functions place different requirements on the species. Thus, a housefly, despite its restricted capacity for sterol conversion, must take up less than
0.5 % of the total sterol in the diet as cholesterol,
i.e. less than 0.1 ~g; the remaining sterol requirement can be statisfied by other sterols [112].
The metabolic pathways leading from the phytosterols to cholesterol have been defined in various insects by tracer experiments and the identification of intermediates. In contrast to previous
assumptions, there are marked differences
between individual insect species and groups
[25, 112]. The removal of the 24-alkyl group
always requires several oxidative and reductive
reaction steps. The metabolic pathway of sitosterol has been examined in more than a dozen
insect species (Fig. 16.4a). Oxidation of the 24ethyl group to a 24-ethylene group produces
fucosterol, which is converted to the 24,28epoxide by the addition of oxygen. The next identifiable intermediate is desmosterol, which can be
reduced to cholesterol. The metabolism of campesterol proceeds in a similar fashion via 24methylene cholesterol and its 24,28-epoxide to
desmosterol and cholesterol (Fig. 16.4a). Stigmasterol is converted via cholesta-5,22,24-trien3~-01 to desmosterol. In the mealworm Tribolium
confusum, the pathway proceeds via cholesta5,7,24-trien-3~-01 to 7-dehydrocholesterol, which
comprises up to 70 % of the total sterol of this
species; the desaturation of C-7 can occur before
or after reduction of the double bond on C-22
(Fig. 16.4b). 7-Dehydrocholesterol, as well as 7dehydrocampesterol and 7-dehydrositosterol predominate in the prothorax gland of Bombyx
mori. All haemolymph sterols are apparently
oxidized in this organ to the 7-dehydro derivatives [101]. Ergosterols can also be converted to
cholesterol by many insects; however, the common cockroach Blatella germanica appears incapable of reducing the C-22 double bond and produces 22-dehydrocholesterol [112].
The Mexican bean beetle Epilachna varivestis
mainly has the saturated sterols cholestanol, campestanol and stigmastanol, in addition to more
than 10 % lathosterol (cholest-7-en-3~-01). Tracer
experiments show that the beetle is unable to
dealkylate phytosterols to cholesterol but produces instead stanols, which are dealkylated to choIe stano I and in part oxidized to lathosterol
(Fig.16.4c) [112, 113]. The ladybird Coccinella
septempunctata, which feeds off aphids but nevertheless belongs to the same family as the bean
beetle (Coccinellidae), appears unable to modify
the sterols, which mainly consist of cholesterol, in
16.2.2 Sterols of the Insects
629
its diet [112]. There are also phytophagous insect
species that lack the ability to convert plant sterols to cholesterol. This is true, for example, for
Oncopeltus fasciatus and other Hemiptera, which
have adapted to the absence of cholesterol to
such an extent that the typical moulting hormone
of these animals, makisterone A, carries a methyl
group on C-24 (Fig. 16.13). The khapra beetle
Trogoderma granarium is also unable to metabolize phytosterols. However, both of these phytophagous species can selectively absorb the cholesterol and campesterol in their diet. The honeybee
Apis mellifera is apparently just as inefficient in
converting phytosterols to cholesterol, but can
accumulate the 24-methyl cholesterol present in,
for example, pollen and pass it on to the young
[112]. The leaf-cutting ants Atta cephalotes cannot
dealkylate the ergosterol predominating in their
fungus diet. Hence, their tissues contain in particular the related compound 24-methylenecholesta-5, 7 ,24(28)-trien-3~-01 as well as 22dihydroergosterol and ergosterol (Fig. 16.4 d)
[99].
As is illustrated by the processes described for
converting phytosterols into cholesterol, insects
can carry out numerous reactions of sterol metabolism but these are not realized by all species
[21, 25, 112]. This includes the reduction of double bonds, in particular at C-5, C-7, C-22 and C24, and the reduction of keto groups. Amongst
the oxidative processes are the introduction of
double bonds and substitution with hydroxy or
oxo groups, which are important for the formation and activation of the ecdysteroids, the biosynthesis of defence substances, and preparation
for the production of glycosides and other conjugates. Cleavage of akyl substituents on C-24,
which arise by oxidation and reduction, is widely
found in the insects. Shortening of the cholesterol
side-chain to ~h CI9 and CI8 steroids occurs during the production of defence substances by the
beetle families Dytiscidae and Silphidae. The
formation of the C21 steroid pregn-5-en-3~,20~diol (Fig. 16.5a) from cholesterol has also been
observed in Manduca sexta [118]. The production
and hydrolysis of sterol esters has been detected
by tracer experiments in many insect species. The
microsomal acyl-Coa : cholesterol acyl transferase
from the gut and fat body of the pre-pupa of the
moth Heliotis zea has been examined in some
detail [7]. Adults of the common cockroach Periplaneta americana have 29 % esterified cholesterol, and the proportion is even higher in insect
eggs. Up to 41 % of the esters in the eggs of the
housefly Musca domestica are completely hydro-
0.5 % of the total sterol in the diet as cholesterol,
i.e. less than 0.1 ~g; the remaining sterol requirement can be statisfied by other sterols [112].
