Lipid hydroperoxides degradation products

Increased electrophoretic mobility of LDL results from modification of apo B by lipid hydroperoxide breakdown products, and usually correlates well with increased degradation of LDL by macrophages (Steinberg et al., 1989 Keaney and Frei, 1994). 

Exposure of cardiolipin to oxygen gas resulted in a substantial loss of the lipid and most of the degradation products were hydroperoxide derivatives. Even though we have not done the comparative experiment, our experience tells us that cardiolipin is more sensitive to oxidative stress than free linoleic acid or trilinolein. Taking into account that mitochondria is the site where reactive oxygen species are often produced, we propose that peroxidation of cardiolipin may easily take place once the intracellular oxidative stress occurs. 

The primary products from autoxidation are hydroperoxides, which are often simply referred to as peroxides. Peroxides are odorless and colorless, but are labile species that can undergo both enzymatic and nonenzymatic degradation to produce a complex array of secondary products such as aliphatic aldehydes, alcohols, ketones, and hydrocarbons. Many of these secondary oxidation products are odiferous and impart detrimental sensory attributes to the food product in question. Being able to monitor and semi-quantitate the development of peroxides by objective means (e.g., PV determination) over time is important for food scientists who want to characterize the quality of an oil or a lipid-containing food product, even though the peroxides themselves are not directly related to the actual sensory quality of the product tested. 

The distillation method (see Alternate Protocol 1) involves recovering malonaldehyde from an acidified food product. It is similar to that of the direct heating approach (see below), except that TBA is reacted only with an aliquot of the distillate. Consequently, physical and chemical interference by extraneous food constituents in the reaction with TBA is minimized because the food is never directly in contact with TBA. Unfortunately, direct heating of a food under acidic conditions enhances the degradation of existing lipid hydroperoxides as malonaldehyde precursors, and generates additional reactive radicals and scission products other than malonaldehyde that can react with TBA (Raharjo and Sofos, 1993). More malonaldehyde or reactive substances will be gener-... 

Peroxide value (PV, also referred to as initial peroxide) The presence of fatty acid hydroperoxides, formed by the oxidative degradation of fatty acids, is a measure of oxidative abuse and degradation of the lipid. Products generated by hydroperoxide degradation will confer a rancid note in edible applications. 

Scheme 3. Formation of hydroperoxides by photoxidation of a lipid with a sensitizer hv is energy in the form of UV iight, sensitizers that are naturally present in photosensitive pigments, their degradation products, or polycyclic aromatic hydrocarbons capable of transferring energy from iight to chemicai moiecuies).

The antioxidant radical produced because of donation of a hydrogen atom has a very low reactivity toward the unsaturated lipids or oxygen therefore, the rate of propagation is very slow. The antioxidant radicals are relatively stable so that they do not initiate a chain or free radical propagating autoxidation reaction unless present in very large quantities. These free radical interceptors react with peroxy radicals (ROO ) to stop chain propagation thus, they inhibit the formation of peroxides (Equation 13). Also, the reaction with alkoxy radicals (RO ) decreases the decomposition of hydroperoxides to harmful degradation products (Equation 14). 

Consequently, in addition to hydroperoxides, a lot of secondary oxidized lipidic compounds, mainly short-chain aldehydes, may appear and represent late markers of lipid peroxidation. As an example, malondialdehyde (MDA, Fig. 6) is known to be the most abundant lipid peroxidation aldehyde whose determination by 2-thiobarbituric acid (TBA) is one ofthe most common assays in lipid peroxidation studies [20]. However, it can be noticed that the TBA assay method [21] is not specific of MDA titration since it also can detect a variety of peroxides and secondary degradation products of lipid peroxidation called... 

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). 

Carotenoids are present in soybeans in a very low concentration (0.8—3.7 ppm), and the main forms are lutein and P-carotene. They are co-extracted with oil but are often removed or degraded by oil refining steps designed to remove the undesirable minor components that contribute to physical and chemical instability and undesirable color, such as degumming to remove PLs, neutralization to remove free fatty acids, bleaching to decompose lipid hydroperoxides, and deodorization to remove volatile oxidation products. 

Amino groups of phospholipids, particularly of phosphatidylethanolamine or serine, react with reducing sugars, osones, and other products of sugar degradation to form brown melanoidins. The pathway via Amadori compounds is similar to the case of amino acids (Utzmann and Lederer, 2001). They thus contribute to the darkening of phospholipid concentrates on storage. Melanoidins are partially bleached by lipid hydroperoxides. 

Lipoxygenase (LOX) converts polyunsaturated fatty acids, such as linoleic and linolenic acids, to lipid hydroperoxides (Figure 2)(52,73,74). The lipid hydroperoxides then form hydroperoxide radicals, epoxides, and/or are degraded to form malondialdehyde. These products are also strongly electrophilic, and can destroy individual amino acids by decarboxylative deamination (e.g., lysine, cysteine, histidine, tyrosine, and tryptophan) cause free radical mediated cross-linking of protein at thiol, histidinyl, and tyrosinyl groups and cause Schiff base formation (e.g., malondialdehyde and lysine aldehyde) (39,49,50,74-78). 

Both LC-MS and MS/MS have permitted greatly improved analyses of various lipid oxidation products in the form of the intact neutral and polar lipid molecules or their partial degradation products, which was not possible by chromatographic methods alone. Thus, the hydroperoxides, epoxides, hydroxides, isoprostanes, and the core aldehydes and acids generated during nonenzymatic peroxidation have been identified in plasma lipoproteins and atheroma samples and have provided a new basis for hypotheses about the origin and progression of vascular disease. 

The following are examples of TLC analyses in which SFE was the method used for sample preparation lipids in wool (analysis by TLC-FID) hpids in fish feed (TLC-FID) the pesticide chlorpyrifos and its degradation products in soil hydrocarbons in heavy petroleum products (TLC-FID) aromatic and ahphatic hydroperoxides in solid matrices (online sample transfer to TLC plates) colored fractions from a Mediterranean brown alga essential oil in foods " and cyanazine from soil (silica gel TLC-densitometry at 220 nm). " ... 

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. 

Furthermore, these hydroperoxides ean start a radical chain reaction, leading to additional aroma-active fat degradation products and enhanced autooxidation. The hydroperoxide degradation can happen spontaneously and may also be catalyzed by enzymes. Finally it should be emphasized that XO contributes to lipid oxidation in milk fat only if an appropriate substrate is present. 

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