426
12 Small Nitrogenous Compounds
of ~-glucuronides and methylated and acetylated
derivatives, and in insects by the formation of ~glucosides, sulphate esters and amino acid conjugates [137]. 3-Hydroxykynurenine is stored by
some insects: for example, in Drosophila in the
Malpighian tubules of the larva, which are not
destroyed during metamorphosis; in Bombyx
mori in the oocytes; and in Drosophila in the yolk
spheres of the eggs. In all cases, these probably
represent reserves for the later synthesis of
ommochromes. This substance has an interesting
fate in the parasitic wasp Habrobracon juglandis:
the 3-hydroxykynurenine taken from the host is
metabolized in the larvae to xanthurenic acid and
is excreted; only the 3-hydroxykynurenine synthesized by the pupa is used for ommochrome
synthesis. Mutants without kynurenine-3monooxygenase activity therefore produce no
ommochrome, even when they are rich in 3hydroxykynurenine during the larval period
[137]. 3-Hydroxykynurenine has also been
detected in gorgonian corals of the genus Eunicella as well as in the urine of the cephalopod
Octopus.
From kynurenine and 3-hydroxykynurenine,
ring closure produces kynurenic acid and xanthurenic acid, which are found in trace amounts in
many insects [137]. The stick insect Carausius
morosus produces only kynurenine, which makes
up about 0.1 % of the faecal dry weight
[137, 202]. In the presence of the relatively
unspecific kynureninase, e.g. in Bombyx mori,
anthranilic acid and 3-hydroxyanthranilic acid
may be produced, and in some cases their glycine
conjugates and ~-glucosides may also be produced. Neither the enzyme nor its products were
found in the dipterans examined [137].
In the vertebrates, tryptophan metabolism
occurs primarily in the liver, whereas insects have
relatively high activities of the relevant enzymes
in the eyes, the epidermis and other organs. In
the honey bee Apis, tryptophan metabolism takes
place only in the eye; eyeless mutants accumulate
tryptophan. A change in the concentrations of
metabolites and in the activities of the enzymes of
tryptophan metabolism in the course of development is often correlated to the programme of
ommochrome synthesis. During metamorphosis,
the body of a holometabolic insect is a closed system, for which a complete metabolite balance
sheet can be drawn up. Thus, in the case of Bombyx mori, about 20 % of the tryptophan consumed is converted to kynurenine, more than
50 % to free and conjugated 3-hydroxykynurenine, about 20 % to ommochromes, and
traces to xanthurenic acid; in contrast, in Phormia 27 % goes to kynurenine, 13 % to 3hydroxykynurenine and 60 % to xanthommatin
[137].
The first enzyme of the kynurenine pathway is
tryptophan-2,3-dioxygenase, known previously as
tryptophan pyrrolase (Fig. 12.9). In the mammals, this is a copper-containing haem protein;
the prosthetic haem can be removed by dialysis
and the apoenzyme reactivated with haematin. In
the fly Protophormia terrae-novae, the coenzyme
cannot be cleaved by dialysis, and in Drosophila
the dialysed enzyme cannot be reactivated with
haematin; this raises the question whether the
insect enzyme is a haem protein. However, the
partially purified enzyme from Protophormia
shows the light-reversible CO inhibition typical of
haem enzymes. The enzymes of insects, bacteria
and mammals all appear to be homotetrameric
with allosteric properties. Tryptophan can bind
both to the catalytic centre and to the allosteric
binding site; this increases the oxygen affinity of
the enzyme. The enzyme activity at constant oxygen partial pressure depends upon the tryptophan
concentration and shows marked sigmoid kinetics; in this way, tryptophan as an essential amino
acid is metabolized only when in excess.
The next enzyme of the kynurenine pathway,
kynurenioe formamidase, has its highest activities
in insects in the fat bodies, Malpighian tubules
and gonads. The pH optima and the Michaelis constants vary greatly with the species
[137]. Kynurenine-3-monooxygenase (kynurenine3-hydrolase) is inactivated by light and was for a
long time difficult to detect. As in the case of
tryptophan-2,3-dioxygenase, this enzyme is also
concentrated in different organs in different species: in Bombyx mori and Calliphora it occurs
especially in the Malpighian tubules, in Schistocera in the eyes and integument, in Apis only in
the eyes, and in Ephestia in the ovaries and the
eggs. In insects, bacteria and mammals, the
enzyme is localized in the outer mitochondrial
membrane [137]. The enzyme from the eggs of
Ephestia kahniella, which has been examined in
detail, is active only with NADPH and not with
NADH; this is in contrast to the mammalian
enzyme. The affinities for substrate and cosubstrate of the enzyme from the eggs of the flour
moth are about ten fold lower than those of the
liver enzyme; in the stick insect, the affinities of
the egg and liver enzymes are approximately the
same. Kynurenioase and kynurenine transaminase can also metabolize 3-hydroxykynurenine.
