202
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
the carotinoid canthaxanthin [38]. Other crustacean lipovitellins have a higher lipid content and
2-11 different apolipoproteins ranging in molecular mass between 45 and 190 kDa [38, 205, 257].
Tracer experiments with isolated organs of the
prawn Penaeus japonicus showed that immunologically identifiable VGs were synthesized by the
ovary but not by the hepato-pancreas. VG synthesis also occurs in the ovary in other decapods
like Palaemon and Pachygrapusus, but occurs in
the fat tissues of amphipods and isopods
[248, 257] and of the tick Ornithodorus moubata
[44].
The vitellin of the polychaete Nereis virens is a
lipoglycoprotein of about 420 kDa which is
coloured green by bilin. It arises in free coelom
cells (eliocytes) and is taken up into the oocytes
(which are also found free in the coelom fluid)
by receptor-mediated endocytosis; the eggmaturation process lasts for about 1 year [74]. It
has also been shown by tracer experiments in the
related species Perinereis cultrifera that VGs secreted by coelomocytes are taken up by egg cells.
The VG has a molecular mass of 530 kDa and
contains two 176-kDa polypeptides. The protein
taken up by the egg cells is progressively reduced
in size from 530 to 390 kDa; a series of five vitellins (VT-1 to VT-5) of decreasing molecular mass
is found in young oocytes but older oocytes contain only VT-5, which has a molecular mass of
390 kDa and about 16 % lipid [9].
In the nematode Caenorhabditis elegans the
yolk contains two different lipoprotein complexes, of which the A-complex contains three
different polypeptides of 170, 115 and 88 kDa
(yp170A, yp115 and yp88) and the B-complex has
only one type (yp170B). These polypeptides arise
from at least three different precursors produced
in the gut cells of the hermaphroditic animal. Two
of these (yp170A and yp170B) are taken up
unchanged into the oocytes, whilst the third is
first cleaved into two fragments (yp115 and yp88).
Caenorhabditis elegans possesses six VG genes,
the coding sequences of which agree 85 % with
the five homologous genes of the closely related
species C. briggsae [230, 283]. The eggs of the sea
urchin Hemicentrotus pulcherrimus contain lipoprotein particles that are 29-48 nm in diameter
with 55-72 %
lipid (predominantly triacylglycerols) and 8-13 % carbohydrate. Denaturation of the particles always produces the same
four polypeptides [59]. VG synthesis in the sea
urchin Stronglyocentrotus purpuratus is quite unique in several respects. There is only one VG
gene and this is expressed in the gut wall and
gonads of both males and females; it has similarities to the VG genes of the vertebrates not only
in its relatively large size of 19 kb but also in the
extreme shortness of the signal sequence and in
various sequence elements of the 5'-NT region
[234].
5.8 Blood Clotting
In the animal kingdom there are three mechanisms for reducing blood losses from damaged
blood vessels:
1. Contraction of the edges of the wound and the
vessels.
2. Blockage of the wound by aggregates of blood
cells.
3. Clotting of the blood fluid.
Here we will consider only the third process,
which has been observed not only in the vertebrates but also in the blood of arthropods and certain
molluscs (oysters) and in the coelom fluids of
sipunculids, brachiopods, echinoids and holothurians. In the case of the invertebrates, biochemical data are available only for the arthropod
groups Xiphosura, Crustacea and Insecta.
5.8.1 Blood Clotting in Vertebrates
The formation of a solid blood clot in all vertebrates involves the polymerization and precipitation
of fibrin; this occurs by partial proteolysis of the
fibrinogen in the blood plasma. The protease
responsible is thrombin, which itself is formed by
proteolysis from its precursor prothrombin. These
two reactions are the last steps in a cascade of
proteolytic processes which each produces an active clotting factor from a precursor (zymogen)
(Fig. 5.1). The cascade has an amplifying effect
and also increases the number of possible regulation points. The activation of the zymogens is
accelerated when they are complexed with protein cofactors, Ca 2 + ions and phospholipids of cell
membranes. Most clotting factors are serine proteases which, like trypsin, attack arginyl bonds;
only the factors Va and VIlla have no enzyme
activity, but they increase the effectiveness of proteases Xa and IXa. The zymogens of the clotting
cascade are homologous in their C-terminal
region (250 amino acids) with the catalytic
domain of the pancreas proteases; their Nterminal sequences, however, are much longer
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
the carotinoid canthaxanthin [38]. Other crustacean lipovitellins have a higher lipid content and
2-11 different apolipoproteins ranging in molecular mass between 45 and 190 kDa [38, 205, 257].
