ZENHANCEMENT OF AGKISTRODON-PISCIVORUS-PISCIVOUUS VENOM PHOSPHOLIPASEA(2) ACTIVITY TOWARD PHOSPHATIDYLCHOLINE VESICLES BY LYSOLECITHIN ANDPALMITIC ACID - STUDIES WITH FLUORESCENT-PROBES OF MEMBRANE-STRUCTURE

The activity of phospholipase A(2) from snake venom to hydrolyze bilay ers of phosphatidylcholines is greatly enhanced by the presence of the hydrolysis products, lysolecithin and fatty acid, in the bilayer. The fluorescence of several probes of membrane structure was used to moni tor changes in bilayer physical properties during vesicle hydrolysis. These changes were compared to emission spectra and fluorescence polar ization results occurring upon direct addition of lysolecithin and/or fatty acid to the bilayer. The excimer to monomer ratio of 1,3-bis(1-p yrene)propane was insensitive to vesicle hydrolysis, suggesting that c hanges in the order of the phospholipid chains were not relevant to th e effect of the hydrolysis products on phospholipase activity. The flu orescence of 6-propionyl-2-(dimethylamino)naphthalene (Prodan) suggest ed that the polarity of the bilayer in the region of the phospholipid head groups increases as the hydrolysis products accumulate in the bil ayer. The fluorescence of 6-dodecanoyl-2-(dimethylamino)naphthalene (L aurdan) confirmed that such effects were restricted to the bilayer sur face. Furthermore, the lysolecithin appeared to be the product most re sponsible for these changes. These results suggested that lysolecithin increases the activity of phospholipase A(2) during vesicle hydrolysi s by disrupting the bilayer surface, making the phospholipid molecules more accessible to the enzyme active site.