The most important electron acceptor in the biosphere is molecular oxygen which, by virtue of its bi-radical nature, readily accepts unpaired electrons to give rise to a series of partially reduced species collectively known as reduced (or ‘reactive’) oxygen species (ROS). These include superoxide (O·-2), hydrogen peroxide (H2O2), hydroxyl radical (HO·) and peroxyl (ROO·) and alkoxyl (RO·) radicals which may be involved in the initiation and propagation of free radical chain reactions and which are potentially highly damaging to cells. Mechanisms have evolved to restrict and control such processes, partly by compartmentation, and partly by antioxidant defences such as chain-breaking antioxidant compounds capable forming stable free radicals (e.g. ascorbate, α-tocopherol) and the evolution of enzyme systems (e.g. superoxide dismutase, catalase, peroxidases) that diminish the intracellular concentration of the ROS. Although some ROS perform useful functions, the production of ROS exceeding the ability of th...

Free Radicals in Biology: Oxidative Stress and the Effects of Ionizing Radiation

Redox and addition chemistry of quinoid compounds and its biological implications

The symptoms of injury in microsomal membranes isolated from crowns of seedlings of Triticum aestivum, L. cultivar Fredrick after a lethal freeze-thaw stress included an increased lipid phase transition temperature, loss of lipid phosphate (lipid-P), and increased free fatty acid levels. However, minimal changes in fatty acid saturation were observed, suggesting minimal amounts of lipid peroxidation. All of these injury symptoms, including the lack of lipid peroxidation, were simulated in vitro by treatment of isolated membranes with oxygen free radicals, generated from either xanthine oxidase (EC 1.1.3.22) or paraquat (l,r-dimethyl-4,4′-bipyridinium dichloride). Further evidence indicating a relationship between free radicals and freezing injury comes from the observation that both protoplasts and microsomal membranes isolated from wheat seedlings, that had been acclimated to induce freezing tolerance, also had increased tolerance of oxygen free radicals, and contained higher lipid-soluble antioxidant levels, than those from non-acclimated seedlings. Lipid-soluble antioxidants accumulated in the crown tissue of the wheat seedling during the acclimation period. Freezing stress accelerated the formation of oxygen free radicals. Membranes isolated from crowns after a freeze–thaw stress tended to produce higher levels of superoxide as shown by the reduction of Tiron (1,2-dihydroxy-l,3-benzenedisulfonic acid). In protoplasts, increased superoxide production coincided with lethal freezing injury. These results are discussed in terms of the possible involvement of oxygen free radicals in mediating aspects of freezing injury to cell membranes.

Free radical and freezing injury to cell membranes of winter wheat

Radicals are frequently formed during the metabolism of ’foreign compounds’ by cells, as perusal of a recent volume clearly indicates.6 Are the radicals important in producing the toxicity of these compounds ? In some cases, one can be fairly sure that the answer is ’yes’. Thus the damage produced to animal and plant tissues by the herbicide paraquat seems to arise because paraquat increases intracellular formation of superoxide (02-) and hydrogen peroxide (H202).7 Phenylhydrazine-derived radicals may well mediate the toxicity of this substance to erythrocytes.~ 4

Free Radicals and Antioxidant Protection: Mechanisms and Significance in Toxicology and Disease:

Complementary hydroxylation assays and stopped-flow e.s.r. techniques have been employed in the investigation of the effect of various iron chelators (of chemical, biological and clinical importance) on hydroxyl-radical generation via the Haber-Weiss cycle and the ascorbate-driven Fenton reaction. Chelators have been identified which selectively promote or inhibit various reactions involved in hydroxyl-radical generation (for example, NTA and EDTA promote all the reactions of both the Haber-Weiss cycle and the ascorbate-driven Fenton reaction, whereas DTPA and phytate inhibit the recycling of iron in these reactions). The biological chelators succinate and citrate are shown to be relatively poor catalysts of the Haber-Weiss cycle, whereas they are found to be effective catalysts of .OH generation in the ascorbate-driven Fenton reaction. It is also suggested that continuous redox-cycling reactions between iron, oxygen and ascorbate may represent an important mechanism of cell death in biological systems.

Model studies of the iron-catalysed Haber-Weiss cycle and the ascorbate-driven Fenton reaction.

Exposure of isolated rat hepatocytes to toxic doses of menadione (2-methyl-1,4-naphthoquinone) results in enhanced formation of active oxygen species, depletion of cellular glutathione and protein thiols, and perturbation of intracellular calcium ion homeostasis. An increase in cytosolic Ca2+ concentration, resulting from inhibition of the plasma membrane Ca2+ translocase by menadione metabolism, appears to be critically involved in the development of cytotoxicity.

Orrenius S

Papers

Oxidative stress studied in intact mammalian cells.