Lipid hydroperoxides decomposition

This test is used for both in vitro and in vivo determinations. It involves reacting thiobarbituric acid (TBA) with malondialdehyde (MDA), produced by lipid hydroperoxide decomposition, to form a red chromophore with peak absorbance at 532 nm (Fig. 10.1). The TBARS reaction is not specific. Many other substances, including other alkanals, proteins, sucrose, or urea, may react with TBA to form colored species that can interfere with this assay. 

Thus, the events form the basis of a chain-reaction process. The lipid hydroperoxide decomposition produces more radicals and noxious aldehydes... 

Analysis of DNA adducts derived from lipid hydroperoxide bifunctional electrophiles has relied primarily on reversed-phase ESI—MS (Blair, 2005). Some of the relatively hydrophobic bifunctional electrophiles that result from lipid hydroperoxide decomposition cannot be analyzed under these conditions because they are poorly ionized. These electrophiles can be converted to their oxime derivatives to improve ESI efficiency however, this conversion results in syn- and anti-oxime isomers and extremely complex EC chromatograms (Lee and Blair, 2000). Therefore, normal-phase EC—APCI/MS has proved to be much more successful for the analysis of lipid-hydroperoxide-derived bifunctional electrophiles (Lee et ah, 2001, 2005b Williams et ah, 2005). 

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. 

O Brien, P.J. (1969). Intracellular mechanisms for the decomposition of a lipid hydroperoxide I. Decomposition of a lipid peroxide by metals ions, haem compounds and nucleophils. Can. J. Biochem. 47, 485-492. 

Haemoglobin-derived haem iron has multiple pro-inflammatory effects resulting from its ability to initiate decomposition of lipid hydroperoxides from PUFAs. In... 

The Toxicity of Lipid Hydroperoxides and Their Decomposition Products... 

Firstly, I will discuss recent evidence supporting the hypothesis that free radicals contribute to important chronic diseases in man and exert an important life-shortening effect. Secondly, I will review data on the toxicity of lipid hydroperoxides and their decomposition products, since lipid hydroperoxides can be a source of free radicals in vivo. And lastly, I will review a system under study in our laboratory in which quantitative data on lipid peroxidation and antioxidants is being obtained using linoleic acid in SDS micelles. 

It is interesting to consider the concentrations of free radicals that result from lipid hydroperoxides in an in vitro model system. For example, my group has been studying the autoxidation of linoleic acid in SDS micelles at 37°C. We initiate the autoxidation by the decomposition of an initiator, as shown in Equation 3. 

The products of lipid oxidation in monolayers were also studied. Wu and coworkers (41) concluded that epoxides rather than hydroperoxides might be the major intermediates in the oxidation of unsaturated fatty acids adsorbed on silica, presumably because of the proximity of the substrate chains on the silica surface. In our work with ethyl oleate, linoleate and linolenate which were thermally oxidized on silica, the major decomposition products found were those typical of hydroperoxide decomposition (39). However, the decomposition patterns in monolayers were simpler and quantitatively different from those of bulk samples. For example, bulk samples produced significantly more ethyl octanoate than those of silica, whereas silica samples produced more ethyl 9-oxononanoate than those of bulk. This trend was consistent regardless of temperature, heating period or degree of oxidation. The fact that the same pattern of volatiles was found at both 60°C and 180°C implies that the same mode of decomposition occurs over this temperature range. 

Food lipids possess an inherent stability to oxidation, which is influenced by the presence of antioxidants and pro-oxidants. After a period of relative stability (induction period), lipid oxidation becomes autocatalytic and rancidity develops. Thus, the typical time-course of autoxidation, as measured by the concentration of hydroperoxides, consists of a lag phase (induction) followed by the rapid accumulation of hydroperoxides, which reaches a maximum and then decreases as hydroperoxide decomposition reactions become more important. The longer the induction period, the more stable the food to oxidation (Lundberg, 1962). 

Babiy, A.V., and Gebicki, J.M., 1999, Decomposition of lipid hydroperoxides chances the uptake of low density lipoprotein by macrophages, Acta Biochim. Pol. 46 31-42. 

Lipid hydroperoxides are fairly stable molecules under physiological conditions, but their decomposition is catalysed by transition metals and metal complexes (O Brien, 1969). Both iron(II) and iron(III) are effective catalysts for hydroperoxide degradation, but the former is more so (Halliwell and Gutteridge, 1984). These include complexes of iron salts with low molecular weight chelates, non-haem iron proteins, free haem, haemoglobin, myoglobin. 

Current information raises questions about the literal application of the classic free radical chain sequence to lipid oxidation. Observed products do not match those predicted Many studies have now shown that hydroperoxides are not exclusive products in early stages and lipid alcohols are not even major products after hydroperoxide decomposition. Product distributions are consistent with multiple pathways that compete with each other and change dominance with reaction conditions and system composition. Rate constants show no strong preference for H abstraction, cyclization, addition, or scission, which partially explains the mixmre of products usually observed with oxidizing lipids. It could be argued that the reactions in Figure 1 accurately describe early processes of lipid oxidation, but LOO rate constants considerably higher for cyclization than for abstraction contradict this. 

As oxidation normally proceeds very slowly at the initial stage, the time to reach a sudden increase in oxidation rate is referred to as the induction period (6). Lipid hydroperoxides have been identified as primary products of autoxidation decomposition of hydroperoxides yields aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, known as secondary oxidation products. These compounds, together with free radicals, constitute the bases for measurement of oxidative deterioration of food lipids. This chapter aims to explore current methods for measuring lipid oxidation in food lipids. 

Oxidation of LDL can be divided into different stages i) initiation of lipid peroxidation ii) propagation of PUFA-mediated lipid peroxidation iii) decomposition of lipid hydroperoxides into reactive aldehydes and ketones, and iv) modification of apo B, leading to recognition of LDL by the macrophage scavenger receptor. 

Analysis of the Decomposition Products of Hydroperoxides. Some authors have monitored formation of some of the decomposition products of the lipid hydroperoxides. Direct spectrophotometric measurements of the formation of oxo-octadecadienoic acids at 280 nm are possible , as are measurements of secondary oxidation products like a-diketones and unsaturated ketones at 268 nm. The formation of various aldehyde products of lipid peroxide decomposition can be monitored by reacting them with 2,4-dinitrophenylhydrazine and, after HPLC separation, measuring at 360-380 mn the DNPH derivatives formed , althongh the sensitivity of this particular technique makes it very susceptible to interference. 

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