate-rich sequences occur in nucleoplasmic protein where
they are required for facilitated translocation of the protein through
the nuclear pore complex (Vancurova et al. 1997
), in
bone sialoproteins where they act as a nucleators of calcium phosphate
formation (Boskey 1995
), and in the secretory
glycoprotein chromogranin A, which plays an intracellular role in
targeting peptide hormones to granules (Hendy et al. 1995
).
Further, with respect to glutamate sequences, the microtubule
cytoskeleton of eukaryotic cells is involved in many essential cellular
functions. Here, the post-translational modification of tubulin by
polyglutamylation (MacRae 1997
) regulates its
interaction with microtube-associated proteins (Bolinnec et al. 1998
). Thus, glutamate may be seen here to play a role in
the formation and function of the cystoskeleton (Audebert et al. 1994
). Finally, the importance of folylpolyglutamates might be
noted, in which these derivatives are intracellular substrates and
regulators of one-carbon metabolism and their synthesis is required
for normal folate retention by cells (Shane 1989
).
Another difference between glutamate and aspartate is that
although both are acidic residues, and have similarly low
pKa and isoelectric points, the extra
methylene group of glutamate invests it with a greater dipolar moment
and therefore a dramatically different hydrophobic effect. Compared
with glycine, whose hydrophobic effect is 1.18 kcal/mol, that of
protonated glutamic acid is 1.73 (higher means more hydrophobic),
whereas that of aspartic acid is 1.13. When deprotonated, glutamate
becomes highly hydrophilic by a fivefold greater factor than aspartate;
thus its amphiphilic nature invests glutamic acid with a unique
position among the polar amino acids as determined both by
physicochemical studies (Karplus 1997
) and by the effect
of residue substitutions in site-directed mutagenic experiments on
protein structure (Bordo and Argos 1991
).
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Reactivity of glutamate |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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) as discussed below.
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All three of the dicarboxylic amino acids form imines (Schiff bases)
with pyridoxal phosphate (PLP), the ligand responsible for
transamination. But the range of tautomers available to serve as
activated or transient states is severely restricted in the case of
aspartate, especially because enolization toward the 2,3-configurations
is unfavored by the electron-withdrawing nature of its
4-keto(carboxylic) group. The 4-methylene group in glutamate and of
amino adipic acid stabilizes this tautomer and its transient
ketimine-stabilized carbanion; as a consequence, the Schiff base
half-life for glutamate vs. aspartate is longer, and therefore the
range of exchangeable substrates is greater for glutamate. Thus,
glutamate will release its nitrogen to transaminating amino acids,
whereas aspartate exchanges its N only with glutamate
(Novogrodsky and Meister 1964
), an observation that has
been confirmed extensively from both a theoretical and a practical
point of view on the mechanism of PLP-mediated enzyme action
(Meister 1990
).
Another very obvious difference, also based on mechanistic
considerations of keto-enol and imine stabilization, lies in the
substrate specificity for -ketoacid reductive amination. The
pH-dependent tautomerization equilibrium between amine and imine is
an important component for the action of glutamate dehydrogenase in the
reaction of glutamate with NAD to produce NADH and ammonia, or of
-ketoglutarate with NADH and ammonia to regenerate glutamate. But
the essential determinant for the course of these catalyzed reactions
is the specific stereoconfiguration of glutamate, alluded to earlier,
with respect to the position of its two carboxyls and relative to the
amine-imine functionality. The reactive cavity of glutamic
dehydrogenase requires a tight fit between two lysines, which lock
glutamate’s carboxyls into place, while a third enzyme residue, a
protonated carboxyl, further positions the -site for reaction with
the redox ligand (Rife and Cleland 1980a
and 1980b
).
Aspartic acid is not a substrate for this dehydrogenase system and
2-aminoadipic acid shows only 0.01 of the activity (Struck and Sizer 1960
), suggesting that the five-carbon structure,
with its concomitant, conjugated keto-enol transients is an
indispensable feature for this all-important regulatory reductase
step. The significance of this metabolic reaction is well illustrated
by the recent identification of a new form of congenital
hyperinsulinemia and hyperammonemia syndrome due to mutations in the
glutamate dehydrogenase gene that impair the control of the activity of
the enzyme (Stanley et al. 1998
).
