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Abstract:
Lipase and colipase are two genetically independent proteins synthesized and secreted by the pancreas. Lipase catalyzes the hydrolysis of dietary triglycerides emulsified in the intestinal lumen. It is activated by interfaces and, to a lesser extent, by micelles and monomeric solutions of glycerides in the presence of an organic solvent. The enzyme is activated due to acceleration of the acyl-enzyme deacylation step after conformational modification of the catalytic site by contact with the interface. Lipase turnover is higher than that of other esterases. The enzyme denaturation rate increases as surface pressure at the interface decreases (as surface tension increases). On the other hand, the enzyme is stabilized by high surface pressure and by colipase. Beyond a certain surface pressure, only lipase is unable to adsorb to the surface. In this case, colipase is indispensable for the anchorage of lipase at the interface. The association colipase with lipase and substrate depends on two distinct sites, each site being formed by an hydrophobic region and by ionizable epsilon-amino and carboxylate groups. The KD of the [lipase.colipase] complex is 10(-7) M; in the presence of substrate, it is 10(-9) M. Colipase is synthesized as pro-colipase. The [ pro-colipase .lipase] complex pre-exists in pancreatic juice and probably in cells of the exocrine pancreas. Trypsin action on pro-colipase leads to the cleavage of the colipase with 96 residues and the N-terminal pentapeptide. The [lipase.colipase] complex obtained is more generally hydrophobic than the previous one. In duodenal contents, this newly formed complex would be stabilized by the pH of the milieu and by free fatty acids appearing in the gastric contents, whether associated or not with the bile lipoprotein complex. The [lipase-colipase] complex, which is also stabilized by the substrate, would be fixed at the triglyceride/water interface, i.e. by passing through the layer of adsorbed amphipathic compounds (bile salts, phospholipids, fatty acids, proteins) owing to their hydrophobic and ionic properties, and by positioning into the interface by ionizable colipase groups.