COMPARISON OF PHOTOSENSITIZED PLASMA-MEMBRANE DAMAGE CAUSED BY SINGLET OXYGEN AND FREE-RADICALS

The efficiency and selectivity of photosensitized damage to membrane f unctions may be influenced strongly by the identity of the initial rea ctive species formed by the photosensitizer. To test this possibility, a phorosensitizer, rose bengal (RE), was used that resides in the pla sma membrane and which generates singlet molecular oxygen (O-1(2).) up on excitation with visible light, and radicals plus O-1(2). upon excit ation with UV radiation. With this approach, O-1(2). and radicals are formed at the same locations in the plasma membrane. The response of t hree plasma membrane functions, namely, proline transport, membrane po tential, and membrane impermeability to charged dye molecules, was ass essed. The efficiencies of the responses in the presence and absence o f oxygen were compared per photon absorbed by RE at two wavelengths; 3 55 nm (UV excitation) and 532 nm (visible excitation). The efficiency of oxygen removal before irradiation was assessed by measuring the RE tripler lifetime. The three membrane functions were inhibited more eff iciently at 355 nm than at 532 nm in the presence of oxygen indicating that the radicals are more effective at initiating damage to membrane components than O-1(2). The ratio of photosensitized effects at the t wo wavelengths in the presence of oxygen was the same for two membrane functions but not for the third suggesting that O-1(2). and radicals initiate a common mechanistic pathway for damage to some membrane func tions but not to others. Removing oxygen reduced the efficiency of 355 nm-induced photosensitization by factors of 1.4 to 7. The sensitivity of the three membrane functions to O-1(2).-initiated damage varied ov er a factor of 50 whereas radical initiated damage only varied by a fa ctor of 15. In summary, these results indicate that radicals and O-1(2 ). formed at the same locations in the plasma membrane vary in their e fficiency and specificity for membrane damage but may, in some cases, operate by a common secondary damage mechanism in the presence of oxyg en.