Lipid peroxidation reactions

While it is generally acknowledged that lipid peroxidation reactions can be extremely complex, involving many components and reaction products, the most important reactions are readily classified according to Scheme 2.1. 

In this section, the general principles of lipid peroxidation reactions, which are well established, are discussed first and specific mechanisms, which may be relevant in vivo, are considered later. 

Different mechanisms to explain the disinfection ability of photocatalysts have been proposed [136]. One of the first studies of Escherichia coli inactivation by photocatalytic Ti02 action suggested the lipid peroxidation reaction as the mechanism of bacterial death [137]. A recent study indicated that both degradation of formaldehyde and inactivation of E. coli depended on the amount of reactive oxygen species formed under irradiation [138]. The action with which viruses and bacteria are inactivated by Ti02 photocatalysts seems to involve various species, namely free hydroxyl radicals in the bulk solution for the former and free and surface-bound hydroxyl radicals and other oxygen reactive species for the latter [139]. Different factors were taken into account in a study of E. coli inactivation in addition to the presence of the photocatalyst treatment with H202, which enhanced the inactivation... 

Jonhnson, H.G., McNee, M.L. and Braughler, J.M. (1987). Inhibitors of metal catalyzed lipid peroxidation reactions inhibit mucus secretion and 15 HETE levels in canine trachea. Prostaglandins Leukotrienes and Med. 30 123-132. 

The amount of TBA reactant can be converted into moles of malondialdehyde by the extinction coefficient of 155 mM cm ozone (22), and the yield of this reaction, or ratio of malondialdehyde/ozone taken-up, is shown in Figure 9. Notice that the yield for ozone-treated linolenic acid varies with time of reaction from about 3% to over 30%. These results differ from those for lipid peroxidation reactions which also give rise to malondialdehyde but have yields of 2-5% (23). 

Cleavage of the carbon bonds during lipid peroxidation reactions results in the formation of aldehydic products such as cytotoxic alkanals and alkenals, as well as alkanes. The breakdown products of lipid peroxidation, alkanals such... 

Central nervous tissue appears to provide an especially avid environment for the occurrence of oxygen-radical generation and lipid-peroxidative reactions due to a high content of polyunsaturated fatty acids. There is now considerable biochemical, physiological and pharmacological data that supports such a connection between radical reactions and secondary CNS tissue injury. 

Braughler, J.M. and Pregenzer, J.F. (1989) The 21-aminosteroid inhibitors of lipid peroxidation reactions with lipid peroxyl and phenoxyl radicals, Free Radical Biol. Med. 7, 125-130. 

These LO or LOO radicals initiate the lipid-peroxidation reaction in which the polyunsaturated fatty acids (mainly 18 2 and 20 4) of the LDL particle are rapidly oxidised to lipid hydroperoxides. 

Enzymic lipid peroxidation reactions are a common reaction in mammalian cells as response to stress from outside. In contrast to plants mammalian cell membranes contain arachidonic acid instead of linolenic acid. The latter is known to generate by LPO processes physiological potent compounds prostaglandines, prostacyclines, thromboxanes and leukotiienes [221]. In addition a great number of other LPO products derived not only from arachidonic acid but also from linoleic acid have been detected in blood, especially low density lipoprotein (LDL) and tissue samples of humans [222-227]. These are increased especially in diseases combined with cell degradation [228], indicating that cell death in plant and mammalian cells is connected with a similar cascade of events. 

As indicated above, the majority of reactive oxygen species are capable of eliciting general lipid peroxidation reactions, where the products described so far do not give any indication of the reactive species involved. In contrast, the hypohalous acids show reactivity with membrane phospholipids, with the generation of specific haloamines and halohydrins. Specifically, hypohalous acids can add across an unsaturated C=C bond within a fatty acid chain to yield alpha beta chlorohydrin and bromohydrin isomers [8] as shown in Scheme 1. 

Step 1 Lipid peroxidation reactions begin after a hydrogen atom is extracted from an unsaturated fatty acid (LH —> L ). Step 2 The lipid radical (L ) then reacts with 02 to form a peroxyl radical (L + 02 —> L—O—O ). Step 3 The radical chain reaction begins when the peroxyl radical extracts a hydrogen atom from another fatty acid molecule (L—O—O + L H —> L—O—OH + L ). Step 4 The presence of a transition metal such as Fe2+ initiates further radical formation (L—O—O—H + Fe2 + — LO4 + HO "... 

Chloroplast thylakoid membranes are composed of an almost equal percentage of protein and lipid. The acyl lipids of the chloroplast membrane are highly unsaturated for example, around 80% of the fatty acid component is the 18 3 unsaturated linolenic acid. The consequences of lipid peroxidation reactions are... 

Yamamoto. S. Enzymatic lipid peroxidation, reactions of mammalians lipoxygenases. Free Radical Biology and Medicine, 10,149-159.1991. 

Because a number of vitamin E deficiency syndromes are responsive to dietary selenium, the possibility of selenium functioning as a lipid antioxidant has been examined. Some very powerful selenium antioxidants are known (Woodbridge, 1959). In the vitamin E-deficient chick, dietary selenite inhibited in vivo and in vitro lipid peroxidation (Zalkin et al., 1960). Further, antioxygenic activity was found for various selenium compounds in simple lipid peroxidation reactions selenoamino acids were weak antioxidants. On the basis of this evidence and the known incorporation of selenium into tissue proteins, Zalkin et al. (1960) suggested that the antioxidants formed in the chick may be selenoamino acids and selenoproteins. Bieri (1961) and Bieri et al. (1961) have reported extensive evidence for inhibition of in vitro lipid peroxidation of liver, kidney, and heart tissue from selenium fed chickens. They also suggested that the lipid antioxidant may be a selenoprotein. Olcott et al. (1961) showed that selenomethionine is a stronger lipid antioxidant than methionine and that selenomethionine decomposes lipid peroxides. 

With the main exception of some nonenzymatic peroxidation reactions on oils or membrane lipids (in which total lipid oxidation is involved), most areas of acyl lipid oxidation in plants are concerned with the fatty acid components, either as free fatty acids or as activated thioester forms. Lipid peroxidation reactions are outside the scope of this chapter but are covered by Galliard and Chan (this volume, Chapter 5). The following discussion will relate mainly to fatty acid oxidation. 

Yamamoto, S. (1991) Enzymatic lipid peroxidation reactions of mammalian lipoxygenases. Free Radical Biol. Med. 10, 149-159. 

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