Oxygen permeability of phosphatidylcholine--cholesterol membranes.

Oxygen transport in phosphatidylcholine-cholesterol membranes has been studied by observing the collision of molecular oxygen with nitroxide radical spin labels placed at various distances from the membrane surface using long-pulse saturation recovery ESR techniques. The collision rate was estimated for tempocholine phosphatidic acid ester, 5-doxylstearic acid, and 16-doxylstearic acid from spin-lattice relaxation times (T1) measured in the presence and absence of molecular oxygen. Profiles of the local oxygen transport parameter across the membrane were obtained as a function of cholesterol mol fraction and temperature in L-alpha-dimyristoylphosphatidylcholine ( Myr2PtdCho) and L-alpha-dioleoylphosphatidylcholine ( Ole2PtdCho) membranes. Membrane oxygen permeability coefficients were estimated from oxygen transport parameter profiles. At approximately 30 degrees C, the oxygen permeability coefficients in the presence and absence of 50 mol % cholesterol are 22.7 and 125.2 cm/s, respectively, for Myr2PtdCho membranes, and 54.7 and 114.2 cm/s, respectively, for Ole2PtdCho membranes (compared with 60-80 cm/s for water layers with the same thicknesses as the membranes). The major results in the liquid-crystalline phase are as follows: (i) In the absence of cholesterol, membranes are not barriers to oxygen transport. (ii) Addition of 50 mol % cholesterol decreases oxygen permeability by a factor of approximately 5 and approximately 2.5 in Myr2PtdCho and Ole2PtdCho membranes, respectively. The resistance to oxygen transport is located in and near the polar headgroup regions in the membrane. (iii) Cholesterol increases oxygen transport in the central regions of Ole2PtdCho membranes.