Tracer experiments with isolated organs of the
prawn Penaeus japonicus showed that immunologically identifiable VGs were synthesized by the
ovary but not by the hepato-pancreas. VG synthesis also occurs in the ovary in other decapods
like Palaemon and Pachygrapusus, but occurs in
the fat tissues of amphipods and isopods
[248, 257] and of the tick Ornithodorus moubata
[44].
The vitellin of the polychaete Nereis virens is a
lipoglycoprotein of about 420 kDa which is
coloured green by bilin. It arises in free coelom
cells (eliocytes) and is taken up into the oocytes
(which are also found free in the coelom fluid)
by receptor-mediated endocytosis; the eggmaturation process lasts for about 1 year [74]. It
has also been shown by tracer experiments in the
related species Perinereis cultrifera that VGs secreted by coelomocytes are taken up by egg cells.
The VG has a molecular mass of 530 kDa and
contains two 176-kDa polypeptides. The protein
taken up by the egg cells is progressively reduced
in size from 530 to 390 kDa; a series of five vitellins (VT-1 to VT-5) of decreasing molecular mass
is found in young oocytes but older oocytes contain only VT-5, which has a molecular mass of
390 kDa and about 16 % lipid [9].
In the nematode Caenorhabditis elegans the
yolk contains two different lipoprotein complexes, of which the A-complex contains three
different polypeptides of 170, 115 and 88 kDa
(yp170A, yp115 and yp88) and the B-complex has
only one type (yp170B). These polypeptides arise
from at least three different precursors produced
in the gut cells of the hermaphroditic animal. Two
of these (yp170A and yp170B) are taken up
unchanged into the oocytes, whilst the third is
first cleaved into two fragments (yp115 and yp88).
Caenorhabditis elegans possesses six VG genes,
the coding sequences of which agree 85 % with
the five homologous genes of the closely related
species C. briggsae [230, 283]. The eggs of the sea
urchin Hemicentrotus pulcherrimus contain lipoprotein particles that are 29-48 nm in diameter
with 55-72 %
lipid (predominantly triacylglycerols) and 8-13 % carbohydrate. Denaturation of the particles always produces the same
four polypeptides [59]. VG synthesis in the sea
urchin Stronglyocentrotus purpuratus is quite unique in several respects. There is only one VG
gene and this is expressed in the gut wall and
gonads of both males and females; it has similarities to the VG genes of the vertebrates not only
in its relatively large size of 19 kb but also in the
extreme shortness of the signal sequence and in
various sequence elements of the 5'-NT region
[234].
5.8 Blood Clotting
In the animal kingdom there are three mechanisms for reducing blood losses from damaged
blood vessels:
1. Contraction of the edges of the wound and the
vessels.
2. Blockage of the wound by aggregates of blood
cells.
3. Clotting of the blood fluid.
Here we will consider only the third process,
which has been observed not only in the vertebrates but also in the blood of arthropods and certain
molluscs (oysters) and in the coelom fluids of
sipunculids, brachiopods, echinoids and holothurians. In the case of the invertebrates, biochemical data are available only for the arthropod
groups Xiphosura, Crustacea and Insecta.
5.8.1 Blood Clotting in Vertebrates
The formation of a solid blood clot in all vertebrates involves the polymerization and precipitation
of fibrin; this occurs by partial proteolysis of the
fibrinogen in the blood plasma. The protease
responsible is thrombin, which itself is formed by
proteolysis from its precursor prothrombin. These
two reactions are the last steps in a cascade of
proteolytic processes which each produces an active clotting factor from a precursor (zymogen)
(Fig. 5.1). The cascade has an amplifying effect
and also increases the number of possible regulation points. The activation of the zymogens is
accelerated when they are complexed with protein cofactors, Ca 2 + ions and phospholipids of cell
membranes. Most clotting factors are serine proteases which, like trypsin, attack arginyl bonds;
only the factors Va and VIlla have no enzyme
activity, but they increase the effectiveness of proteases Xa and IXa. The zymogens of the clotting
cascade are homologous in their C-terminal
region (250 amino acids) with the catalytic
domain of the pancreas proteases; their Nterminal sequences, however, are much longer