Finally, a similar argument about stabilization of resonant activated
intermediates applies to the action of glutamate decarboxylase
(O’Leary and Koontz 1980
), which supplies
4-aminobutyric acid. The latter is not only a neurotransmitter, but the
reaction affords a mechanism for the recycling of the glutamate carbon
skeleton into succinate. Aspartic acid and 2-aminoadipic acid are not
substrates in this type of transformation, possibly because of their
inability to sustain the requisite activated states (O’Leary et al. 1981
).
Reactivity of the 4-carbon.
The ability of glutamate to enolize into a conjugated system, aided by
enzymes and biological metal cations, is a unique mechanistic feature
that allows only glutamic acid to support carbonium ion and/or
stabilized radical formations at the 4-carbon. These are the activated
intermediate states that permit trapping of CO2
in the formation of carboxyglutamate acid (Vermeer 1990
), homolytic rearrangement to 3-methyl-aspartate under the
action of B-12—dependent enzymes (Marsh and Ballou 1998
) and alkylation, as an entry point for biosynthesis and
natural products. This property accounts for the numerous vitamin
K–dependent proteins, which contain post-translationally
carboxylated glutamic residues that are involved in blood coagulation,
the proteins in calcified tissues such as osteocalcin (a negative
regulator of bone formation) and matrix Gla protein (a calcification
inhibitor), as well as a number of others such as nephrocalcin and
plaque Gla proteins, whose functions remain unclear (Ferland 1998
). These proteins are characterized by an N-terminal
domain of ~40 amino acids in which the majority of glutamate residues
are carboxylated. This domain has a high affinity for calcium.
Reactivity of the 5-carbon.
Enolization at the 4,5-position, including resonance tautomer
stabilization by the electronic properties of the 3-methylene,
undoubtedly endows glutamate with the unique ability to sustain redox
reactions at the 5-carboxyl, affording the transient glutamic
semialdehyde species, which is the precursor for proline and ornithine
synthesis and the intermediate for the reverse flux of these latter
amino acids into glutamate as part of the metabolic elaboration of the
arginine cycle (Jones 1985
, Wakabayashi 1995
). Aspartate cannot sustain redox at the 4-carboxyl, which
would not be resonance stabilized, just as its 2-position is not
stabilized in the contexts already described. 2-Aminoadipic acid may
substitute for glutamate in these reactions, but only in the context of
inborn errors of metabolism and pipepolic acid formation in
nonmammalian systems.
The reactivity at the 5-carbon of glutamate is also characterized
by the well-known fact that this position readily forms amides.
Glutamate and aspartate each yield glutamine and asparagine,
respectively. Further, it is really not possible to consider glutamate
metabolism in the mammalian organism in any comprehensive way without
bringing glutamine into the picture. This would introduce a large area
of contemporary amino acid metabolic research, but this is beyond the
remit of this presentation. A number of reviews on glutamine function
and metabolism might be consulted for further details (Curthoys and Watford 1995
, Newsholme and Calder 1997
,
Souba 1991
, 1993
and 1997
). This property of
-glutamyl transamination endows the glutamate/glutamine pair with a
significant role in nitrogen trafficking within the body, especially
the transfer of nitrogen between peripheral tissues, notably muscle,
possibly lung, and the splanchnic region. Also, it has been proposed
recently that glutamate serves as a vehicle for the transport of
nitrogen, arising from the catabolism of N-rich amino acids, out of
the central nervous system (Lee et al. 1998
).
Additionally, the spectrum of terminal amidations is greatly
augmented, in the case of glutamate, by its ability to form a
thermodynamically stabilized, five-member lactam, (5-oxoproline).
This then becomes an active species in the -glutamyl cycle
(Beutler 1989
, Meister 1988
) as a
substrate in its own right, or as an activated intermediate in the
mechanism of transpeptidation. Aspartate cannot form a lactam, which
would be a very unfavorable four-member and strained ring
structure. The six-member lactam ring of 2-aminoadipic acid, in
contrast, is a thermodynamically stable end product. Therefore, it does
not participate in the -glutamyl reactions for which 5-oxoproline is
the thermodynamically optimal substrate.
In summary, it might be evident from these considerations of the reactivity of the glutamate molecule that it offers a number of features/properties not shared by its homologs. These make it a favorable choice, if not the only one, for facilitating metabolic processes that play major roles in the nitrogen economy of the host and in reference to structure-function relationships of a variety of key cellular proteins.
