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U.S. Department of Agriculture/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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KEY WORDS: • amino acid nutrition • metabolism • dietary requirement • function
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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): "We have emphasized on several occasions... that the classification of an amino acid like arginine or glutamic acid as dispensable or indispensable is purely a matter of definition."
I wish to consider this "matter of definition" by examining the terms from a nutritional, metabolic and functional perspective.
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Nutritional definitions of indispensable and dispensable amino acids |
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TOPABSTRACTINTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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) was, "One which cannot be synthesized by the animal
organism out of materials ordinarily available to the cells
at a speed commensurate with the demands for normal
growth." The key phrases in this definition, and phrases that were, in fact, italicized by the authors, are "ordinarily available," "at a speed" and "normal growth." Each is an important qualifier.
The phrase "ordinarily available" is important because a number of
nutritionally essential amino acids, e.g., the branched-chain amino
acids, phenylalanine and methionine, can be synthesized by
transamination of their analogous 
-keto acids. However, these keto
acids are not normally part of the diet and hence are not "ordinarily
available to the cells." The phrase "at a speed" is important
because there are circumstances in which the rate of synthesis of an
amino acid can be constrained, e.g., by the availability of appropriate
quantities of metabolic nitrogen. Indeed, the rate of synthesis becomes
of specific importance when we consider a group of amino acids,
exemplified by arginine, cysteine, proline and perhaps glycine, that
are frequently described as conditionally essential. For example,
Womack and Rose (1947)
made the important point that the
degree to which arginine could be regarded as indispensable was very
much a function of the quantities of its natural precursors, proline
and glutamate, in the diet. Finally, the phrase "normal growth" is
critical in two respects. First, it serves to emphasize that the
definitions were originally constructed in the context of growth. For
example, it is possible to show (Table 1
) that the ingestion of diets completely devoid of glutamate, which in
some ways can be regarded as the doyen of dispensable amino acids,
leads to a small but statistically significant slower rate of growth.
Second, constraining the definition of essentiality to growth does not
encompass the importance of some amino acids to pathways of disposal
other than protein deposition, a subject that I discuss later.
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Amino acid biosynthesis |
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TOPABSTRACTINTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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). In this regard, it is very important to note that the loss of the
ability to carry out these biosyntheses appeared early in evolution and
is a common feature of the metabolism of eukaryotic organisms in
general, and not just of mammals. However, within this view, the
important term is de novo synthesis. This is because some indispensable
amino acids can be synthesized from precursors that are structurally
very similar. For example, methionine can be synthesized both by
transamination of its keto acid analogue and by remethylation of
homocysteine. In this sense, then, the mammal is capable of
synthesizing leucine, isoleucine, valine, phenylalanine and methionine.
However, this is not new synthesis, because the branched-chain keto
acids and homocysteine were originally derived from branched-chain
amino acids and methionine, respectively. According to this restricted
metabolic definition of essentiality, threonine and lysine (and perhaps
tryptophan) are the only truly essential amino acids.
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). Interestingly, there is a reciprocal relationship between the
contribution of endogenous synthesis to the plasma flux of a given
nonessential amino acid and the degree to which the intestine
metabolizes the dietary amino acids in first pass (see Reeds et al. 1996
, Stoll et al. 1998
).
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Conditionally essential amino acids |
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TOPABSTRACTINTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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First, the synthesis of these amino acids (Table 4
) requires the provision of another amino acid, either as the carbon
donor or as a donor of an accessory group, such as the sulfur group of
cysteine. Thus, the ability of the organism to synthesize a given
conditionally essential amino acid is set by the availability of its
amino acid precursor, a point that was emphasized by Rose in his
studies of the interactions among glutamate, proline and arginine
nutrition. In some cases, e.g., the maintenance of the glycine supply
of the milk-fed mammal, the demand for the conditionally essential
amino acid synthesis (Jackson et al. 1981
) necessitates
an increase in the synthesis of its precursor, in this case, serine.
