186
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
hynchus keta, a dimeric protein with 16-kDa subunits that is synthesized in the coelomic epithelium and the mesovarium. The function of CFSP
is not known; immunologically similar proteins
are found in all Oncorhynchus species but not in
other salmon [160].
All plasma proteins are already present in the
embryo, e.g. in humans from the 6th week, but in
relative amounts that ate different to those found
in the adult. In pig foetuses, however, about onethird of the plasma proteins are not cross-reactive
with the antisera against adult plasma [137].
Thus, there are specific foetal proteins, of which
a-fetoprotein and fetuin have received particular
attention. The synthesis of the majority of plasma
proteins occurs in the liver; in embryos, the yolksac is also always involved. Whilst in humans,
plasma protein synthesis in the yolk-sac ceases at
about 12 weeks, it continues in rats and chickens
up to the moment of birth or hatching, respectively. In contrast to the human and chicken yolksacs, that of the rat produces no albumin;
investigations in other species have unfortunately
not been carried out. Many vertebrates produce
specific plasma proteins when in certain physiological states. For example, the body reacts to
infection, inflammation or tissue damage by producing acute-phase proteins. Also relevant here
are the mammalian pregnancy proteins, and the
precursors of the yolk proteins that appear during
the development of yolked eggs.
5.1.2 Plasma Proteins of Invertebrates
By means of high-resolution electrophoretic
methods, more than 100 different polypeptides
with molecular masses of 15-200 kDa have been
demonstrated in the haemolymph of the mussel
Mytilus edulis [245]. In insect haemolymph,
10-30 protein fractions can be electrophoretically
separated and grouped, according to function,
into the vitellogenins, lipoproteins, hormonebinding proteins, storage proteins, defence proteins and enzymes. The haemolymph proteins show
species-specific changes during each developmental stage. In the haemolymph of older larvae,
however, there are usually only a few protein
types, in particular the larval haemolymph proteins (LHPs) and the lipid-transporting lipophorins. Their concentration is low (10-20 mglml) at
the beginning of each developmental stage but
reaches high values at each ecdysis, e.g. in Lepidoptera 60-100 mglml and in bluebottle larvae as
much as 200 mglml [87,217]. Synthesis of the
haemolymph proteins occurs mainly in the fat
bodies; it has been shown, however, in tracer
experiments with caterpillars of the butterfly Calpodes ethlius that the mid-gut and epidermis cells
are involved in the synthesis of arylphorins and
other haemolymph proteins [197, 198].
In animal species with haemoglobin or haemocyanin freely dissolved in the plasma, these two
substances make up the largest fraction of the
plasma proteins. Other non-respiratory proteins
are found in many chelicerates and decapod crustaceans but, as yet, little is known about either
their structure or function. There is no real basis
for the assumption that they are mainly copperfree "apohaemocyanins". The coelomic fluid of
the annelids and the haemolymph of the molluscs
[3] also contain up to 20 proteins that are detectable by gel electrophoresis (30-60 are seen on
2-D electrophoregrams) but about which little is
known.
5.2 Serum Albumin and «-Fetoprotein
Albnmin is the most easily identifiable plasma
protein of the higher vertebrates due to its solubility at low ionic strength, low molecular mass,
high charge, particular binding characteristics
and paucity of carbohydrates. At a concentration
of 35-50 mglml, it makes up about 60 % of
human plasma protein; it also forms the largest
fraction in the lower vertebrates. Because of its
relatively high concentration and low molecular
mass, albumin is the main factor determining the
colloid-osmotic pressure of the plasma. In the
tadpoles of Rana catesbeiana, its concentration is
less than 1 mglml and only increases markedly
during metamorphosis. Serum albumins have
also been isolated from non-mammalian species;
they appear to correspond broadly in their molecular masses and electrophoretic behaviour to
those of the mammals and man [30, 57, 92, 154].
In the agnathans and cartilaginous fish no plasma
protein has so far been found which corresponds
to albumin in its physicochemical and functional
properties, such as the ability to bind fatty acids
[73]. The so-called pre-albumins of many vertebrates appear to have nothing in common with real
albumins, except a high electrophoretic mobility.
