PHOSPHOLIPASE A(2) AND COBRA VENOM CYTOTOXIN V(C)5 INTERACTIONS AND MEMBRANE-STRUCTURE

The hydrolytic activity and interaction of acidic and neutral phosphol ipase A(2) (PLA(2)) with large unilamellar liposomes treated with cobr a venom cytotoxin V(c)5 (CT V(c)5) were studied to more fully understa nd the modulating effects of cationic membrane-active peptides on PLA( 2). Studies were done by fluorescence displacement, EPR spin probes, a nd P-31-NMR. The results showed that CT V(c)5 inhibits PLA(2) activity on phosphatidylcholine liposomes. Enzymatic activity of both acidic a nd neutral PLA(2)'s were enhanced on liposomes containing cardiolipin and pretreated with cytotoxin. The cytotoxin, however, inhibited enzym e lipid hydrolysis if these same liposomes were first treated with aci dic PLA(2). The highest enzymatic activity was found on substrates wit h nonbilayer lipid packing. Using EPR of spin labeled enzymes, it was shown that CT V(c)5 inhibited binding of acidic PLA(2) to liposomes an d caused displacement of acidic PLA(2) from liposomes. No direct inter action was found between CT V(c)5 and neutral PLA(2). It is suggested that cytotoxin perturbs packing of lipid molecules in liposomes contai ning cardiolipin and is responsible for increased catalysis, whereas d irect interaction between CT V(c)5 and acidic PLA(2) inhibits enzyme a ctivity. It is concluded that variability in substrate composition and the chemical nature of both PLA(2) and cationic peptide determine whe ther enzyme activity is affected by substrate packing or by direct enz yme-peptide interaction. Models of interactions of PLA(2) with CT V(c) 5 and phospholipid membranes are presented.