tica to penetrate the gut wall of its host [162, 188],
function as digestive enzymes in blood-sucking
fleas, beetles and crustaceans [151, 184], and are
involved in embryo development in various sea
urchins and the brine shrimp Artemia as well as in
insect metamorphosis [197,290]. A proteinase, in
several forms of 27-31 kDa, has been isolated
from the trematode Schistosoma mansoni, and
this corresponds to the vertebrate cathepsin B in
its activity with synthetic low molecular weight
substrates; it is responsible for the digestion of
haemoglobin from the host's blood. S. mansoni
possesses multiple genes for this enzyme [30, 49].
Cysteine proteinases with particular properties
are found in the paragonimid Paragonimus ohirai; they are inhibited not only by the usual thiolprotease inhibitors, such as antipain and phydroxymercuriphenylsulfonate (p-HMPS), but
also by urantitrypsin and soybean trypsin inhibitor
[298]. The recently discovered Ca 2 + -dependent
neutral proteases (CANPs) also belong to the
family of cysteine proteinases. Because of their
homology to the plant proteinase papain, these
enzymes have been given the name calpains by
the IUB enzyme commission. Calpains have been
detected not only in the muscles and other organs
of vertebrates but also in molluscs, crustaceans
and insects [211]. The vertebrate enzymes are
heterodimers of an 80-kDa catalytic subunit and
a 28-kDa chain which is cleaved off during activation. On the basis of their calcium affinities,
one can distinguish !-l-calpain (type I), which
requires only 1-70 !-lmol Ca 2 +/l for half-maximum
activity, and m-calpain, which requires
0.4-0.6 mmol Ca 2 +/l. The breast muscle of the
chicken contains a third "high m-calpain" type
which needs 3.8 mmol Ca 2 +/l [294]. The catalytic
chain of m-calpain from chicken muscle has been
sequenced via its cDNA. Four domains can be
recognized in the sequence of 705 amino acids.
The catalytic domain II (positions 81-320) has
30 % sequence agreement with other cysteine
proteinases, and the regulatory domain IV
resembles Ca2+ -binding proteins like calmodulin
and troponin C; the functions of domains I and
III are not yet known. The sequence agreement
with proteins of other super-families suggests that
calpains arose by gene fusion. The calpains apparently have no general proteolytic function but
specific proteolytic activities that have not yet
been described in detail. A sudden increase
in intracellular Ca 2 + concentration, as effected
by the secondary messenger inositol-l,4,5,trisphosphate, leads to the activation of this
enzyme [18, 212].
3.3.4 Aspartate Proteinases
95
3.3.4 Aspartate Proteinases
Characteristic of the aspartate proteinases is a pH
optimum below 6 (acidic proteinases), the presence of two especially reactive aspartate residues,
and inhibition by the pepstatin of Streptomyces
(Table 3.5). Two classes of acidic proteinases are
found in the mammalian stomach: the pepsins
and gastricsins, which serve to digest proteins in
the adults; and the chymosins, which precipitate
casein in the stomachs of young breast-feeding
animals. The chymosins show such low general
proteolytic activity that undigested milk immunoglobulins can be harvested from young animals. The gene and protein sequences have been
determined for pepsinogens and prochymosins in
various mammals, the chicken and tuna fish
[101, 117, 129, 130, 216,273]. The various stomach proteinases agree in about 40 % of amino acid
positions and are clearly homologous. Comparison of the three enzyme types shows that pepsinogen C (progastricsin) arose during evolution
before the separation of prochymosin and pepsinogen A (Table 3.6). Not only do cathepsins D
and E, renin and other intracellular aspartate
proteinases of other species belong to the same
super-family, but it also includes the fungal
enzyme penicillopepsin, which has 24 % similarity to the stomach proteinases. The region around
the active aspartate residue is particularly conserved.
The aspartate proteinases often show a high
degree of heterogeneity. This may be due to the
existence of several genes but also involves posttranslational modifications such as phosphorylation, glycosylation or partial autolysis. Thus, in
the pig stomach, for example, there are four different pepsins and pepsinogens. The predominant
pepsin A is mainly secreted in the region of the
fundus and has a pH optimum of 2; pepsins Band
C are released primarily into the antro-pyloric
part of the stomach, have a somewhat narrower
specificity than pepsin A, and have a pH optimum of 3; pepsin D is the phosphate-free form of
pepsin A. The so-called gastricsin of primates corresponds to pepsin C of the pig [267]. Multiple
pepsins are known in other mammals including
man and the chicken, but the homologies of the
individual types in different species are uncertain.
