Promotion of Peroxidation and Hydroperoxidation of Lipids

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Polyunsaturated fatty acids (PUFAs) are abundant in cellular membranes and in low-density lipoproteins (LDL) (Dekkers et al. 1996). The PUFAs allow for fluidity of cellular membranes. A free radical prefers to steel electrons from the lipid membrane of a cell, initiating a free radical attack on the cell known as lipid peroxidation. Lipid peroxidation, a well-established mechanism of cellular injury in plants and animals, is used as an indicator of oxidative stress in cells and tissues. Lipid peroxides are unstable and decompose to form a complex series of compounds including reactive carbonyl compounds. Polyunsaturated fatty acid peroxides generate malondialdehyde (MDA) and 4-hydroxyalkenals (HAE) upon decomposition, and the measurement of MDA and HAE has been used as an indicator of lipid peroxidation. Reactive oxygen species target the carbon–carbon double bond of polyunsaturated fatty acids. The double bond on the carbon weakens the carbon–hydrogen bond allowing for easy dissociation of the hydrogen by a free radical. A free radical will steal the single electron from the hydrogen associated with the carbon at the double bond. In turn this leaves the carbon with an unpaired electron and hence becomes a free radical. In an effort to stabilize the carbon-centered free radical, molecular rearrangement occurs. The newly arranged molecule is called a conjugated diene (CD). The CD then very easily reacts with oxygen to form a peroxyl radical. The peroxyl radical steals an electron from another lipid molecule in a process called propagation. This process then continues in a chain reaction (Halliwell and Gutteridge 1985).