Correlation between the phospholipids domains of the target cell membrane and the extent of Naja kaouthia PLA2-induced membrane damage: Evidence of distinct catalytic and cytotoxic sites in PLA2 molecules

Two phospholipase A2 (PLA2) enzymes (NK-PLA2-A and NK-PLA2-B) were purified from the venom of the monocled cobra Naja kaouthia. The molecular weights of NK-PLA2-A and NK-PLA2-B, as estimated by mass spectrometry, were 13,619 and 13,303 Da respectively. Both phospholipases were highly thermostable, had maximum catalytic activity at basic pH, and showed preferential hydrolysis of phosphatidylcholine. Intravenous injection of either PLA2 up to a dose of 10 mg/kg body weight was non-toxic to mice and did not show neurotoxic symptoms. The N. kaouthia PLA2s displayed anticoagulant and cytotoxic activity, but poor hemolytic activity. Both the PLA2s were more toxic to Sf9 and Tn cells compared to VERO cells. NK-PLA2 exhibited selective lysis of wild-type baculovirus-infected Sf9 cells compared to normal cells. Amino acid modification studies and heating experiments suggest that separate sites in the NK-PLA2 molecules are responsible for their catalytic, anticoagulant and cytotoxic activities.