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Comments on the physiology of glutamate |
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reviewed this topic two decades ago. To limit our discussion we will
review briefly the two following aspects of the regulation of glutamate
metabolism: 1) blood and tissue glutamate concentrations and
2) factors that affect glutamine/glutamate turnover,
including the fate of dietary glutamate. These represent two areas of
investigation in which significant advances have been made since the
First International Glutamate Symposium (Filer et al. 1979
). Further and more elaborate presentations of advances are
given elsewhere in these proceedings. |
Blood and tissue concentrations |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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) (Forslund et al. 2000
, Maher et al. 1984
, Matthews and Campbell 1992
). This also
holds for their relative concentrations in the free amino acid pool of
muscle (Table 3
) with the muscle/plasma concentration gradient for both glutamate and
glutamine being much greater than for most other amino acids, except
taurine (Bergstrom et al. 1974
).
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) (Bergstrom et al. 1990
). In contrast, the
concentrations of plasma glutamate and glutamine in blood drawn from
subjects in the postabsorptive state appear to be affected by the
chronic level of dietary protein intake, especially when comparing
concentration changes within the supramaintenance range of intake
(Table 2)
. In our recent studies, conducted in collaboration with the
Uppsala group (Forslund et al. 2000
), we observed
(Fig. 6
) that a high protein intake [2.5 g protein/(kg body wt · d)]
results in a profound and significant fall in the 24-h concentration of
plasma glutamine and, in our hands, to a lesser extent in plasma
glutamate. The metabolic basis for and functional significance of this
response in circulating glutamine and glutamate levels to a high
protein intake is not clear. However, it raises the question of
whole-body glutamic acid/glutamine kinetics and how these respond
to changes in dietary protein intake.
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Glutamate and glutamine kinetics |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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. Since that time, direct estimates have been made of glutamate and
glutamine flux, using tracer techniques. An example of these is also
given in Table 4
. The following two points might be made: 1)
it seems that the tracer-derived values underestimate the flux of
glutamate and glutamine due to protein turnover. This is probably due
to the fact that measurement of the plasma glutamate and glutamine
isotopic abundances overestimates those in the extravascular pools and
thus the flux will be underestimated. On the other hand, Van Acker et al. (1998)
concluded that a slow equilibration of a
plasma glutamine tracer with the large muscle glutamate pool leads to
overestimates of whole-body glutamine flux when the tracer infusion
periods are relatively short (less than ~11 h). 2) There
is considerable de novo synthesis of both glutamate and glutamine,
which Munro (1979)
could not take into account for lack
of available data at that time.
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, Matthews and Campbell (1992)
reported a significant
decline in both glutamate and glutamine de novo synthesis rates when
dietary protein intake was increased from a level sufficient to meet
requirements [0.8 g protein/(kg · d)] to a high intake of
2.2 g protein/(kg · d). This finding presumably helps to
explain the fall in plasma glutamate/glutamine levels with high protein
feeding. However, the site(s) at which this response occurs (possibly
muscle; Nurjhan et al. 1995
) and the biochemical
mechanisms involved remain to be defined. Furthermore, it will be
important to understand how peripheral and visceral tissues/organs
interact to achieve the new steady-state changes in the plasma
amino acid concentrations.
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Glutamate uptake by intestinal tissues |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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, Windmueller Spaeth (1974
and 1975)
and
Neame and Wiseman (1958)
, it was clear to Munro
in 1979 that the intestinal tissues were responsible for a significant
metabolism of dietary glutamate and glutamine. Since then, elegant
stable isotope studies have extended our understanding of the
quantitative handling by the intestine of dietary glutamate and
glutamine, with a confirmation that little or no dietary glutamate
enters either the systemic (Matthews et al. 1993
) or the
portal blood supply; it is significantly oxidized in the splanchnic
region (Battezzati et al. 1995
) and probably in the
intestinal mucosa (Reeds et al. 1996
), where it serves
as a significant energy yielding substrate. Additionally, a significant
fate of glutamate is glutathione synthesis (Reeds et al. 1997
). These findings are discussed in detail elsewhere in
these proceedings but they are highlighted here as exciting examples of
our understanding about the quantitative aspects of the physiology of
glutamate metabolism. A net effect of the extensive intestinal
metabolism of glutamate is the achievement of relatively stable plasma
concentrations of glutamate and glutamine throughout the fasting and
fed periods of the 24-h day (Fig. 6)
(glutamate data not shown). |
Summary and Conclusion |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONPrebiotic and evolutionary...Properties of glutamateOverall structural motif and...Reactivity of glutamateComments on the physiology...Blood and tissue concentrationsGlutamate and glutamine kineticsGlutamate uptake by intestinal...Summary and ConclusionREFERENCES |
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1.
