Platelet LDL receptor recognizes with the same apparent affinity both oxidized and native LDL. Evidence that the receptor-ligand complexes are not internalized.

We have recently demonstrated that the platelet low-density lipoprotein (LDL) receptor is immunologically different from the "classic" receptor of nucleated cells. We undertook the current studies to investigate the interaction of this receptor with oxidized LDL and to determine whether an endocytosis-mediated response is involved in the binding of LDL to platelets. The platelet LDL receptor recognized with the same affinity both native and oxidized LDL particles (IC50, 0.045 and 0.054 g/L; Kd, 45.8 and 65.9 nmol/L, respectively). The Hill coefficients of the displacement of 125I-LDL binding were -1.10 and -1.05 for unlabeled native and oxidized LDL, respectively, thereby suggesting a single set of binding sites. To ascertain whether human platelets bind oxidized LDL, we performed ligand binding assays with 125I-oxidized LDL. Saturation curves of 125I-oxidized LDL binding at 22 degrees C showed that human platelets bound these modified particles to a class of saturable binding sites, numbering approximately 3895 +/- 241 per platelet with a dissociation constant (Kd) of 96.2 +/- 10.3 nmol/L. Displacement experiments showed that 125I-oxidized LDL binding was inhibited with the same affinity by both oxidized and native LDL (IC50, 0.055 and 0.065 g/L; Kd, 88 and 64 nmol/L, respectively). The Hill coefficients of the displacement of the 125I-oxidized LDL binding were -1.02 and -1.07 for unlabeled oxidized and native LDL, respectively, suggesting that a single set of binding sites is implicated. Moreover, oxidized LDL- at a protein concentration of 0.5 g/L enhanced ADP- and collagen-induced platelet aggregation in a manner similar to native LDL.(ABSTRACT TRUNCATED AT 250 WORDS)