Antibodies having markedly different effects on enzymatic activity and induction of acetylcholine release by two presynaptically-acting phospholipase A2 neurotoxins

Phospholipase A2 activity-independent membrane-damaging effect of notexin

To elucidate whether the phospholipase A(2) (PLA(2)) activity of notexin was exclusively associated with the manifestation of its pharmacological activities, the interaction of notexin with phospholipid liposomes was explored by fluorescence and CD measurement underlying the conditions of depriving its PLA(2) activity. Although a higher membrane-damaging activity was noted with Ca(2+)-bound notexin, abolishment of PLA(2) activity by EDTA and Sr(2+) could not diminish the membrane-damaging activity of notexin. Fluorescence-quenching studies and CD measurement indicated that Ca(2+)-bound, Sr(2+)-bound or metal-free notexin did not adopt the same conformation upon binding with phospholipids. Regardless of the presence of Ca(2+), Sr(2+) or EDTA, self-quenching assay with rhodamine-labeled notexin revealed that the toxin pertained to form oligomer when it bound with liposomes. Although Lys-modified notexin retained full PLA(2) activity, a notable decrease in membrane-damaging activity was observed. These results indicate that notexin could directly cause a leakage of membrane via a PLA(2) activity-independent manner, and implicate that another biological event contributes to the activity of notexin in vivo.

Neurotoxic effects of venoms from seven species of Australasian black snakes (Pseudechis): efficacy of black and tiger snake antivenoms

1. Pseudechis species (black snakes) are among the most widespread venomous snakes in Australia. Despite this, very little is known about the potency of their venoms or the efficacy of the antivenoms used to treat systemic envenomation by these snakes. The present study investigated the in vitro neurotoxicity of venoms from seven Australasian Pseudechis species and determined the efficacy of black and tiger snake antivenoms against this activity. 2. All venoms (10 microg/mL) significantly inhibited indirect twitches of the chick biventer cervicis nerve-muscle preparation and responses to exogenous acetylcholine (ACh; 1 mmol/L), but not to KCl (40 mmol/L), indicating activity at post-synaptic nicotinic receptors on the skeletal muscle. 3. Prior administration of either black or tiger snake antivenom (5 U/mL) prevented the inhibitory effects of all Pseudechis venoms. 4. Black snake antivenom (5 U/mL) added at t90 (i.e. the time-point at which the original twitch height was reduced by 90%) significantly reversed the effects of P. butleri (28+/-5%), P. guttatus (25+/-8%) and P. porphyriacus (28+/-10%) venoms. Tiger snake antivenom (5 U/mL) added at the t90 time-point significantly reversed the neurotoxic effects of P. guttatus (51+/-4%), P. papuanus (47+/-5%) and P. porphyriacus (20+/-7%) venoms. 5. We show, for the first time, the presence of neurotoxins in the venom of these related snake species and that this activity is differentially affected by either black snake or tiger snake antivenoms.

The efficacy of two antivenoms against the in vitro myotoxic effects of black snake (Pseudechis) venoms in the chick biventer cervicis nerve-muscle preparation

Neurotoxicity is rarely seen following human systemic envenoming by Australasian black snakes (genus Pseudechis) with myotoxicity being the most prominent feature following bites by some species. This study investigated the in vitro myotoxicity of venoms from seven species of Australasian Pseudechis and determined the efficacy of CSL black and tiger snake antivenoms. All Pseudechis venoms (10 microg/ml) significantly inhibited direct twitches of the chick biventer cervicis nerve-muscle preparation (p<0.05, one-way ANOVA). Prior addition of black snake antivenom (5 U/ml) prevented the inhibitory effects of all Pseudechis venoms (p<0.05, one-way ANOVA), except Pseudechis butleri. Prior addition of tiger snake antivenom (5 U/ml) prevented the venom-induced reduction in direct twitches to Notechis scutatus venom and all Pseudechis venoms (p<0.05, one-way ANOVA), with the exception of Pseudechis australis and Pseudechis colletti venoms. Black or tiger snake antivenom (5 U/ml) added 1 h after the addition of venom inhibited further reduction of direct twitches by N. scutatus and most Pseudechis venoms, but did not significantly restore twitch height. PLA2 activity was found in all venoms with the following rank order: Pseudechis porphyriacus>P. australis>Pseudechis papuanus>P. butleri>Pseudechis guttatus> or =Pseudechis pailsii>P. colletti>N. scutatus. The results of the present study suggest that Australasian Pseudechis venoms possess variable myotoxic activity. The ability of black or tiger snake antivenom to prevent or inhibit further venom-induced effects varied across the genus.

