Download Glutamine and the bowel

Glutamine and the bowel
報告者：張靜怡
日期：93.02.12
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Introduction ~ Intestine

Functions

digestion
nutrition absorption
fermentation



traditional view
performs a number of critical physiologic functions
→ complex, multicelllular organ
1. Secretory cell
2. Immune cell
3. Neuroendocrine cell
4. Absorptive enterocytes
Intestinal mucosa
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Introduction ~ Intestine

Secretory cell + Immune cell
→ intestinal tissues are involved in immune surveillance and in
generating endocrine responses to the in lumenal environment.
(Burrin et al. 2000)

Neuroendocrine cell
→ these regulatory roles are supported by an intrinsic intestinal neural system.
(Kudsk 2000)
Functionally related to the central neural pathways
Thus, the intestinal is one partner in a central-peripheral system
that senses both the antigenic and the nutritional environment.
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Introduction ~ Intestine

The most metabolically active tissues in the body

the portal drained viscera ( stomach, intestine, pancreas and spleen)

Weight < 6% of body weight
but can be responsible for
1. up to 50% of the whole-body turnover of some essential amino acids
(Stoll et al.1998, Yu et al. 1992 and 1995)
2. 10 ~ 20% of whole-body energy expenditure
(van Goudoever et al. 2000)
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Introduction ~ Glutamine

In 1974, Windmueller and Spaeth published the first of a series
of highly influential papers
(Windmueller and Spaeth1974, 1975, 1976 and 1980)
→ demonstrated that amino acids, especially nonessential amino acids have an
important role in the intestine.
→ made the crucial observation that the intestine removes as much as 25% of
the systemic flux of glutamine.
→ intestinal glutamine metabolism：
1. Contribute a nutritionally important portion of intestinal energy generation.
2. As precursor：synthesis of ornithine, cotrulline, proline and arginine.
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Introduction ~ Glutamine

The 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)
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General questions

Is glutamine essential for intestinal function ?

To what extent does this relate to its metabolic role ?

What is the importance of glutamine as a biosynthetic
precursor ?

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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Glutamine
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Glutamine

Functional pathways : (Table 1.)
1. The amide nitrogen of glutamine is utilized in support of
purine, pyrimidine and amino sugar synthesis.
2. The carbon chain and -amino group of glutamine enter
pathways that lead to the synthesis of other amino acids
especially proline, ornithine and arginine. (Wu 1998)
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Glutamine Metabolism

The biochemical division reflects an intracellular
compartmentalization :
1. purine, pyrimidine and amino sugar synthesis are
cytoplasmic activities.
2. the metabolism of carbon skeleton of glutamine is
initiated by its deamidation by mitochondrial
phosphate-dependent glutaminase.
(Curthoys and Watford 1995)
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Glutamine Metabolism

Is Glutamine necessary for mucosal metabolism ?
1. The fact that the amide group of glutamine is important specifically
for nucleic and mucin glycosylation implies strongly that the presence
of glutamine is obligatory for the maintenance of proliferative and
secretory activity.
2. The utilization of the carbon skeleton of glutamine involves its initial
conversion to glutamate and free ammonia. ---------- two questions
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Glutamine Metabolism

two questions :
1. Is the high fractional rate of glutamine oxidation in mucosa simply a
reflection of the necessary oxidation of the glutamate released from the
deamidation of glutamine ?
2. What extent can glutamate substitute for glutamine in the pathways
that involve the utilization of the carbon and amino nitrogen moieties ?
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Is Glutamine necessary for mucosal metabolism ?
Studied finding :
1.
Even thought glutamine starvation of isolated intestine mucosal cell
lines does substantially inhibit their proliferation, there is evidence
to suggest that in regard to the support of small intestinal mucosal
mass and protein synthesis, glutamate is as effective as glutamine.
(Horvath et al. 1996 , Hasebe et al. 1999)
2.
Those pathways of mucosal intermediary metabolism that utilize the
carbon skeleton of glutamine can apparently utilize glutamate
equally well.
in vitro (Wu 1997, Wu et al. 2000 Wu and Reeds, unpublished data ; Fig. 1)
in vivo ( Brunton et al. 1999)
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Is Glutamine necessary for mucosal metabolism ?
3.
The effectiveness of glutamate as a precursor for mucosal
glutathione synthesis. (Reeds et al. 1997)
4.
In humans, the first-pass fractional extraction of dietary glutamate is
grater than that of eternal glutamine . (Matthewset al. 1997)
→ the metabolism of glutamine taken up from the mesenteric artery is more
extensive than that of glutamine absorbed from the intestinal lumen.
5.
In pigs, more visceral CO2 was generated from glutamate and
glucose metabolism than from glutamine. (Stoll et al. 1999) (Fig. 2)
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Is Glutamine necessary for mucosal metabolism ?
Glutamate and glutamine are
interchangeable as important substrates
for the mucosal cellular system.
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New roles for gluatmine in the gut

