Termination lipid oxidation

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. 

Rubbo, H., Parthasarathy, S., Barnes, S., Kirk, M., Kalyanaraman, B., and Freeman, B. A., 1995, Nitric oxide inhibition of lipoxygenase-dependent liposome and low-density lipoprotein oxidation termination of radical chain propagation reactions and formation of nitrogen-containing oxidized lipid derivatives, Arch. Biochem. Biophys. 324 15-25. 

Part of the problem stems from considering lipid oxidation as precisely following classic free radical chain reactions. To be sure, lipids do oxidize by a radical chain mechanism, and they show initiation, propagation, and termination stages... 

Termination is one of those nebulous handwaving terms used to imply that a process is coming to a close. In hpid oxidation, termination is an even fuzzier concept in that, from a practical standpoint, the lipid oxidation chains probably never fully stop. In addition, a specific radical may be terminated and form some product, but if this occurs by H abstraction or rearrangement, another radical is left behind so the chain reaction continues. Net oxidation slows down when H abstractions or other radical quenching processes exceed the rate of new chain production, but it would be difficult indeed to totally stop the entire radical chain reaction. Thus, in the discussion below, termination refers to an individual radical, not the overall reaction. 

Figure 13. Effects of oxygen and temperature on termination processes in lipid oxidation. Adapted (114).

What is important in the context of termination reactions is that radicals formed in nonlipid molecules combine with lipid radicals to generate co-oxidation products (Reactions 77 and 78) that provide footprints of LOOH reactions (389) and should not be ignored in consideration of lipid oxidation kinetics, mechanisms. 

Lipid oxidation in both food systems and biological tissues exhibit the same temporal three-stage pattern of initiation, propagation, and termination... 

Mitochondria have a highly permeable outer membrane and a protein-enriched Inner membrane that Is extensively folded. Enzymes In the Inner mitochondrial membrane and central matrix carry out the terminal stages of sugar and lipid oxidation coupled to ATP synthesis. 

Freezing is one of the most common preservation methods to maintain the quality of fish and shellfish. Although freezing or frozen storage is able to prevent microbial spoilage effectively, it cannot terminate chemical deteriorations, which mainly involve protein denaturation and lipid oxidation of the products. 

Lipid oxidation is an autocatalysed free-radical chain reaction which is normally divided into three phases initiation, propagation and termination (Figure 3.33). 

During the last steps of lipid oxidation, the fatty acid chains breakdown to give aldehydes (hexanal, propanal, malondialdehyde), depending on the lipid structure. These compounds react with thiobarbituric acid to give coloured compounds the measurement of which at 535 nm can be used to follow the oxidation process in its terminal phase [91]. In addition, hexanal, which is an important decomposition product of n-6 polyunsaturated fatty acid peroxidation in rat liver samples, human red blood cell membranes, and human LDL (low density lipoproteins), can be measured by headspace gas chromatography [92]. Malondialdehyde, another important decomposition product, can also be analysed by GC (Gas Chromatography) [93], and, after reaction with urea to give 2-hydroxypyrimidine, by HPLC [94]. 

The term interfacial oxidation refers to the complex interaction between constituents in multiphase lipid systems in either promoting or inhibiting lipid oxidation. Interfacial oxidation is a surface reaction dependent on the rate of oxygen diffusion and its interactions with unsaturated lipids, metal initiators, radical generators and terminators, all of which are distributed in different compartments of colloidal systems. 

Lipid oxidation is catalysed by transition metals that form compounds in many oxidation states, due to the relatively low reactivity of unpaired d electrons. These compounds, which include mainly iron, copper, manganese, nickel, cobalt and chromium, are reduced by adopting one electron. The last three elements are indeed fairly active, but the level of their active forms is so low that these metals are almost of no significance. Other metals, as free ions or some undissociated salts or complexes, act as catalysts directly or indirectly in the initiation, propagation and termination phase of autoxidation reaction. 

