The mechanism of potentiation of the presynaptic effect of phospholipase A2 by the crotapotin component of the crotoxin complex

Molecular evolution and structure-function relationships of crotoxin-like and asparagine-6-containing phospholipases A2 in pit viper venoms

Some myotoxic or neurotoxic PLA2s (phospholipases A2) from pit viper venoms contain characteristic N6 substitutions. Our survey of the venoms of more than ten pit viper genera revealed that N6-PLA2s exist only in limited Asian pit vipers of two genera, Protobothrops and Gloydius, and exist as either monomers or the basic subunits of heterodimers in some New World pit vipers. For the newly identified N6-PLA2s, the neuromuscular blocking activities were assayed with the chick biventer cervicis neuromuscular tissue, whereas the increased serum creatine kinase level assessed their myotoxicities. The purified N6-PLA2s from Protobothrops mangshanensis and Gloydius intermedius saxatilis were found to be presynaptic neurotoxins. In contrast, all N6-PLA2s from the venoms of Sistrurus miliarius strackeri, S. m. barbouri, Crotalus viridis viridis, C. lepidus lepidus, Cerrophidion godmani and Bothreichis schlegelii were myotoxins without neurotoxicity even in the presence of crotoxin A. Crotoxin-like complexes were for the first time purified from the venoms of Sitrurus catenatus tergeminus, C. mitchelli mitchelli, C. horridus atricaudatus, C. basiliscus and C. durissus cumanensis. The cDNAs encoding six novel N6-PLA2s and subunits of the crotoxin-like complex from S. c. tergeminus were cloned and fully sequenced. Phylogeny analysis showed that two structural subtypes of N6-PLA2s with either F24 or S24 substitution have been evolved in parallel, possibly descended respectively from species related to present-day Protobothrops and Gloydius. Calmodulin binds all the N6-PLA2s but crotoxin A may inhibit its binding to crotoxin B and to other neurotoxic N6-PLA2s. Structure-activity relationships at various regions of the PLA2 molecules were extensively discussed.

Crotoxin, the major toxin from the rattlesnake Crotalus durissus terrificus, inhibits 3H-choline uptake in guinea pig ileum

We examined the effect of crotoxin, the neurotoxic complex from the venom of the South American rattlesnake Crotalus durissus terrificus, on the uptake of 3H-choline in minces of smooth muscle myenteric plexus from guinea pig ileum. In the concentration range used (0. 03-1 microM) and up to 10 min of treatment, crotoxin decreased 3H-choline uptake by 50-75% compared to control. This inhibition was time dependent and did not seem to be associated with the disruption of the neuronal membrane, because at least for the first 20 min of tissue exposure to the toxin (up to 1 microM) the levels of lactate dehydrogenase (LDH) released into the supernatant were similar to those of controls. Higher concentrations of crotoxin or more extensive incubation times with this toxin resulted in elevation of LDH activity detected in the assay supernatant. The inhibitory effect of crotoxin on 3H-choline uptake seems to be associated with its phospholipase activity since the equimolar substitution of Sr2+ for Ca2+ in the incubation medium or the modification of the toxin with p-bromophenacyl bromide substantially decreased this effect. Our results show that crotoxin inhibits 3H-choline uptake with high affinity (EC25 = 10 +/- 5 nM). We suggest that this inhibition could explain, at least in part, the blocking effect of crotoxin on neurotransmission.

Gabaergic-benzodiazepine system is involved in the crotoxin-induced anxiogenic effect

