Lipid autoxidation pathways

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). 

Volatile compounds formed by anabolic or catabolic pathways include fatty acid derivatives, terpenes and phenolics. In contrast, volatile compounds formed during tissue damage are typically formed through enzymatic degradation and/ or autoxidation reactions of primary and/or secondary metabolites and includes lipids, amino acids, glucosinolates, terpenoids and phenolics. 

N-Heterocyclics. The reaction of primary amines with the carbonyl products derived from lipid oxidation is a major pathway in lipid-protein interactions. Formation of Schiff s base intermediates followed by cyclization and rearrangement can yield imines, pyridines and pyrroles (5,15,30,31). For example, 2-pentylpyridine may result from the reaction of ammonia with 2,4-decadienal, one of the principle aldehydes from the autoxidation of linoleate (5). 

Reduced transition metal complexes are traditionally implicated as the initiators for the autoxidation of fats, lipids, and foodstuffs. However, whether this involves direct activation of O2 or of reduced dioxygen (02 -, H00% and HOOH) is unclear. Reduced metal plus HOOH yields HO via Fenton chemistry and probably is the pathway for initiation of autoxidation in many systems. Although there has been an expectation that one or more of the intermediates from the autoxidation of reduced transition metals (equation 112) can act as the initiator for the autoxidation of organic substrates, direct experimental evidence has not been presented. 

Ketones Aliphatic ketones formed by autoxidation of lipids also contribute to the flavor of oils and food products. For example, Guth and Grosch (13) identified l-octen-3-one as one of the odor-active compounds in reverted soybean oil. This compound was described as metallic and mushroom-like. The reaction pathway for the formation of l-octen-3-one from the linoleate-10-hydroperoxide via the p-scission route is illustrated in Figure 2. 10-Hydroperoxide of linoleate is not the usual hydroperoxide formed by autoxidation of linoleate however, it is one of the major hydroperoxides formed by the photosensitized oxidation (singlet oxygen reaction) of linoleate (14). 

Lipid oxidation is a major pathway for the formation of volatile compounds in roasted tree nuts. After the enzymes in the nuts (e.g., lipoxygenase) are inactivated by the high temperatures used for roasting, autoxidation becomes the principal source of lipid breakdown [66]. Lipid degradation reactions are not necessarily deleterious to flavor [67], and some aldehydes and ketones (e.g., n-aldehydes and 2-aIkanones) produced during roasting may impart desirable sweet, fruity, and pungent aroma notes to roasted tree nuts. 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

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