Lipid peroxidation autoxidation products

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. 

A characteristic of lipid products, particularly those with unsaturated lipids is peroxide formation with oxi-dation. Free radicals such as ROO, RO, and OH can damage the drug and induce toxicity. Lipid peroxides may also form due to autoxidation, which increases with unsaturation level. Hydrolysis of the... 

The antioxidant activity of a series of lignans was studied by Faure et al. [232] in a rat brain homogenate autoxidation test, and lipid peroxidation was evaluated from the luminescence intensity and thiobarbituric reactive product accumulation. Isopregomisin (0.7 pM) was the most active lignan, followed by guayacasin (1.1 pM) and dihydroguaiaretic acid (2.8 pM). The difference in the effect can be explained by the degree of methylation, which increases the activity. 

It is well established that liver microsomes in the presence of NADPH and in the absence of chelator are capable of oxidizing cholesterol and other 3P-hydroxy 5-unsaturated steroids yielding the common autoxidation products 3-8 [16]. In this case, the cholesterol oxidation is secondary to the enzymatic NADPH-dependent lipid peroxidations. The enzyme-catalyzed reaction is required, however, merely to reduce Fe i to Fe which in turn catalyzes ordinary autoxidation. It has also been shown by EPR studies using spin traps that radicals are involved in these conversions [16]. The major radicals detected were lipid peroxyl radicals and superoxide, whereas only small amounts of hydroxyl radicals were... 

The first stage of autoxidation of a pure oil is easily traceable by applying conventional wet chemistry, for example determination of the peroxide value. Peroxides are known to be heat labile compounds, which undergo breakdown at elevated temperatures to form simple hydrocarbons. Thus ethane and pentane are the predominant breakdown products of linolenate and linoleate peroxides, respectively (Evans et al., 1967). Scholz and Ptak (1966) and Evans et al. (1969) proposed a gas chromatographic method to measure rancidity in edible oils whereby the undiluted oil is injected directly into the hot injector (250°C). At these temperatures lipid peroxides are... 

As a reasonable biogenetie pathway for the enzymatic conversion of the polyunsaturated fatty acid 3 into the bicyclic peroxide 4, the free radical mechanism in Equation 3 was postulated 9). That such a free radical process is a viable mechanism has been indicated by model studies in which prostaglandin-like products were obtained from the autoxidation of methyl linolenate 10> and from the treatment of unsaturated lipid hydroperoxides with free radical initiators U). 

These initial steps are highly exothermic, which accelerates the autoxidation cycle to an explosive rate to give CO, CO2, and H2O as the stable products. Within biological matrices the autoxidation and peroxidation of lipids and fats from foodstuffs are important examples of oxygen radical... 

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). 

Fats or raw materials that serve as a source for fatty acids are frequently employed in process flavourings. During the flavour reaction, thermal peroxidation of lipids such as triglycerides, fatty acids and phospholipids occurs. This non-enzymatic lipid oxidation, also called autoxidation, leads to a very complex mixture of reaction products, and has to be regarded separately from the enzymatic lipid oxidation which occurs at low temperature and is catalysed by lipoxygenases. 

Neff, W.E., Frankel, E.N. and Weisleder, D. High-pressure liquid chromatography of autoxidized lipids II. Hydroperoxy-cyclic peroxides and other secondary products from methyl linolenate. Lipids 16, 439-448 (1981). 

Frankel, E.N., Neff, W.E. and Selke, E. Analysis of autoxidized fats by gas chromatography-mass spectrometry. VIII. Volatile thermal decomposition products of hydroperoxy cyclic peroxides. Lipids 18, 353-357 (1983). 

Havrilla, C.M., Hachey, D.L. and Porter, N.A. Coordination (Ag+) ionspray-mass spectrometry of peroxidation products of cholesterol linoleate and cholesterol arachidonate High-performance liquid chromatography-mass spectrometry analysis of peroxide products from polyunsaturated lipid autoxidation. J. Am. Chem. Soc. 122, 8042-8055 (2000). 

Lipid oxidation in foods is a complex chain of reactions that first consist of the introduction of a functional group containing two concatenated oxygen atoms (peroxides) into unsaturated fatty acids, in a free-radical chain reaction, that afterward gives rise to secondary oxidation products. Different pathways for lipid oxidation have been described radical mechanism or autoxidation, singlet oxygen-mediated mechanism or photooxidation, and enzymatic oxidation. 

Lipid autoxidation is generally believed to involve a free- radical chain mechanism (1) initiation steps that lead to free radicals (R ), (2) propagation of the free radicals (R -I-O2 —> ROO, ROO -1-RH — ROOH-I-R ), and (3) termination steps R -H R R—R, R- ROO- ROOR, ROO ROO O 2 ROOR (or alcohol and carbonyl compound). The oxidation of lipids results in peroxides as primary oxidation products, which in turn degrade further to secondary oxidation products, including aldehydes, ketones, epoxides, hydroxy compounds, carboxylic acids, oligomers, and polymers. 

---