The importance of bile salt for the reactivation of pancreatic lipase by colipase

Abstract

This study shows the importance of bile salt in the reactivation of pancreatic lipase by colipase. An emulsion of tributyrin was used as substrate for lipase. Addition of phosphatidylcholine or fatty acids (oleic acid, decanoic acid and butyric acid) caused inhibition of lipase to various degrees. The inhibition was greater with increased hydrophobicity of the compound added. Colipase alone increased lipase activity only marginally in the presence of phosphatidylcholine or oleic acid. It caused a higher increase in activity in the presence of decanoic acid; with butyric acid the activity was increased to a maximal level. In the presence of both bile salt (over critical micellar concentration) and colipase, lipase reached almost maximal activity in the different systems. The binding of lipase to colipase was not affected by butyric acid, K d being around 10 −6 M. It was increased by decanoic acid and oleic acid, K d being 10 −7 M and 10 −8 M, respectively. At the same time, the activity of lipase/colipase against tributyrin was lower in the presence of oleic acid than in the presence of butyric acid. Thus, reactivation of lipase by colipase in these systems was not only related to the binding between the two proteins, but there was also a competition between the triacylglycerol surface and the mixed salt/amphiphile micelles in solution for the lipase-colipase complex. It is concluded that colipase, which by itself is weakly surface-active, needs bile salt to displace some of the amphiphilic compounds at the triacylglycerol surface, thereby lowering the substrate packing, after which colipase could bind to the interface. Lipase is bound to colipase either in solution or at the surface. Once at the surface the activity of lipase is dependent on the quality of the interface.