Binding of divalent and trivalent cations with crotoxin and with its phospholipase and its non-catalytic subunits: effects on enzymatic activity and on the interaction of phospholipase component with phospholipids

We have studied the interaction of divalent and trivalent cations with a potent phospholipase A 2 neurotoxin, crotoxin, from Crotalus durissus terrificus venom. The pharmacological action of crotoxin requires dissociation of its catalytic subunit (component B) and of its non-enzymatic chaperone subunit (component A), then the binding of the phospholipase subunit to target sites on cellular membranes and finally phospholipid hydrolysis. In this report, we show that the phospholipase A 2 activity of crotoxin and of component B required Ca 2+ and that other divalent cations (Sr 2+ , Cd 2+ and Ba 2+ ) and trivalent lanthanide ions are inhibitors. The lowest phospholipase A 2 activity was observed in the presence of Ba 2+ , which proved to be a competitive inhibitor of Ca 2+ . The binding of divalent cations and trivalent lanthanide ions to crotoxin and to its subunits has been examined by equilibrium dialysis and by spectrofluorimetric methods. We found that crotoxin binds two divalent cations per mole with different affinities; the site presenting the highest affinity ( K d in the mM range) is involved in the activation (or inhibition) of the phospholipase A 2 activity and must therefore be located on component B, the other site ( K d higher than 10 mM) is probably localized on component A and does not play any role in the catalytic activity of crotoxin. We also observed that crotoxin component B binds to vesicular and micellar phospholipids, even in the absence of divalent cations. The affinity of this interaction either does not change or else increases by an order of magnitude in the presence of divalent cations.