Colipase-induced reorganization of interfaces as a regulator of lipolysis

Abstract

Colipase is a 10 kDa protein cofactor that allows pancreatic triacylglycerol lipase (PTL) to function in the presence of endogenous inhibitors, like diacyl phosphatidylcholine. From structural studies of lipase–colipase it has been shown that colipase forms a 1:1 complex with lipase, increases the hydrophobic surface area of lipase presented to the interface and exposes the active site. This has led to the idea that colipase serves to anchor lipase at inhibitor-rich interfaces, thereby allowing lipase to function catalytically. Early studies, however, present a more complicated picture of lipolysis and indicate a role for the products of lipolysis in the anchoring and activation of lipase. Using techniques of surface chemistry, we have studied the adsorption of pancreatic lipase and colipase to defined lipid monolayers comprised of SOPC and model lipolysis reactants, i.e . fatty acid and 1,2-diacylglycerol. At physiologically relevant lipid packing densities, lipase does not adsorb to phosphatidylcholine unless fatty acid is also present. Moreover, in phosphatidylcholine-rich interfaces containing fatty acid, as might be encountered in vivo, lipase adsorption is greatly enhanced by the presence of colipase. To determine why this occurs, colipase adsorption to monolayers of phosphatidylcholine and lipolysis reactants were compared. We found that colipase interacts more strongly with interfaces comprised of or containing lipolysis reactants than those of phosphatidylcholine. Analysis of the interaction stoichiometry indicates that ∼25 reactant acyl chains are affected by the presence of colipase in the interface and suggests that colipase laterally reorganizes the interface, concentrating lipolysis reactants in its vicinity. To test this hypothesis directly, the fluorescence properties of BODIPY-containing lipids in monolayers were characterized in the presence and absence of colipase. Preliminary results support the reorganization hypothesis. Overall, the studies reviewed herein support the idea that colipase laterally concentrates reactants of lipolysis in its vicinity, creating a reactant-rich nanodomain to which lipase adsorbs.