Multiple chymosins are also found; in calves
there are two chymosins (A and B) with different
sequences, and form C arises by autolysis. In
pups of the seal Pagophilus groenlandicus there
are four chymosins which, surprisingly, also
appear in the adults [246]. In man, there are
function as digestive enzymes in blood-sucking
fleas, beetles and crustaceans [151, 184], and are
involved in embryo development in various sea
urchins and the brine shrimp Artemia as well as in
insect metamorphosis [197,290]. A proteinase, in
several forms of 27-31 kDa, has been isolated
from the trematode Schistosoma mansoni, and
this corresponds to the vertebrate cathepsin B in
its activity with synthetic low molecular weight
substrates; it is responsible for the digestion of
haemoglobin from the host's blood. S. mansoni
possesses multiple genes for this enzyme [30, 49].
Cysteine proteinases with particular properties
are found in the paragonimid Paragonimus ohirai; they are inhibited not only by the usual thiolprotease inhibitors, such as antipain and phydroxymercuriphenylsulfonate (p-HMPS), but
also by urantitrypsin and soybean trypsin inhibitor
[298]. The recently discovered Ca 2 + -dependent
neutral proteases (CANPs) also belong to the
family of cysteine proteinases. Because of their
homology to the plant proteinase papain, these
enzymes have been given the name calpains by
the IUB enzyme commission. Calpains have been
detected not only in the muscles and other organs
of vertebrates but also in molluscs, crustaceans
and insects [211]. The vertebrate enzymes are
heterodimers of an 80-kDa catalytic subunit and
a 28-kDa chain which is cleaved off during activation. On the basis of their calcium affinities,
one can distinguish !-l-calpain (type I), which
requires only 1-70 !-lmol Ca 2 +/l for half-maximum
activity, and m-calpain, which requires
0.4-0.6 mmol Ca 2 +/l. The breast muscle of the
chicken contains a third "high m-calpain" type
which needs 3.8 mmol Ca 2 +/l [294]. The catalytic
chain of m-calpain from chicken muscle has been
sequenced via its cDNA. Four domains can be
recognized in the sequence of 705 amino acids.
The catalytic domain II (positions 81-320) has
30 % sequence agreement with other cysteine
proteinases, and the regulatory domain IV
resembles Ca2+ -binding proteins like calmodulin
and troponin C; the functions of domains I and
III are not yet known. The sequence agreement
with proteins of other super-families suggests that
calpains arose by gene fusion. The calpains apparently have no general proteolytic function but
specific proteolytic activities that have not yet
been described in detail. A sudden increase
in intracellular Ca 2 + concentration, as effected
by the secondary messenger inositol-l,4,5,trisphosphate, leads to the activation of this
enzyme [18, 212].
3.3.4 Aspartate Proteinases
95
3.3.4 Aspartate Proteinases
Characteristic of the aspartate proteinases is a pH
optimum below 6 (acidic proteinases), the presence of two especially reactive aspartate residues,
and inhibition by the pepstatin of Streptomyces
(Table 3.5). Two classes of acidic proteinases are
found in the mammalian stomach: the pepsins
and gastricsins, which serve to digest proteins in
the adults; and the chymosins, which precipitate
casein in the stomachs of young breast-feeding
animals. The chymosins show such low general
proteolytic activity that undigested milk immunoglobulins can be harvested from young animals. The gene and protein sequences have been
determined for pepsinogens and prochymosins in
various mammals, the chicken and tuna fish
[101, 117, 129, 130, 216,273]. The various stomach proteinases agree in about 40 % of amino acid
positions and are clearly homologous. Comparison of the three enzyme types shows that pepsinogen C (progastricsin) arose during evolution
before the separation of prochymosin and pepsinogen A (Table 3.6). Not only do cathepsins D
and E, renin and other intracellular aspartate
proteinases of other species belong to the same
super-family, but it also includes the fungal
enzyme penicillopepsin, which has 24 % similarity to the stomach proteinases. The region around
the active aspartate residue is particularly conserved.
The aspartate proteinases often show a high
degree of heterogeneity. This may be due to the
existence of several genes but also involves posttranslational modifications such as phosphorylation, glycosylation or partial autolysis. Thus, in
the pig stomach, for example, there are four different pepsins and pepsinogens. The predominant
pepsin A is mainly secreted in the region of the
fundus and has a pH optimum of 2; pepsins Band
C are released primarily into the antro-pyloric
part of the stomach, have a somewhat narrower
specificity than pepsin A, and have a pH optimum of 3; pepsin D is the phosphate-free form of
pepsin A. The so-called gastricsin of primates corresponds to pepsin C of the pig [267]. Multiple
pepsins are known in other mammals including
man and the chicken, but the homologies of the
individual types in different species are uncertain.
Multiple chymosins are also found; in calves
there are two chymosins (A and B) with different
sequences, and form C arises by autolysis. In
pups of the seal Pagophilus groenlandicus there
are four chymosins which, surprisingly, also
appear in the adults [246]. In man, there are