Presynaptically acting snake venom phospholipase A2 enzymes attack unique substrates

Analysis of fatty acids released by crotoxin in rat brain synaptosomes

Crotoxin, the main toxin of Crotalus durissus terrificus venom, exerts its lethal effect by blocking neurotransmission at the neuromuscular junction level through a triphasic mechanism. This effect seems to depend on its phospholipasic activity, suggesting that the mechanism of neurotransmission blockage may be related to fatty acids release in specific sites of the nervous terminal. In this work, we purified the fatty acids released by crotoxin's activity and this outline was compared with other phospholipases A(2), including CB, a subunit of crotoxin. Our results show a higher release of palmitate and arachidonate by crotoxin when compared to other phospholipases A(2). Since palmitate has a role in protein acylation processes and arachidonate participates in signal transduction events, these mechanisms may be related to the neurotoxic actions of crotoxin.

Beta-bungarotoxin is a potent inducer of apoptosis in cultured rat neurons by receptor-mediated internalization

The neurotoxic phospholipase A(2), beta-bungarotoxin (beta-BuTx), is a component of the snake venom from the Taiwanese banded krait Bungarus multicinctus. beta-BuTx affects presynaptic nerve terminal function of the neuromuscular junction and induces widespread neuronal cell death throughout the mammalian and avian CNS. To analyse the initial events of beta-BuTx-mediated cell death, the toxin was applied to cultured rat hippocampal neurons where it induced neuronal cell death in a concentration-dependent manner (EC(50) approximately equal to 5 x 10(-13) M) within 24 h. Fluorescence labelled beta-BuTx was completely incorporated by neurons within < 10 min. Binding and uptake of beta-BuTx, as well as induction of cell death, were efficiently antagonized by preincubation with dendrotoxin I, a blocker of voltage-gated potassium channels devoid of phospholipase activity. Binding of beta-BuTx was selective for neurofilament-positive cells. As evident from intense annexin-V and TUNEL stainings, application of beta-BuTx induced apoptotic cell death exclusively in neurons, leaving astrocytes unaffected. No evidence was obtained for any contribution of either caspases or calpains to beta-BuTx-induced apoptosis, consistent with the inability of the inhibitors Z-Asp-DCB and calpeptin, respectively, to protect neurons from beta-BuTx-induced cell death. These observations indicate that induction of cell death by beta-BuTx comprises several successive phases: (i) binding to neuronal potassium channels is the initial event, followed by (ii) internalization and (iii) induction of apoptotic cell death via a caspase-independent pathway.

Isolation and enzymatic characterization of a basic phospholipase A2 from Bothrops jararacussu snake venom

A novel basic phospholipase A2 (PLA2) isoform was isolated from Bothrops jararacussu snake venom and partially characterized. The venom was fractionated by HPLC ion-exchange chromatography in ammonium bicarbonate buffer, followed by reverse-phase HPLC to yield the protein Bj IV. Tricine SDS-PAGE in the presence or absence of dithiothreitol showed that Bj IV had a molecular mass of 15 and 30 kDa, respectively. This enzyme was able to form multimeric complexes (30, 45, and 60 kDa). Amino acid analysis showed a high content of hydrophobic and basic amino acids as well as 14 half-cysteine residues. The N-terminal sequence (DLWSWGQMIQETGLLPSYTTY...) showed a high degree of homology with basic D49 PLA2 myotoxins from other Bothrops venoms. Bj IV had high PLA2 activity and produced moderate myonecrosis in skeletal muscle, but showed no neuromuscular activity in mouse phrenic nerve-diaphragm preparations. Bj IV showed allosteric enzymatic behavior, with maximal activity at pH 8.2 and 35-45 degrees C. Full PLA2 activity required Ca2+ but was inhibited by Cu2+ and Zn2+, and by Cu2+ and Mg2+ in the presence and absence of Ca2+, respectively. Crotapotins from Crotalus durissus terrificus rattlesnake venom significantly inhibited the enzymatic activity of Bj IV. The latter observation suggested that the binding site for crotapotin in this PLA2 was similar to that in the basic PLA2 of the crotoxin complex from C. d. terrificus venom. The presence of crotapotin-like proteins capable of inhibiting the catalytic activity of D49 PLA2 could partly explain the low PLA2 activity of Bothrops venoms.

Lysine 49 phospholipase A2 proteins

The structures of several K49 PLA2 proteins have been determined and these differ as a group in several regions from the closely related D49 PLA2 enzymes. One outstanding difference is the presence of a high number of positively charged residues in the C-terminal region which combined with the overall high number of conserved lysine residues gives the molecule an interfacial adsorption surface which is highly positively charged compared to the opposite surface of the molecule. Although some nucleotide sequences have been reported, progress in obtaining active recombinant proteins has been slow. The K49 proteins exert several toxic activities, including myotoxicity, anticoagulation and edema formation. The most studied of these activities is myotoxicity. The myotoxicity induced by the K49 PLA2 proteins is histologically similar to that caused by the D49 PLA2 myotoxins, with some muscle fiber types possibly more sensitive than others. Whereas it is clear that the K49 PLA2 myotoxins lyse the plasma membrane of the affected muscle cell in vivo, the exact mechanism of this lysis is not known. Also, it is not known whether the toxin is internalized before, during or after the initial lysis or ever. The K49 PLA2 toxins lyse liposomes and cells in culture and in the latter, the PLA2 myotoxins exert at least two distinct mechanisms of action, neither of which is well-characterized. While the K49 PLA2 proteins are enzymatically inactive on artificial substrates, the toxins cause fatty acid production in cell cultures. Whether the fatty acid release is due to the enzymatic activity of the K49 PLA2 or stimulation of tissue lipases, is unknown. While there may be a role for fatty acid production in one mechanism of myotoxicity, a second mechanism appears to be independent of enzymatic activity. Although we are beginning to understand more about the structure of these toxins, we still know little about the precise mechanism by which they interact with the skeletal muscle cell in vivo.

Ability of wedelolactone, heparin, and para-bromophenacyl bromide to antagonize the myotoxic effects of two crotaline venoms and their PLA2 myotoxins

We examined the ability of wedelolactone, heparin and para-bromophenacyl bromide to antagonize the myotoxic activity in mice of venoms from Crotalus viridis viridis and Agkistrodon contortrix laticinctus and two phospholipase A2 myotoxins, CVV myotoxin and ACL myotoxin, isolated from them. Myotoxicity was measured by the increase in plasma creatine kinase (CK) activity at two hours and histological changes in extensor digitorum longus muscle (EDL) at three hours after injection of the test solution. Both heparin and wedelolactone independently reduced the myotoxic effect of both crude venoms and both myotoxins, but wedelolactone was more effective. Wedelolactone plus heparin reduced the myotoxic effect of CVV myotoxin more than either antagonist alone. The PLA2 inhibitor, para-bromophenacyl bromide (pBPB), reduced the myotoxic effect of both myotoxins more than either wedelolactone or heparin. On the other hand, the myotoxic effect of polylysine was not reduced by either wedelolactone or para-bromophenacyl bromide, but it was reduced by heparin. These results indicate that wedelolactone, para-bromophenacyl bromide and heparin are antagonists of these two phospholipase A2 myotoxins, and that antagonism by the first two compounds may be due to a more specific interaction with these proteins than that by the latter.

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.