OXYGEN PERMEABILITY OF THYLAKOID MEMBRANES - ELECTRON-PARAMAGNETIC-RESONANCE SPIN-LABELING STUDY

Oxygen transport in thylakoid membranes of spinach chloroplasts (Spina cia oleracea) has been studied by observing the collisions of molecula r oxygen with spin labels, using line broadening electron paramagnetic resonance (EPR) spectroscopy. Stearic acid spin labels were used to p robe the local oxygen diffusion-concentration product. The free radica l moiety was located at various distances from the membrane surface, a nd collision rates were estimated from linewidths of the EPR spectra m easured in the presence and absence of molecular oxygen. The profile o f the local oxygen diffusion-concentration product across the membrane determined at 20 degrees C demonstrates that this product, at all mem brane locations, is higher than the value measured in water. From the profile of the oxygen diffusion-concentration product, the membrane ox ygen permeability coefficient has been estimated using the procedure d eveloped earlier (W.K. Subczynski, J.S. Hyde, A. Kusumi, Proc. Natl. A cad. Sci. USA 86 (1989) 4474-4478). At 20 degrees C, the oxygen permea bility coefficient for the lipid portion of the thylakoid membrane was found to be 39.5 cm s(-1). This value is 20% higher than the oxygen p ermeability coefficient of a water layer of the same thickness as the thylakoid membrane. The high permeability coefficient implies that the oxygen concentration difference across the thylakoid membrane generat ed under the illumination of the leaf by saturating actinic light is n egligible, smaller than 1 mu M. (C) 1998 Elsevier Science B.V. All rig hts reserved.