PORPHYRIN-SENSITIZED PHOTOREACTIONS IN THE PRESENCE OF ASCORBATE: OXIDATION OF CELL MEMBRANE LIPIDS AND HYDROXYL RADICAL TRAPS

Abstract— Photooxidation reactions in ascorbate (AH)-containing erythrocyte membrane suspensions have been studied in broad perspective by simultaneously monitoring lipid peroxidation in the membrane compartment and formation of hydrogen peroxide (H₂O₂) and hydroxyl radical (OH) in the aqueous compartment. Non-bound uroporphyrin (UP) and membrane-bound protoporphyrin (PP) were used as sensitizers. Photoreduction of UP to the radical anion (UP^-) was detected by electron spin resonance when UP/AH/membrane mixtures were irradiated anaerobically. Aerobic irradiation resulted in a strong AH^--stimulation of lipid peroxidation, H₂O₂ formation, and OH^- generation (detected with 2-deoxyribose (DOR) and the spin trap 5,5-dimethyl-l-pyrroline-N-oxide). Use of diagnostic agents (e.g. catalase, desferrioxamine, mannitol) revealed that OH^- is involved in light-stimulated DOR oxidation, but not in lipid peroxidation. Similar irradiation in the presence of PP resulted in far greater lipid peroxidation than observed with UP, but less DOR oxidation, and insignificant accumulation of H₂O₂. This suggests that photoreduction of membrane-bound PP is less efficient, possibly due to hindered access of AH^-.