In this reaction scheme, the steady-state concentration of peroxyl radicals will be a direa function of the concentration of the transition metal and lipid peroxide content of the LDL particle, and will increase as the reaction proceeds. Scheme 2.2 is a diagrammatic representation of the redox interactions between copper, lipid hydroperoxides and lipid in the presence of a chain-breaking antioxidant. For the sake of clarity, the reaction involving the regeneration of the oxidized form of copper (Reaction 2.9) has been omitted. The first step is the independent decomposition of the Upid hydroperoxide to form the peroxyl radical. This may be terminated by reaction with an antioxidant, AH, but the lipid peroxide formed will contribute to the peroxide pool. It is evident from this scheme that the efficacy of a chain-breaking antioxidant in this scheme will be highly dependent on the initial size of the peroxide pool. In the section describing the copper-dependent oxidation of LDL (Section 2.6.1), the implications of this idea will be pursued further. 

Mouse peritoneal macrophages that have been activated to produce nitric oxide by 7-interferon and lipopolysac-charide were shown to oxidize LDL less readily than unactivated macrophages. Inhibition of nitric oxide synthesis in the same model was shown to enhance LDL oxidation (Jessup etal., 1992 Yates a al., 1992). It has recently been demonstrated that nitric oxide is able to inhibit lipid peroxidation directly within LDL (Ho etal., 1993c). Nitric oxide probably reacts with the propagating peroxyl radicals thus terminating the chain of lipid peroxidation. The rate constant for the reaction between nitric oxide and peroxyl radicals has recently been determined to be 1-3 X10 M" s (Padmaja and Huie, 1993). This... 

Esterbauer et cil. (1992) have studied the in vitro effects of copper on LDL oxidation and have shown that there are three distinct stages in this process. In the first part of the reaction, the rate of oxidation is low and this period is often referred to as the lag phase the lag phase is apparently dependent on the endogenous antioxidant content of the LDL, the lipid hydroperoxide content of the LDL particle and the fatty acid composition. In the second or propagation phase of the reaction, the rate of oxidation is much faster and independent of the initial antioxidant status of the LDL molecule. Ultimately, the termination reactions predominate and suppress the peroxidation process. The extensive studies of Esterbauer et al. have demonstrated the relative importance of the endogenous antioxidants within the LDL molecule in protecting it from oxidative modification. 

The importance of vitamin E for maintenance of lipid integrity in vivo is emphasized by the fact that it is the only major lipid-soluble chain-breaking antioxidant found within plasma, red cells and tissue cells. Esterbauer etal. (1991) have shown that the oxidation resistance of LDL increases proportionately with a-tocopherol concentration. In patients with RA, synovial fluid concentrations of a-tocopherol are significantly lower relative to paired serum samples (Fairburn et al., 1992). The low level of vitamin E within the inflamed joint implies it is being consumed via its role in terminating lipid peroxidation and this will be discussed further in Section 3.3. 

Esterbauer et al. (1991) have demonstrated that /3-carotene becomes an effective antioxidant after the depletion of vitamin E. Our studies of LDL isolated from matched rheumatoid serum and synovial fluid demonstrate a depletion of /8-carotene (Section 2.2.2.2). Oncley et al. (1952) stated that the progressive changes in the absorption spectra of LDL were correlated with the autooxidation of constituent fatty acids, the auto-oxidation being the most likely cause of carotenoid degradation. The observation that /3-carotene levels in synovial fluid LDL are lower than those of matched plasma LDL (Section 2.2.2) is interesting in that /3-carotene functions as the most effective antioxidant under conditions of low fOi (Burton and Traber, 1990). As discussed above (Section 2.1.3), the rheumatoid joint is both hypoxic and acidotic. We have also found that the concentration of vitamin E is markedly diminished in synovial fluid from inflamed joints when compared to matched plasma samples (Fairburn etal., 1992). This difference could not be accounted for by the lower concentrations of lipids and lipoproteins within synovial fluid. The low levels of both vitamin E and /3-carotene in rheumatoid synovial fluid are consistent with the consumption of lipid-soluble antioxidants within the arthritic joint due to their role in terminating the process of lipid peroxidation (Fairburn et al., 1992). 

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