An activation-collision mechanism for cholesterol transfer between membranes

Area of Science:

• Biochemistry
• Cell Biology
• Membrane Biophysics

Background:

• Cholesterol is a vital membrane component.
• Its transfer mechanism between membranes has been debated.
• Aqueous diffusion is a widely accepted model.

Purpose of the Study:

• To investigate the mechanism of cholesterol transfer between biological membranes.
• To challenge the prevailing aqueous diffusion model.
• To elucidate the kinetics and pathway of sterol transfer.

Main Methods:

• Experiments using radiolabeled cholesterol ([3H]cholesterol).
• Varying acceptor particle sizes (ghosts, vesicles, liposomes, lipoproteins).
• Kinetic analysis of transfer rates under different conditions (plasma presence, dilution).

Main Results:

• Cholesterol transfer rate inversely correlated with acceptor particle size.
• Plasma accelerated transfer despite acting as a competitor.
• Transfer kinetics were not simple second-order and not limited by unstirred layers.
• Cholesterol in retinal rod discs was not in equilibrium with plasma lipoproteins.

Conclusions:

• Cholesterol transfer requires a complex mechanism, not aqueous diffusion.
• A two-step model (activation followed by collision) is proposed.
• This model explains existing and new kinetic data for sterol and phospholipid transfer.