100
3 The Structural Variety and Metabolism of Proteins
sized exclusively in the liver; in contrast, in the
related species M. cairoli the concentration of
hybrids, but only the M. cairoli gene is expressed
in the kidney; thus, a cis-active element appears
to be responsible for the tissue specificity. The
inhibitor synthesized in the liver is released into
the blood plasma, whereas that in the kidneys
enters the urine [15]. Horse plasma contains a
pre-
In man, there is only one gene for
and rat [10, 106]. The serpins involved in the
regulation of blood clotting are AT-III,
similar specificities, PAI-l and PAI-2 have completely different structures; PAI-2, like the
closely related ovalbumin, is secreted without
cleavage of a signal sequence [261, 299]. Gliae
cells from rat brain secrete the proteinase inhibitor nexin, which promotes neurite growth. Rat
nexin agrees in 84 % of its sequence with human
nexin, 32 % with AT-III and 25 % with
inhibitor, contrapsin, which, despite the difference in specificity, is more similar to human
inhibitors from blood plasma but also those from
snake toxins (Fig. 3.8), which will be dealt with in
Chapter 9. These polypeptides are, without
exception, toxic; several of them, e.g. the dendrotoxins from the black mamba, Dendroapsis
polylepsis, have lost their proteinase inhibitory
activity [60]. The chelonianin from the eggs of a
Red Sea turtle species appears to be the result of
gene fusion (exon shuffling); the two domains
belong to separate protein super-families, one to
the Kunitz family and the other to a previously
unknown family [134].
In the egg albumin of birds, the ovomucoid
which belongs to the family of secretory pancreas
trypsin inhibitors (PSTI family, Kazal type) is of
particular interest from a comparative biochemistry point of view. The inhibitory effect of the
ovomucoids on trypsin (T), on the one hand, and
on chymotrypsin, subtilisin and elastase (CSE) ,
on the other hand, differs greatly between species. In the chicken, one T is bound per molecule
of inhibitor, in the golden pheasant it is one C, in
the turkey one T and one CSE, and in ducks there
are two T's and one CSE. The explanation for this
curious variation lies in the fact that the ovomucoid chain consists of three domains whose reactive sites are T-specific, CSE-specific or neutral,
according to the nature of Pl. It is particularly
clear in this case that the amino acid PI of the
reactive centre varies considerably in its evolution. In the completely sequenced ovomucoid of
the chicken, the three domains occupy positions
1-64, 65-130 and 131-186 [135]. The sequences
of the third domain in 125 bird species have been
compared. 45-Asn is always completely or partially glycosylated, except in the ostrich, which has
a carbohydrate-free serine at this position. In the
pheasants, the ovomucoid molecule carries the
additional amino acids 134-Val and 135-Ser; this
variation stems from inaccurate splicing of an
intronlexon boundary [150]. Other proteinase
inhibitors of the Kazal type also contain several
domains (between one and six), each with a reactive centre (many-headed inhibitors) (Table 3.7).
The two-headed inhibitors, for example, are
quite well known; these were discovered in
canine saliva but are also found in various cat species [223].
