Product activation of pancreatic lipase. Lipolytic enzymes as probes for lipid/water interfaces.

During the action of pancreatic lipase and colipase on racemic 1,2-didodecanoylglycerol monolayers in the absence of bile salts, biphasic kinetics was observed under conditions of high lipid packing. Similar kinetics has earlier been reported using phospholipid-emulsified triolein droplets (Borgström, B. (1980) Gastroenterology 78, 954-962). These kinetics are characterized by a lag time tau d, dependent on products accumulation at the substrate/water interface. This lag time is differentiated from the previously described enzyme concentration independent lag time tau i in systems of low lipid packing (Verger, R., Mieras, M. C. E., and de Haas, G. H. (1973) J. Biol. Chem. 248, 4023-4034). Both tau i and tau d reflect a rate-limiting step due to the slow enzyme penetration into the substrate interface. The variation of tau d under different conditions (change in pH and concentration of Ca2+, enzyme, bovine serum albumin, and lipolytic products) lead us to propose a model for the product activation during lipolysis. We will discuss the use of the pancreatic lipase-colipase system to probe the lipid packing of emulsified triglyceride particles and lipoproteins using tau d as a reference value.