Adsorption of pancreatic (pro)phospholipase A2 to various physiological substrates.

The adsorption of pancreatic phospholipase was studied in vitro in the presence of egg yolk lipoprotein emulsion, Intralipid emulsion, and milk fat globules. When the emulsions are incubated with bile salts, the latter dissociate a considerable fraction of the phospholipids initially associated with the emulsions, leading to the coexistence of an emulsified phase and a phase of mixed micelles. After the addition of pancreatic phospholipase A2, gel filtration shows that the enzyme was more than 90% bound to mixed micelles, regardless of the type of emulsion used. Comparable results were obtained by replacing the bile salts with human gallbladder bile. In parallel, pancreatic zymogen was never found to be bound to any of the lipid structures present (emulsion or mixed micelles). When the catalytic site of pancreatic phospholipase A2 was blocked with 4-bromophenacylbromide, there was no fixation on mixed micelles. Fixation was restored by the presence of lysolecithins and fatty acids in the incubation mixtures. The partial transformation of all emulsified substrates to mixed micelles by bile salts in vivo would thus lead to optimum activity of pancreatic phospholipase A2.