Complete sequences have been obtained,
either directly or via cDNA, for human, bovine,
sheep, pig, rat and mouse albumins and the two
albumins of the clawed frog Xenopus laevis. The
albumins consist of a single polypeptide chain of
5 Plasma Proteins, Yolk Proteins and Metal-Binding Proteins
hynchus keta, a dimeric protein with 16-kDa subunits that is synthesized in the coelomic epithelium and the mesovarium. The function of CFSP
is not known; immunologically similar proteins
are found in all Oncorhynchus species but not in
other salmon [160].
All plasma proteins are already present in the
embryo, e.g. in humans from the 6th week, but in
relative amounts that ate different to those found
in the adult. In pig foetuses, however, about onethird of the plasma proteins are not cross-reactive
with the antisera against adult plasma [137].
Thus, there are specific foetal proteins, of which
a-fetoprotein and fetuin have received particular
attention. The synthesis of the majority of plasma
proteins occurs in the liver; in embryos, the yolksac is also always involved. Whilst in humans,
plasma protein synthesis in the yolk-sac ceases at
about 12 weeks, it continues in rats and chickens
up to the moment of birth or hatching, respectively. In contrast to the human and chicken yolksacs, that of the rat produces no albumin;
investigations in other species have unfortunately
not been carried out. Many vertebrates produce
specific plasma proteins when in certain physiological states. For example, the body reacts to
infection, inflammation or tissue damage by producing acute-phase proteins. Also relevant here
are the mammalian pregnancy proteins, and the
precursors of the yolk proteins that appear during
the development of yolked eggs.
5.1.2 Plasma Proteins of Invertebrates
By means of high-resolution electrophoretic
methods, more than 100 different polypeptides
with molecular masses of 15-200 kDa have been
demonstrated in the haemolymph of the mussel
Mytilus edulis [245]. In insect haemolymph,
10-30 protein fractions can be electrophoretically
separated and grouped, according to function,
into the vitellogenins, lipoproteins, hormonebinding proteins, storage proteins, defence proteins and enzymes. The haemolymph proteins show
species-specific changes during each developmental stage. In the haemolymph of older larvae,
however, there are usually only a few protein
types, in particular the larval haemolymph proteins (LHPs) and the lipid-transporting lipophorins. Their concentration is low (10-20 mglml) at
the beginning of each developmental stage but
reaches high values at each ecdysis, e.g. in Lepidoptera 60-100 mglml and in bluebottle larvae as
much as 200 mglml [87,217]. Synthesis of the
haemolymph proteins occurs mainly in the fat
bodies; it has been shown, however, in tracer
experiments with caterpillars of the butterfly Calpodes ethlius that the mid-gut and epidermis cells
are involved in the synthesis of arylphorins and
other haemolymph proteins [197, 198].
In animal species with haemoglobin or haemocyanin freely dissolved in the plasma, these two
substances make up the largest fraction of the
plasma proteins. Other non-respiratory proteins
are found in many chelicerates and decapod crustaceans but, as yet, little is known about either
their structure or function. There is no real basis
for the assumption that they are mainly copperfree "apohaemocyanins". The coelomic fluid of
the annelids and the haemolymph of the molluscs
[3] also contain up to 20 proteins that are detectable by gel electrophoresis (30-60 are seen on
2-D electrophoregrams) but about which little is
known.
5.2 Serum Albumin and «-Fetoprotein
Albnmin is the most easily identifiable plasma
protein of the higher vertebrates due to its solubility at low ionic strength, low molecular mass,
high charge, particular binding characteristics
and paucity of carbohydrates. At a concentration
of 35-50 mglml, it makes up about 60 % of
human plasma protein; it also forms the largest
fraction in the lower vertebrates. Because of its
relatively high concentration and low molecular
mass, albumin is the main factor determining the
colloid-osmotic pressure of the plasma. In the
tadpoles of Rana catesbeiana, its concentration is
less than 1 mglml and only increases markedly
during metamorphosis. Serum albumins have
also been isolated from non-mammalian species;
they appear to correspond broadly in their molecular masses and electrophoretic behaviour to
those of the mammals and man [30, 57, 92, 154].
In the agnathans and cartilaginous fish no plasma
protein has so far been found which corresponds
to albumin in its physicochemical and functional
properties, such as the ability to bind fatty acids
[73]. The so-called pre-albumins of many vertebrates appear to have nothing in common with real
albumins, except a high electrophoretic mobility.
Complete sequences have been obtained,
either directly or via cDNA, for human, bovine,
sheep, pig, rat and mouse albumins and the two
albumins of the clawed frog Xenopus laevis. The
albumins consist of a single polypeptide chain of