Abstract
Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. Simulation of the small angle X-ray scattering curves and comparison with published data suggested that this disrupted shell organization represents an intermediate state in the transition from isotropic to layered packing of the lipid. Thus, the results revealed, with 3D images, the lipid packing in the dynamic process of the LDL lipid-core phase transition.
Highlights
Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core
Recent work has shown that the phase transition of the LDL lipid core occurs over a timescale in the range of ف10ms [17]
With this issue in mind, we attempted to minimize the time that the sample grid experienced a sub-phase transition temperature before being frozen: The incubation chamber was set at 53°C, which is ف16°C above the point that the LDL lipid core goes from a disordered to an ordered state (Fig. 1). [The LDL particle is stable and free from thermal denaturation at this temperature for a period of time [18].] The distance between the ethane surface and the 53°C chamber was minimized to a few millimeters and the door separating them only opened ف2 s before the grid was plunged in the liquid ethane
Summary
Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. More recent work from electron cryo-microscopy (cryo-EM) studies have shown in threedimensional (3D) images that the LDL particle exhibits a disk shape instead of a sphere at temperatures below the phase transition, and the cholesterol ester forms a flat layered density [12,13,14] as opposed to concentric shells.
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