Possible roles of bile lipids and colipase in lipase adsorption

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Abstract
The adsorption isotherms of bile salts, phospholipids, and cholesterol were determined with siliconized glass beads. The molar fractions of cholesterol, phospholipid, and bile polypeptide fractions increased simultaneously and considerably on the surface of the beads in comparison to the corresponding fractions found in bile. The composition of the adsorbed film is approximately 1 cholesterol: 2 phospholipid: 3 bile salt molecules. The performed complex of [porcine pancreatic] lipase, [bovine] colipase, and bile lipids behaves as an entity which determines lipase adsorption. The modification of the interface quality of a lipid substrate by a detergent is not per se the reason for the lack of lipase adsorption. A model is proposed according to which lipolysis under physiological conditions would occur in 2 steps requiring 2 cofactors. Colipase would be necessary for the formation of the lipase-bile lipoprotein complex, and bile lipids would be required to direct the adsorption of this lipolytic entity toward the emulsified substrate.