12.2 Nutritional Aspects of Amino Acid Metabolism
409
an increase in volume. The immediate volume
regulation that ensues relies upon reductions in
the osmotically active substances of the cell by
the mechanisms mentioned above.
12.2 Nutritional Aspects
of Amino Acid Metabolism
The amino acid metabolism of animals is characterized by the facts that nitrogen for the synthesis
of amino acids can only be used at the oxidation
level of the amino group, and certain amino acids
(essential amino acids) cannot be synthesized de
novo. Thus, the nutrients must contain sufficient
amino acids to replace those lost by excretion and
to supply those required for further growth; they
must include all the essential amino acids in at
least the amounts needed for protein synthesis.
As the cell has no significant reserves of essential
amino acids, a nutritional deficiency of just one of
these amino acids causes the limitation of protein
synthesis and degradation of the excess of
remaining amino acids. Although the nonessential amino acids are in fact all dispensable,
feeding of a mixture of only essential amino acids
results in a suboptimal nitrogen balance; at least
one of the non-essential amino acids must be supplied.
1Wo methods are available for the determination of amino acid requirements. In the classical
diet method, animals are kept, or even reared, on
a chemically defined diet for long periods and are
examined for signs of deficiency: reduced growth,
delayed development, the occurrence of malformations, reduced longevity, increased nitrogen
excretion, alterations in the blood amino acid
spectrum, reduced incorporation of 14C-leucine
into protein, etc. In the 14C method, a suitable
labelled nutrient, such as 14C-glucose, is administered and the radioactivity of isolated amino acids
is determined; only non-essential amino acids are
labelled. The latter method is used for animals
that cannot be maintained on a chemically
defined diet, e.g. many parasites, blood-suckers,
microphages, or highly specialized plant eaters.
The results from the two methods may be quite
different. It is often the case that the capacity to
synthesize an amino acid is insufficient in a certain stage of the life cycle; such "semi-essential"
or "growth-promoting" amino acids are obviously
recognized only by the diet method. For animals
with yolk-rich eggs, the results of diet experiments may be confused by the transfer of essential amino acids from the yolk proteins to the
developing animal. The supply of essential amino
acids by symbiotic microorganisms may lead to
false results, even in 14C experiments, especially
when the labelled substrate is supplied as feed
and not injected into the bloodstream.
Numerous investigations, in particular with
mammals, insects and protozoans, have shown
that histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and
valine are all apparently essential. Thus, the ability to synthesize these amino acids was already
lost very early in evolution. Only a few animal
groups are able to make arginine, and even here
the availability of arginine for protein biosynthesis is so limited by arginase activity that supply of
this amino acid is growth stimulating, e.g. in
many mammals and human babies. In several animal groups, insufficient amino acid metabolism
creates further essential or semi-essential amino
acids: some ciliates and the beetle Trogoderma
granarium require tyrosine in addition to phenylalanine, i.e. the hydroxylation of phenylalanine is
insufficient. Proline is semi-essential for many
insects, e.g. for Culex pipiens, some lines of Phormia regina and Cochliomyia hominivorax, and
perhaps for Bombyx mori. Crayfish of the genus
Astacus, the freshwater prawn Macrobranchium
rosenbergii and the mosquito Culex pipiens lack
the ability to make asparagine from aspartic acid;
the amide must be present in the diet. Some
insects cannot meet their requirements for
sulphur-containing amino acids with methionine
alone and also require cysteine; examples include
the butterfly Argyrotaenia velutinana and several
aphids. Glycine and serine in the diet improve the
development and survival of Culex pipiens. The
domestic cat is unable to make taurine and a deficiency leads to severe symptoms, such as degeneration of the retina; humans and other primates
also require taurine in their diet [46, 82,227].
So long as the appropriate aminotransferases
are present, the essential amino acids can all be
replaced by the analogous keto acids, although
these are always only partly aminated. o-amino
acids can be used after they have been deaminated by o-amino acid oxidases. In this regard,
there are species-specific differences: many
insects can make use of o-histidine, o-methionine
and o-phenylalanine; Aedes uses o-tryptophan
and o-leucine; and Tribolium uses o-lysine. None
of the o-amino acids is growth stimulating for
Bombyx mori; in fact, some are weakly inhibitory. In the Peking duck, o-methionine is also
unable to replace completely the L-enantiomer
409
an increase in volume. The immediate volume
regulation that ensues relies upon reductions in
the osmotically active substances of the cell by
the mechanisms mentioned above.
