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Department of Animal Sciences, University of Illinois, Urbana, IL 61801 and * U.S. Department of Agriculture/ARS Children’s Nutrition Research Center, Baylor College of Medicine, Houston, TX
3To whom correspondence should be addressed. E-mail: preeds{at}uiuc.edu.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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KEY WORDS: • glutamine metabolism • intestine • glutamate
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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). These
regulatory roles are supported by an intrinsic intestinal neural system
(Kudsk 2000
) that is separate from, but functionally
related to the central neural pathways. Thus, the intestine is one
partner in a central-peripheral system that senses both the
antigenic and the nutritional environment and thereby modifies the host
response.
The host pays a metabolic price for these critical intestinal
functions. The portal drained viscera (the stomach, intestine, pancreas
and spleen) are among the most metabolically active tissues in the
body. For example, although these tissues collectively never account
for >6% of body weight, they can be responsible for up to 50% of the
whole-body turnover of some essential amino acids (Stoll et al. 1998
, Yu et al. 1992 and 1995
) and between
10 and 20% of whole-body energy expenditure (van Goudoever et al. 2000
). For these reasons alone, the examination of the
substrates that are used by the intestinal tissues and the potential
for nutrient regulation of the intestine’s multiple functions is a
subject worthy of intensive study.
In 1974, Windmueller and Spaeth published the first of a series of
highly influential papers (Windmueller and Spaeth 1974, 1975, 1976 and 1980
) in which they demonstrated that amino acids,
especially nonessential amino acids have an important metabolic role in
the intestine. However, from the perspective of the present discussion,
they made the crucial observation that the intestine removes as much as
25% of the systemic flux of glutamine. Their measurements of
intestinal glutamine metabolism also showed that glutamine metabolism
could not only contribute a nutritionally important portion of
intestinal energy generation, but that the amino acid was the precursor
for a number of important metabolic pathways, especially those leading
to the synthesis of ornithine, citrulline, proline and arginine [see
Wu (1998)
for further discussion]. Although they
studied the metabolism of other amino acids, especially glutamate and
aspartate (Windmueller and Spaeth 1980
), their
observations of glutamine metabolism have had a substantial influence
on a number of aspects of clinical nutrition and have spawned a
substantial literature on the role of glutamine in the bowel
(Hall et al. 1996
, Smith and Wilmore 1990
).
In this paper we wish to survey aspects of this literature and discuss a number of general questions as follows: 1) Is glutamine essential for intestinal function? 2) To what extent does this relate to its metabolic role? 3) What is the importance of glutamine as a biosynthetic precursor? 4) Is glutamine supplementation of the nutrient mixture presented to patients of any metabolic or clinical benefit?
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Glutamine, glutamate and mucosal metabolism |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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). In the first, the amide nitrogen of glutamine is utilized
in support of purine, pyrimidine and amino sugar synthesis. In the
second, the carbon chain and 
-amino group of glutamine
enter pathways that lead to the synthesis of other amino acids, notably
proline, ornithine and arginine (Wu 1998
). This
biochemical division of glutamine metabolism also reflects an
intracellular compartmentalization, in as much as purine, pyrimidine
and amino sugar synthesis are cytoplasmic activities, whereas the
metabolism of the carbon skeleton of glutamine is initiated by its
deamidation by mitochondrial phosphate-dependent glutaminase
(Curthoys and Watford 1995
).
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There are, as we can ascertain, no in vivo data to identify directly
whether glutamine amide nitrogen is an obligatory requirement for
mucosal health. For example, even though glutamine starvation of
isolated intestinal mucosal cell lines does substantially inhibit their
proliferation, there is evidence to suggest that in regard to the
support of small intestinal mucosal mass (Horvath et al. 1996
) and protein synthesis (Hasebe et al. 1999
), glutamate is as effective as glutamine. Moreover, those
pathways of mucosal intermediary metabolism that utilize the carbon
skeleton of glutamine can apparently utilize glutamate equally well. In
fact it seems that both in vitro (Wu 1997
, Wu et al. 2000
, Wu and Reeds, unpublished data; Fig. 1
) and in vivo (Brunton et al. 1999
), proline is a more effective
substrate for arginine, ornithine and polyamine synthesis than either
glutamine or glutamate. The same apparently applies to the
effectiveness of glutamate as a precursor for mucosal glutathione
synthesis (Reeds et al. 1997
). Moreover, in humans, the
first-pass fractional extraction of dietary glutamate is greater
than that of enteral glutamine (Matthews et al. 1993
),
suggesting that the metabolism of glutamine taken up from the
mesenteric artery is more extensive than that of glutamine absorbed
from the intestinal lumen. Limited evidence in pigs suggests much the
same conclusion (Stoll et al. 1999
), and in these
studies, substantially more visceral CO2 was
generated from glutamate and glucose metabolism than from glutamine
(Fig. 2
). At this stage then, we would conclude that from a strictly metabolic
perspective, glutamate and glutamine are interchangeable as important
substrates for the mucosal cellular system.
