Recombinant Vipera ammodytes ammodytes Phospholipase A2, ammodytoxin A (ATXA)

Snake venom phospholipase A2 (PLA2) that acts as a presynaptic neurotoxin, an inhibitor of blood coagulation, and has been found to bind with high affinity to intracellular proteins. The response of indirectly stimulated neuromuscular preparations to ammodytoxin (Atx) is triphasic. The first phase, the transient inhibition of the acetylcholine (ACh) release, starts soon after the addition of Atx and lasts for several minutes. This phase is probably independent of Atx enzymatic activity. The effect may be due to the specific binding of the toxin to presynaptic receptors. These receptors, called N-type receptors, are still unidentified. It is noteworthy that a neuronal isoform of the M-type PLA2 receptor (R180) has been identified as a high-affinity receptor for Atx in neuronal plasma membranes. It was demonstrated however that this receptor is not essential for expression of neurotoxicity by Atx. The second phase corresponds to an augmentation of neurotransmitter release. A peak is reached 10-20 min after exposure of the preparation to Atx and is followed by a gradual reduction. In this phase, the enzymatic activity of Atx of the mammalian is not significant. It is speculated that the increased release of neurotransmitter in this phase is induced by the interference of Atx with voltage-gated potassium channels. Measurements of ionic currents showed however that voltage-gated potassium channels are not affected by Atx. The third phase of the response of neuromuscular preparations to Atx, which corresponds to a complete and irreversible paralysis, is clearly dependent on the hydrolytic activity of the toxin. In addition to its presynaptic neurotoxicity, Atx shows an anticoagulant activity by binding with high affinity to activated coagulation factor X (F10) thus inhibiting the formation of the prothrombinase complex (FX/FV) and its activity (IC(50) is 20 nM). Surprisingly, Atx was discovered to bind intracellular proteins such as calmodulin (CaM) (IC(50) is 6 nM), 14-3-3 proteins gamma (YWHAG) and epsilon (YWHAE) (by similarity with AtxC), as well as R25 (by similarity with AtxC), a mitochondrial integral membrane protein found in cerebral cortex. These findings raised a doubt about the dogma of the exclusively extracellular action of PLA2s, defended by the potential instability of these molecules in the reducing environment of the eukaryotic cytosol coupled with their possible inability to act as enzymes in this cellular compartment, due to too low concentration of calcium ions. This hypothesis was challenged efficiently by demonstrating the internalization of AtxA into a culture cells, but still remains to be directly demonstrated in vivo. PLA2 catalyzes the calcium-dependent hydrolysis of the 2-acyl groups in 3-sn-phosphoglycerides.