Modification of a phospholipid-stabilised emulsion interface of bile: effect on pancreatic lipase activity

Lipase is activated by binding to an insoluble emulsified or aggregated substrate. The extent of binding is related to the physicochemical as well as the compositional structure of the interface, the quality of the interface. ‘Quality’ is as yet undefined but thought to contain contributions from electrostatic interactions, orientation of substrate, and hydration forces. To investigate the electrostatic and compositional factors we have used olive oil-in-water emulsions prepared with phosphatidylcholine and four bile salts of varying hydrophobicities. By measurement of the droplet zeta potential we have monitored semi-quantitatively the incorporation of bile salts within the interface. No correlation was found between droplet surface charge as monitored by the zeta potential and lag phase. The duration of the observed lag phase was found to be inversely related to the degree of incorporation of the bile salts. Simultaneously there was evidence of lipase binding to monomeric bile salts, reducing its availability for adsorption. Calcium ions reduced the surface charge but there was no correlation with lag phase duration. The evidence presented here agrees with a more specific role for calcium ions, i.e., the formation of a new catalytically active enzyme complex, (enzyme)–(mixed micelle)–(calcium ion).