Oxygen toxicity, protecting cells

Catalase is one of the oldest known enzymes and was developed when life became aerobic, i.e. when organisms started to use oxygen. When a cell uses oxygen in its metabolism HP is very often produced as a by-product. However HP is toxic to the cells, so they need some defence mechanism. Catalase was invented by the evolution to protect living cells from HP. 

The O2 molecule is essential to all aerobic forms of life, but many anaerobic organisms (e.g. anaerobic bacteria such as Clostridia spp.) are killed after only brief exposures to molecular O2. However, it is well established that even aerobic organisms, including man and other animals, show signs of oxygen toxicity when exposed to O2 tensions above those normally found in air (i.e. >21% O2). Such toxicity does not normally occur because aerobic cells possess protective enzymes that prevent either the formation or the accumulation of oxygen metabolites. It is only when these protective systems be-... 

Since free radical accumulation was proposed to mediate Ap toxicity,27 we studied the effects of EGCG on intracellular reactive oxygen species (ROS) using DCF assay. As expected, a 24-hour exposure to Ap, 42 resulted in a small but significant increase in DCF fluorescence (+18% relative to control), which was reduced by EGCG (10 xM) and by EC (101xM), a tea catechin that failed to protect cells. 

Aerobic organisms are protected from oxygen toxicity by three enzymes glutathione peroxidase, catalase, and superoxide dismutase. Superoxide dismutases are metal-loenzymes (the cytoplasmic enzyme contains Cu + and Zn +, whereas the mitochondrial enzyme contains Mn +l widely distributed in aerobic cells. The role of superoxide dismutases in preventing oxygen toxicity is still controversial. For example, some aerobic cells (e.g., adipocytes and some bacteria) lack superoxide dismutase, whereas some strict anaerobes possess this enzyme. 

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