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, Wu et al. 1997
). Moreover, in the case of
these two amino acids, the available evidence suggests that dietary, as
opposed to systemic, amino acid precursors are obligatory
(Beaumier et al. 1995
, Berthold et al. 1995
, Brunton et al. 1999
, Murphy et al. 1996
, Stoll et al. 1999
). It follows from this
that alterations in either intestinal metabolism or the route of
nutrition can have a critical bearing on the ability of the organism to
synthesize these amino acids. This is shown strikingly by the problems
of arginine and ammonia homeostasis that accompany total parenteral
nutrition (Brunton et al. 1999
).
Third, most evidence suggests that even in the presence of abundant
quantities of the appropriate precursors, the quantities of
conditionally essential amino acids that can be synthesized may be
quite limited (Beaumier et al. 1995
, Berthold et al. 1995
, Castillo et al. 1993
, Fukagawa et al. 1996
, Jaksic et al. 1987
), so it can be
argued that there are circumstances, especially stressful
circumstances, under which the metabolic demands for the amino acids
rise to values that are beyond the biosynthetic capacity of the
organism. Such appears to be the case with regard to the proline
nutrition of burned individuals (Jaksic et al. 1991
).
Moreover, in immature individuals, such as low-birth-weight infants, it
is possible that conditionally essential amino acid synthesis may be
limited by a frank lack of enzymic activity (Gaull et al. 1972
).
These comments do, however, have to be tempered with caution, because
it seems likely that the metabolism of some conditionally essential
amino acids is tightly compartmentalized and hence that isotopic
measurements in the plasma pool may give a quantitatively misleading
impression of the scale of biosynthesis. This apparently applies to
proline, arginine and cysteine metabolism, because estimates of their
rate of synthesis from parallel measurements of intake and body
proteolysis do not agree with estimates based on isotopic incorporation
from labeled precursors (cf Beaumier et al. 1995
and
Berthold et al. 1995
for arginine; Jaksic et al. 1987
and Berthold et al. 1995
for proline).
Furthermore, there is also evidence to suggest that newly synthesized
conditionally essential amino acids may be used within their cells of
origin and hence do not equilibrate with the plasma pool (Miller et al. 1996
). Nevertheless, even with these uncertainties, it
appears that the synthesis of these amino acids can become limiting for
growth and other physiological functions and that an absolute, as
opposed to a relative, dietary requirement can be defined.
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Amino acids and physiological function |
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TOPABSTRACTINTRODUCTIONNutritional definitions of...Amino acid biosynthesisConditionally essential amino...Amino acids and physiological...REFERENCES |
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). In other words, the amino acid requirements for the support of
protein deposition in the human infant differ from those of other
mammals only to the degree to which their respective rates of protein
deposition differ.
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), and >90% of the total amino acid
requirement, even of the young child, is associated with the
maintenance of body protein stores (i.e., nitrogen equilibrium).
Formulating the amino acids needs for "maintenance" is difficult
and is still the subject of controversy (see Young and Borgonha 2000
).
At least as important as the technical and experimental difficulties
associated with the measurements of maintenance amino acid needs
(Fuller and Garlick, 1994
) is the problem of identifying
the processes that consume amino acids close to nitrogen equilibrium. A
portion of these needs is, of course, directly associated with protein
metabolism and reflects two related factors: that amino acids released
from tissue protein degradation are unlikely to be recycled with
complete efficiency, and that the presence of finite concentrations of
free amino acids inevitably leads to some degree of catabolism. There
also is increasing evidence that a significant portion of the needs for
some essential amino acids may reflect the <100% efficient recycling
of intestinal secretions (Fuller et al. 1994
,
Fuller and Reeds, 1998
. This aspect of basal or
maintenance amino acid needs is amenable to direct measurement,
although some technical aspects of these measurements, notably those
associated with intestinal protein metabolic function, pose
difficulties (see Fuller and Reeds, 1998
). However, as
more information has accrued, it has become increasingly clear that
amino acids are involved (and hence consumed) in a number of
physiological functions that are not directly related to protein
metabolism itself.
