Superoxide anion permeability of phospholipid membranes and chloroplast thylakoids

The permeability of phospholipid membranes to the superoxide anion (O₂^?) was determined using soybean phospholipid vesicles containing FMN in the internal space. The efflux of O₂^? generated by the illumination of FMN was so slow that more than 90% of the radicals were spontaneously disproportionated within the vesicles before they could react with cytochrome c at the membrane exterior. The amount of diffused O₂^? was proportional to the intravesicular concentration of O₂^? over a range from 1 to 10 μm which was deduced from its disproportionation rate. The permeability coefficient of the phospholipid bilayer for O₂^? was estimated to be 2.1 × 10^?6 cm s^?1 at pH 7.3 and 25 ° C. Superoxide dismutase trapped inside vesicles was not reactive with extravesicular O₂^? unless Triton X-100 was added. O₂^? generated outside spinach chloroplast thylakoids did not interact with superoxide dismutase or cytochrome c which had been enclosed in the thylakoids. Thus, chloroplast thylakoids also showed little permeability to O₂^?.