On the Function of Bile Salts and Proteins as Cofactors of Lipase

Pancreatic lipase is rapidly and irreversibly inactivated at a hexadecane-water interphase. The unfolded protein is only very slowly, in the course of hours or days, released into the aqueous phase. Bile salts prevent the denaturation of the lipase. Bovine albumin also protects the enzyme. Denaturation of lipase occurs also at the interphase of the substrates, tributyrin or olive oil, and water. Kinetic studies show that taurocholate and albumin prevent but cannot reverse the unfolding of the enzyme. The accelerating effect that these agents have on lipolysis can be explained on this basis. It is concluded that it is one of the functions of bile salts, and of proteid cofactors, to protect the native structure of lipase and to keep the oil-water interphase free from blockage by unfolded proteins. Despite its preferential specificity for insoluble triglycerides, lipase is not better adapted to an existence at the oil-water interphase than other enzymes. The enzyme requires isolation from rather than binding to the hydrophobic moieties of its substrates, and it relies on the help of surfactants for the prevention of hydrophobic bonding and unfolding. This interpretation is in harmony with previous evidence for the lack of lipophilic bonding in the substrate-enzyme complex.