Oxidation lipid hydroperoxides, decomposition

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. 

Other products, such as unsaturated ketones (Stark and Forss 1962), saturated and unsaturated alcohols (Hoffman 1962 Stark and Forss 1964, 1966), saturated and unsaturated hydrocarbons (Forss et al 1967 Horvat et al. 1965 Khatri 1966), and semialdehydes (Frankel et al 1961), have been observed in the decomposition of hydroperoxides of oxidized lipid systems. 

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). 

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. 

The primary oxidation products (hydroperoxides) are unstable and susceptible to decomposistion. A complex mixture of volatile, nonvolatile, and polymeric secondary oxidation products is formed through decomposition reactions, providing various indices of lipid oxidation (5). Secondary oxidation products include aldehydes, ketones, alcohols, hydrocarbons, volatile organic acids, and epoxy compounds, among others. Methods for assessing lipid oxidation based on their formation are discussed in this section. 

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. 

A commonly used technique to follow lipid oxidation is to monitor the formation of malondialdehyde (MDA), a water-soluble compound produced from lipid hydroperoxide and endoperoxide decomposition. The formation of the MDA can be followed by measuring it directly via HPLC - , however most often it is reacted with thiobarbituric acid (TBA) with a 1 2 stoichiometry (equation 26), and formation of the product is followed spectrophotometrically at 535 nm or followed by fluorescence . ... 

The initial products derived from lipid peroxidation are subject to decomposition reactions, which may be facilitated by the presence of metals and other one-elec-tron-donating species. In particular, the lipid hydroperoxide and bicyclic endoper-oxide products are susceptible to degradation. Bicyclic endoperoxides undergo acid-catalyzed ring-scission across the endoperoxide to yield the three-carbon dial-dehyde, malondialdehyde (MDA) (Figure 5.1) [2]. MDAis also produced enzymatically from the endoperoxide-metabolizing enzyme thromboxane synthase [3, 4]. MDA is an abundant product of lipid peroxidation and reacts with DNA nucleophiles (see discussion below). 

Carbonyl compounds in oxidized lipids are the secondary oxidation products resulting from the decomposition of the hydroperoxides. They can be quantified by the reaction with 2,4-dinitrophenylhydrazine and the resulting colored hydrazones are measured spectrophotometrically at 430-460 nm. The carbonyl value is directly related to sensory evaluation, because many of the carbonyl molecules are those responsible for off-flavor in oxidized oil. The anisidine value is a measure of carbonyl compounds that have medium molecular weight and are less volatile (Frankel 1998). It can be used to discover something about the prior oxidation or processing history of an oil. 

Undesirable changes in nutritional quality of foods are initiated by the autoxidation or enzymic oxidation of unsaturated lipids to lipid hydroperoxides. Lipid hydroperoxides and their products of decomposition can react with food components, such as amino acids, proteins and certain other biochemicals. These reactions and the potential role of hydroperoxides in causing mutagenicity are reviewed. 

FIGURE 8.3 Formation of secondary products by hydroperoxide decomposition. (From Frankel, E.N. 1998. Lipid Oxidation, The OUy Press, Dundee, U.K., Chapter 4. With permission.)... 

Decomposition of Lipid Hydroperoxides ( Off-Flavor Oxidation Products)...387... 

A large body of scientific evidence suggests that the progressive loss of food palatability during lipid oxidation is due to the production of short chain compounds from the decomposition of lipid hydroperoxides. The volatile compounds produced from the oxidation of edible oils are influenced by the composition of the hydroperoxides and positions of oxidative cleavage of double bonds in the fatty acids. ... 

---