O

FIGURE 8.4 Under normal situations, 98% of oxygen reduction is catalyzed by the cytochrome oxidase system in mitochondria. (From Angel, M.F., Plast. Reconstruc. Surg., 79, 990, 1987. With permission.)

Typically, oxygen radicals are short lived, and their effects are limited to a radius of a few microns. However, reaction with polyunsaturated fatty acids in the cell membranes generates more stable lipid peroxides that may exert their damage at a distance from the original site. Furthermore, polyunsaturated fatty acids and their phospholipid esters are readily oxidized by a chain reaction mechanism. Once an initiating lipid radical is generated, it attacks another lipid to form a second lipid radical; thus, this lipid peroxidation cycle is propagated21 (Figure 8.5; see also Chapter 3).

Two types of defense systems protect aerobic organisms from oxygen radicals. One system includes the preventative antioxidants, such as catalase and peroxidase, which decompose substrates without the generation of oxygen radicals (Table 8.3).

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