Importance of phospholipids, pancreatic phospholipase A2, and fatty acid for the digestion of dietary fat: in vitro experiments with the porcine enzymes.

Long chain triglycerides emulsified with phospholipid are not directly available for hydrolysis by pancreatic lipase in vitro even in the presence of bile salts and colipase. The inhibition can be overcome by pancreatic phospholipase A2. There is a limited hydrolysis of the phospholipid during this period. The inhibition is explained by the finding that lipase does not bind to triglyceride emulsified by phospholipid but remains in the aqueous phase. A limited hydrolysis of the phospholipid by phospholipase A2 results in the binding of lipase to the substrate interface and a rapid rate of hydrolysis of the triglyceride. With time the inhibition of lipase activity can also be overcome by pancreatic lipase. A lag phase is seen before the accelerated hydrolysis of triglyceride reaches a high rate. The length of the lag phase is dependent on factors such as lipase and colipase concentration, pH, Ca++, and concentration of bile salt. During the lag phase no significant hydrolysis of phospholipid occurs. The primary factor is the binding of colipase to the substrate interface. Fatty acid present in the oil phase or produced from it by a limited hydrolysis of phospholipid by phospholipase A2 or triglyceride by lipase, changes the properties of the interface so that colipase can bind and thereby lipase via its binding to colipase. The milieu of small intestinal content favors the concerted action of several factors to make dietary triglyceride available for an effective hydrolysis by pancreatic lipase.