Interaction of phospholipase A2 from Naja melanoleuca snake venom with monomeric substrate analogs. Activation of the enzyme by protein-protein or lipid-protein interactions?

Unlike porcine pancreatic phospholipase A2, the enzyme from Naja melanoleuca does not display biphasic kinetic behaviour at substrate concentrations around the critical micelle concentration. This snake venom enzyme was further investigated by direct binding studies using n-tridecylphosphocholine. Binding of this substrate analog to the enzyme was monitored by using equilibrium gel filtration, equilibrium dialysis and ultraviolet difference spectroscopy. It is concluded that, in the presence of submicellar concentrations of n-tridecylphosphocholine, a lipid-protein complex is formed consisting of about 4 protein and 36 lipid molecules. Ca2+ ions are required for the formation of this complex. A model is proposed which describes the formation of this type of complex. These lipid-protein aggregates are held responsible for the non-hyperbolic kinetic behaviour of the snake venom enzyme towards monomeric substrates.