An acidic phospholipase A2 (RVVA-PLA2-I) purified from Daboia russelli venom exerts its anticoagulant activity by enzymatic hydrolysis of plasma phospholipids and by non-enzymatic inhibition of factor Xa in a phospholipids/Ca2+ independent manner

A homodimeric acidic PLA2 (RVVA-PLA2-I) of 58.0 kDa molecular weight purified from Russell’s viper (Daboia russelli) venom demonstrated dose-dependent catalytic, strong anticoagulant and indirect hemolytic activities and inhibited blood coagulation cascade in both enzymatic and non-enzymatic mechanisms. In in vitro condition, RVVA-PLA2-I showed preferential hydrolysis of phosphatidylcholine with a Km and Vmax values of 0.65 mM and 28.9 μmol min−1, respectively. Biochemical study and GC-analysis of plasma phospholipids hydrolysis by PLA2 revealed that anticoagulant activity of RVVA-PLA2-I was partly attributed by the enzymatic hydrolysis of pro-coagulant phospholipids PC, followed by PS. The spectrofluorometric and gel-filtration analyses documented binding of RVVA-PLA2-I with activated factor X and PC; however, it does not bind with factor Va, prothrombin and thrombin. Therefore, this anticoagulant PLA2 inhibits the blood coagulation cascade non-enzymatically by binding with coagulation factor Xa, even in the absence of phospholipids and Ca2+ and thus slows down the blood coagulation by partially inhibiting the prothrombin activation. Chemical modification of essential amino acids present in the active site, neutralization with Azadirachta indica leaves extract (AIPLAI) and heat-inactivation study reinforce the association of catalytic and anticoagulant activity of RVVA-PLA2-I and also throw a light on its non-enzymatic mechanism of anticoagulant action.

► RVVA-PLA2-I, an acidic PLA2, purified from Daboia russelli venom of eastern India origin, demonstrated strong anticoagulant activity suggesting its important role in pathogenesis post Russell's viper envenomation.
► This is the first report of occurrence of a 28.5 kDa homodimeric snake venom anticoagulant PLA2 from Russell's viper venom which is a Factor Xa inhibitor.
► Preferential hydrolysis of phosphatidylcholine (PC) over phosphatidylserine (PS) and phosphatidylethanolamine by RVVA-PLA2-I contradicts some earlier findings suggesting that strong anticoagulant activity may not always be restricted to those PLA2 showing preferential hydrolysis of PS.
► The association of catalytic and anticoagulant activity of RVVA-PLA2-I is reinforced and a non-enzymatic mechanism of inhibition of Factor Xa is demonstrated thereby suggesting a dual mechanism of rendering an overall anticoagulant effect.