644
16 Sterols and Steroids
Diptera [40, 63]. Little is known about the other
enzymes of ecdysteroid synthesis; the 25- and 22hydroxylases are apparently microsomal, and the
2-hydroxylase is a mitochondrial cytochrome P450 monooxygenase, but without the characteristic inhibition by CO [49, 58].
The best-known enzyme of ecdysteroid biosynthesis is the 2O-monooxygenase that hydroxylates
ecdysone to 20-hydroxyecdysone. This is not present in the prothorax glands but is found in the fat
body, the Malpighian tubules, the follicle epithelium and other tissues. It is a typical cytochrome
P-450 monooxygenase with microsomal and mitochondrial forms in species-specific proportions
[109]. It is still a matter of debate whether ecdysone or 2O-hydroxyecdysone is the actual moulting hormone. It was shown in 1973 that the two
hormones bring about different puff patterns in
the giant chromosomes of Chironomus. In 1978,
experiments with Drosophila showed that the two
hormones have the same effects but that 20hydroxyecdysone is 50- to 100-fold more active
than ecdysone. Three cell lines of Drosophila
melanogaster with very different capacities for the
hydroxylation of ecdysone showed the same
response to the two moulting hormones. Depending upon the test model, ecdysone and 20hydroxyecdysone can have very different, even
complementary, effects. It may be that a hormonal effect of ecdysone that still functions in primitive arthropods has been lost during evolution, so
that in some groups it is now essentially only a
pro-hormone for 20-hydroxyecdysone [65, 132].
Makisterone A differs from 20-hydroxyecdysone only by having a methyl group on
C-24 , i.e. it is a CZ8 ecdysteroid (Fig. 16.13). In
the insects, this ecdysteroid has so far been
detected only in several species of Hemiptera,
Diptera and Hymenoptera, and in the crustaceans in the brachyuran Callinectes sapidus; it is,
however, probably widely distributed in the arthropods. The insects referred to include Drosophila melanogaster, Apis mellifera and various
phytophagous Hemiptera from the families Pentatomidae (e.g. Oncopeltus fasciatus and Dysdercus cingulatus) and Alydidae (Megalotomus quinquespinosus). The diet of all these species contains almost no cholesterol and, in addition, they
lack the ability to produce cholesterol from
ingested Cz8 and CZ9 steroids by cleavage of the
24-alkyl residue. Hence, they synthesize their
moulting hormones from 24-methylsterols. However, in the course of evolution they have turned
the problem to their advantage and adapted to
this rather unusual moulting hormone; in some
Hemiptera makisterone A is tenfold more active
than 20-hydroxyecdysone. Even the predatory
pentatomid Podisus maculiventris, which undoubtedly obtains sufficient cholesterol in its
diet, still relies upon makisterone A [30, 31,
65, 132]. Ponasterone A (Fig. 16.13) was discovered in 1979 in the brachyurans Callinectes sapidus, Carcinus maenas and Gecarcinus lateralis.
This is converted to 20-hydroxyecdysone by 25hydroxylation and to inokosterone by 26hydroxylation. Both end products are found as
natural hormonal components in crustaceans but
in some insects, e.g. Bombyx mori, they are
derived from exogenous ponasterone A [68].
In some adult insects, ecdysteroids that are
synthesized in the follicle cells of the ovary are
accumulated in species-specific proportions in the
haemolymph or eggs; e.g. in the migratory locust
Locusta migratoria ecdysteroids occur in the
ovary and haemolymph or eggs in the ratio 50 : 1,
and in the wax moth Galleria melonella in the
ratio 1 : 1. Compared with the relatively simple
ecdysteroid spectrum of the larval stages, the
composition of ecdysteroids in the females and in
the eggs is very complex. Here, there are not only
large quantities of synthesis intermediates, such
as 2,22-dideoxyecdysone and 2-deoxyecdysone in
Bombyx mori, but also inactivation products,
such as 20,26-dihydroxyecdysone in Carausius
morosus, ecdysone-22-phosphate in Bombyx
mori, 26-hydroxyecdysone-26-phosphate in Manduca sexta and ecdysonic acid in Schistocerca gregaria [32,35, 86, 119]. The majority of ecdysteroids in the ovary and eggs are always conjugated.
The insects mainly contain ecdysteroid-22phosphate and also 26-phosphate [119], but, in
contrast, the ticks have 22-fatty acid esters
[22, 23], Twenty-two long-chain fatty acyl esters
of ecdysone have recently been discovered in
freshly laid eggs of the house cricket Acheta
domestica [128]. The most unusual conjugate has
been found in freshly laid eggs of Locusta migratoria (Fig. 16.14); here, the 22-0H of ecdysone is
bound via a phosphate residue to N 6 -isopentenyl
adenosine (cytokinin), which functions in plants
as a growth regulator but is also found as the
phosphate in mammalian cells [120]. During
embryogenesis in Schistocerca gregaria and other
orthopterans, the conjugates are gradually hydrolyzed, and the released ecdysteroids are further
metabolized to inactive end products. Thus, in
these primitive insects, the ecdysteroids of the
ovary are involved in the development of the
embryo, whereas in the more highly developed
insects they regulate the production of yolk pro-
16 Sterols and Steroids
Diptera [40, 63]. Little is known about the other
enzymes of ecdysteroid synthesis; the 25- and 22hydroxylases are apparently microsomal, and the
2-hydroxylase is a mitochondrial cytochrome P450 monooxygenase, but without the characteristic inhibition by CO [49, 58].
