Lipid peroxy radicals

Lipid peroxides Lipid peroxy radicals Lipid alkoxyl radicals Aldehydes... 

Vitamin E is an effective scavenger of lipid peroxy radicals and is efficient at protecting unsaturated fatty acids against lipid peroxidation. The chemistry of vitamin E is rather complex as there are eight compounds, four tocopherols and four tocotrienols, which exhibit vitamin E activity. The relative bioactivity of the various compounds varies considerably, from 1.0 for a-tocopherol to 0.03 for 8-tocopherol. In milk, a-tocopherol accounts for virtually all of vitamin E, although very small amounts of (3-tocopherols and y-tocopherols are present. Also, the concentration of a-tocopherol in milk fat varies widely, with the level in samples of commercial butter ranging from 18 to 35 pg/g fat (MAFF, 1999). These concentrations equate to a low level of vitamin E bioactivity, 0.025 to 0.05 IU/g fat. 

In lipid peroxidation, unsaturated fatty acids are peroxidized in the biological membranes of cells and their organelles. This leads to chain breaks in fatty acids with insertion of hydrophilic groups and cis-trans-iso-merization. Membrane-bound proteins are damaged. Lipid radicals (L ) are transformed into unstable lipid-peroxy radicals (LOO ). Lipid peroxidation markers include malondialdehyde and 4-hydroxynonenal. (18, 38, 79, 97) (s. figs. 2.19 21.12)... 

Once formed, polyunsaturated fatty acids (PUFA) can undergo double bond rearrangement to produce a diene conjugated molecule, which enhances stability of the radical intermediate. In the presence of oxygen, chain propagation can occur via lipid peroxy radical formation (LOO ), and initiation of an autocatalytic cycle as outlined below in Eqs. 11 to Eq. 13 [76] ... 

Lipid peroxy radicals will react further to yield cyclic peroxides, cyclic endoperoxide and finally to form aldehydes, including malondialdehyde [77] or other end-products such as isoprostanes or pentane and ethane [78,79]. 

A. Membrane Attack Formation of Lipid and Lipid Peroxy Radicals... 

Vitamin E is an efficient antioxidant and nonenzymatic terminator of free radical chain reactions, and has little pro-oxidant activity. When Vitamin E donates an electron to a lipid peroxy radical, it is converted to a free radical form that is stabilized by resonance. If this free radical form were to act as a pro-oxidant and abstract an electron from a polyunsaturated lipid, it would be oxidizing that lipid and actually propagate the free radical chain reaction. The chemistry of vitamin E is such that it has a much greater tendency to donate a second electron and go to the fully oxidized form. 

Peroxy radicals Lipid peroxy radicals generated by Reaction A are important in propagating other radicals by H-abstraction (reverse of Reaction A). Weakly bonded hydrogens are particularly susceptible to abstraction by the peroxy radical. The radical generated in this way can become oxidized via Reaction C. 

Peroxy radicals can react by yet other competing routes. For example, evidence for lipid peroxy radical combination through a tetraoxide has been reported recently (8). Such tetraoxides could generate singlet oxygen and nonradical products by the Russell mechanism (9) as shown in Reaction D. 

Lipid peroxidation in microsomal fraction initiated by hydroperoxides or iron/ascorbate, as well as lipid peroxidation of isolated hepatocytes under oxygenation, has been studied extensively by Cadenas et al. (1981) and Cadenas and Sies (1982). The available evidence points to 02 formation by the Russell mechanism (Russell 1957, Howard and Ingold 1968). Secondary lipid-peroxy radicals react via a transient tetroxide to yield an alcohol, a carbonyl and oxygen, the carbonyl or oxygen being in an electronically excited state. 

The composition and content of the different tocopherol components in plant tissue vary considerably, ranging from extremely low levels found in potato tubers to high levels found in oil seeds. a-Tocopherol is the predominant form in photosynthetic tissues and is mainly localized in plastids. The particular enrichment in the chloroplast membranes is probably related to the ability of tocopherols to quench or to scavenge reactive oxygen species and lipid peroxy radicals by physical or chemical means. In this way, the photosynthetic apparatus can be protected from oxygen toxicity and lipid peroxidation. In nonphotosynthetic tissues, 7-tocopherol frequently predominates and can be involved in the prevention of autoxidation of polyunsaturated fatty acids. 

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