Interactions between fatty acids and lipoprotein lipase: specific binding and complex formation.

Abstract

Lipoprotein lipase is the extrahepatic lipase responsible for the hydrolysis of triglycerides in chylomicrons and very low-density lipoproteins. Its enzymic activity and its location on the surface of endothelial cells are affected by the presence of fatty acids, implying that the protein possesses binding sites for that ligand. In this study, we examine the binding of fatty acids to LpL and describe factors that must be considered when the dissociation constant of the acceptor-ligand equilibrium is close to the critical micelle concentration of the fatty acid. The interaction of fatty acids with lipoprotein lipase (LpL) was studied by two methods. A new direct method, based on the LpL-induced increase in the apparent critical micelle concentration of the sodium soap of the fatty acid, indicates the presence of multiple high-affinity binding sites. In the second method, the specific binding of fatty acids to LpL was measured by quantitating the blue shift in the tryptophan fluorescence of LpL that occurs upon binding the ligand. Both methods suggest the existence of 4-6 fatty acid binding sites on LpL with a dissociation constant on the order of 10(-6)-10(-7) M. Further analysis of the blue shift indicates that at higher concentrations of fatty acid, large complexes are formed consisting of 260-310 molecules of fatty acid per LPL monomer. In contrast, no large complexes are formed with fatty acids that form crystals above their solubility limit.