Audebert S., Koulakoff A., Berwald-Netter Y., Gros F., Denoulet P., Eddé B. Developmental regulation of polyglutamylated - and ß-tubulin in mouse brain neurons. J. Cell Sci. 1994;107:2313-2322[Abstract]
2.
Bailey J. RNA-directed homochirality. FASEB J 1998;12:503-507
3. Bailey J., Chrysostomou A., Hough J. H., Gledhill T. M., McCall A., Clark S., Ménard F., Tamura M. Circular polarization in star-formation regions: implications for biomolecular homochirality 1998 Science (Washington DC) 281 672–674.
4.
Battezzati A., Brillon D. J., Matthews D. E. Oxidation of glutamic acid by the splanchnic bed in humans. Am. J. Physiol. 1995;269:E269-E276
5. Beutler E. Nutritional and metabolic aspects of glutathione. Annu. Rev. Nutr. 1989;9:287-302[Medline]
6.
Bergstrom J., Furst P., Noree L. O., Vinnars E. Intracellular free amino acid concentration in human muscle tissue. J. Appl. Physiol. 1974;36:693-697
7. Bergstrom J., Furst P., Vinnars E. Effect of a test meal, with and without protein, on muscle and plasma free amino acids. Clin. Sci. (Lond.) 1990;79:331-337[Medline]
8. Blomstrand E., Anderson S., Hassmen P., Ekblom B., Newsholme E. A. Effect of branched chain-amino acid and carbohydrate supplementation on the exercise-induced change in plasma and muscle concentration of amino acids in human subjects. Acta Physiol. Scand. 1995;153:87-96[Medline]
9. Blomstrand E., Ek S., Newsholme E. A. Influence of ingesting a solution of branched-chain amino acids on plasma and muscle concentrations of amino acids during prolonged submaximal exercise. Nutrition 1996;12:485-490[Medline]
10. Böhler C., Nielsen P. E., Orgel L. E. Template switching between PNA and RNA oligonucleotides. Nature (Lond.) 1995;376:578-581[Medline]
11. Bolinnec Y., Moudjou M., Fougult J. P., Desbruyères E., Eddé B., Bornens M. Glutamylation of centreole and cycloplasmic tubulin in proliferating non-neuronal cells. Cell Motil. Cytoskelet. 1998;39:223-232[Medline]
12. Bonner W. A. The origin and amplificiation of biomolecular chirality. Origins Life Evol. Biosph. 1991;21:59-111[Medline]
13. Bordo D., Argos A. Suggestions for "safe" residue substitutions in site-directed mutagenesis. J. Mol. Biol. 1991;217:721-729[Medline]
14. Boskey A. L. Osteopontin and related phosphorylated sialoproteins: effects on mineralization. Ann. N.Y. Acad. Sci. 1995;760:249-256[Abstract]
15.
Carter P., Welbourne T. C. Glutamate transport asymmetry in renal metabolism. Am. J. Physiol. 1998;274:E877-E884
16. Cayley S., Lewis B. A., Guttman H. J., Record M. T., Jr Characterization of the cytoplasm of Escherichia coli K-12 as a function of external osmolority. Implications for protein-DNA interactions in vivo. J. Mol. Biol. 1991;222:281-300
17. Chyba C., Sagan C. Engodenous production, exogenous delivery and impact-shock synthesis of organic molecules: an inventory for the origins of life. Nature (Lond.) 1992;355:125-132
18. Curthoys N. P., Watford M. Regulation of glutaminase activity and glutamine metabolism. Annu. Rev. Nutr. 1995;15:133-159[Medline]
19.
dePouplana L. B., Turner R. J., Steer B., Schimmel P. Genetic code origins: tRNAs older than their synthesases?. Proc. Natl. Acad. Sci. U.S.A. 1998;95:11295-11300
20. Engel M. H., Macko S. A. Isotopic evidence for extraterrestrial non-racemic amino acids in the Murchinson meteorite. Nature (Lond.) 1997;389:265-268[Medline]
21. Ferland G. The vitamin K-dependent proteins: an update. Nutr. Rev. 1998;56:223-230[Medline]
22. Filer L. J., Jr Garattini S. Kare M. R. Reynolds W. A. Wurtman R. J. eds. Glutamic Acid: Advances in Biochemistry and Physiology 1979 Raven Press New York, NY.
