Autoxidation lipid, mechanism

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

Human ceruloplasmin inhibits lipid autoxidation induced by ascorbate or inorganic Fe It is considered an acute-phase protein with a beneficial effect in inflammation . It was suggested that ceruloplasmin acts as a scavenger of OJ radicals, as it inhibited the reduction of Fe(III)-cytochrome c and of nitroblue tetrazolium in the presence of xanthine oxidase, acetaldehyde, and dioxygen as an OJ-generating system A mechanism without reduction of Cu , similar to that... 

The literature dealing with the autoxidation mechanism involved in lipid deterioration has been concerned with investigations on pure unsaturated fatty acids and their esters. The reactions involved, however, are representative of those occurring in lipids and lipid-containing food products. 

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. 

For review articles on mechanisms of autoxidation of polyunsaturated lipids see Ref. 158. 

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

Figure 3. Mechanisms of lipid autoxidation and formation of products.

Pryor WA, Stanley JP, Blair E. Autoxidation of polyunsaturated fatty acids 11. A suggested mechanism for the formation of TBA-reactive materials from prostaglandin-Uke endoperoxides. Lipids 1976 11 370. 

During this process, the position and geometry of the double bond may change. The hydroperoxide mixtures produced by autoxidation and photooxidation are not the same, indicating that different mechanisms are involved. Free radical oxidation can be promoted or inhibited. Deliberate promotion speeds the polymerization of drying oils, and strenuous efforts are made to inhibit the onset of rancidity in edible oils. Frankel has recently reviewed this topic in depth (41) see also (1) for an extensive discussion of oxidation of food lipids. 

Since the early 1960s, the understanding of autoxidation of unsaturated lipids and antioxidative mechanisms have advanced significantly as a result of development of... 

In fats and oils, the process of oxidation is similar to that oxidation of any other unsaturated organic material and requires an initiation process, in order to generate free radicals from the substrate. As antioxidants inhibit oxidation or autoxidation process, the mechanism(s) involved need(s) to be discussed. Figure 1 explains the relationship of antioxidant activity and oxidation of a lipid as examined by a typical evaluation method. 

The majority of compounds that are subject to autoxidation are unsaturated substances or highly condensed polymers, and the target of ROS is commonly the diene fimctionality—or double bonds. Autoxidation of unsaturated lipids is a good model to demonstrate this mechanism. The initial reaction between molecular oxygen and a polyunsaturated fatty acid (PUFA) occurs as RH -l- O2 —> ROOH, which involves the movement of a double bond as well as the insertion... 

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. ... 

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