Functions, structures and Triton X-100 effect for the catalytic subunits of heterodimeric phospholipases A2 from Vipera nikolskii venom

Phospholipases A2 (PLA2s) from snake venoms have diverse pharmacological functions including neurotoxicity, and more studies are necessary to understand relevant mechanisms. Here we report the different crystal structures for two enzymatically active basic subunits (HDP-1P and HDP-2P) of heterodimeric neurotoxic PLA2s isolated from Vipera nikolskii venom. Structural comparisons with similar PLA2s clearly show some flexible regions which might be important for the catalytic function and neurotoxicity. Unexpectedly, Triton X-100 molecule bound in the hydrophobic channel of HDP-1P and HDP-2P was observed, and its binding induced conformational changes in the Ca2+ binding loop. Enzymatic activity measurements indicated that Triton X-100 decreased the activity of PLA2, although with comparatively low inhibitory activity. For the first time exocytosis experiments in pancreatic β cells were used to confirm the presynaptic neurotoxicity of relevant snake PLA2. These experiments also indicated that Triton X-100 inhibited the influence of HDP-1P on exocytosis, but the inhibition was smaller than that of MJ33, a phospholipid-analogue inhibitor of PLA2. Our studies performed at a cellular level are in good agreement with earlier findings that enzymatic activity of the snake presynaptic PLA2 neurotoxins is essential for effective block of nerve terminals.