Before passing to a discussion of these pathways, it is critical to
emphasize two additional points. First, at protein intakes that are
just sufficient to maintain body protein equilibrium, metabolic
nitrogen itself, rather than any single amino acid, may be the limiting
nutrient. In other words, because nitrogen is in short supply, the
ability of the organism to synthesize amino acids may become
compromised to the extent that nonessential amino acid intake could
become limiting. This might be particularly applicable to conditions
associated with the consumption of low quantities of so-called high
quality proteins (i.e., proteins that are well balanced for protein
deposition and hence with a high indispensable amino acid/dispensable
amino acid ratio). Second, there is now evidence to show that the adult
human is capable of lowering the catabolism of any single amino acid
close to zero if that amino acid is strongly limiting (Raguso et al. 1999
). However, the rate of catabolism of the amino acid
observed under this circumstance is much lower than that found when
protein as a whole is the limiting dietary nutrient. One explanation
for this observation is that under protein-free feeding conditions,
the free amino acid pool derives exclusively from tissue proteolysis so
that all amino acids are equally limiting. The consequence is that the
utilization of any single amino acid in the support of a nonprotein
process automatically limits the ability of the organism to recycle all
others back into the protein stores of the body. The questions that
arise are: What are these nonprotein pathways of consumption, and what
is their quantitative impact on amino acid needs in general? The short
answer to both questions is simple: There is not sufficient current
information to provide accurate answers. Nevertheless, it is possible
to hypothesize which pathways could be the most important at the level
of overall physiological function.
To develop these hypotheses, it is useful to consider those functions
that are necessary to maintain health. This is not a new approach, as
its usefulness was clearly appreciated by some of the founders of
nutritional science. For example, Voit (1902), as quoted
by Lusk (1922)
, wrote the following:
"I therefore maintain my "older" point of view, that of pure metabolism... the more unifying development will be possible as one investigates what substances are destroyed under different circumstances... and how much of the different materials must be fed to maintain the body in condition."
In my opinion, four systems are critical for the body to be
"maintained in condition": the intestine, to maintain absorptive
and protective function; the immune system and other aspects of
defense; the skeletal musculature system; and the central nervous
system. Within each system, critical metabolic roles for some specific
amino acids can be identified (Table 6
).
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is that with the exception of the involvement of phenylalanine
and tryptophan in the maintenance of the adrenergic and serotonergic
neurotransmitter systems and of methionine as a methyl group donor for
the synthesis of creatine, the necessary precursors are nonessential or
conditionally essential amino acids. That being so, it is tempting to
argue that the ability to maintain the synthesis of these amino acids
is of sufficiently high functional priority that under conditions in
which protein (nitrogen) is limiting, essential amino acids are used to
maintain these pathways. In Table 7
I attempted to compare estimates of the turnover or losses of some
critical end products with the kinetics of their precursor amino acids.
This comparison suggests that some pathways, e.g., taurine and nitric
oxide production, have little quantitative impact on the nutrition of
the precursors, whereas others are of much greater importance. Thus,
creatine synthesis (as estimated from creatinine excretion) and the
turnover of glutathione (estimated from measurements in the plasma and
red blood cells) have a substantial impact on the utilization of some
precursors, especially if the rate of product synthesis is set against
the intake or net synthesis of the precursor. The continuing synthesis
of these two end products, one involved in energy transduction in both
the musculature and central nervous system and the other a critical
factor in detoxification mechanisms, appears to have a substantial
potential impact on the nutritional status of the individual. Indeed,
data obtained in protein-depleted pigs (Jahoor et al. 1995
) and in asymptomatic HIV-infected patients
(Jahoor et al. 1999
) demonstrate that the supply of
protein and cysteine can measurably alter the ability of the organism
to maintain glutathione synthesis.
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FOOTNOTES |
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2 This work is a publication of the USDA/ARS
Children’s Nutrition Research Center, Department of Pediatrics, Baylor
College of Medicine and Texas Children’s Hospital, Houston, TX. The
work was supported in part by federal funds from the U.S. Department of
Agriculture Agricultural Research Service, Cooperative Agreement No.
58-6258-6001. 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. I am very grateful to Leslie
Loddeke for her skillful editing of the manuscript.
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REFERENCES |
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