196
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
spectroscopic investigations have shown that the
phospholipids in the lipophorin particles of
Locusta are located on the surface [124]. Sterols
are always present but never sterol esters. The
hydrocarbon content varies markedly between
species, reaching 28 % in Periplaneta but only
1.4 % in Philosamia. The hydrocarbon composition corresponds approximately to that of the
cuticular lipids [125]. Dependent on the type of
nutrition, the lipophorins may contain more or
less carotinoids, and are accordingly more or less
intensively yellow. Juvenile hormones are also
bound to the lipophorins [121].
Comparative investigations of a total of seven
insect orders show that each lipophorin particle
contains at least two apolipoprotein molecules:
an apoLp-1 of 230-250 kDa and an apoLp-II of
70-85 kDa [20, 121, 215]. Antisera against the
apoLp-II of Manduca show cross-reactivity with
the apoLp-II of all other species, but apoLp-l, on
the other hand, shows no immunological similarity with other apoLp-1 [215]. Differences in sensitivity to protease attack indicate that apoLp-I is
more exposed than apoLp-II at the particle surface [62]. An additional, small apoLp-III of
17-20 kDa is found in the lipid-rich lipophorins
of the Lepidoptera, Orthoptera and Hemiptera
[220]. The apoLp-III gene of Manduca sexta consists of four exons which code for a signal
sequence of 15 amino acids, a pro-sequence of 5
amino acids, which is cleaved off during maturation, and a mature protein of 165 amino acids.
The amino acid sequence and the spatial structure
of apoLp-III has also been determined for the
migratory locust Locusta migratoria. Although
there is only 29 % sequence similarity between
these two proteins, the Manduca apoLp-III combines with the Locusta lipophorins to give a complex which is active on isolated fat bodies
[49, 107, 121]. All lipophorins contain mannose
and glucosamine [121,215] but the N-bound carbohydrate chains have been examined in detail
only in Locusta. In this case, apoLp-1 carries six
different oligosaccharides with the general formulae of Mans-9GlcNAez and GlclMan9G1cNAez; the
smaller apoLp-II has only three oligosaccharides
[181]. The associated apoLp-III is a glycoprotein
in the Orthoptera but carbohydrate-free in the
Lepidoptera and Hemiptera [98].
The cyclic alterations in the Jipophorins during
transport of lipids from the fat bodies to the flight
muscles (lipid shuttle) have been particularly well
investigated in the butterflies Manduca sexta and
Acherontia atropos [104, 121, 218, 247]. The apolipoproteins are formed in the fat bodies. Isolated
fat bodies secrete a lipoprotein, for example in
Manduca, that differs significantly from that
found in the haemolymph: the density is
1.24-1.28 compared with at the highest 1.15 gJ
cm 3 , and the ratio phospholipid : diacylglycerol is
8.3 instead of at the highest 0.9. Only in animals
on a fat-free diet can such a protein be found in
the haemolymph. Normally, the lipoprotein
formed in the fat body, consisting mainly of apolipoproteins and phospholipids, is immediately
loaded with diacylglycerols [203]. The haemolymph of the resting animal contains 9- to 16-nmlarge HDLp particles that are poor in diacylglycerols and are known as N eHow due to their strongly
visible carotinoid coloration. Owing to the effect
of the adipokinetic hormone (AKH), secreted by
the corpora cardiaca at the beginning of flight,
diacylglycerol is increasingly released from the fat
bodies into the haemolymph and loaded onto the
lipophorin particles. Their density is consequently reduced, and the continuously expanding
particles are stabilized by the incorporation of
apoLp-III (protein ~), which is available in large
amounts in the haemolymph. In actively flying
animals, one finds mainly lipid-rich LDLp particles of 20-50 nm which contain as many as 16
apoLp-III and are known as lipophorin A + [48].
Following contact of the LDLp (lipophorin
A +) with the flight muscle, the diacylglycerols are
hydrolysed by a lipoprotein lipase and apoLp-III
is released; the low-lipid HDLp (lipophorin
N ellow ) that remains can then be reloaded with
diacylglycerols. Flight muscle lipoprotein lipase
has been isolated from the migratory locust and
found to be very different from the mammalian
enzyme (p. 662). The characteristic changes in the
lipophorin system that occur in the course of
development from the larva to the pupa and
eventually to the adult animal have been examined in detail in Manduca sexta and Bombyx
mori [169]. The haemolymph of M. sexta contains
specific lipid transfer particles (LTP) which catalyse transfer of lipids between lipoproteins of different density. The LTP is a VHDL particle of
1.4 . 10 6 Da, spherical with a long tail, and made
up from the glycoproteins apoLTP-1 (320 kDa),
apoLTP-II (85 kDa) and apoLTP-III (55 kDa)
with 14 % lipid. The LTP catalyses lipid transfer
from a donor LDLp to an acceptor LDLp, resulting in an HDLp and a VLDLp; the LTP, however,
can also use other lipoproteins as substrate,
including even some from vertebrates [219, 221].