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New roles for glutamine in the gut |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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) published immunocytochemical evidence for the presence of
glutamine synthase in the mucosa, and James et al. (1998)
demonstrated the presence of the enzyme by direct
biochemical measurements. In addition, there is isotopic evidence both
in vivo (Reeds et al.1996
) and in vitro (Fig. 1)
for
active glutamine synthesis in the intestinal mucosa. Of particular
importance is the recent evidence (DeMarco et al. 1999
)
showing that even in the presence of physiologic concentrations of
extracellular glutamine, inhibition of glutamine synthesis with
methionine sulfoxamine substantially inhibits the proliferation of IEC6
cells and the appearance of differentiation markers in Caco-2 cells
(Weiss et al. 1999
). Indeed, those authors developed the
provocative argument that endogenously synthesized glutamine may play a
specific, but as yet uncharacterized role in the mucosa.
Taking these observations in conjunction with the ability of glutamate
to substitute for glutamine in a number of biosynthetic pathways raises
the possibility that glutamine is playing a regulatory rather than a
biochemical role in these cells. This idea has received significant
support from recent research by Rhoads and his colleagues
(Blikslager et al. 1999
, Rhoads et al. 1997 and 2000
). The results obtained in these experiments are
particularly intriguing because they have shown the following:
1) not only does glutamine specifically activate protein
kinases that are known to be involved in cell cycle regulation, but
2) glutamine metabolism is apparently necessary for these
regulatory actions. Unfortunately, the mechanism underlying the effects
of glutamine on intracellular regulatory kinases as well as the pathway
of glutamine metabolism involved remain unidentified.
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Does glutamine make a difference? |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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) and intestinal protective
function (Stechmiller et al. 1997
) that accompany
disease and stress, it is plausible to argue that glutamine
supplementation may be of therapeutic benefit in the support of the
intestinal mucosa and immune systems (Wilmore and Shabert 1998
) under conditions of disease and trauma. Not surprisingly,
there have been many attempts to use glutamine supplementation to
ameliorate the metabolic changes that accompany stress in general and
intestinal disease in particular [reviewed by Elia and Lunn (1997)
, Fürst (2000)
and Sacks (1999)
]. We think that it is fair to say that not only does the literature report confusing and variable success but also serves to emphasize the following. First, it is crucial to identify the desired end point of glutamine supplementation. Second, it is equally important to define the nature of the stress or disease that it is hoped will be ameliorated with glutamine. Third, it is more than likely that the route of glutamine supplementation (parenteral or enteral) influences the response and, finally, other aspects of the nutritional support of the patient are of extreme importance.
Among these considerations, we would argue that the question of end
point is the most crucial, and the effects of glutamine on nitrogen
metabolism provide a good example of this. Thus, in the eight studies
of parenteral glutamine supplementation identified by Sacks (1999)
in which measurements of nitrogen balance were made,
there were uniform increases in circulating glutamine concentrations
and improved nitrogen balance. On the other hand, clinically
demonstrable benefit was not a uniform finding. Conversely, in 18
studies of enteral glutamine supplementation (Fürst 2000
), there are no reports of significantly improved nitrogen
balance, but a number of reports of improved morbidity. Even so, the
benefits have not been uniform (Elia and Lunn 1997
). By
and large, we would conclude that effects of glutamine supplements on
mucosal mass, even in animal models, have been equivocal at best.
Despite this rather negative conclusion, the emergence of new roles for
glutamine suggests other areas in which glutamine supplements may prove
to be of benefit. In this regard, one promising area is the putative
role of glutamine in amino sugar synthesis. This role has two potential
implications. First, by influencing the synthesis of components of the
extracellular matrix, glutamine may be one factor in the maintenance of
mucosal structure, especially the maintenance of tight junctions
(Panigrahi et al. 1997
). Second, by being a potential
precursor for N-acetylglucosamine and
N-acetylglactosamine synthesis, glutamine could play a
critical role in intestinal mucin synthesis and hence in the
maintenance of the passive barrier to bacterial ingress (Khan et al. 1999
). In this regard, in one of the most interesting
recent papers concerned with enteral glutamine supplementation of the
diets of low-birth-weight infants (Neu et al. 1997
), the
supplement had no effect on either circulating glutamine concentrations
or infant growth but was associated with changes in immune cell subtype
distribution that were compatible with the idea that the supplement had
lowered the overall immune challenge presented to the infants. Whether
this reflected the maintenance of tight junctions and mucin synthesis
(Khan et al. 1999
) or whether it reflected interactions
with locally generated cytokines (Kudsk et al. 2000
) is
not known at this time.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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FOOTNOTES |
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2 Supported in part by federal funds from U.S.
Department of Agriculture, Agricultural Research Service Cooperative
Agreement no 58–6250-6001, Cooperative State Research, Education and
Extension Service grant 98–35206 and by National Institutes of Health
Grant RO1-HD33920. The contents of this publication do not necessarily
reflect the views or policies of the U.S. Department of Agriculture,
nor does mention of trade names, commercial products, or organizations
imply endorsement by the U.S. government.
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LITERATURE CITED |
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TOPABSTRACTINTRODUCTIONGlutamine, glutamate and mucosal...New roles for glutamine...Does glutamine make a...CONCLUSIONSLITERATURE CITED |
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