Lipid oxidation initiation

Figure 2. Formation of ferryl iron in initiation and catalysis of lipid oxidation Reaction schemes lor formation of hypervalent iron states by FeF and Fe complexes and subsequent reactions leading to radicals that can initiate lipid oxidation. L, metal ligand R, alkyl or acyl group. Fe + sequence (71, 73) Fe sequence (81), adapted.

The principal light-absorbing groups of lipids are double bonds, peroxide 0—0 bonds, and carbonyls the last two are most important. The primary mechanism by which ultraviolet radiation initiates lipid oxidation is actually indirect, mediated through homolytic scission of any preformed hydroperoxides to generate the true initiators— LO, HO, and RO —that abstract hydrogens from lipid molecules and form the ab initio L. ... 

A second source of UV-induced radicals to initiate lipid oxidation is excitation of carbonyl compounds (88). The carbonyl n ti transition (340 kJ/mol) occurs when light is absorbed at 350 nm and lower wavelengths (87). 

Figure 3. Heme-catalyzed formation of species that can initiate lipid oxidation generation and reaction of ferryl iron complexes [Fe/ = O, Fe (OH)]. Adapted (143, 160) used with permission.

TABLE 2. Lifetimes and Hydrogen Abstraction Rates of Various Radicals that Initiate Lipid Oxidation. 

In some cases reduction of ferric iron can be accomplished by enzymically utilizing electrons from NADH and, to a lesser extent, NADPH through an enzymic system associated with both the sarcoplasmic reticulum and mitochondria. Ferrous iron can activate molecular oxygen by producing superoxide. Superoxide may then undergo dismutation spontaneously or by the action of SOD and produce the hydrogen peroxide that can interact with another atom of ferrous iron to produce the hydroxyl radical. The hydroxyl radical can initiate lipid oxidation. It is generally accepted that ferrous iron is the reactive form of iron in oxidation reaction. Since it is likely that most iron ordinarily exists in the cell as ferric iron, the ability to reduce ferric to ferrous iron is critical. 

The many methods to initiate lipid peroxidation in vitro, such as azo initiators, metal ions, pulse radiolysis, photoinitiation (Type I), enzymes (oxidases), to mention a few, have been reviewed . However, as Bucala emphasized in a review ", oxidation initiation is a pivotal first step and there is little understanding of how initiation proceeds in vivo. Transition metal ions, iron or copper, are frequently used to initiate lipid oxidation, but free (unchelated) redox-active transition metals are virtually absent from biological systems" and appear to have little bearing on known pathological processes ". 

The ferry 1-Fe" is slowly reduced to the met-form. If the globin structures of deoxy-, oxy-or met-forms of myoglobin are disturbed, they are reversibly or irreversibly converted into hemochrome-Fe + or hemichrome-Fe. The role of these denatured species in lipid oxidation of meat is not clear. When ferric hemin is detached from the globin, the ferryl ion can initiate lipid oxidation by hydrogen abstraction to produce a lipid radical plus a proton. Iron can also be released from hemin in the presence of hydroperoxides to participate in lipid oxidation processes. Ascorbic acid and other reducing compounds in muscle cytosol effectively inhibit lipid oxidation promoted by ferryl ions in membranes. 

Whether lipid oxidation in muscle foods is catalysed by the iron redox cycle or by formation of the ferryl ions is not clear. However, ferrous ions react with lipid hydroperoxides much faster than with hydrogen peroxide. As shown above, if the reaction of metmyoglobin with hydroperoxides produces ferryl radicals capable of initiating lipid oxidation, it is necessary to prevent the formation of metmyoglobin or methemoglobin. At acidic pH, ferric myoglobin can initiate lipid oxidation in the presence of lipid hydroperoxides. 

SM van Ruth, JP Roozen, M Posthumus, FJHM Jansen. Volatile composition of sunflower oil-in-water emulsions during initial lipid oxidation Influence of pH. J Agric Food Chem 47 4365 369, 1999. 

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