23. Forslund, A. H., Hambraeus, L., van Beurden, H., Holmbäch, V., El-Khoury, A. E., Hjorth, G., Olsson, R., Stridsberg, M., Wide, L., Akerfeldt, T., Regan, M. & Young, V. R. (2000) The inverse relationship between intake and certain plasma free amino acid concentrations in healthy men, with physical exercise. Am. J. Physiol. (in press).
24. Giacometti T. Free and bound glutamate in natural products. Filer L. J., Jr Garattini S. Kare M. R. Reynolds W. A. Wurtmaned R. J. eds. Glutamic Acid: Advances in Biochemistry and Physiology 1979:25-34 Raven Press New York, NY.
25. Graham T. E., MacLean D. A. Ammonia and amino acid metabolism in skeletal muscle: Human, rodent and canine models. Med. Sci. Sports Exerc. 1998;30:34-46[Medline]
26. Harada K. Synthesis of amino acids and peptides under possible prebiotic conditions. Weinstein B. eds. Chemistry and Biochemistry of Amino Acids, Peptides, and Proteins 1974;vol. 2:297-351 Marcel Dekker New York, NY.
27. Hendy G. N., Bevan S., Mattei M.-G., Mouland A. J. Chromogranin A. Clin. Investig. Med. 1995;18:47-65[Medline]
28.
Henriksson J. Effect of exercise on amino acid concentrations in skeletal muscle and plasma. J. Exp. Biol. 1991;160:149-165
29. Hill A. R., Bohler C., Orgel L. E. Polymerization on the rocks: negatively-charged alpha-amino acids. Origins Life Evol. Biosph. 1998;28:235-243[Medline]
30.
Huber C., Wächterschauser G. Peptides by activation of amino acids with CO on (NiFe)S surfaces; implications for the origin of life. Science (Washingnton, DC) 1998;281:670-672
31. Jones M. E. Conversion of glutamate to ornithine and proline: pyrroline-5-carboxylate, a possible modulator of arginine requirements. J. Nutr. 1985;115:509-515
32. Karplus P. A. Hydrophobicity regained. Protein Sci 1997;6:1302-1307[Abstract]
33. Kohn W. D., Kay C. M., Hodges R. S. Protein destabilization by electrostatic repulsions in the two-stranded alpha-helical coiled-coil/leucine zipper. Protein Sci 1995a;4:237-250[Abstract]
34.
Kohn W. D., Monera O. D., Kay C. M., Hodges R. S. The effects of interhelical electrostatic repulsions between glutamic acid residues in controlling the dimerization and stability of two-stranded alpha-helical coiled-coils. J. Biol. Chem. 1995b;270:25495-25506
35.
Kozlov I. A., Pitsch S., Orgel L. E. Oligomerization of activated D- and L-guanosine mononucleotides on templates containing D- and L-deoxcytidylate residues. Proc. Natl. Acad. Sci. U.S.A. 1998;95:13448-13452
36.
Lee W.-J., Hawkins R. A., Vina J. R., Peterson D. R. Glutamine trasnport by the blood-brain barrier: a possible mechanism for nitrogen removal. Am. J. Physiol. 1998;274:C1101-C1107
37.
MacLean D. A., Spriet L. L., Hultman E., Graham T. E. Plasma and muscle amino acid and ammonia responses during prolonged exercise in humans. J. Appl. Physiol. 1991;70:2095-2103
38. MacRae T. H. Tubulin post-translational modifications; enzymes and their mechanisms of action. Eur. J. Biochem. 1997;244:265-278[Medline]
39.
Maher T. J., Glaeser B. S., Wurtman R. J. Diurnal variations in plasma concentrations of basic and neutral amino acids and in red cell concentrations of aspartate and glutamate: effects of dietary protein intake. Am. J. Clin. Nutr. 1984;39:722-729
40. Man E. H., Bada J. L. Dietary D-amino acids. Annu. Rev. Nutr. 1987;7:209-225[Medline]
41.
Marchini J. S., Cortiella J., Hiramatsu T., Chapman T. E., Young V. R. Requirements for indispensable amino acids in humans: longer-term amino acid study with support for the adequacy of the MIT amino acid requirement pattern. Am. J. Clin. Nutr. 1993;58:670-683
42. Marsh E. N., Ballou D. P. Coupling of cobalt-carbon bond homolysis and hydrogen atom abstraction in adenosylcobalamin-dependent glutamate mutase. Biochemistry 1998;37:11864-11872[Medline]
43.