Chemical modification of arginine residues of Notechis scutatus scutatus notexin

Notexin, a presynaptic phospholipase A2 (PLA2) neurotoxin isolated from Notechis scutatus scutatus venom, was inactivated by arginine-specific reagents, phenylglyoxal and 1,2-cyclohexanedione. Kinetic analyses of the modification reaction revealed that the inactivation of notexin followed pseudo-first order kinetics and the loss of PLA2 activity was correlated with the incorporation of one molecule of modification reagent per toxin molecule. However, the results of amino acid analysis and sequence determination revealed that two arginine residues at positions 43 and 79 of notexin were modified simultaneously. Modification of the arginine residues was accompanied with a decrease in the ability to inhibit the indirectly evoked contraction of chick biventer cervicis muscle and bind with synaptic membranes. The secondary structure of the toxin molecule did not significantly change after modification with phenylglyoxal as revealed by the CD spectra. The modified derivative retained its affinity for Ca2+, indicating that the modified arginine residues did not participate in Ca2+ -binding. Together with the notion that Arg-43 and Arg-79 of notexin are located in the proximity of its catalytic site and toxic site, respectively, our results suggest that modification of Arg-43 and Arg-79 should differently contribute to the observed decrease in the PLA2 activity and neurotoxic effect of notexin.

Venom phospholipase A2-induced impairment of glutamate uptake: an indirect and nonselective effect related to phospholipid hydrolysis

In a nominally calcium-free medium, a toxic phospholipase A2, paradoxin, PDX (1-200nM) was able to significantly decrease glutamate uptake by rat hippocampal mini-slices. Under the same experimental conditions, PDX could also inhibit the reuptake of choline and dopamine, suggesting a nonselective action. Furthermore, we found no evidence of competition between PDX and [3H]L-Aspartate described as a marker of glutamate carrier proteins. A direct blockage of glutamate uptake by binding to the glutamate transporters is thus unlikely to occur. Implication of the free fatty acids (FFAs), or their metabolites, was clearly shown by the total suppression of PDX effect on reuptake in a medium inhibiting its catalytic activity (EGTA/Sr2+ buffer). Moreover, analysis of the FFAs liberated showed a significant increase in polyunsaturated fatty acid (PUFA) levels. Arachidonic acid (AA) concentration reached in the water phase, though in the low micromolar range, may be especially relevant in explaining this effect. Much higher concentrations are found in the membranes and may also participate in the action on reuptake. Evidence for the involvement of FFAs was also provided by the antagonistic, although partial, action of bovine serum albumine (BSA, 1%). Finally, free radicals or eicosanoids did not seem to play a significant role given the persistence of inhibition in the presence of NDGA (1 microM) or indomethacin (10 microM), inhibitors of the two major AA metabolic pathways. Altogether, PDX-induced uptake impairment may thus be related to the direct action of AA and other PUFAs on the glutamate transporter, as well as through less selective actions.

Chemical modification of notexin from Notechis scutatus scutatus (Australian tiger snake) venom with pyridoxal-5'-phosphate

Notexin from Notechis scutatus scutatus snake venom was subjected to Lys modification with pyridoxal 5'-phosphate (PLP), and one major modified derivative was purified on a cation-exchanger SP-8HR column. The results of amino acid analysis and sequence determination revealed that only 2 Lys residues at positions 82 and 115 out of 11 Lys residues in notexin were modified. The incorporation of PLP into the protein was accompanied by the loss of 53% lethal toxicity, but the modified notexin showed an about 1.2-fold increase in enzymatic activity. However, the secondary structure of the toxin molecule did not significantly change after modification with PLP as revealed by the CD spectra, and the antigenicity of PLP derivative remained unchanged. The modified derivative retained its affinity for Ca2+, indicating that the modified Lys residues did not participate in Ca2+ binding. These results indicate that modification of Lys residues causes a differential effect on the enzymatic activity and lethal toxicity of notexin, and suggest that notexin might possess two functional sites, one responsible for the catalytic activity and the other associated with its lethal effect.

