Fatty acid hydroperoxide dependent

Significance of Fatty Acid Hydroperoxide-Dependent PAH Oxidation... 

Hydroperoxide-dependent peroxygenase (epoxygenase). This enzyme, detected in soybeans [7] and broad beans [8], controls the fatty acid hydroperoxide-dependent epoxidation of unsaturated fatty acids. Hydroperoxide molecule plays a role of oxygen donor for epoxidation. This pathway was proposed as the source of natural epoxides of linoleate, vemolic and coronaric acids, occurring in different amounts in many seed oils. 

Labeque, R. and Marnett, L.J. (1988). Reaction of hematin with allylic fatty acid hydroperoxides identification of products and implications for pathways of hydroperoxide-dependent epoxidation of 7,8-dihydroxy-7,8-dihydrobenzo [a]pyrene. Biochemistry 27, 7060-7070. 

Figure 9. Stereochemical differences between fatty acid hydroperoxide- and mixed-function oxidase-dependent oxidation of ( ) -BP-7,8-dihydrodiol.

In plant tissues, various enzymes convert the hydroperoxides produced by LOX to other products, some of which are important as flavor compounds. These enzymes include hydroperoxide lyase, which catalyzes the formation of aldehydes and oxo acids hydroperoxide-dependent peroxygenase and epoxygenase, which catalyze the formation of epoxy and hydroxy fatty acids, and hydroperoxide isomerase, which catalyzes the formation of epoxyhydroxy fatty acids and trihydroxy fatty acids. LOX produces flavor volatiles similar to those produced during autoxidation, although the relative proportions of the products may vary widely, depending on the specificity of the enzyme and the reaction conditions. 

Glutathione (GSH) peroxidase (EC 1.11.1.9) is one of several selenoproteins that contain a unique selenocysteine residue at the active site (1). This enzyme catalyzes the GSH-dependent reduction of both hydrogen peroxide and organic hydroperoxides, including fatty acid hydroperoxides formed during lipid peroxidation. GSH peroxidase activity has been measured in both protozoan and helminth parasites (11,12). The enzyme from Schistosoma mansoni has been cloned (13). The presence of the unique selenocysteine codon (UGA) confirms that it is a selenoprotein. Although GSH peroxidase activity appears to be absent from Hymenolepsis dimimta and M. expansa... 

In addition to reaction with H2O2, the selenium-dependent GSH-Px is postulated to act together with phospholipase A, in converting potentially harmful phospholipid hydroperoxides (LOOH) to free fatty acid alcohols (ROH) via production of lysophospholipids and free fatty acid hydroperoxides (ROOH Eq. (19)) (van Kuijk et al., 1987). 

Howe, G.A., G.I. Lee, A. Itoh, L. Li, and A.E. DeRocher (2000). Cytochrome P450-depend-ent metabolism of oxylipids in tomato. Cloning and expression of allene oxide synthase and fatty acid hydroperoxide lyase. Plant Physiol. 123, 711-724. 

Terpenic hydrocarbons are stable in the absence of air, but are easily oxidised in air, especially at higher temperatures. Their autoxidation proceeds by similar mechanisms as autoxidation of unsaturated fatty acids and depends greatly on the hydrocarbon structure. The primary autoxidation products are hydroperoxides. In branched hydrocarbons, the hydroperoxyl group mainly occurs in the secondary or tertiary carbon adjacent to the quaternary carbon of the double bond. The final autoxidation products are usually epoxides, alcohols and ketones. The primary site of attack in olefins is the carbon adjacent to the double bond, as in monounsaturated... 

Many species of plants have hydroperoxide lyase enzymes that cleave fatty acid hydroperoxides into two fragments (11,12). If the substrate is a 13-hydroperoxy fatty acid, then the products are 12-oxo-cls-9-dodecenoic acid and either hexanal or cls-3-hexenal, depending on whether the hydroperoxide was derived from linoleic or linolenic acid, respectively (Fig. 2). 

The primary oxidation products are fatty acid hydroperoxides (FAHPO) secondary products are mainly mono- and tri-hydroxy fatty acids. The amounts and proportions of oxidation products vary, depending upon the storage time of the milling products. Table 1 shows that hydrated stored wholemeal (in this case dough) contains more oxidation products (mainly FAHPO) than a dough from freshly-milled wholemeal. 

Esterbauer et cil. (1992) have studied the in vitro effects of copper on LDL oxidation and have shown that there are three distinct stages in this process. In the first part of the reaction, the rate of oxidation is low and this period is often referred to as the lag phase the lag phase is apparently dependent on the endogenous antioxidant content of the LDL, the lipid hydroperoxide content of the LDL particle and the fatty acid composition. In the second or propagation phase of the reaction, the rate of oxidation is much faster and independent of the initial antioxidant status of the LDL molecule. Ultimately, the termination reactions predominate and suppress the peroxidation process. The extensive studies of Esterbauer et al. have demonstrated the relative importance of the endogenous antioxidants within the LDL molecule in protecting it from oxidative modification. 

Reaction yields depend on the nature of the substrate. Linseed oil contains two polyunsaturated fatty acids 50% linolenic acid and 18% linoleic acid. The corresponding hydroperoxides are obtained with low yields. 

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