Binding of lipoprotein lipase to apolipoprotein B-containing lipoproteins

The binding of lipoprotein lipase (LPL) to different lipoproteins and to a lipid emulsion was studied. After incubating the same amount of I-125-labelled LPL with VLDL, LDL or a lipid emulsion containing no apolipoproteins, we separated the free enzyme from the lipoprotein-bound LPL by gel filtration and by lipoprotein precipitation with phosphotungstic acid. By the former method we observed that all these types of lipid particles bound LPL indicating that the lipid moiety accounts for the LPL-lipoprotein interaction, This binding of LPL to lipoproteins was disrupted by high salt concentrations. When balanced by the apolipoprotein B content, it was observed that a significantly higher amount of I-125-labelled LPL co-eluted with VLDL than with LDL in gel permeation. The K-d values for binding of LPL to lipoproteins were estimated by use of lipoprotein precipitation. The obtained K-d values, both in the absence and in the presence of human lipoprotein deficient serum, were lower for VLDL than for LDL indicating a higher affinity of LPL for VLDL than for LDL. We finally compared binding capacity of LPL to VLDL subfractions with different apo E content. For this, we used apo E-poor (VLDL-B) and apo E-rich (VLDL-D) subfractions separated by heparin-Sepharose chromatography. We found that I-125-labelled LPL co-eluted to a similar extent with both subfractions on gel filtration, and the estimated K-d values from lipoprotein precipitation were not statistically different. Taken together, our results indicate that the lipid moiety, probably the phospholipids, accounts for the LPL-lipoprotein interaction; differences in size, the presence of C apolipoproteins or the conformation of apo B may be responsible for the higher affinity of LPL for VLDL than for LDL herein observed.