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The ability of different lipoprotein Lpa preparations to compete with LDL-binding to the B/E-receptor was investigated by ligand blot and filter assays. Lpa was purified from donors with various genetic polymorphic forms by affinity chromatography using lysine-Sepharose or specific immunoadsorbers. These preparations were free of "LDL-like" material. Part of Lpa was reduced and freed from specific apoa antigen yielding "Lpa-." 125I-labeled low density lipoproteins (LDL) were incubated with B/E-receptor preparations from bovine adrenal cortex or from human skin fibroblasts, and the competition with unlabeled LDL, Lpa, Lpa-, apoa, and apoE-free HDL was studied by a ligand blot or filter assay technique. The following results were obtained. 1) LDL and Lpa- were equally potent in displacing 125I-labeled from B/E-receptor in the ligand blot and the filter assay. Lpa + ( = Lpa) also displaced LDL but to a much lesser degree: 50% displacement was observed with LDL and Lpa- at a 1-fold excess, whereas a 7.5-fold excess was required of Lpa +. 2) Apoa, as well as apoE-free HDL, did not compete with LDL binding. 3) Competition experiments using B/E-receptors from bovine adrenal cortex or from human skin fibroblasts were comparable. 4) There was no difference in the behavior of Lpa isolated from the two affinity chromatography methods. 5) Lpa of different genetic variants behaved virtually identically. The results are discussed from the point of view of the in vivo metabolism of Lpa.
Steyrer et al. (Sun,) studied this question.