In mammals, kynureninase catalyses the conver-
12 Small Nitrogenous Compounds
of ~-glucuronides and methylated and acetylated
derivatives, and in insects by the formation of ~glucosides, sulphate esters and amino acid conjugates [137]. 3-Hydroxykynurenine is stored by
some insects: for example, in Drosophila in the
Malpighian tubules of the larva, which are not
destroyed during metamorphosis; in Bombyx
mori in the oocytes; and in Drosophila in the yolk
spheres of the eggs. In all cases, these probably
represent reserves for the later synthesis of
ommochromes. This substance has an interesting
fate in the parasitic wasp Habrobracon juglandis:
the 3-hydroxykynurenine taken from the host is
metabolized in the larvae to xanthurenic acid and
is excreted; only the 3-hydroxykynurenine synthesized by the pupa is used for ommochrome
synthesis. Mutants without kynurenine-3monooxygenase activity therefore produce no
ommochrome, even when they are rich in 3hydroxykynurenine during the larval period
[137]. 3-Hydroxykynurenine has also been
detected in gorgonian corals of the genus Eunicella as well as in the urine of the cephalopod
Octopus.
From kynurenine and 3-hydroxykynurenine,
ring closure produces kynurenic acid and xanthurenic acid, which are found in trace amounts in
many insects [137]. The stick insect Carausius
morosus produces only kynurenine, which makes
up about 0.1 % of the faecal dry weight
[137, 202]. In the presence of the relatively
unspecific kynureninase, e.g. in Bombyx mori,
anthranilic acid and 3-hydroxyanthranilic acid
may be produced, and in some cases their glycine
conjugates and ~-glucosides may also be produced. Neither the enzyme nor its products were
found in the dipterans examined [137].
In the vertebrates, tryptophan metabolism
occurs primarily in the liver, whereas insects have
relatively high activities of the relevant enzymes
in the eyes, the epidermis and other organs. In
the honey bee Apis, tryptophan metabolism takes
place only in the eye; eyeless mutants accumulate
tryptophan. A change in the concentrations of
metabolites and in the activities of the enzymes of
tryptophan metabolism in the course of development is often correlated to the programme of
ommochrome synthesis. During metamorphosis,
the body of a holometabolic insect is a closed system, for which a complete metabolite balance
sheet can be drawn up. Thus, in the case of Bombyx mori, about 20 % of the tryptophan consumed is converted to kynurenine, more than
50 % to free and conjugated 3-hydroxykynurenine, about 20 % to ommochromes, and
traces to xanthurenic acid; in contrast, in Phormia 27 % goes to kynurenine, 13 % to 3hydroxykynurenine and 60 % to xanthommatin
[137].
The first enzyme of the kynurenine pathway is
tryptophan-2,3-dioxygenase, known previously as
tryptophan pyrrolase (Fig. 12.9). In the mammals, this is a copper-containing haem protein;
the prosthetic haem can be removed by dialysis
and the apoenzyme reactivated with haematin. In
the fly Protophormia terrae-novae, the coenzyme
cannot be cleaved by dialysis, and in Drosophila
the dialysed enzyme cannot be reactivated with
haematin; this raises the question whether the
insect enzyme is a haem protein. However, the
partially purified enzyme from Protophormia
shows the light-reversible CO inhibition typical of
haem enzymes. The enzymes of insects, bacteria
and mammals all appear to be homotetrameric
with allosteric properties. Tryptophan can bind
both to the catalytic centre and to the allosteric
binding site; this increases the oxygen affinity of
the enzyme. The enzyme activity at constant oxygen partial pressure depends upon the tryptophan
concentration and shows marked sigmoid kinetics; in this way, tryptophan as an essential amino
acid is metabolized only when in excess.
The next enzyme of the kynurenine pathway,
kynurenioe formamidase, has its highest activities
in insects in the fat bodies, Malpighian tubules
and gonads. The pH optima and the Michaelis constants vary greatly with the species
[137]. Kynurenine-3-monooxygenase (kynurenine3-hydrolase) is inactivated by light and was for a
long time difficult to detect. As in the case of
tryptophan-2,3-dioxygenase, this enzyme is also
concentrated in different organs in different species: in Bombyx mori and Calliphora it occurs
especially in the Malpighian tubules, in Schistocera in the eyes and integument, in Apis only in
the eyes, and in Ephestia in the ovaries and the
eggs. In insects, bacteria and mammals, the
enzyme is localized in the outer mitochondrial
membrane [137]. The enzyme from the eggs of
Ephestia kahniella, which has been examined in
detail, is active only with NADPH and not with
NADH; this is in contrast to the mammalian
enzyme. The affinities for substrate and cosubstrate of the enzyme from the eggs of the flour
moth are about ten fold lower than those of the
liver enzyme; in the stick insect, the affinities of
the egg and liver enzymes are approximately the
same. Kynurenioase and kynurenine transaminase can also metabolize 3-hydroxykynurenine.
In mammals, kynureninase catalyses the conver-