In the case of only a few invertebrate serineproteinase inhibitors is there enough information
to allow their classification into known or new
families (Table 3.7). The proteinase inhibitors
from the slime of the vineyard snail, Helix pomatia, are amongst the best known; the Kunitz-type
inhibitor (K) has in fact been sequenced. There
are three to four different inhibitors in the egg
albumin glands of the investigated pulmonate
snails, and the spectrum of activity of these
against different proteinases varies with the species [198]. In the insects, the inhibitors from larval haemolymph of the moths Bombyx mori and
Manduca sexta have received particular attention
(Table 3.7). In B. mori, at least 15 chymotrypsin
inhibitors (SCI) can be separated electrophoretically, and SCI-I, SCI-II and SCI-III have been
sequenced and identified as belonging to the
Kunitz type [54]. In M. sexta, four 47-kDa serpins
with different specificities have been identified,
of which the elastase inhibitor has been
sequenced. In addition, the haemolymph con-
3 The Structural Variety and Metabolism of Proteins
sized exclusively in the liver; in contrast, in the
related species M. cairoli the concentration of
hybrids, but only the M. cairoli gene is expressed
in the kidney; thus, a cis-active element appears
to be responsible for the tissue specificity. The
inhibitor synthesized in the liver is released into
the blood plasma, whereas that in the kidneys
enters the urine [15]. Horse plasma contains a
pre-
In man, there is only one gene for
and rat [10, 106]. The serpins involved in the
regulation of blood clotting are AT-III,
similar specificities, PAI-l and PAI-2 have completely different structures; PAI-2, like the
closely related ovalbumin, is secreted without
cleavage of a signal sequence [261, 299]. Gliae
cells from rat brain secrete the proteinase inhibitor nexin, which promotes neurite growth. Rat
nexin agrees in 84 % of its sequence with human
nexin, 32 % with AT-III and 25 % with
inhibitor, contrapsin, which, despite the difference in specificity, is more similar to human
inhibitors from blood plasma but also those from
snake toxins (Fig. 3.8), which will be dealt with in
Chapter 9. These polypeptides are, without
exception, toxic; several of them, e.g. the dendrotoxins from the black mamba, Dendroapsis
polylepsis, have lost their proteinase inhibitory
activity [60]. The chelonianin from the eggs of a
Red Sea turtle species appears to be the result of
gene fusion (exon shuffling); the two domains
belong to separate protein super-families, one to
the Kunitz family and the other to a previously
unknown family [134].
In the egg albumin of birds, the ovomucoid
which belongs to the family of secretory pancreas
trypsin inhibitors (PSTI family, Kazal type) is of
particular interest from a comparative biochemistry point of view. The inhibitory effect of the
ovomucoids on trypsin (T), on the one hand, and
on chymotrypsin, subtilisin and elastase (CSE) ,
on the other hand, differs greatly between species. In the chicken, one T is bound per molecule
of inhibitor, in the golden pheasant it is one C, in
the turkey one T and one CSE, and in ducks there
are two T's and one CSE. The explanation for this
curious variation lies in the fact that the ovomucoid chain consists of three domains whose reactive sites are T-specific, CSE-specific or neutral,
according to the nature of Pl. It is particularly
clear in this case that the amino acid PI of the
reactive centre varies considerably in its evolution. In the completely sequenced ovomucoid of
the chicken, the three domains occupy positions
1-64, 65-130 and 131-186 [135]. The sequences
of the third domain in 125 bird species have been
compared. 45-Asn is always completely or partially glycosylated, except in the ostrich, which has
a carbohydrate-free serine at this position. In the
pheasants, the ovomucoid molecule carries the
additional amino acids 134-Val and 135-Ser; this
variation stems from inaccurate splicing of an
intronlexon boundary [150]. Other proteinase
inhibitors of the Kazal type also contain several
domains (between one and six), each with a reactive centre (many-headed inhibitors) (Table 3.7).
The two-headed inhibitors, for example, are
quite well known; these were discovered in
canine saliva but are also found in various cat species [223].
In the case of only a few invertebrate serineproteinase inhibitors is there enough information
to allow their classification into known or new
families (Table 3.7). The proteinase inhibitors
from the slime of the vineyard snail, Helix pomatia, are amongst the best known; the Kunitz-type
inhibitor (K) has in fact been sequenced. There
are three to four different inhibitors in the egg
albumin glands of the investigated pulmonate
snails, and the spectrum of activity of these
against different proteinases varies with the species [198]. In the insects, the inhibitors from larval haemolymph of the moths Bombyx mori and
Manduca sexta have received particular attention
(Table 3.7). In B. mori, at least 15 chymotrypsin
inhibitors (SCI) can be separated electrophoretically, and SCI-I, SCI-II and SCI-III have been
sequenced and identified as belonging to the
Kunitz type [54]. In M. sexta, four 47-kDa serpins
with different specificities have been identified,
of which the elastase inhibitor has been
sequenced. In addition, the haemolymph con-