Hydroperoxides of unsaturated fatty acids

Hydroperoxides of unsaturated fatty acids formed by autoxidation are very unstable and break down into a wide variety of volatile flavor compounds as well as nonvolatile products. It is widely accepted that hydroperoxide decomposition involves homolytic cleavage of the -OOH group, giving rise to an alkoxy radical and a hydroxy radical (5). 

A great deal of work has been done on the separation of hydroperoxides of unsaturated fatty acids by HPLC and columns of silica gel (reviewed by the author [168]). As the separation is carried out anaerobically at room temperature, little harm can come to the samples. Conjugated double bonds usually present in such fatty acids permit sensitive and specific detection by UV spectrophotometry at 235 nm. The technique has been used with other oxygenated fatty acids, and as an example, epoxy fatty acids were isolated from the total fatty acids of a seed oil by HPLC with a column of Partisir silica gel with hexane-diethyl ether (9 1, v/v) as the mobile phase [871],... 

Figure 3.60 Mechanism of action of iyases to 13-hydroperoxides of unsaturated fatty acids.

The hydroperoxidation of unsaturated fatty acids performed by lipoxygenases is an important process involved in pathways leading to the inflammatory response. The lipoxygenase nomenclature derives from their positional specificity with respect to arachidonic acid substrates in mammalian organisms. The well studied plant Hpoxygenase-1 from soybeans is, in this respect, a 15-lipoxygenase. Crystal structure data for the inactive Fe(II) form in soybeans... 

In searching the literature, only three oxygenases were found in the fatty acid category which were considered adequately studied as to be included in this review. These are (1) lipoxidase, which performs the hydroperoxidation of unsaturated fatty acids (2) long chain fatty acid peroxidase, which catalyzes the ct-oxidation and decarboxylation of C12-C18 fatty acids and (3) a fatty acid hydroxylase, which initiates a two step reaction resulting in the formation of a monounsaturated fatty acid. 

Autoxida.tlon. The autoxidation (7) of unsaturated fatty acids in phosphoHpids is similar to that of free acids. Primary products are diene hydroperoxides formed in a free-radical process. 

Applications of peroxide formation are underrepresented in chiral synthetic chemistry, most likely owing to the limited stability of such intermediates. Lipoxygenases, as prototype biocatalysts for such reactions, display rather limited substrate specificity. However, interesting functionalizations at allylic positions of unsaturated fatty acids can be realized in high regio- and stereoselectivity, when the enzymatic oxidation is coupled to a chemical or enzymatic reduction process. While early work focused on derivatives of arachidonic acid chemical modifications to the carboxylate moiety are possible, provided that a sufficiently hydrophilic functionality remained. By means of this strategy, chiral diendiols are accessible after hydroperoxide reduction (Scheme 9.12) [103,104]. 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

Fatty acid hydroperoxides can be separated from each other and other lipids by MEKC followed by FLD according to equation 34 with 106a, using as catalyst microperoxidase-11 immobilized on the wall of a small capillary coupled at the end of the electrophoresis track. MEKC with DA-UVD can be applied for separation of unsaturated fatty acids from the mixture of hydroperoxides obtained on oxidation with 102 . ... 

SCHEME 20. Application of oxidative ozonolysis for structural analysis of unsaturated fatty acid hydroperoxides... 

The irradiation of unsaturated fatty acids in foods predominantly results in the formation of a hydroperoxyl radical and then the formation of a hydroperoxide. The hydroperoxides are generally unstable in foods and break down to form mainly carbonyl compounds, many of which have low odor threshold, and contribute to the rancid notes often detected when fat-rich foods are irradiated [18]. In the absence of air, their formation is limited. 

The arachidonic acid cascade is a biological free radical oxidation of unsaturated fatty acids leading to formation of the prostaglandins (equation 102). Cyclization of a peroxy radical intermediate 66 leading to endoperoxide 67 was proposed as a pathway for this process, and this was demonstrated in chemical model systems, in which the peroxyl radical 66 was generated by hydrogen abstraction from the hydroperoxide corresponding to 66. 

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

Oxidative cleavage of unsaturated fatty acids by lipoxygenase and hydroperoxide lyase... 

Lipoxygenase (LOX) is a non-haem, iron-containing dioxygenase that catalyses the regioselective and enantioselective dioxygenation of unsaturated fatty acids containing at least one (Z,Z)-l,4-pentadienoic system. For instance, LOX from soy converts linoleic acid to the (S)-13-hydroperoxide [1]. 

The hydroperoxides formed in the autoxidation of unsaturated fatty acids are unstable and readily decompose. The main products of hydroperoxide decomposition are saturated and unsaturated aldehydes. The mechanism suggested for the formation of aldehydes involves cleavage of the isomeric hydroperoxide (I) to the alkoxyl radical (II), which undergoes carbon-to-carbon fission to form the aldehyde (III) (Frankel et al. 1961). 

Lipid peroxidation (Figure 14.5) is the initiating reaction in a cascade of events, starting with the oxidation of unsaturated fatty acids to form lipid hydroperoxides, which then break down to yield a variety of end products, mainly aldehydes, which can go on to produce toxicity in distal tissues. For this reason cellular damage results not only from the breakdown of membranes such as those of the endoplasmic reticulum, mitochondria, and lysosomes but also from the production of reactive aldehydes that can travel to other tissues. It is now thought that many types of tissue injury, including inflammation, may involve lipid peroxidation. 

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. 

Milk is characterized as having a pleasing, slightly sweet taste with no unpleasant after-taste (Bassette et al., 1986). However, its bland taste makes it susceptible to a variety of flavor defects. Autoxidation of unsaturated fatty acids gives rise to unstable hydroperoxides, which decompose to a wide range of carbonyl products, many of which can contribute to off-flavors in dairy products. The principal decomposition products of hydroperoxides are saturated and unsaturated aldehydes (Frankel et al., 1961), with lesser amounts of unsaturated ketones (Stark and Forss, 1962), saturated and unsaturated hydrocarabons (Forss et al., 1961), semialdehydes (Frankel et al., 1961) and saturated and unsaturated alcohols (Hoffman, 1962 Stark and Forss, 1966). 

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