Discrete subspecies of human low density lipoproteins are heterogeneous in their interaction with the cellular LDL receptor.

The low density lipoproteins (LDL) of human plasma consist of a series of discrete particle subspecies of distinct physicochemical, immunological, and hydrodynamic properties. Such structural differences are intimately linked to the metabolic heterogeneity of circulating LDL in vivo. The current studies were designed to evaluate and compare the interaction of discrete LDL subspecies from normolipidemic subjects with the LDL receptor. Plasma LDL of d 1.019-1.063 g/ml from healthy males were fractionated into 15 subspecies of defined physicochemical characteristics by isopycnic density gradient ultracentrifugation as described earlier (Chapman et al., J. Lipid Res. 1988. 29: 442-458). The major LDL subspecies, LDL-5 to LDL-10, exhibited an overall range in density from 1.0244 to 1.0435 g/ml; individual subspecies increased in density by increments of 0.027 (LDL-5), 0.026 (LDL-6), 0.030 (LDL-7), 0.031 (LDL-8), 0.035 (LDL-9), and 0.042 g/ml (LDL-10), respectively. Taken together, these subspecies accounted for approximately 70% of the total mass of LDL of d 1.019-1.063 g/ml; their cholesterol: protein ratios decreased from 1.70 to 1.12 and particle size from 275 to 260 A with increase in density. ApoB-100 was the unique protein component in subspecies 5-8, with trace amounts (less than 0.2% of apoLDL) of both apoA-I and apoE in subspecies 9 and 10. The interaction of individual LDL subspecies with the LDL receptor on cultured human U-937 monocyte-like cells was compared by determining receptor binding affinities at 4 degrees C. Scatchard analysis of specific binding curves demonstrated a single class of binding site for each subspecies. The lowest dissociation constants were displayed by LDL subspecies 6 (Kd 5.71 nM), 7 (Kd 5.24 nM) and 8 (Kd 4.67 nM), while subspecies 5, 9, and 10 displayed significantly higher Kd values (8.35, 7.20, and 6.87 nM, respectively). Competitive displacement studies at 4 degrees C, in which unlabeled subspecies from the same gradient series competed for binding with 125I-labeled LDL subspecies, confirmed the relative binding affinities of these subspecies. As the hydrophobic lipid core of LDL undergoes a thermotropic transition at approximately 37 degrees C, which may in turn influence the surface structure of the particle, internalization and degradation studies were performed at 37 degrees C. No effect of temperature was detectable; again, LDL subspecies at each extreme of the density distribution (LDL-5 and LDL-10) displayed significantly lower binding affinities for the LDL receptor than that from the peak region (LDL-7).(ABSTRACT TRUNCATED AT 400 WORDS)