12.2 Nutritional Aspects
of Amino Acid Metabolism
The amino acid metabolism of animals is characterized by the facts that nitrogen for the synthesis
of amino acids can only be used at the oxidation
level of the amino group, and certain amino acids
(essential amino acids) cannot be synthesized de
novo. Thus, the nutrients must contain sufficient
amino acids to replace those lost by excretion and
to supply those required for further growth; they
must include all the essential amino acids in at
least the amounts needed for protein synthesis.
As the cell has no significant reserves of essential
amino acids, a nutritional deficiency of just one of
these amino acids causes the limitation of protein
synthesis and degradation of the excess of
remaining amino acids. Although the nonessential amino acids are in fact all dispensable,
feeding of a mixture of only essential amino acids
results in a suboptimal nitrogen balance; at least
one of the non-essential amino acids must be supplied.
1Wo methods are available for the determination of amino acid requirements. In the classical
diet method, animals are kept, or even reared, on
a chemically defined diet for long periods and are
examined for signs of deficiency: reduced growth,
delayed development, the occurrence of malformations, reduced longevity, increased nitrogen
excretion, alterations in the blood amino acid
spectrum, reduced incorporation of 14C-leucine
into protein, etc. In the 14C method, a suitable
labelled nutrient, such as 14C-glucose, is administered and the radioactivity of isolated amino acids
is determined; only non-essential amino acids are
labelled. The latter method is used for animals
that cannot be maintained on a chemically
defined diet, e.g. many parasites, blood-suckers,
microphages, or highly specialized plant eaters.
The results from the two methods may be quite
different. It is often the case that the capacity to
synthesize an amino acid is insufficient in a certain stage of the life cycle; such "semi-essential"
or "growth-promoting" amino acids are obviously
recognized only by the diet method. For animals
with yolk-rich eggs, the results of diet experiments may be confused by the transfer of essential amino acids from the yolk proteins to the
developing animal. The supply of essential amino
acids by symbiotic microorganisms may lead to
false results, even in 14C experiments, especially
when the labelled substrate is supplied as feed
and not injected into the bloodstream.
Numerous investigations, in particular with
mammals, insects and protozoans, have shown
that histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and
valine are all apparently essential. Thus, the ability to synthesize these amino acids was already
lost very early in evolution. Only a few animal
groups are able to make arginine, and even here
the availability of arginine for protein biosynthesis is so limited by arginase activity that supply of
this amino acid is growth stimulating, e.g. in
many mammals and human babies. In several animal groups, insufficient amino acid metabolism
creates further essential or semi-essential amino
acids: some ciliates and the beetle Trogoderma
granarium require tyrosine in addition to phenylalanine, i.e. the hydroxylation of phenylalanine is
insufficient. Proline is semi-essential for many
insects, e.g. for Culex pipiens, some lines of Phormia regina and Cochliomyia hominivorax, and
perhaps for Bombyx mori. Crayfish of the genus
Astacus, the freshwater prawn Macrobranchium
rosenbergii and the mosquito Culex pipiens lack
the ability to make asparagine from aspartic acid;
the amide must be present in the diet. Some
insects cannot meet their requirements for
sulphur-containing amino acids with methionine
alone and also require cysteine; examples include
the butterfly Argyrotaenia velutinana and several
aphids. Glycine and serine in the diet improve the
development and survival of Culex pipiens. The
domestic cat is unable to make taurine and a deficiency leads to severe symptoms, such as degeneration of the retina; humans and other primates
also require taurine in their diet [46, 82,227].
So long as the appropriate aminotransferases
are present, the essential amino acids can all be
replaced by the analogous keto acids, although
these are always only partly aminated. o-amino
acids can be used after they have been deaminated by o-amino acid oxidases. In this regard,
there are species-specific differences: many
insects can make use of o-histidine, o-methionine
and o-phenylalanine; Aedes uses o-tryptophan
and o-leucine; and Tribolium uses o-lysine. None
of the o-amino acids is growth stimulating for
Bombyx mori; in fact, some are weakly inhibitory. In the Peking duck, o-methionine is also
unable to replace completely the L-enantiomer