Lipid molecules, autoxidation

The hydrocarbon chains of fatty acids often contain one or more cis double bonds. These unsaturated lipids are especially prone to autoxidation. An example is provided by the following equation, which shows an autoxidation reaction of the ester of a-linoleic acid. Note that this is a chain reaction, so one initiator can cause numerous lipid molecules to be oxidized. 

The most widely used antioxidants are free radical scavengers that remove reactive radicals formed in the initiation and propagation steps of autoxidation. A number of natural or synthetic phenols can compete, even at low concentrations, with lipid molecules as hydrogen donors to hydroperoxy and alkoxy radicals, producing hydroperoxides and alcohols and an unreactive radical. (3-carotene reacts with per-oxy radicals, producing a less-reactive radical. These stabilized radicals do not initiate or propagate the chain reaction. 

Scheme 1. Possible reactions of the autoxidation process. R is an alkyl group of an unsaturated lipid molecule. H is an a-methylenic hydrogen atom easily detachable because of the activating influence of the neighboring double bond or bonds. RO is alkoxy radical, ROO" is peroxy radical, and is an initiator.

Autoxidation is the process in foods and bulk lipids, which leads to rancidity. Rancidity is the spoiled off-flavor obtained by subjective organoleptic appraisal of food. Autoxidation is the oxidative deterioration of unsaturated fatty acids via an autocatalytic process consisting of a free radical mechanism. This indicates that the intermediates are radicals (odd electron species) and that the reaction involves an initiation step and a propagation sequence, which continues until the operation of one or more termination steps. Autoxidation of lipid molecules is briefly described by reactions 1-3. ... 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

In the coming years, it is clear that the toxicity of the products of lipid autoxidation will come under intense scrutiny that is, not only are the free radicals that are produced during the autoxidation toxic, but some of the stable molecules also are, and these products can explain the occurrence of damage at some distance from the site of autoxidation (fil). 

Lipid peroxidation is a special form of autoxidation, where the primary target molecules are polyunsaturated fatty acids and their derivatives [15]. These compounds are more susceptible to peroxidation than cholesterol due to the presence of double-allylic hydrogens, which are easily extracted as a result of stabilization of the radical formed. Cholesterol, a major component in cell membranes together with the polyunsaturated membrane lipids, becomes a second target of the radical chain reaction leading to the formation of compounds 1-8 [15]. 

The basic chemistry of enzyme catalyzed oxidation of food lipids such as in cereal products, or in many fruits, and vegetables is the same as for autoxidation, but the enzyme lipoxygenase (LPX) is very specific for the substrate and for the method of oxidation." Lipoxygenases are globulins with molecular weights ranging from 0.6-1 x 10 Da, containing one iron atom per molecule at the active site. 

The mechanism of the antioxidant effect of vitamin E is similar to the effect of other lipophilic antioxidants. Tocopherols react with a number of free radicals including active oxygen species. One tocopherol molecule can react with two hydroperoxyl radicals. Autoxi-dation of lipids is inhibited by reaction of tocopherols (abbreviated as T-OH) with hydroperoxyl lipid radicals (R-O-O ) with the formation of hydroperoxides (R O-OH) and radicals of tocopherols (tocopheroxyl radicals, T O ). This reaction interrupts the radical chain autoxidation reaction of Hpids during the propagation phase ... 

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