The metabolic pathways leading from the phytosterols to cholesterol have been defined in various insects by tracer experiments and the identification of intermediates. In contrast to previous
assumptions, there are marked differences
between individual insect species and groups
[25, 112]. The removal of the 24-alkyl group
always requires several oxidative and reductive
reaction steps. The metabolic pathway of sitosterol has been examined in more than a dozen
insect species (Fig. 16.4a). Oxidation of the 24ethyl group to a 24-ethylene group produces
fucosterol, which is converted to the 24,28epoxide by the addition of oxygen. The next identifiable intermediate is desmosterol, which can be
reduced to cholesterol. The metabolism of campesterol proceeds in a similar fashion via 24methylene cholesterol and its 24,28-epoxide to
desmosterol and cholesterol (Fig. 16.4a). Stigmasterol is converted via cholesta-5,22,24-trien3~-01 to desmosterol. In the mealworm Tribolium
confusum, the pathway proceeds via cholesta5,7,24-trien-3~-01 to 7-dehydrocholesterol, which
comprises up to 70 % of the total sterol of this
species; the desaturation of C-7 can occur before
or after reduction of the double bond on C-22
(Fig. 16.4b). 7-Dehydrocholesterol, as well as 7dehydrocampesterol and 7-dehydrositosterol predominate in the prothorax gland of Bombyx
mori. All haemolymph sterols are apparently
oxidized in this organ to the 7-dehydro derivatives [101]. Ergosterols can also be converted to
cholesterol by many insects; however, the common cockroach Blatella germanica appears incapable of reducing the C-22 double bond and produces 22-dehydrocholesterol [112].
The Mexican bean beetle Epilachna varivestis
mainly has the saturated sterols cholestanol, campestanol and stigmastanol, in addition to more
than 10 % lathosterol (cholest-7-en-3~-01). Tracer
experiments show that the beetle is unable to
dealkylate phytosterols to cholesterol but produces instead stanols, which are dealkylated to choIe stano I and in part oxidized to lathosterol
(Fig.16.4c) [112, 113]. The ladybird Coccinella
septempunctata, which feeds off aphids but nevertheless belongs to the same family as the bean
beetle (Coccinellidae), appears unable to modify
the sterols, which mainly consist of cholesterol, in
16.2.2 Sterols of the Insects
629
its diet [112]. There are also phytophagous insect
species that lack the ability to convert plant sterols to cholesterol. This is true, for example, for
Oncopeltus fasciatus and other Hemiptera, which
have adapted to the absence of cholesterol to
such an extent that the typical moulting hormone
of these animals, makisterone A, carries a methyl
group on C-24 (Fig. 16.13). The khapra beetle
Trogoderma granarium is also unable to metabolize phytosterols. However, both of these phytophagous species can selectively absorb the cholesterol and campesterol in their diet. The honeybee
Apis mellifera is apparently just as inefficient in
converting phytosterols to cholesterol, but can
accumulate the 24-methyl cholesterol present in,
for example, pollen and pass it on to the young
[112]. The leaf-cutting ants Atta cephalotes cannot
dealkylate the ergosterol predominating in their
fungus diet. Hence, their tissues contain in particular the related compound 24-methylenecholesta-5, 7 ,24(28)-trien-3~-01 as well as 22dihydroergosterol and ergosterol (Fig. 16.4 d)
[99].
As is illustrated by the processes described for
converting phytosterols into cholesterol, insects
can carry out numerous reactions of sterol metabolism but these are not realized by all species
[21, 25, 112]. This includes the reduction of double bonds, in particular at C-5, C-7, C-22 and C24, and the reduction of keto groups. Amongst
the oxidative processes are the introduction of
double bonds and substitution with hydroxy or
oxo groups, which are important for the formation and activation of the ecdysteroids, the biosynthesis of defence substances, and preparation
for the production of glycosides and other conjugates. Cleavage of akyl substituents on C-24,
which arise by oxidation and reduction, is widely
found in the insects. Shortening of the cholesterol
side-chain to ~h CI9 and CI8 steroids occurs during the production of defence substances by the
beetle families Dytiscidae and Silphidae. The
formation of the C21 steroid pregn-5-en-3~,20~diol (Fig. 16.5a) from cholesterol has also been
observed in Manduca sexta [118]. The production
and hydrolysis of sterol esters has been detected
by tracer experiments in many insect species. The
microsomal acyl-Coa : cholesterol acyl transferase
from the gut and fat body of the pre-pupa of the
moth Heliotis zea has been examined in some
detail [7]. Adults of the common cockroach Periplaneta americana have 29 % esterified cholesterol, and the proportion is even higher in insect
eggs. Up to 41 % of the esters in the eggs of the
housefly Musca domestica are completely hydro-