Fatty acid flip-flop in phospholipid bilayers is extremely fast

Area of Science:

• Biochemistry
• Membrane Biophysics
• Cell Biology

Background:

• The transport of fatty acids (FA) across cellular membranes is crucial for various biological processes.
• Understanding the rate of FA movement across phospholipid bilayers is key to elucidating their transport mechanisms.
• Previous studies indicated spontaneous FA movement across bilayers but lacked millisecond-level resolution.

Purpose of the Study:

• To measure the rate of fatty acid (FA) movement across phospholipid bilayers with millisecond time resolution.
• To investigate the influence of vesicle size and fatty acid properties on transbilayer movement.
• To re-evaluate the flip-flop kinetics of fatty acid analogues like (anthroyloxy)stearic acid.

Main Methods:

• Utilized stopped-flow fluorometry to achieve millisecond time resolution for FA flip-flop assays.
• Employed small unilamellar vesicles (SUVs, ~25 nm) and large unilamellar vesicles (LUVs, ~100 nm).
• Studied the flip-flop rates of various saturated and unsaturated fatty acids (lauric, myristic, palmitic, oleic, stearic) and (anthroyloxy)stearic acid.

Main Results:

• In SUVs, >80% flip-flop of studied fatty acids was completed within 5-10 ms at neutral pH.
• In LUVs, fatty acids showed measurable flip-flop rates with a half-life (t 1/2) of 23 ± 12 ms.
• Flip-flop rate was independent of fatty acid chain length or structure; (anthroyloxy)stearic acid demonstrated rapid flip-flop.

Conclusions:

• Fatty acids translocate rapidly across phospholipid bilayers, with kinetics measurable in the millisecond range.
• The rate of flip-flop is not dependent on fatty acid chain length or structure.
• These findings support the model that proteins are not required for fatty acid translocation across cell membranes.