Figure 3. Effect of LDL concentration on aggregation, fusion, and lipid droplet formation.

Single-donor human LDLs contained 0.1–1.0 mg/ml apoB (shown by numbers) in 20 mM Na phosphate, pH 7.5. The samples were (A) heated from 4°C to 98°C at a rate of 11°C/h to record the melting data or (B) incubated at 85°C to record the kinetic data. The increase in particle size upon LDL aggregation, fusion, and lipid droplet formation was monitored by turbidity at 320 nm. Kinetic data at each LDL concentration were approximated by a sigmoidal function, V(t) = V₀ + (V₁−V₀){1+ exp[(t−t_1/2)/k)}, where V₀ and V₁ are the initial and final values of turbidity, t_1/2 is the midpoint, and k is the rate constant of the reaction. The plot of k versus apoB concentration is nonlinear (C); polynomial fitting suggests second-order reaction (gray line). These results show that increasing LDL concentration greatly increases the rate and the extent of LDL fusion and lipid droplet formation.