Phospholipid asymmetry and transmembrane diffusion in photoreceptor disc membranes

The phospholipid asymmetry and flip-flop rate in rod outer segment (ROS) disc membranes has been investigated by both a spin-label reduction method and a novel electron-electron double resonance (ELDOR) method. At 39 degrees C in the absence of ATP, spin-labeled derivatives of phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylserine (PS) undergo rapid transmembrane diffusion, with a half-time of less than 10 min. At equilibrium, approximately 53% of the spin-labeled PC, 37% of the spin-labeled PE, and 18% of the spin-labeled PS reside in the inner (intradiscal) monolayer. The rapid transmembrane diffusion of phospholipids is partially inhibited by N-ethylmaleimide, suggesting that the process is mediated by proteins in the disc membrane.