Effect of 3,4-diaminopyridine and tetraethylammonium on the presynaptic blockade caused by beta-bungarotoxin

Snake-venom phospholipase A2 neurotoxins. Potentiation of a single-chain neurotoxin by the chaperon subunit of a two-component neurotoxin

The venoms from Crotalinae and Viperinae snakes contain only two kinds of phospholipase A2 neurotoxins (beta-neurotoxins): single-chain beta-neurotoxins, such as agkistrodotoxin and ammodytoxin-A, and dimeric beta-neurotoxins, which, in the case of the best studied ones, crotoxin-like toxins, consist of the non-covalent association of a phospholipase A2 (CB) and a non-enzymatic chaperon (CA). Possible evolutionary relationships of these beta-neurotoxins have been investigated by analyzing whether CA could behave as a chaperon toward agkistrodotoxin and ammodytoxin, as it does in the crotoxin complex. CA increased the lethal potency of agkistrodotoxin and modified its pharmacological effect on Torpedo synaptosomes. Sedimentation experiments proved that CA can form an heterocomplex with agkistrodotoxin. Agkistrodotoxin prevented the binding to CA of an anti-CA mAb which recognizes an epitope at the zone of interaction between crotoxin subunits, suggesting the association of CA and agkistrodotoxin implicated the same zone. A 10-fold molar excess of CA over ammodytoxin modified the effect of ammodytoxin on acetylcholine release but did not increase the lethal potency of ammodytoxin. Sedimentation experiments showed CA and ammodytoxin can form an heterocomplex which is less stable than CA.agkistrodotoxin. Ammodytoxin A did not compete with the anti-CA mAb. These observations are in good agreement with the sequence similarities between CB and agkistrodotoxin (80%) and ammodytoxin A (60%).

Differential effects of presynaptic phospholipase A2 neurotoxins on Torpedo synaptosomes

The effects of several phospholipase A2 neurotoxins from snake venoms were examined on purely cholinergic synaptosomes from Torpedo electric organ. The noncatalytic component A of crotoxin had no effect, whereas its phospholipase component B, used alone or complexed to component A, elicited a rapid and dose-dependent acetylcholine (ACh) release and a depolarization of the preparation. Subsequent ACh release evoked by high K+ levels or calcium ionophore was identical to the control after the action of component A but reduced after the action of crotoxin or of component B. These effects were not observed when the phospholipase A2 activity of the toxin was blocked either by replacing Ca2+ by Ba2+ (respectively, activator and inhibitor of phospholipase A2) or by alkylation of component B with p-bromophenacyl bromide. beta-Bungarotoxin, another very potent phospholipase A2 neurotoxin, induced release of little ACh, did not affect ionophore-evoked ACh release, but significantly reduced depolarization-induced ACh release. The single-chain phospholipase A2 neurotoxin agkistrodotoxin behaved like crotoxin component B. A nonneurotoxic phospholipase A2 from mammalian pancrease induced release of an amount of ACh similar to that released by crotoxin but did not affect the evoked responses. The obvious differences in effect of the various neurotoxins suggest that they exert their specific actions on the excitation-secretion coupling process at different sites or by different mechanisms.

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.

Protease inhibitor homologues from mamba venoms: facilitation of acetylcholine release and interactions with prejunctional blocking toxins

1 Five polypeptides, which were isolated from elapid snake venoms and which are structurally related to protease inhibitors, were tested for action on isolated biventer cervicis nerve-muscle preparations of the chick. 2 Dendrotoxin from the Eastern green mamba (Dendroaspis angusticeps) and toxins K and I from the black mamba (Dendroaspis polylepis polylepis) increased to indirect stimulation without affecting responses to exogenous acetylcholine, carbachol of KCl. 3 The two other protease inhibitor homologues, HHV-II from Ringhals cobra (Hemachatus haemachatus) and NNV-II from Cape cobra (Naja nivea) did not increase responses to nerve stimulation. Trypsin inhibitor from bovine pancreas also had no facilitatory effects on neuromuscular transmission. 4 The facilitatory toxins from mamba venoms interacted with the prejunctional blocking toxins, beta-bungarotoxin, crotoxin and notexin, but not with taipoxin. The blocking effects of beta-bungarotoxin were reduced by pretreatment with the mamba toxins, whereas the blocking actions of crotoxin and notexin were enhanced. 5 The results indicate that protease inhibitor homologues from mamba venoms form a new class of neurotoxin, which acts to increase the release of acetylcholine in response to motor nerve stimulation. 6 From the interaction studies it is concluded that the facilitatory toxins bind to motor nerve terminals at sites related to those occupied by the prejunctional blocking toxins. However, differences in interactions with individual toxins suggest that there must be several related binding sites on the nerve terminals.

Presynaptic toxicity of the histidine-modified, phospholipase A2-inactive, beta-bungarotoxin, crotoxin and notexin

Beta-Bungarotoxin, crotoxin and notexin were treated extensively with p-bromophenacyl bromide in order to modify the histidine group of the active site and to greatly decrease the phospholipase A2(PLA) catalytic activity. They were studied for their residual presynaptic effects on the mouse isolated phrenic nerve-diaphragm and chick biventer cervicis muscle preparations. The modified toxins still had 1.7-5.2% PLA activity, which was inhibited by Sr2+, as is the enzyme activity of the native toxins. The neuromuscular blocking activity of these modified toxins, which was reduced 30-60 fold in the mouse diaphragm, was due to a presynaptic effect, as judged from the unchanged amplitude of m.e.p.p.s in the blocked preparations. In contrast to the native toxins, however, the presynaptic effect of modified beta-bungarotoxin and notexin was neither antagonized by Sr2+ nor accelerated by increasing the rate of nerve stimulation, indicating that the presynaptic effects of the modified beta-bungarotoxin and notexin are not likely to be due to their PLA enzyme activity. The modified toxins retained a much greater fraction of their early presynaptic effects in comparison to their enzymatic and neuromuscular blocking activities, although the time-course of the early effects was delayed. The results indicate that the modified neurotoxins per se have their own presynaptic effects, which are unrelated to the PLA enzyme activities.