The best-known enzyme of ecdysteroid biosynthesis is the 2O-monooxygenase that hydroxylates
ecdysone to 20-hydroxyecdysone. This is not present in the prothorax glands but is found in the fat
body, the Malpighian tubules, the follicle epithelium and other tissues. It is a typical cytochrome
P-450 monooxygenase with microsomal and mitochondrial forms in species-specific proportions
[109]. It is still a matter of debate whether ecdysone or 2O-hydroxyecdysone is the actual moulting hormone. It was shown in 1973 that the two
hormones bring about different puff patterns in
the giant chromosomes of Chironomus. In 1978,
experiments with Drosophila showed that the two
hormones have the same effects but that 20hydroxyecdysone is 50- to 100-fold more active
than ecdysone. Three cell lines of Drosophila
melanogaster with very different capacities for the
hydroxylation of ecdysone showed the same
response to the two moulting hormones. Depending upon the test model, ecdysone and 20hydroxyecdysone can have very different, even
complementary, effects. It may be that a hormonal effect of ecdysone that still functions in primitive arthropods has been lost during evolution, so
that in some groups it is now essentially only a
pro-hormone for 20-hydroxyecdysone [65, 132].
Makisterone A differs from 20-hydroxyecdysone only by having a methyl group on
C-24 , i.e. it is a CZ8 ecdysteroid (Fig. 16.13). In
the insects, this ecdysteroid has so far been
detected only in several species of Hemiptera,
Diptera and Hymenoptera, and in the crustaceans in the brachyuran Callinectes sapidus; it is,
however, probably widely distributed in the arthropods. The insects referred to include Drosophila melanogaster, Apis mellifera and various
phytophagous Hemiptera from the families Pentatomidae (e.g. Oncopeltus fasciatus and Dysdercus cingulatus) and Alydidae (Megalotomus quinquespinosus). The diet of all these species contains almost no cholesterol and, in addition, they
lack the ability to produce cholesterol from
ingested Cz8 and CZ9 steroids by cleavage of the
24-alkyl residue. Hence, they synthesize their
moulting hormones from 24-methylsterols. However, in the course of evolution they have turned
the problem to their advantage and adapted to
this rather unusual moulting hormone; in some
Hemiptera makisterone A is tenfold more active
than 20-hydroxyecdysone. Even the predatory
pentatomid Podisus maculiventris, which undoubtedly obtains sufficient cholesterol in its
diet, still relies upon makisterone A [30, 31,
65, 132]. Ponasterone A (Fig. 16.13) was discovered in 1979 in the brachyurans Callinectes sapidus, Carcinus maenas and Gecarcinus lateralis.
This is converted to 20-hydroxyecdysone by 25hydroxylation and to inokosterone by 26hydroxylation. Both end products are found as
natural hormonal components in crustaceans but
in some insects, e.g. Bombyx mori, they are
derived from exogenous ponasterone A [68].
In some adult insects, ecdysteroids that are
synthesized in the follicle cells of the ovary are
accumulated in species-specific proportions in the
haemolymph or eggs; e.g. in the migratory locust
Locusta migratoria ecdysteroids occur in the
ovary and haemolymph or eggs in the ratio 50 : 1,
and in the wax moth Galleria melonella in the
ratio 1 : 1. Compared with the relatively simple
ecdysteroid spectrum of the larval stages, the
composition of ecdysteroids in the females and in
the eggs is very complex. Here, there are not only
large quantities of synthesis intermediates, such
as 2,22-dideoxyecdysone and 2-deoxyecdysone in
Bombyx mori, but also inactivation products,
such as 20,26-dihydroxyecdysone in Carausius
morosus, ecdysone-22-phosphate in Bombyx
mori, 26-hydroxyecdysone-26-phosphate in Manduca sexta and ecdysonic acid in Schistocerca gregaria [32,35, 86, 119]. The majority of ecdysteroids in the ovary and eggs are always conjugated.
The insects mainly contain ecdysteroid-22phosphate and also 26-phosphate [119], but, in
contrast, the ticks have 22-fatty acid esters
[22, 23], Twenty-two long-chain fatty acyl esters
of ecdysone have recently been discovered in
freshly laid eggs of the house cricket Acheta
domestica [128]. The most unusual conjugate has
been found in freshly laid eggs of Locusta migratoria (Fig. 16.14); here, the 22-0H of ecdysone is
bound via a phosphate residue to N 6 -isopentenyl
adenosine (cytokinin), which functions in plants
as a growth regulator but is also found as the
phosphate in mammalian cells [120]. During
embryogenesis in Schistocerca gregaria and other
orthopterans, the conjugates are gradually hydrolyzed, and the released ecdysteroids are further
metabolized to inactive end products. Thus, in
these primitive insects, the ecdysteroids of the
ovary are involved in the development of the
embryo, whereas in the more highly developed
insects they regulate the production of yolk pro-