Interfacial interactions between proteins and mammalian lipases

The effects of proteins, both endogenous and exogenous, on the activity of lipases against water soluble and water insoluble substrates have been reviewed. The enzymes considered are pancreatic and gastric lipases, and lipoprotein, bile-salt-stimulated human milk and pancreatic carboxyl ester lipases. A brief account is given of the function of each enzyme and of the physical properties of the interacting proteins, which include albumins, lysozymes, globulins and immunoglobulins, myoglobin, transferrins, alpha-lactalbumin and melittin. With few exceptions (for example, the effect of colipase on pancreatic lipase), the interaction of proteins with lipases which act at the lipid-water interface of water insoluble substrates results in deactivation of enzymic activity. It seems that the amphiphilic nature of proteins allows them to aggregate at interfaces, thereby altering the nature of the interface and decreasing accessibility of the substrate to the enzyme. This discussion gives consideration to association of the proteins with the enzyme or the interface and to whether the interactions with specific binding sites or interfacial inactivation are responsible for the observations. However, the effect of proteins on lipases acting against water soluble substrates varies from protein to protein. Activation of enzyme-activity occurs if the interacting proteins are able to act as acyl transfer agents and thus introduce another catalytic hydrolysis pathway into the reaction mechanism. Inhibition may be caused by specific interactions between the protein and the enzyme or the substrate.