The behavioral effects of crotoxin (CTX), the major component of Crotalus durissus terrificus venom, were studied in rats submitted to the open field, holeboard, and social interaction tests. CTX (100, 250, and 500 microg/kg, i.p.) was administered 2 h before the tests. In the open field, CTX reduced ambulation (250 microg/kg) and rearing (250 and 500 microg/kg) and increased grooming (100 and 250 microg/kg) and freezing (250 microg/kg). In the holeboard and social interaction, all the CTX doses evaluated decreased, respectively, head dip and head dipping, and social interaction time. The CTX-induced behavioral alterations could be attributed to its neuromuscular transmission blockade, but this possibility was ruled out because CTX (250 and 500 microg/kg, i.p., 2 h before the rotarod test) was unable to modify the rotarod performance of rats. The involvement of the benzodiazepine receptor in the CTX-induced behavioral alterations was investigated through the pretreatment (30 min before the tests, i.p.) of the animals with diazepam (1.2 mg/kg), or flumazenil (4 and 10 mg/kg). Both diazepam and flumazenil antagonized the CTX-induced behavioral alterations in the open field, holeboard, and social interaction tests. This study demonstrated that: (1) CTX is an anxiogenic compound; and (2) the gabaergic-benzodiazepine system may play a role in the CTX-induced anxiogenic effect.

Reduction of crotoxin-induced neuromuscular blockade by gamma radiation

A comparative study between crotoxin and gamma irradiated crotoxin was performed on the indirectly evoked twitches and tetani of sciatic nerve-extensor digitorum longus muscle of rats. Crotoxin (3 to 14 microg/ml) decreased the amplitude of twitches and induced a slight tetanic fade, and irradiated crotoxin did not significantly affect either twitch amplitude or tetanic tension. Since gamma radiation reduced the neurotoxicity of crotoxin it may be useful for the production of anticrotalic serum.

Amino acid sequences of a heterodimeric neurotoxin from the venom of the false horned viper (Pseudocerastes fieldi)

The main toxic component of the venom of the false horned viper, Pseudocerastes fieldi, is a heterodimeric neurotoxin composed of a basic subunit, Cb II, and one of two acidic subunits, either Cb I alpha or Cb I beta. The nontoxic acidic subunit increases the toxicity of the basic subunit. Both subunits have phospholipase A2 (PLA2) amino acid sequences. Cb I alpha and Cb I beta themselves are inactive towards phosphatidylcholine and when complexed with Cb II promote a delay in the onset of phospholipase activity of Cb II. Cb I alpha and Cb I beta do hydrolyze the synthetic substrate, 3-octanoyloxy-4-nitrobenzoic acid, but at < 1% the rate of Cb II. Comparisons of the amino acid sequences of Cb II and Cb I alpha with the corresponding acidic and basic subunits of other heterodimeric neurotoxins show high amino acid sequence identity. Some of the amino acids which are different between the acidic and basic subunits are in highly conserved sequences in their respective types of PLA2. This suggests that these amino acid changes in the conserved regions are important for the structure and function of the heterodimeric proteins.

Presynaptic effects of snake venom toxins which have phospholipase A2 activity (beta-bungarotoxin, taipoxin, crotoxin)

The presynaptic effects of beta-bungarotoxin, crotoxin and taipoxin were studied in the mouse phrenic nerve-diaphragm preparation (27 degrees C). The phospholipase A2 activity, assayed by pH-stat titration, was reduced to 4-10% at 27 degrees C compared with that at 37 degrees C. The late neuromuscular blocking activity was also reduced by more than three fold for all toxins. In contrast, the early biphasic response to the toxins, i.e. immediate depression followed by facilitation, was not delayed. The evoked quantal release of acetylcholine was enhanced by all toxins at low Ca2+-concentrations during the phase of facilitation, without an increase of the maximal release. At the late phase of treatment with beta-bungarotoxin and taipoxin, the curve relating the quantal contents of endplate potentials with Ca2+-concentration was shifted parallel to the right at low Ca2+, but marked depression of the maximal release occurred at high Ca2+. When diaminopyridine was added at the time of the late phase block by beta-bungarotoxin, the quantal release could still be enhanced at low Ca2+-concentrations, even beyond control; however, the maximal release was not simultaneously restored. It is concluded that the late phase block, but not the early biphasic response, is due to an enzymatic action and the release mechanism is abolished when the hydrolysis of membrane phospholipids proceeds to a certain critical level.

