322
9 Toxic Proteins and Peptides
solved substrates, and are Ca z + dependent. Many
snake venoms contain several isoenzymes with
widely differing isoelectric points (pI 4-11) and
different quaternary structures; only the basic
isoenzymes are toxic. They are presynaptically
neurotoxic on skeletal muscles through the inhibition of acetylcholine release, and are also often
myonecrotic, destroying muscle fibres and causing the appearance of myoglobin in the urine
[30] ; they are amongst the most dangerous animal
toxins. The more than 40 well-known phospholipase Az snake venoms are polypeptides of
108-140 amino acids with 4, 6 or 7 disulphide
bridges; they can occur in solution as monomers,
homodimers, heterodimers or even larger aggregates [38, 42]. They are homologous amongst
themselves and with the enzymes from the mammalian pancreas; the sequence agreement is particularly high in the region of the active centre
and the Ca2+ -binding site. The enzymes from
bovine pancreas and the venom of the rattlesnake
Crotalus atrox are very similar in their spatial
structure. The Ca2+ -binding 49-Asp as well as 28Tyr, 30-Gly and 32-Gly are all involved in the
catalytic mechanism; the venom of the copperhead snake Agkistrodon piscivorus, however,
contains in addition to the 49-Asp enzyme a further phospholipase Az with 49-Lys, which has an
altered catalytic mechanism but is otherwise
homologous to the other isoenzymes. Both the
pancreas enzymes and the venom enzymes of the
elapids and the sea snakes contain a characteristic
disulphide bridge, 11-Cys-69-Cys; this is missing
in the venom enzymes of the Crotalidae and Viperidae, which instead have a disulphide bridge
associated with an extended C-terminus [42].
Snake venoms often contain several toxic phospholipases A z, e.g. the sand viper Vipera ammodytes has three monomeric enzymes referred to as
ammodytoxins A-C [53]. In addition, many
venoms contain complex toxins composed of different, but homologous, subunits. Thus, the
"vipoxin" from V. ammodytes consists of a toxic,
basic phospholipase A z and a non-toxic acidic
phospholipase inhibitor which agree in 62 % of
their 122 amino acids [41]. The "crotoxin" of the
North American rattlesnake Crotalus durissimus
was discovered in 1938 as the first presynapticacting neurotoxin, but its structure has only
recently been completely determined. In this
case, there is also a basic subunit, CB, of 122
amino acids which is enzymically active and toxic;
the acidic subunit, however, consists of three peptides of 38,35 and 14 amino acids linked by a total
of seven disulphide bridges. These result from the
partial hydrolysis of a 122-amino-acid precursor,
CA, which is homologous to CB and even has an
identical signal sequence of 16 amino acids [2, 7].
The most complicated structure and with the
highest known toxicity (1 !-tglkg mouse Lv.) is
found in the "textilotoxin" of the Australian common brown snake Pseudonaja textilis; this toxin is
made up of three peptide chains, A-C, and a
homodimer DD [52]. The venom of the North
American rattlesnake contains a further toxin,
"crotamin", which has as yet undefined effects on
the musculature. This is a basic polypeptide of 42
amino acids with three disulphide bridges, and
has no sequence similarity to any other known
toxin. Other Crotalus species have similar toxins
[36, 63]. The "taipoxin" of the Australian "taipan" Oxyuranus scutellatur is made up of three
subunits which differ greatly from each other but
are all homologous with other phospholipases A z.
The basic a subunit, which is the only one showing any sort of enzyme activity, is weakly toxic in
the purified form; the combination of the three
subunits is 500-fold more toxic, with an LDso of
2.1 !-tg/kg [37].
9.1.2 Enzymes and Other Protein
Components
The poisonous secretions of the snakes always
contain a large number of protein components
with different pharmacological effects or biological activities; often more than 25 protein fractions may be distinguished by electrophoresis or
chromatography. Venom secretions with similar
toxicities from different populations, or even
from different individuals, may show large differences in electrophoretic pattern and enzyme
activity. The effects of enzyme components
include local capillary damage and tissue necrosis
(from phospholipases, proteases, hyaluronidases), inhibition or promotion of blood clotting
(from specific proteases, phospholipases), reduction in blood pressure, and pain (from kininreleasing enzymes).
