422
12 Small Nitrogenous Compounds
oxoglutarate and introduced into the citric acid
cycle. The amino group of glutamic acid is mostly
transferred by an alanine transaminase to give
pyruvate. As alanine formation can also take
place in isolated mitochondria, pyruvate cannot
arise from the cytosolic glycolysis system; it is
mainly produced from malate by the NADdependent malic enzyme which is particularly active in the mitochondria (see Table 15.7a (4),
p.582). The regulation of malate metabolism is
not yet clear; malate dehydrogenase, e.g. in the
mitochondria of Leptinotarsa, has a tenfold
higher activity than malic enzyme, which in addition is inhibited by ATP and pyruvate during proline oxidation [260]. There are species-specific
differences in the fate of the amino nitrogen
released from glutamic acid: in the flight muscles
of Leptinotarsa just as much alanine is produced
as proline is degraded; in Glossina about 20 % is
released as ammonia by the action of glutamate
dehydrogenase, and in Popillia it is 50 % [260].
The proline content of the haemolymph is kept
constant through the synthesis of proline in fat
bodies. In Glossina and Leptinotarsa, proline
synthesis in the fat bodies makes use in particular
of triacylglycerol and the alanine released by
muscle; less so of glutamic acid, aspartic acid or
leucine; and hardly at all of glucose. Thus, the
system muscle-haemolymph-fat body allows the
use of the fatty acids of the fat bodies for the
energy metabolism of the flight muscles. A comparison of the enzyme constitution of Glossina fat
bodies with that of the fat bodies of other flies
clearly illustrates the specific adaptation towards
proline as a substrate for energy metabolism
[126, 261]. In a similar fashion, proline is oxidized
in preference to other substrates in isolated sarcosomes from the leg muscle of the dung beetle
Heliocopris dilloni. The enzyme constitution here
and in other Scarabaeidae also shows that proline
is the preferred energy source for the leg muscles.
Tracer experiments with the fly Aldrichina have
shown that only arginine is used for proline biosynthesis by the larvae, whilst glutamic acid is
used more than arginine in the adults. The ratelimiting enzyme of proline synthesis from arginine is ornithine-~-transaminase, which converts
ornithine to glutamic acid semialdehyde and is
much more active in the larvae than in the adults.
As in the rat, this enzyme is mitochondrial, and
its activity decreases in the order fat body > gut
> Malpighian tubules> muscles.
The enzymes of proline metabolism have not
been completely described, even for the mammals. However, it is known that the insects have a
mitochondrial proline oxidase, which uses atmospheric oxygen to oxidizes proline to L\ I-pyrroline5-carboxylate; they also have a particle-bound
L\1-pyrroline-5-carboxylate reductase, which reduces L\1-5-carboxylate to proline. The only
enzyme of proline metabolism to have been isolated from invertebrates is the L\1-pyrroline-5carboxylate reductase from Drosophila melanogaster; the main difference between this and the
rat enzyme is a higher activity with NADPH than
with NADH [88].
12.5.2 Sulphur-Containing Amino Acids
Of the two sulphur-containing standard amino
acids, methionine is always essential, whereas
cysteine can be synthesized by most animals via
the cystathionine pathway (Fig. 12.7) [57]. In this
case, the sulphur is obtained from methionine
and the C skeleton from serine (transsulphation). 14C-serine, or its precursor 14C_
formiate, is incorporated into cysteine in the sawfly Acantholyda nemoralis but only into cystathionine in the stick insect Carausius morosus. This
suggests that the sawfly has a functioning cystathionine pathway, whereas in the stick insect the
pathway is blocked at the cystathionine step. A
fully functional cystathionine pathway is also present in several dipterans and lepidopterans, but is
missing in other lepidopterans and some aphids
[224]. During human embryo development, there
is no active cystathionase and cysteine is therefore required [57]. Mammals can resynthesize
methionine from homocysteine; the required
methyl-group donor varies with the species: in
the rat it is mainly betaine (trimethylglycine), and
in sheep 5-methyltetrahydrofolate [275].
