Peroxidation (auto-oxidation) of lipids exposed to oxygen forming peroxides is responsible not only for deterioration of foods (rancidity), but also for damage to tissues in vivo, where it may be a cause of cancer, inflammatory diseases, atherosclerosis, and aging. The deleterious effects are considered to be caused by free radicals, molecules that have unpaired valence electrons, making them highly reactive. Free radicals containing oxygen (eg, ROO^•, RO^•, OH^•) are termed reactive oxygen species (ROS). These are produced during peroxide formation from fatty acids containing methylene interrupted double bonds, that is, those found in the naturally occurring polyunsaturated fatty acids (Figure 1). Lipid peroxidation is a chain reaction in which free radicals formed in the initiation stage in turn generate more (propagation), and thus it has potentially devastating effects. The processes of initiation and propagation can be depicted as follows:

1. Initiation: A free radical (X^•) reacts with a polyunsaturated fatty acid forming the first fatty acid radical (R^•1).

X^• + R¹H→R^•1+XH

2. Propagation: The unstable fatty acid radical R^•1 reacts with oxygen to produce a peroxyl radical (R¹OO^•) which then attacks another fatty acid to form a new fatty acid radical R^•2 and a peroxide of R¹. R^•2 can then react with fatty acid R³H and so on in a chain reaction. Thus one free radical may be responsible for the peroxidation of a very large number of polyunsaturated fatty acid molecules.

 R^•1 + O₂ →R¹OO^•

R¹OO^• +R²H→R¹OOH+R^•2, etc.

 The process may be terminated at the third stage.

 3. Termination: The chain reaction terminates when two radicals react to form a non-radical product (ROOR or RR).

 ROO^•+ROO^•→ROOR+O₂

 ROO^•+R^•→ROOR

 R^• + R^• →RR

Fig1. Lipid peroxidation. The reaction may be initiated by an existing free radical (X^•), by light, or by metal ions. Malondialdehyde is only formed by fatty acids with three or more double bonds and is used as a measure of lipid peroxidation together with ethane from the terminal two carbons of ω3 fatty acids and pentane from the terminal five carbons of ω6 fatty acids.

Antioxidants are used to control and reduce lipid per oxidation, both by humans in their activities and in nature. Propyl gallate, butylated hydroxyanisole (BHA), and butylated hydroxytoluene (BHT) are antioxidants used as food additives. Naturally occurring antioxidants include vitamin E (tocopherol), which is lipid soluble, and urate and vitamin C, which are water soluble. β-Carotene is an antioxidant at low PO₂. Antioxidants fall into two classes: (1) preventive antioxidants, which reduce the rate of chain initiation (stage 1 above) and (2) chain-breaking antioxidants, which interfere with chain propagation (stage 2 above). Preventive antioxidants include catalase and other peroxidases such as glutathione peroxidase that react with ROOH; selenium, which is an essential component of glutathione peroxidase and regulates its activity, and chelators of metal ions such as ethylenediaminetetraacetate (EDTA) and diethylenetriaminepentaacetate (DTPA). In vivo, the principal chain-breaking antioxidants are superoxide dismutase, which acts in the aqueous phase to trap superoxide free radicals (O2 ^•–), urate, and vitamin E, which acts in the lipid phase to trap ROO^• radicals.

Peroxidation is also catalyzed in vivo by heme compounds and by lipoxygenases  found in platelets and leukocytes. Other products of auto-oxidation or enzymic oxidation of physiologic significance include oxysterols (formed from cholesterol) and the prostaglandin-like iso-prostanes (formed from the peroxidation of polyunsaturated fatty acids such as arachidonic acid) which are used as reliable markers of oxidative stress in humans.

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