Oxidation of unsaturated lipids

N.A. Porter, S.E. Caldwell, K.A. Mills, Mechanisms of free radical oxidation of unsaturated lipids, Lipids, 30, 277 290 (1995). 

It is well known that neutrophils, monocytes, macrophages, and other phagocytes produce superoxide upon activation with various stimuli and therefore, are potential initiators of lipid peroxidation. In 1985, Carlin and Arfors [75,76] showed that leukocytes initiate the oxidation of unsaturated lipids. Surprisingly, the leukocyte-initiated peroxidation of linoleic acid was not inhibited by SOD and, therefore, apparently was not initiated by superoxide, while liposome peroxidation was mediated by superoxide. No convincing explanations were given. 

LPO causes non-enzymic oxidation of unsaturated lipids, probably acting through its haem group the heat-denatured enzyme is more active than the native enzyme. 

On the other hand, unsaturated lipids may act as secondary mediators of the MnP/Mn system, in a mechanism known as lipid peroxidation, which is a free radical chain-reaction proceeding via the oxidation of unsaturated lipids [32, 33]. Thiols may also undergo oxidation by MnP/Mn and produce thiyl radicals, which in turn mediate in the oxidation of a variety of compounds [34]. 

Chan, H.W.S. 1987. The mechanism of auto-oxidation. In Auto-oxidation of Unsaturated Lipids (H.W.C. Chan, ed.), pp. 1-16, Academic Press, London. 

The oxidation of unsaturated lipids by radical reactions (Section 15.11)... 

Antioxidant activity of a given compound is assessed as resistance to oxidation of lipids in the presence of that particular compound. Therefore, most of the methods described and used to assess antioxidant activity follow oxidation and the stages of oxidation of unsaturated lipid substrates. Many techniques have been developed to determine the antioxidant efficacy of the compounds of interest, but all of these have to be employed and interpreted carefully. Frankel (19, 46) has listed following parameters that are fairly important in choosing methods to evaluate antioxidants. 

Porter, N.A., Caldwell, S.E., and Mills, K.A. 1995. Mechanisms of Free Radical Oxidation of Unsaturated Lipids. Lipids. 30 277-290. 

Undesirable changes in nutritional quality of foods are initiated by the autoxidation or enzymic oxidation of unsaturated lipids to lipid hydroperoxides. Lipid hydroperoxides and their products of decomposition can react with food components, such as amino acids, proteins and certain other biochemicals. These reactions and the potential role of hydroperoxides in causing mutagenicity are reviewed. 

Foods of animal origin are suspected to contain some amount of COP formed by autoxidation. Cholesterol autoxidation is a well-established free radical process that involves the same chemistry that occurs for the oxidation of unsaturated lipids. Cholesterol contains one double bond at the carbon-5 position therefore, the weakest points in its structure are at the carbon-7 and carbon-4 positions. However, due to the possible influence of the hydroxyl group at carbon-3 and the tertiary carbon atom at carbon-5, the carbon-1 position is rarely attacked by molecular oxygen, and therefore the abstraction of an allylic hydrogen predominantly occurs at carbon-7 and gives rise to a series of A and B ring oxidation products. In the chain reaction,... 

Grosch, W. (1987) Reactions of hydroperoxides - products of low molecular weight, in Aut-oxidation of Unsaturated Lipids (ed. H. W. S. Chan), Academic Press, London, pp. 95-139. 

The direct oxidation of unsaturated lipids by triplet oxygen ( O ) is spin forbidden because the lipid ground state of singlet multiplicity has an opposite spin direction from that of oxygen of triplet multiplicity. This spin barrier between lipids and oxygen can be readily overcome in the presence of initiators that can produce radicals by different mechanisms. 

The mechanism of initiation of lipid oxidation has been debated for many years. The most likely initiation process is the metal-catalysed decomposition of preformed hydroperoxides. The thermal oxidation of unsaturated lipids is usually autocatalytic and involves initiation by decomposition of hydroperoxides, which is generally considered metal-catalysed because it is very difficult or nearly impossible to eliminate trace metals that act as potent catalysts for reactions (3) and (4). 

This is the most widely occurring oxidation and describes the first stages of the oxidation of unsaturated lipids producing hydroperoxides as the fundamental primary products. Because this step is slow and rate-determining, hydrogen abstraction from unsaturated lipids becomes selective for the most weakly bound hydrogen. 

To exclude oxidation of unsaturated lipids during handling the addition of anti-oxidants (hydrochinone, butylated hydroxy toluene, butyl hydrochinone) is recommended (Lewis 1958, Neudoeeffee and Lea 1966). 

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