Presynaptically acting snake venom phospholipase A2 enzymes attack unique substrates

Synaptosomes were incubated with bovine serum albumin (BSA) to examine whether the presynaptic action of snake venom phospholipase A2 (PLA2) toxins is due either to the release of fatty acids resistant to extraction by BSA or to the liberation of a specific fatty acid type. In the presence of BSA (0.5% or 1.0%) two PLA2 enzymes from Naja naja atra and Naja naja kaouthia snake venoms that do not have a predominant presynaptic action at the neuromuscular junction (PS-) did not stimulate acetylcholine (ACh) release from synaptosomes. In contrast, two PLA2 enzymes (beta-bungarotoxin, scutoxin) that do have a predominant presynaptic action at the neuromuscular junction (PS+) did stimulate ACh release. BSA did not antagonize PS- enzymes by more efficiently extracting the fatty acids produced by these enzymes relative to PS+ enzymes. While absolute amounts of total and unsaturated fatty acid produced overlapped for the PS- and PS+ enzymes, the two PS+ enzymes produced a significantly greater absolute amount and relative percentage of palmitic acid (16:0) than did either of the PS- enzymes. However, the levels of free palmitic acid remaining in the synaptosomes where they would exert effects on ACh release were similar for the N. n. kaouthia PLA2 (PS-) and beta-bungarotoxin (PS+). Therefore, the total (supernatant plus synaptosomal) amount of palmitic acid produced per se did not account for stimulation of ACh release, since the greater amounts produced by the PS+ enzymes were removed from the synaptosomes by BSA. The production of higher levels of palmitic acid suggests either that PS+ enzymes gain access to sites containing phospholipid substrates unavailable to the PS- enzymes, or that they have a different substrate preference. These findings suggest new possibilities for the mechanism of PS+PLA2 action, including site-directed enzymatic activity and protein acylation.

No role for enzymatic activity or dantrolene-sensitive Ca2+ stores in the muscular effects of bothropstoxin, a Lys49 phospholipase A2 myotoxin

The role of low levels of phospholipase A2 (PLA2) activity and intracellular Ca2+ stores in the pharmacological action of bothropstoxin (BthTX), a myotoxic Lys49 PLA2 homologue isolated from the venom of Bothrops jararacussu, was investigated. We examined the muscular effects of BthTX in the mouse diaphragm and its PLA2 activity in radiolabeled human and rat primary cultures of skeletal muscle. Although it is a Lys49 PLA2 homologue, BthTX had a low, but easily detectable, level of enzymatic activity relative to two Asp49 PLA2 enzymes from Naja naja kaouthia and Naja naja atra venoms, and this activity was reduced by about 85% in the presence of Sr2+ (4.0 mM). However, the replacement of 1.8 mM Ca2+ by 4 mM Sr2+ did not alter the BthTX-induced contracture and blockade of the muscle twitch tension. In addition, Sr2+ decreased by 50% the time required to cause 50% paralysis, and evoked approximately a four-fold increase in the number of spontaneous spikes. In isolated sarcoplasmic reticulum preparations, BthTX opened the intracellular Ca2+ release channel (ryanodine receptor) and lowered the threshold of Ca(2+)-induced Ca2+ release by a second, as yet unidentified, mechanism. However, in intact muscle, dantrolene, an antagonist of some forms of intracellular Ca2+ release, had no effect on the actions of BthTX. These findings do not support any role for the low levels of PLA2 activity, or dantrolene-sensitive intracellular Ca2+ stores, in the action of BthTX. The mechanism whereby Sr2+ stimulates the pharmacological activity of BthTX remains to be clarified.

Bovine serum albumin does not completely block synaptosomal cholinergic activities of presynaptically acting snake venom phospholipase A2 enzymes

Bovine serum albumin (BSA), which binds fatty acids, was used to test the contribution of free fatty acid to the presynaptic toxicity of phospholipase A2 (PLA2) enzymes. The effects of BSA on inhibition of [14C]choline uptake and stimulation of [14C]acetylcholine (ACh) release in synaptosomes by PLA2 enzymes that do not have a predominant presynaptic action at the neuromuscular junction (PS-) were compared with those on the cholinergic actions of PLA2 enzymes that do have a predominant presynaptic action at the neuromuscular junction (PS+). The inhibition of choline uptake by the Naja naja atra PLA2, a PS- PLA2, was completely antagonized by BSA (0.5%); whereas that by beta-bungarotoxin, a PS+ PLA2, was unaffected by BSA. The inhibition of choline uptake by two other PS+ PLA2 toxins (scutoxin and pseudexin) was partially antagonized by BSA. The effects of the PLA2 enzymes were antagonized in the same manner by BSA whether on Na(+)-dependent or on Na(+)-independent choline uptake. Likewise, the stimulation of ACh release by two PS- PLA2 enzymes (from Naja naja atra and Naja naja kaouthia snake venoms) was completely blocked by BSA; whereas that by beta-bungarotoxin was unaffected and that by scutoxin and pseudexin was only partially antagonized by BSA. The results suggest that the PS- PLA2 enzymes are completely dependent on fatty acid production for their cholinergic toxicity and that BSA can be used to investigate further the neurotoxic mechanisms of PS+ PLA2 enzymes in synaptosomes.