Cysteine is the starting point for the synthesis
of glutathione, coenzyme A and taurine. The synthesis of taurine from cysteine in animals can follow one of several routes, e.g. via cysteamine,
cysteine sulphinic acid or cysteinic acid
(Fig. 12.8). Cysteamine does not apparently arise
simply by the decarboxylation of cysteine but, in
mammals, is formed mainly indirectly via coenzyme A [57]. The formation of cysteamine from lanthionine is possible in mammals but, in view of
the low concentration of the latter amino acid, is
of little importance. Some insects have large
amounts of lanthionine but the synthesis of cysteamine in these cases has not been investigated
[57]. Hypotaurine, the intermediate of taurine
synthesis via cysteamine and cysteine sulphinic
acid, has been detected in Porifera, Cnidaria,
12 Small Nitrogenous Compounds
oxoglutarate and introduced into the citric acid
cycle. The amino group of glutamic acid is mostly
transferred by an alanine transaminase to give
pyruvate. As alanine formation can also take
place in isolated mitochondria, pyruvate cannot
arise from the cytosolic glycolysis system; it is
mainly produced from malate by the NADdependent malic enzyme which is particularly active in the mitochondria (see Table 15.7a (4),
p.582). The regulation of malate metabolism is
not yet clear; malate dehydrogenase, e.g. in the
mitochondria of Leptinotarsa, has a tenfold
higher activity than malic enzyme, which in addition is inhibited by ATP and pyruvate during proline oxidation [260]. There are species-specific
differences in the fate of the amino nitrogen
released from glutamic acid: in the flight muscles
of Leptinotarsa just as much alanine is produced
as proline is degraded; in Glossina about 20 % is
released as ammonia by the action of glutamate
dehydrogenase, and in Popillia it is 50 % [260].
The proline content of the haemolymph is kept
constant through the synthesis of proline in fat
bodies. In Glossina and Leptinotarsa, proline
synthesis in the fat bodies makes use in particular
of triacylglycerol and the alanine released by
muscle; less so of glutamic acid, aspartic acid or
leucine; and hardly at all of glucose. Thus, the
system muscle-haemolymph-fat body allows the
use of the fatty acids of the fat bodies for the
energy metabolism of the flight muscles. A comparison of the enzyme constitution of Glossina fat
bodies with that of the fat bodies of other flies
clearly illustrates the specific adaptation towards
proline as a substrate for energy metabolism
[126, 261]. In a similar fashion, proline is oxidized
in preference to other substrates in isolated sarcosomes from the leg muscle of the dung beetle
Heliocopris dilloni. The enzyme constitution here
and in other Scarabaeidae also shows that proline
is the preferred energy source for the leg muscles.
Tracer experiments with the fly Aldrichina have
shown that only arginine is used for proline biosynthesis by the larvae, whilst glutamic acid is
used more than arginine in the adults. The ratelimiting enzyme of proline synthesis from arginine is ornithine-~-transaminase, which converts
ornithine to glutamic acid semialdehyde and is
much more active in the larvae than in the adults.
As in the rat, this enzyme is mitochondrial, and
its activity decreases in the order fat body > gut
> Malpighian tubules> muscles.
The enzymes of proline metabolism have not
been completely described, even for the mammals. However, it is known that the insects have a
mitochondrial proline oxidase, which uses atmospheric oxygen to oxidizes proline to L\ I-pyrroline5-carboxylate; they also have a particle-bound
L\1-pyrroline-5-carboxylate reductase, which reduces L\1-5-carboxylate to proline. The only
enzyme of proline metabolism to have been isolated from invertebrates is the L\1-pyrroline-5carboxylate reductase from Drosophila melanogaster; the main difference between this and the
rat enzyme is a higher activity with NADPH than
with NADH [88].
12.5.2 Sulphur-Containing Amino Acids
Of the two sulphur-containing standard amino
acids, methionine is always essential, whereas
cysteine can be synthesized by most animals via
the cystathionine pathway (Fig. 12.7) [57]. In this
case, the sulphur is obtained from methionine
and the C skeleton from serine (transsulphation). 14C-serine, or its precursor 14C_
formiate, is incorporated into cysteine in the sawfly Acantholyda nemoralis but only into cystathionine in the stick insect Carausius morosus. This
suggests that the sawfly has a functioning cystathionine pathway, whereas in the stick insect the
pathway is blocked at the cystathionine step. A
fully functional cystathionine pathway is also present in several dipterans and lepidopterans, but is
missing in other lepidopterans and some aphids
[224]. During human embryo development, there
is no active cystathionase and cysteine is therefore required [57]. Mammals can resynthesize
methionine from homocysteine; the required
methyl-group donor varies with the species: in
the rat it is mainly betaine (trimethylglycine), and
in sheep 5-methyltetrahydrofolate [275].
Cysteine is the starting point for the synthesis
of glutathione, coenzyme A and taurine. The synthesis of taurine from cysteine in animals can follow one of several routes, e.g. via cysteamine,
cysteine sulphinic acid or cysteinic acid
(Fig. 12.8). Cysteamine does not apparently arise
simply by the decarboxylation of cysteine but, in
mammals, is formed mainly indirectly via coenzyme A [57]. The formation of cysteamine from lanthionine is possible in mammals but, in view of
the low concentration of the latter amino acid, is
of little importance. Some insects have large
amounts of lanthionine but the synthesis of cysteamine in these cases has not been investigated
[57]. Hypotaurine, the intermediate of taurine
synthesis via cysteamine and cysteine sulphinic
acid, has been detected in Porifera, Cnidaria,