Expression of apolipoprotein B epitopes in lipoproteins. Relationship to conformation and function

The immunochemical properties of apolipoprotein (apo) B have been studied in very low density lipoprotein (VLDL)1 (Sf 100 to 400), VLDL2 (Sf 60 to 100), VLDL3 (Sf 20 to 60), different intermediate density lipoprotein (IDL), and low density lipoprotein (LDL) subfractions isolated from patients with type IV hypertriglyceridemia. In these lipoproteins, we characterized the association of apo B with other apolipoproteins and the expression and immunoreactivity of several apo B epitopes close to the apo B receptor binding sites (3F5, 4G3, 3A8, and 5E11) and of other epitopes located on the apo B100-B48 common region (1D1 and 2D8). Immunoprecipitation showed that the proportion of lipoprotein particles expressing each apo B epitope increased from VLDL1 to LDL2; this was more apparent with 3A8 and 5E11 than with 3F5. The VLDL that were negative for apo E epitopes (60% or more of the total) were enriched in apo C. The lipoprotein particles containing apo E and/or apo C-III decreased progressively from VLDL1 (30% and 85%, respectively) to LDL2 (10% and 25%, respectively). Similar observations were made for apo C-I and apo D, demonstrating that apolipoprotein heterogeneity is greatest in the lightest lipoproteins. By competitive radioimmunoassay, the epitope for 4G3 was equally immunoreactive in each lipoprotein subclass, and the affinity constant (Ka) of 4G3 for different lipoproteins showed little variation. In contrast, both immunoreactivity and Ka of 3A8 and 5E11 increased progressively and significantly with the increasing density of the lipoprotein subclasses. This phenomenon is correlated with the increasing binding affinity of apo B in these lipoprotein subclasses to the LDL receptor of fibroblasts. We conclude that, as the apo B-containing lipoproteins become smaller, the conformation of specific regions of apo B is modified: in the receptor binding domain, the conformation of epitope 4G3, which is mapped between residues 2980 and 3080, remains constant, while that of 3A8 and 5E11 (residues 3441 to 3568) changes progressively. We propose the theory that the change in conformation in the domain spanning residues 3441 and 3568 allows the maximum expression of epitopes 3A8 and 5E11 and of the receptor binding site.