[Quenching of singlet oxygen with chlorophylls and porphyrins]

The rate constants (kox) of irreversible oxidation of chlorophylls and porphyrins by 1 delta g state of oxygen (1O2) have been determined. It has been shown that kox values are 1-5 orders of magnitude lower than the values of rate constants of quenching 1O2 (kq) with the same pigments. The correlation between kox and kq has been found. Its analysis allowed to conclude that oxidation proceeds via intermediate formation of a charge transfer complex between the pigments and 1O2. A kinetic scheme of the processes leading to physical and chemical quenching 1O2 with chlorophylls and porphyrins is proposed, its mathematical analysis is carried out. The results of this analysis can be used for a search of substances capable of effective quenching 1O2 without destruction of the quenchers. It is concluded that chlorophylls protect photosynthetic membranes from the destruction by 1O2, and some synthetic metal-porphyrins can be used as good protectors of different systems against photooxidation.

Damage of photoreceptor membrane lipids and proteins induced by photosensitized generation of singlet oxygen

It has been shown that illumination of rod outer segment suspension in the presence of photosensitizers (methylene blue lambda greater than or equal to 620 nm; retinal 370 less than or equal to lambda less than or equal to 390 nm) results in chemical modification of the lipid and protein components of the photo-receptor membranes. This modification can be registered by accumulation of lipid peroxidation (LPO) products as well as oligomerization of rhodopsin and a decrease of rhodopsin thermal stability. These effects are prevented by 'O2-quenchers and free radical scavengers. It has been found that the electric activity (ERG) of isolated frog retina is inhibited due to photosensitized generation of 'O2 which can be overcome by preliminary addition of 'O2-quenchers and free radical scavengers to the incubation medium. The LPO products are accumulated in the retinae of rats exposed to high intensity light in vivo. It is concluded that 'O2 and LPO are involved in light-induced damage of the retina.

[Oxidation of NADH by singlet oxygen generated by triplet flavin]

Mechanisms of NADH oxidation in the presence of flavines in the light under aerobic and anaerobic conditions has been studied by spectrophotometry method and by registering the kinetics of changes in pH, luminescence and oxygen content in solution. It is shown that singlet oxygen O2* generated by triplet flavine may be an agent that immediately oxidizes NADH under aerobic conditions. The rate constant of the reaction of O2* with NADH accounts for about 1.9 . 10(9) M-1 s-1 and maximum quantum yield of NADH photooxidation in the presence of flavine mononucleotide is 0.7. During aerobic oxidation of NADH the flavine molecules function as photosensitizers rather than as carriers of reduced equivalents.

Hydrogen peroxide causes the fatal injury to human fibroblasts exposed to oxygen radicals

Oxygen radicals are suspected as being a cause of the cellular damage that occurs at sites of inflammation. The phagocytic cells that accumulate in areas of inflammation produce superoxide, hydrogen peroxide, hydroxyl radical, and probably singlet oxygen in the extracellular fluid. The mechanism by which these oxygen molecules kill cells is unknown. To determine which of the oxygen species is responsible for the cellular killing, we exposed human fibroblasts in culture to oxygen radicals generated by the enzymatic action of xanthine oxidase upon acetaldehyde. Using the amount of chromium-51 released from labeled fibroblasts as an index of cellular death, we found that cells were protected only by interventions that reduce hydrogen peroxide concentration. Agents that inactivate superoxide, hydroxyl radical, and singlet oxygen were ineffective in limiting oxygen radical-induced cellular death.