[Interaction between LDL receptor and lipoproteins containing apo B].

The physiocochemically defined lipoproteins such as VLDL, LDL are comprised of subpopulations with different lipid and apolipoprotein composition. In order to determine the respective roles of different apolipoproteins (B, C-III, E) in their metabolism, four species (LpB, LpB:E, LpB: C-III and LpB: C-III: E) have been separated by sequential immunoaffinity chromatography. We examined the binding characteristics of each lipoprotein to HeLa cells and expressed the results in relation to the number of moles of apo B. LpB particles which contained apo B as their sole apolipoprotein had lower affinity for the LDL receptor that did total LDL but an apparently higher number of binding sites. The presence of apo E of phenotype E3 or E4 on one particle increased the affinity for the receptor. The apparent number of binding sites decreased probably due to the fact that a particle containing multiple copies of apo E bound to more than one molecule of receptor. Interaction with several LDL receptors would also explain the higher binding affinity which we observed. When the apo E phenotype was E2/E2, the LpB: E particle did not bind to the receptor. We showed also that apo C-III, when present, diminished the binding of apo B containing lipoproteins. These data suggest that apolipoproteins E and C-III impaired the interaction of apo B with the LDL receptor. It is likely that in LpB: E only apo E (in the case of E3 or E4 phenotype) participates in the LDL receptor binding.