Decomposition products of lipid

Some of the decomposition products of lipid peroxidation, such as aldehydes, are extremely cytotoxic and it has been shown that these are formed within atherosclerotic... 

A mechanism has been reported for the formation of trithlolane from the reaction of aldehydes with hydorgen sulfide (51). The identification of 3-methyl-5-butyl-l,2,4-trithiolane and 3-methyl-5-pentyl-1,2,4-trithiolane in food flavor suggests that pentanal and hexanal were Involved in the formation of these compounds (Figure 5). Pentanal and hexanal are major thermal and oxidative decomposition products of lipids. 

Analysis Measure relatively low levels of oxidation (below 1%) and include measurement of initial or primary products of lipid oxidation (e.g., hydroperoxides, conjugated dienes) as well as secondary decomposition products of lipid oxidation (e.g., carbonyls, volatiles, dialdehydes). 

More recent studies determined hexanal and other volatile compounds by headspace gas chromatography (HS-GC) to measure lipid oxidation in meat. Although hexanal data may sometimes be in agreement with the results of the non-specific TEA method, the sensitive HS-GC method is more desirable because it determines specific decomposition products of lipid hydroperoxides. The same factors that influence lipid oxidation in meat such as pH, metal catalysts and antioxidants also affect and confound the interpretation of the results of the colorimetric TEA method. The TEA method is, therefore, not recommended to determine oxidation of meat and other complex foods because the degree to which non-lipid oxidation and degradation products, including browning reaction products, contribute to the TEA color remains unclear. 

Decomposition of the primary products of lipid oxidation generates a complex mixture including saturated and unsaturated aldehydes such as hexanal. Hexanal is the most commonly measured end product of lipid oxidation, and both sensory and physicochemical methods are used for its determination. Where other antioxidant activity tests may be nonspecific, physicochemical measurement of hexanal offers the advantage of analyzing a single, well-defined end product. 

Yonei, S. Furui, H. (1981) Lethal and mutagenic effects of malondialdehyde, a decomposition product of peroxidized lipids, on Escherichia coli with different DNA-repair capacities. Mutat. Res., 88, 23-32... 

Frankel, E.N. and Gardner, H.W. 1989. Effect of a-tocopherol on the volatile thermal decomposition products of methyl linoleate hydroperoxides. Lipids 24 603-608. 

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. 

The basic mechanism of autoxidation at elevated temperatures is similar to that of room-temperature oxidation, i.e., a free radical chain reaction involving the formation and decomposition of hydroperoxide intermediates. Although relative proportions of the isomeric hydroperoxides, specific for oleate, linoleate and linolenate, vary with oxidation temperatures in the range 25°C -80°C, their qualitative pattern is the same (. Likewise, the major decomposition products isolated from fats oxidized over wide temperature ranges are those reflecting autoxidation of their constituent fatty acids (2 -6). The mechanisms and products of lipid oxidation have been extensively studied. The reader is referred to the numerous monographs, reviews and research articles available in the literature (1,A,7,8,9,10,11). 

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. 

Volatile compounds generated by model systems of zeln, corn amylopectin and corn oil extruded at barrel temepratures of 120°C and 165°C were analyzed by GC and GC/MS. The largest quantities of lipid oxidation products were detected in systems containing all three components. In each system, the quantity of 2,4-deca-dienal was low relative to the quantities of hexanal, heptanal and benzaldehyde. Identification of the Maillard reaction products, 2-methyl-3(or 6)-pentyl-pyrazine, 2-methyl-3(or 6)-hexylpyrazine and 2,5-di-methyl-3-pentylpyrazine, suggested that lipid-derived aldehydes might be involved in the formation of substituted pyrazines. 4-Methylthiazole was identified as a major decomposition product of thiamin when corn meal containing 0.5% thiamin was extruded at a final temperature of 180°C. 

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. 

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). 

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]. 

Products of Lipid Peroxide Decomposition. In view of the potential problems with direct lipid analysis, a simpler and more sensitive assay, the detection of thiobarbituric acid-reacting materials (TBARM), was used to detect DPE injury to membranes... 

Two reactions used in steroid chemistry were modified by Bennett for histochemical use. Frozen sections of either unfixed or formalin-fixed tissue were used, with no differences reported in their reactivity (see, however, Section V.2) the sections were 80 to 100 microns in thickness. In the first method, sections were treated with phenylhydrazine hydrochloride (1 %) in acetate buffer, pH 6 to 6.5, overnight. The formation of yellow phenylhydrazones indicated the presence of carbonyl groups. The pH of the solution was kept low enough to prevent extensive accumulation of the decomposition products of phenylhydrazine, which are yellow and soluble in lipid. In order to avoid reaction with ascorbic acid the sections were first oxidized briefly with iodine or indophenol. Since dehydro-ascorbic acid, which is formed by the oxidation of ascorbate, also forms phenylhydrazones, it is doubtful that this procedure had any value. However, since ascorbic acid and its oxidation product are soluble in most aqueous mixtures, they probably would not remain in sections as ordinarily treated. 

Frankel, E.N., Neff, W.E. and Selke, E. Analysis of autoxidized fats by gas chromatography-mass spectrometry VII. Volatile thermal decomposition products of pure hydroperoxides from autoxidized and photosensitized oxidized methyl oleate, linoleate, and linolenate. Lipids 16, 279-285 (1981). 

---