Comparative studies of lipase and phospholipase A2 acting on substrate monolayers

The kinetic aspects of lipolysis by pancreatic lipase and phospholipase A2 from different sources have been compared using monomolecular films of short chain lipids as the substrates. Phosphatidylcholine monolayers, in contrast to phosphatidylethanolamine and phosphatidylglycerol monolayers, were resistant to hydrolysis by pancreatic lipase. The induction time, measured during pre-steady state conditions, increased abruptly for a given value of the surface pressure. This appears to be due to a degree of lipid packing above which the enzyme no longer can penetrate the lipid film. The existence of an optimum in the velocity versus surface pressure profile is the result of at least two counterbalancing factors. As the surface pressure increases, the amount of enzyme present in the interface decreases, whereas the minimal specific activity of the enzyme increases. From this study with monolayers we can conclude that activity of lipolytic enzymes used as tools for probing biological membranes will be greatly influenced by the physiochemical nature of the membrane-water interface. Thus, studies such as this one which can measure the penetrating ability of various lipolytic enzymes can be useful in deriving a better understanding of biological membrane structure.