Differences in low density lipoprotein (LDL) receptor-binding affinity among LDL particles of different size were examined in competitive binding assays in human skin fibroblasts and LDL (d=1.020 to 1.050 g/mL) from subjects with a predominance of large (≥272 Å), medium (259 to 271 Å), and small (≤257 Å) LDL. Among 57 normolipidemic subjects with LDL cholesterol (-C) levels <160 mg/dL, binding affinity was reduced by 16% in those with predominantly large LDL and by 14% in those with small LDL compared with most subjects who had a predominance of medium-size LDL and in all LDL size subgroups in 66 subjects with LDL-C ≥160 mg/dL. Differences in LDL receptor-binding affinity were further investigated by using LDL density subfractions (I, d=1.026 to 1.032 g/mL; II, d=1.032 to 1.038 g/mL; and III, d=1.038 to 1.050 g/mL) from three subjects with predominantly large (pattern A) and small (pattern B) LDL particles. The binding affinity (Kd) of LDL-II was similar for patterns A and B (9.2±1.4 and 9.4±0.7, respectively) and 30% lower in LDL-III from both groups (P<.05). The binding affinity of LDL-I in pattern A (12.6±1.5 μg/mg) was lower (P<.05) than that in LDL-II and LDL-I from pattern B (8.0±2.4 μg/mg). After incubation with a monoclonal antibody that specifically blocked the LDL receptor-binding domain of apoE, LDL-I from two pattern B subjects showed substantially lower binding affinity (Kd=20.0 and 19.2 μg/mg) than in pattern A (Kd=13.2 and 14.2 μg/mg), a result consistent with our finding of a higher apoE content in pattern B LDL-I (P<.001). Thus, factors associated with variations in particle size and apoE content in LDL subclasses in normolipidemic subjects contribute to the differences in LDL receptor binding that may result in differing metabolic behavior in vivo.
