INTERACTIONS BETWEEN FATTY-ACIDS AND LIPOPROTEIN-LIPASE - SPECIFIC BINDING AND COMPLEX-FORMATION

Lipoprotein lipase is the extrahepatic lipase responsible for the hydr olysis of triglycerides in chylomicrons and very low-density lipoprote ins. Its enzymic activity and its location on the surface of endotheli al cells are affected by the presence of fatty acids, implying that th e protein possesses binding sites for that ligand. In this study, we e xamine the binding of fatty acids to LpL and describe factors that mus t be considered when the dissociation constant of the acceptor-ligand equilibrium is close to the critical micelle concentration of the fatt y acid. The interaction of fatty acids with lipoprotein lipase (LpL) w as studied by two methods. A new direct method, based on the LpL-induc ed increase in the apparent critical micelle concentration of the sodi um soap of the fatty acid, indicates the presence of multiple high-aff inity binding sites. In the second method, the specific binding of fat ty acids to LpL was measured by quantitating the blue shift in the try ptophan fluorescence of LpL that occurs upon binding the ligand. Both methods suggest the existence of 4-6 fatty acid binding sites on LpL w ith a dissociation constant on the order of 10(-6)-10(-7) M. Further a nalysis of the blue shift indicates that at higher concentrations of f atty acid, large complexes are formed consisting of 260-310 molecules of fatty acid per LPL monomer. In contrast, no large complexes are for med with fatty acids that form crystals above their solubility limit.