Most of the phospholipases from snake venom
cleave 2-acyl residues from different phospholipids, Le. they are of type A z. They have haemolytic effects and several of them have synergistic
effects on toxin activity. A phospholipase B found
in the venom of the elapid Pseudechis australus
attacks the 1-acyl group in lysophosphatide, and
several phospholipases A z also have a similar,
albeit weak, activity. Although the phospholipases Az are acidic or basic, according to their
9 Toxic Proteins and Peptides
solved substrates, and are Ca z + dependent. Many
snake venoms contain several isoenzymes with
widely differing isoelectric points (pI 4-11) and
different quaternary structures; only the basic
isoenzymes are toxic. They are presynaptically
neurotoxic on skeletal muscles through the inhibition of acetylcholine release, and are also often
myonecrotic, destroying muscle fibres and causing the appearance of myoglobin in the urine
[30] ; they are amongst the most dangerous animal
toxins. The more than 40 well-known phospholipase Az snake venoms are polypeptides of
108-140 amino acids with 4, 6 or 7 disulphide
bridges; they can occur in solution as monomers,
homodimers, heterodimers or even larger aggregates [38, 42]. They are homologous amongst
themselves and with the enzymes from the mammalian pancreas; the sequence agreement is particularly high in the region of the active centre
and the Ca2+ -binding site. The enzymes from
bovine pancreas and the venom of the rattlesnake
Crotalus atrox are very similar in their spatial
structure. The Ca2+ -binding 49-Asp as well as 28Tyr, 30-Gly and 32-Gly are all involved in the
catalytic mechanism; the venom of the copperhead snake Agkistrodon piscivorus, however,
contains in addition to the 49-Asp enzyme a further phospholipase Az with 49-Lys, which has an
altered catalytic mechanism but is otherwise
homologous to the other isoenzymes. Both the
pancreas enzymes and the venom enzymes of the
elapids and the sea snakes contain a characteristic
disulphide bridge, 11-Cys-69-Cys; this is missing
in the venom enzymes of the Crotalidae and Viperidae, which instead have a disulphide bridge
associated with an extended C-terminus [42].
Snake venoms often contain several toxic phospholipases A z, e.g. the sand viper Vipera ammodytes has three monomeric enzymes referred to as
ammodytoxins A-C [53]. In addition, many
venoms contain complex toxins composed of different, but homologous, subunits. Thus, the
"vipoxin" from V. ammodytes consists of a toxic,
basic phospholipase A z and a non-toxic acidic
phospholipase inhibitor which agree in 62 % of
their 122 amino acids [41]. The "crotoxin" of the
North American rattlesnake Crotalus durissimus
was discovered in 1938 as the first presynapticacting neurotoxin, but its structure has only
recently been completely determined. In this
case, there is also a basic subunit, CB, of 122
amino acids which is enzymically active and toxic;
the acidic subunit, however, consists of three peptides of 38,35 and 14 amino acids linked by a total
of seven disulphide bridges. These result from the
partial hydrolysis of a 122-amino-acid precursor,
CA, which is homologous to CB and even has an
identical signal sequence of 16 amino acids [2, 7].
The most complicated structure and with the
highest known toxicity (1 !-tglkg mouse Lv.) is
found in the "textilotoxin" of the Australian common brown snake Pseudonaja textilis; this toxin is
made up of three peptide chains, A-C, and a
homodimer DD [52]. The venom of the North
American rattlesnake contains a further toxin,
"crotamin", which has as yet undefined effects on
the musculature. This is a basic polypeptide of 42
amino acids with three disulphide bridges, and
has no sequence similarity to any other known
toxin. Other Crotalus species have similar toxins
[36, 63]. The "taipoxin" of the Australian "taipan" Oxyuranus scutellatur is made up of three
subunits which differ greatly from each other but
are all homologous with other phospholipases A z.
The basic a subunit, which is the only one showing any sort of enzyme activity, is weakly toxic in
the purified form; the combination of the three
subunits is 500-fold more toxic, with an LDso of
2.1 !-tg/kg [37].
9.1.2 Enzymes and Other Protein
Components
The poisonous secretions of the snakes always
contain a large number of protein components
with different pharmacological effects or biological activities; often more than 25 protein fractions may be distinguished by electrophoresis or
chromatography. Venom secretions with similar
toxicities from different populations, or even
from different individuals, may show large differences in electrophoretic pattern and enzyme
activity. The effects of enzyme components
include local capillary damage and tissue necrosis
(from phospholipases, proteases, hyaluronidases), inhibition or promotion of blood clotting
(from specific proteases, phospholipases), reduction in blood pressure, and pain (from kininreleasing enzymes).
Most of the phospholipases from snake venom
cleave 2-acyl residues from different phospholipids, Le. they are of type A z. They have haemolytic effects and several of them have synergistic
effects on toxin activity. A phospholipase B found
in the venom of the elapid Pseudechis australus
attacks the 1-acyl group in lysophosphatide, and
several phospholipases A z also have a similar,
albeit weak, activity. Although the phospholipases Az are acidic or basic, according to their