Matthews D. E., Campbell R. G. The effect of dietary protein intake on glutamine and glutamate nitrogen metabolism in humans. Am. J. Clin. Nutr. 1992;55:963-970
44.
Matthews D. E., Marano M. A., Campbell R. G. Splanchnic bed utilization of glutamine and glutamate in humans. Am. J. Physiol. 1993;264:E848-E854
45.
Meister A. Glutathione metabolism and its selective modificton. J. Biol. Chem. 1988;263:17205-17208
46. Meister A. On the transamination of enzymes. Ann. N.Y. Acad. Sci. 1990;585:13-31[Medline]
47. Meldrum B. S. Glutamate as a neurotransmitter in the brain: review of physiology and pathology. J. Nutr. 2000;130:1007S-1015S
48. Melendez-Hevia E., Waddell T. G., Cascante M. The puzzle of the Krebs citric acid cycle: assembling the pieces of chemically feasible reactions, and opportunism in the design of metabolic pathways during evolution. J. Mol. Evol. 1996;43:293-303[Medline]
49. Munro H. N. Factors in the regulation of glutamate metabolism. Filer L. J., Jr Garattini S. Kare M. R. Reynolds W. A. Wurtmaned R. J. eds. Glutamic Acid: Advances in Biochemistry and Physiology 1979:55-68 Raven Press New York. NY.
50. Neame K. D., Wiseman G. The alanine and oxo acid concentrations in mesentric blood during the absorption of L-glutamate acid by the small intestine of dog, cat and rabbit in vivo. J. Physiol. 1958;140:148-155[Medline]
51. Newsholme E. A., Calder P. C. The proposed role of glutamine in some cells of the immune system and speculative consequences for the whole animal. Nutrition 1997;13:728-730[Medline]
52.
Nitta I., Kamada Y., Nodu H., Ueda T., Watanabe K. Reconstitution of peptide band formation with Escherichia coli 23S ribosomal RNA domains. Science (Washington, DC) 1998;281:666-669
53. Novogrodsky A., Meister A. Specificity and resolution of glutamate-aspartate transaminase. Biochim. Biophys. Acta 1964;81:605-608
54. Nurjhan N., Bucci A., Perriello G., Stumvoll M., Dailey G., Bier D. M., Toft I., Jensen T. G., Gerich J. E. Glutamine: a major gluconeogenic precursor and vehicle for intraorgan carbon transport in man. J. Clin. Investig. 1995;95:272-277
55. O’Hara P. J., Sheppard P. O., Thøgersen H., Venezia D., Haldeman B. A., McGrane V., Hovamed K. M., Thomsen D., Gilbert T. L., Mulvihill E. R. The ligand-binding domain in metabotropic glutamate receptors is related to bacterial periplasmic binding proteins. Neuron 1993;11:41-52[Medline]
56. O’Leary M. H., Koontz S. W. Coenzyme binding site of glutamate decarboxylase. Biochemistry 1980;19:3400-3406[Medline]
57. O’Leary M. H., Yamada H., Yapp C. J. Multiple isotope effect probes of glutamate decarboxylase. Biochemistry 1981;20:1476-1481[Medline]
58. Orgel L. E. Polymerization on the rocks: theoretical introduction. Origins Life Evol. Biosph. 1998;28:227-234[Medline]
59. Piccirilli J. A. RNA seeks its maker. Nature (Lond.) 1995;376:548-549[Medline]
60. Reeds P. J., Burrin D. G., Jahoor F., Wykes L., Henry J., Frazer E. M. Enteral glutamate is almost completely metabolized in first pass by the gastrointestinal tract of infant pigs. Am. J. Phyisol. 1996;270:E413-E418
61.
Reeds P. J., Burrin D. G., Stoll B., Jahoor F., Wykes L., Henry J., Frazer M. E. Enteral glutamate is the preferential source for mucosal glutathione synthesis in fed piglets. Am. J. Physiol. 1997;273:E408-E415
62. Rife J. E., Cleland W. W. Kinetic mechanism of glutamate dehydrogenase. Biochemistry 1980a;19:2321-2328[Medline]
63. Rife J. E., Cleland W. W. Determination of the chemical mechanism of glutamate dehydrogenase from pH studies. Biochemistry 1980b;19:2328-2333[Medline]
64. Sekura R., Van derWerf P., Meister A. Mechanism and significance of the mammalian pathway for elimination of D-glutamate, inhibition of glutathione synthesis by D-glutamate. Biochem. Biophys. Res. Commun. 1976;71:11-18
