5.6.2 Plasma Lipoproteins of Insects and Other Invertebrates
195
22-amino-acid repeats which build amphiphilic ahelices. ApoD differs from the other small apolipoproteins in many significant properties. The
169-amino-acid apoD chain has an a-helix portion of less than 5 % and no periodicity. The
highest concentration of apoD mRNA is found in
the medulla of the adrenal body, and it is high in
the kidney, pancreas and small intestine, but low
in the liver. The apoD gene has a very different
exon-intron structure compared with that of the
genes of the other apolipoproteins [147]. Apparently, apoD does not belong to the protein superfamily of the apolipoproteins but, together with
the retinol-binding protein, to that of the a2uglobulins (Table 5.2). The genes of human apolipoproteins are dispersed on different chromosomes: apoA-II, apoB-l00/48 and apoD are found
as single copies on chromosomes 1, 2 and 3,
respectively; apoA-I, apoC-III and apoA-IV
form a gene cluster on chromosome 11, in the
same order as is found in the chicken; and apoE,
apoC-1 and apoC-II form a cluster on chromosome 9. With the exception of apoD, the genes of
all the small apolipoproteins have a similar structure with one intron each in the 5'-NT (nontranslated) region, the signal region and the
region coding for the mature protein; the first
intron is missing in the gene for apoA-IY. The
apoD gene contains no intron in the signal region
but three introns in the remaining coding region.
The gene of apoB-l00, with a total length of
43 kb, is unexpectedly short when one considers
that it includes 4564 co dons in its 29 exons,
including the 27 signal-coding triplets and the
stop-codons; in fact, exons 26 and 29, with 7572
and 1906 bp, respectively, are exceptionally long
[147].
The protein or gene sequences are known for
all human apolipoproteins and for many of those
from the chicken, rat, mouse, guinea-pig, rabbit
and dog [56, 147, 161 280]. Sequence comparisons indicate relatively high rates of evolution.
For example, human and rat apoA-IV differ at
38 % of positions, and rabbit apoA-1 differs by
20 % from that of the dog, by 32 % from human
and by 40 % from rat [123, 279]. The rate of
evolution of the apolipoproteins A-II, C-II and CIII is approximately three times higher than that
of ~-globin (see Table 4.12), and that of A-I and
A-IV is about twice as high; the apoE rate is also
higher than that of ~-globin, but that of apoB is
markedly lower. Evolution apparently began with
an apolipoprotein similar to apoC-I and resulted
consecutively in the forms C-II, C-III, A-II, E, AIV and A-I [147]. Many of the apolipoproteins
found in lower vertebrates correspond to those of
man in their electrophoretic mobility but should
not necessarily be assumed to be identical. Up to
now, only two apolipoproteins of lower vertebrates have been sequenced (via cDNA) , namely
those from the HDL of the sea lamprey Petromyzon marin us ; they show no significant sequence
similarity to any apolipoprotein class of mammals. Including the signal peptide, pro-peptide
and mature protein, LALl consists of 21 + 8 + 76
amino acids, and LAL2 of 23 + 168 amino acids
[202]. A close relationship between the lipoproteins of the lower and higher vertebrates is, however, indicated by immunological cross-reactivity
between the VLDL and LDL of trout, chicken
and guinea-pig.
5.6.2 Plasma Lipoproteins of Insects
and Other Invertebrates
Lipid transport in insects is fundamentally different from that in vertebrates. In the latter, the triacylglycerols form the non-polar nucleus of lipoprotein particles and their fatty acids can be
released only by radical reshaping of the particle.
In insects, fatty acids are transported as strongly
polar diacylglycerols (DAGs) arranged close to
the surface of the lipoprotein particles, from
whence they can be easily removed. Thus, in contrast to the vertebrates, the lipid transport particles of insects are reusable transport proteins. In
the migratory locust Locusta migratoria, the halflives of apolipoproteins and DAGs is 5-6 days
and 2-3 h, respectively [101]. The lipidtransporting proteins of insects were originally
termed "diacylglycerol-carrying proteins"; after it
became known that they were also involved in
lipid resorption in the intestine and in the transport of sterols, hydrocarbons and carotinoids,
Chino in 1981 suggested the name Iipophorins.
The lipophorin systems of representatives from
various different insect orders have been examined in detail [72, 89, 106, 168,229,247, 261].