Isolation and characterization of the main toxic fraction from the venom of the false horned viper (Pseudocerastes fieldi)

The venom of Pseudocerastes fieldi was subjected to gel filtration on Sephadex G-75. Most of the protein and lethality of the venom were eluted in a major symmetrical peak (C). The lethality of this peak is confined to a basic protein fraction, Cb (pI greater than 9.5) separable by DEAE-cellulose chromatography. Two proteins with molecular sizes close to 16,000 daltons were isolated from this fraction by preparative acidic gel electrophoresis in the presence of Triton X-100. One of the proteins (CbII) is lethal to mice (LD50 = 1 mg/kg) and shows phospholipase A activity as well as direct hemolytic activity. The other protein (CbI) does not reveal any known biological activity. However, upon recombination of the two a synergistic lethal activity is evident (the LD50 of the mixture = 0.25 mg/kg). It is suggested that CbI may be a specifier which potentiates the toxicity of the phospholipase A at the target site.

A study on the interaction of crotapotin with crotoxin phospholipase A2, notexin and other presynaptic neurotoxins

1 Crotapotin, the acidic subunit of crotoxin, greatly potentiated the presynaptic effect of isolated basic phospholipase A (PLA) of crotoxin in both mouse diaphragm and chick biventer cervicis muscles whereas the myotoxic effect was not affected significantly.2 In contrast to crotoxin PLA, the presynaptic effects of notexin and notechis-5, self-active single chain toxins, were antagonized by crotapotin while actions of beta-bungarotoxin were not affected.3 By assaying PLA activity, crotoxin PLA was found to be unstable in physiological salt solution, especially when in contact with muscle, due to massive non-specific binding to and destruction by the muscle.4 The decline of crotoxin PLA was greatly reduced by the presence of crotapotin but not by another acidic protein, volvatoxin A(2), or heparin.5 Notechis-5 was found to be stable even when in the presence of muscles.6 [(3)H]-acetylated crotoxin PLA, which retained about 40% of its original enzyme and presynaptic blocking activities, also bound rapidly to the mouse diaphragm on incubation and this binding was greatly hindered by the simultaneous addition of crotapotin.7 The prevention of binding of crotoxin PLA by crotapotin occurred mostly at those sites where the binding was easily dissociable on washing. No antagonism of binding occurred at the firmly binding site.8 The binding of [(3)H]-acetylated crotapotin was much less than that of crotoxin PLA, and interestingly, the binding was increased by the latter, suggesting that crotapotin may be first bound to the diaphragm together with crotoxin PLA.9 No specific binding at the endplate zone was found either for crotoxin PLA or for crotapotin.10 It is concluded that crotapotin potentiates the presynaptic effect of crotoxin PLA by curtailing its non-specific affinity with muscles, minimizing its dispersal and destruction en route to the nerve terminal, but not by acting as an affinity probe for the nerve terminal.

Inhibition by neurotoxic phospholipases A2 of synaptosomal uptake of gamma-aminobutyric acid

A comparison has been made of the abilities of several neurotoxic and nontoxic phospholipases A2 from snake venoms to inhibit the intake of gamma-aminobutyric acid into synaptosomes from rat cerebral cortex. The neurotoxic phospholipase A2 inhibited GABA uptake more than the nontoxic enzymes did. However, there was a poor correlation between the measured specific enzyme activity of a phospholipase A2 and its ability to inhibit the uptake of GABA.

Correlation of enzymatic activity and anticoagulant properties of phospholipase A2

Some highly purified phospholipases A from the venom of viperidae, crotalidae and elapidae were found to hve anticoagulant properties. All phospholipases which exhibited anticoagulant properties are characterized by a high isoelectric point, but not all strongly basic phospholipases are anticoagulant. Anticoagulant phospholipases hydrolyse highly packed monomolecular films of phospholipids without any lag time while non-anticoagulant phospholipases present considerable induction times indicative of a low penetrating power. When the ester linkages in the procoagulant lipids were replaced by the non-hydrolysable ether bonds, the mixture retained its clotting ability even in the presence of phospholipases, thus suggesting that anticoagulant phospholipases prevent clot formation by hydrolysis of phospholipids. This was confirmed by chemical modification of phospholipases, viz. alkylation of the active-centre histidine with 1-bromo-octan-2-one. This modification yielded proteins which had lost their anticoagulant properties but which retained a high affinity for phospholipids.