The extracellular glutamine :
1. removed from the arterial circulation by the
intestinal tissues.
2. The mucosal cells in both the crypt and villous
compartments are simultaneously synthesizing
glutamine.
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New roles for gluatmine in the gut
1. In 1996, Neu and Shenoy published :
→ immunocytochemical evidence for the presence of glutamine
synthase in the mucosa.
2. In 1998, James et al. demonstrated :
→ the presence of the enzyme by direct biochemical measurement.
3. Isotopic evidence both in vivo (Reeds et al. 1996) and in vitro (Fig. 1)
→ active glutamine synthesis in the intestinal mucosa.
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New roles for gluatmine in the gut
4.
Even in the presence of physiologic concentrations of
extracellular glutamine, inhibition of glutamine synthesis with
methionine suflfoxamine substantially inhibits the
proliferation of IEC6 cells (DeMarco et al. 1999) and the appearance
of differentiation markers in Caco-2 cells (Weiss et al. 1999).
Endogenously synthesized glutamine may play a specific,
but as yet uncharacterized role in the mucosa.
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New roles for gluatmine in the gut

Observations :
the ability of glutamate to substitute for glutamine in a number
of biosynhetic pathways raises the possibility that glutamine is
playing a regulatory rather than a biochemical role in these
cells.
(Blikslager et al. 1999 , Rhoads et al. 1997 and 2000)
1. glutamine specifically activate protein kinases that are known to be
involved in cell cycle regulation.
2.glutamine metabolism is apparently necessary for these regulatory
actions.
Mechanism unknown1
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Dose glutamine make difference ?

Evidences :
1. glutamine in the bowel is taken with the alterations in
interorgan flow of glutamine.
(Souda and Austgen 1990)
2. intestinal protective unction that accompany disease and stress .
(Stechmiller et al. 1997)
3. glutamine supplementation may be of therapeutic benefit in the support of the
intestinal mucosa and immune systems under conditions of disease and trauma.
(Wilmore and Shabert 1998)
glutamine supplementation use for stress un general and intestinal
disease in particular
(reviewed by Elia and Lunn ( 1997) ,Fűrst (2000) and Sacks(1999) )
？
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？
1.
It is crucial to identify the desired end point of glutamine
supplementation.
2.
It is equally important to define the nature of the stress
or disease that is hoped will be improved with glutamine.
3. It is more than likely that the route of glutamine
supplementation (parenteral or enteral) influences the
response.
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For end point &
the effects of glutamine on nitrogen metabolism

8 studies of parenteral glutamine supplementation (Sacks 1999) :
→ measurement of nitrogen balance were made, there were uniform
increases in circulating glutamine concentrations and improved
nitrogen balance. ~ human study

18 studies of enteral glutamine supplementation (,Fűrst 2000 ) :
→ there are no reports of significantly improved nitrogen balance, but a
number of reports of improved morbidity . ~ animal study
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Dose glutamine make difference ?

For amino sugar synthesis ：
1. glutamine may be one factor in the maintenance of mucosal structure,
especially the maintenance of tight junction.
(Panigrahi et al. 1997)
2. a 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)

For immune cell change
(Neu et al. 1999)

For cytokines generate
(Kudsk et al. 1999)
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Conclusion

Systemic glutamine supports the function of the intestinal
mucosal system.

If glutamine does play a physiologic role in the bowel, it is
not compellingly related to its intermediary metabolism.
→ in fact, glutamate and proline, especially derived from the diet, can
readily substitute for many of the metabolic roles of glutamine,
including energy generation and amino acid synthesis.
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Conclusion

The mucosal cells not only utilize extracellular glutamine
but also synthesize the amino acid.
Given that inhibition of glutamine synthesis inhibits both proliferation
and differentiation of mucosal cell cultures.
→ Glutamine may be performing some more subtle regulatory role.
→ demonstration that glutamine will activate a number of genes
associated with cell cycle progression in the mucosal cells.
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Conclusion
Both the mechanisms underlying glutamine’s function
and whether glutamine supplementation uniformly
benefits mucosal health remain, at best, equivocal.
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Reference

Peter J. Reeds , Doiglas G Burrin (2001) Glutamine and
The Bpwel. J Nutr. 131 : 2505S-2508S

Reginald H. Garrett . (1995) Biochemistry, pp836-837.
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