They appear under the electron microscope as
globular particles with a diameter of 13-16 nm,
corresponding to molecular masses of 500700 kDa [221]. Their lipid content is normally 4050 % but varies with the metabolic state. According to the increase in density with decreasing lipid
content, the lipophorins are placed into the
classes LDLp, HDLp and VHDLp [20]. Diacylglycerols and phospholopids are the predominant
lipids; triacylglycerols are only found in trace
amounts. Nuclear magnetic resonance (NMR)
195
22-amino-acid repeats which build amphiphilic ahelices. ApoD differs from the other small apolipoproteins in many significant properties. The
169-amino-acid apoD chain has an a-helix portion of less than 5 % and no periodicity. The
highest concentration of apoD mRNA is found in
the medulla of the adrenal body, and it is high in
the kidney, pancreas and small intestine, but low
in the liver. The apoD gene has a very different
exon-intron structure compared with that of the
genes of the other apolipoproteins [147]. Apparently, apoD does not belong to the protein superfamily of the apolipoproteins but, together with
the retinol-binding protein, to that of the a2uglobulins (Table 5.2). The genes of human apolipoproteins are dispersed on different chromosomes: apoA-II, apoB-l00/48 and apoD are found
as single copies on chromosomes 1, 2 and 3,
respectively; apoA-I, apoC-III and apoA-IV
form a gene cluster on chromosome 11, in the
same order as is found in the chicken; and apoE,
apoC-1 and apoC-II form a cluster on chromosome 9. With the exception of apoD, the genes of
all the small apolipoproteins have a similar structure with one intron each in the 5'-NT (nontranslated) region, the signal region and the
region coding for the mature protein; the first
intron is missing in the gene for apoA-IY. The
apoD gene contains no intron in the signal region
but three introns in the remaining coding region.
The gene of apoB-l00, with a total length of
43 kb, is unexpectedly short when one considers
that it includes 4564 co dons in its 29 exons,
including the 27 signal-coding triplets and the
stop-codons; in fact, exons 26 and 29, with 7572
and 1906 bp, respectively, are exceptionally long
[147].
The protein or gene sequences are known for
all human apolipoproteins and for many of those
from the chicken, rat, mouse, guinea-pig, rabbit
and dog [56, 147, 161 280]. Sequence comparisons indicate relatively high rates of evolution.
For example, human and rat apoA-IV differ at
38 % of positions, and rabbit apoA-1 differs by
20 % from that of the dog, by 32 % from human
and by 40 % from rat [123, 279]. The rate of
evolution of the apolipoproteins A-II, C-II and CIII is approximately three times higher than that
of ~-globin (see Table 4.12), and that of A-I and
A-IV is about twice as high; the apoE rate is also
higher than that of ~-globin, but that of apoB is
markedly lower. Evolution apparently began with
an apolipoprotein similar to apoC-I and resulted
consecutively in the forms C-II, C-III, A-II, E, AIV and A-I [147]. Many of the apolipoproteins
found in lower vertebrates correspond to those of
man in their electrophoretic mobility but should
not necessarily be assumed to be identical. Up to
now, only two apolipoproteins of lower vertebrates have been sequenced (via cDNA) , namely
those from the HDL of the sea lamprey Petromyzon marin us ; they show no significant sequence
similarity to any apolipoprotein class of mammals. Including the signal peptide, pro-peptide
and mature protein, LALl consists of 21 + 8 + 76
amino acids, and LAL2 of 23 + 168 amino acids
[202]. A close relationship between the lipoproteins of the lower and higher vertebrates is, however, indicated by immunological cross-reactivity
between the VLDL and LDL of trout, chicken
and guinea-pig.
5.6.2 Plasma Lipoproteins of Insects
and Other Invertebrates
Lipid transport in insects is fundamentally different from that in vertebrates. In the latter, the triacylglycerols form the non-polar nucleus of lipoprotein particles and their fatty acids can be
released only by radical reshaping of the particle.
In insects, fatty acids are transported as strongly
polar diacylglycerols (DAGs) arranged close to
the surface of the lipoprotein particles, from
whence they can be easily removed. Thus, in contrast to the vertebrates, the lipid transport particles of insects are reusable transport proteins. In
the migratory locust Locusta migratoria, the halflives of apolipoproteins and DAGs is 5-6 days
and 2-3 h, respectively [101]. The lipidtransporting proteins of insects were originally
termed "diacylglycerol-carrying proteins"; after it
became known that they were also involved in
lipid resorption in the intestine and in the transport of sterols, hydrocarbons and carotinoids,
Chino in 1981 suggested the name Iipophorins.
The lipophorin systems of representatives from
various different insect orders have been examined in detail [72, 89, 106, 168,229,247, 261].
They appear under the electron microscope as
globular particles with a diameter of 13-16 nm,
corresponding to molecular masses of 500700 kDa [221]. Their lipid content is normally 4050 % but varies with the metabolic state. According to the increase in density with decreasing lipid
content, the lipophorins are placed into the
classes LDLp, HDLp and VHDLp [20]. Diacylglycerols and phospholopids are the predominant
lipids; triacylglycerols are only found in trace
amounts. Nuclear magnetic resonance (NMR)