The fat-body ,cells of Manduca carry on the surface a receptor of approximately 120 kDa which
binds lipoproteins more strongly the more dia-
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
spectroscopic investigations have shown that the
phospholipids in the lipophorin particles of
Locusta are located on the surface [124]. Sterols
are always present but never sterol esters. The
hydrocarbon content varies markedly between
species, reaching 28 % in Periplaneta but only
1.4 % in Philosamia. The hydrocarbon composition corresponds approximately to that of the
cuticular lipids [125]. Dependent on the type of
nutrition, the lipophorins may contain more or
less carotinoids, and are accordingly more or less
intensively yellow. Juvenile hormones are also
bound to the lipophorins [121].
Comparative investigations of a total of seven
insect orders show that each lipophorin particle
contains at least two apolipoprotein molecules:
an apoLp-1 of 230-250 kDa and an apoLp-II of
70-85 kDa [20, 121, 215]. Antisera against the
apoLp-II of Manduca show cross-reactivity with
the apoLp-II of all other species, but apoLp-l, on
the other hand, shows no immunological similarity with other apoLp-1 [215]. Differences in sensitivity to protease attack indicate that apoLp-I is
more exposed than apoLp-II at the particle surface [62]. An additional, small apoLp-III of
17-20 kDa is found in the lipid-rich lipophorins
of the Lepidoptera, Orthoptera and Hemiptera
[220]. The apoLp-III gene of Manduca sexta consists of four exons which code for a signal
sequence of 15 amino acids, a pro-sequence of 5
amino acids, which is cleaved off during maturation, and a mature protein of 165 amino acids.
The amino acid sequence and the spatial structure
of apoLp-III has also been determined for the
migratory locust Locusta migratoria. Although
there is only 29 % sequence similarity between
these two proteins, the Manduca apoLp-III combines with the Locusta lipophorins to give a complex which is active on isolated fat bodies
[49, 107, 121]. All lipophorins contain mannose
and glucosamine [121,215] but the N-bound carbohydrate chains have been examined in detail
only in Locusta. In this case, apoLp-1 carries six
different oligosaccharides with the general formulae of Mans-9GlcNAez and GlclMan9G1cNAez; the
smaller apoLp-II has only three oligosaccharides
[181]. The associated apoLp-III is a glycoprotein
in the Orthoptera but carbohydrate-free in the
Lepidoptera and Hemiptera [98].
The cyclic alterations in the Jipophorins during
transport of lipids from the fat bodies to the flight
muscles (lipid shuttle) have been particularly well
investigated in the butterflies Manduca sexta and
Acherontia atropos [104, 121, 218, 247]. The apolipoproteins are formed in the fat bodies. Isolated
fat bodies secrete a lipoprotein, for example in
Manduca, that differs significantly from that
found in the haemolymph: the density is
1.24-1.28 compared with at the highest 1.15 gJ
cm 3 , and the ratio phospholipid : diacylglycerol is
8.3 instead of at the highest 0.9. Only in animals
on a fat-free diet can such a protein be found in
the haemolymph. Normally, the lipoprotein
formed in the fat body, consisting mainly of apolipoproteins and phospholipids, is immediately
loaded with diacylglycerols [203]. The haemolymph of the resting animal contains 9- to 16-nmlarge HDLp particles that are poor in diacylglycerols and are known as N eHow due to their strongly
visible carotinoid coloration. Owing to the effect
of the adipokinetic hormone (AKH), secreted by
the corpora cardiaca at the beginning of flight,
diacylglycerol is increasingly released from the fat
bodies into the haemolymph and loaded onto the
lipophorin particles. Their density is consequently reduced, and the continuously expanding
particles are stabilized by the incorporation of
apoLp-III (protein ~), which is available in large
amounts in the haemolymph. In actively flying
animals, one finds mainly lipid-rich LDLp particles of 20-50 nm which contain as many as 16
apoLp-III and are known as lipophorin A + [48].
Following contact of the LDLp (lipophorin
A +) with the flight muscle, the diacylglycerols are
hydrolysed by a lipoprotein lipase and apoLp-III
is released; the low-lipid HDLp (lipophorin
N ellow ) that remains can then be reloaded with
diacylglycerols. Flight muscle lipoprotein lipase
has been isolated from the migratory locust and
found to be very different from the mammalian
enzyme (p. 662). The characteristic changes in the
lipophorin system that occur in the course of
development from the larva to the pupa and
eventually to the adult animal have been examined in detail in Manduca sexta and Bombyx
mori [169]. The haemolymph of M. sexta contains
specific lipid transfer particles (LTP) which catalyse transfer of lipids between lipoproteins of different density. The LTP is a VHDL particle of
1.4 . 10 6 Da, spherical with a long tail, and made
up from the glycoproteins apoLTP-1 (320 kDa),
apoLTP-II (85 kDa) and apoLTP-III (55 kDa)
with 14 % lipid. The LTP catalyses lipid transfer
from a donor LDLp to an acceptor LDLp, resulting in an HDLp and a VLDLp; the LTP, however,
can also use other lipoproteins as substrate,
including even some from vertebrates [219, 221].
The fat-body ,cells of Manduca carry on the surface a receptor of approximately 120 kDa which
binds lipoproteins more strongly the more dia-