LIPOPROTEIN-LIPASE ASSOCIATION WITH LIPOPROTEINS INVOLVES PROTEIN-PROTEIN INTERACTION WITH APOLIPOPROTEIN-B

Lipoprotein lipase (LPL) hydrolyzes chylomicron and very low density l ipoprotein (VLDL) triglycerides and potentiates the cellular uptake of lipoproteins. These LPL-lipoprotein associations could involve only p rotein-lipid interaction, or they could be modulated by apolipoprotein s (ape), ApoB is the major protein component of chylomicrons, VLDL, an d low density lipoprotein (LDL), ApoB100, a large glycoprotein with a molecular mass of 550 kDa, is composed of several functional domains, A carboxyl terminal region of the protein is the ligand for the LDL re ceptor, There are several hydrophobic domains that are believed to be important in lipid binding, The relatively hydrophilic amino-terminal region of apoB, however, has no known function, Using solid phase assa ys we quantified LPL lipoprotein complex formation, On a molar basis, severalfold greater amounts of LPL bound to LDL and VLDL than to high density lipoprotein at all the concentrations of LPL tested (0.9-55 nM ). To assess the roles of LDL protein versus lipid, we performed compe tition and ligand blotting experiments, LDL and an amino-terminal frag ment of apoB competed better for I-125-LPL binding to LDL than did lip id emulsion particles, Delipidation of LDL-coated plates did not alter LPL binding, On ligand blots, LPL bound to amino-terminal fragments o f apoB generated by thrombin digestion but not to apoA1, apoE, or carb oxyl-terminal fragments of apoB, Further evidence for LPL interaction with the amino-terminal region of apoB was obtained using anti-apoB mo noclonal antibodies, Antibodies directed against the amino-terminal re gions of apoB blocked LPL interaction with LDL, whereas those against the carboxyl-terminal region of apoB did not inhibit LPL interaction w ith LDL, Thus, we conclude that a specific interaction between LPL and the amino-terminal region of apoB may facilitate LPL association with circulating lipoproteins.