Asymmetric permeability of the membrane of egg phosphatidylcholine-cholesterol vesicles to 2,2,6,6 tetramethyl piperidinyl-l-oxycholine.

The rates of uptake and release of 2,2,6,6 tetramethyl piperidinyl-l-oxycholine (Tempo-choline) for vesicles made of dipalmitoyl phosphatidylcholine (DPPC) and of egg phosphatidylcholine-cholesterol mixtures were measured by ESR and found to have interesting temperature-dependences. In the former case, both rates exhibit a sharp maximum at the critical temperature of phase transition of the bilayer membrane. In the latter case, the permeability of the membrane to Tempo-choline is asymmetric with respect to uptake and release: uptake is appreciable at temperatures higher than 66 degrees C, while release is observable only at temperatures higher than 80 degrees C. The asymmetric permeability is explained in terms of the asymmetric distribution of cholesterol between the outer and inner membranes of the vesicle.