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

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.

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.