Oxidation 12-lipoxygenase, table

Lipoxygenases, which catalyse the oxidation of unsaturated fatty acids containing the cis,cis-l,4-pentadiene moiety to the corresponding 1 -hydroperoxy-f rans,ds-2,4-diene (Table 2.3), are widely distributed in plants and animals. The mammalian... 

The lipoxygenase and peroxidase enzymes also have a negative impact on the oxidative state of the bran (Table 9). Further degradation of the oil occurs as reflected in an increase in peroxide and thiobarbituric acid value and a decrease in iodine value. Both lipoxygenase and peroxidase enzymes are inactivated with lipase inactivation. 

In addition to the enzymically-derived volatile aroma compounds (Table I), low levels of autoxidatively-derived carbonyls can also be detected in harvested fish held a day on ice, and these volatiles are listed in Table II. The oxidatively-derived carbonyls modify the fresh plant-like aromas of fresh fish by providing oxidized-oil-like, staling fish-type odor notes (3-5, 9). The formation of hexanal in freshly harvested fish appears to be enzymic because the concentration of this compound can be diminished by lipoxygenase inhibitors (25). However, when fresh fish are stored on ice or are held under frozen storage, hexanal concentrations also increase because of autoxidative processes (40). 

Table III. Effect of a lipoxygenase-specific inhibitor on the occurrence of enzymically- and oxidatively-derived volatiles in fish.

Besides the design of synthetic molecules, several papers reported the presence of specific enzymatic reactions able to produce also in the cells compounds able to act as reversible FAAH inhibitors (Maccarrone and Finazzi-Agro, 2004a). In particular, it has been found that oxidative metabolites of AEA generated by various lipoxygenases, i.e. the hydroxyanandamides (HAEAs see 12-OH-AEA in Table 4.1), are powerful inhibitors of FAAH (van der Stelt et al., 2002). Instead, derivatives of AEA generated by cyclooxygenase-2, and termed prosta-mides, have been recently shown to be inelfective on FAAH activity (Matias et al., 2004). 

The fish muscle prooxidative system, both enzymatic and nonenzymatic, is more active than that in homeotherms in addition, susceptibility of lipids to UV-catalyzed oxidation is higher (see Chapter 8, Table 8.1). Enzymic NADH-Fe systems were more active than nonenzymic ascorbate-Fe systems in lipid oxidation of cod sarcoplasmic reticulum (Soyer and Hultin, 2000). The most active lipoxygenases (LOX) are in the gill and skin of fish (Hsieh et al., 1988). Fish gill tissue contains two separate LOX (12- and 15-LOX), each being active toward PUFA (AA, EPA, DHA) but exhibiting different hydroperoxide addition sites. LOX from sardine skin were... 

As shown in table 2 the enzyme displayed maximal activity with hexanal, but still showing high activity with nonanal, an alternative intermediate in the oxidation of linoleate through the lipoxygenase pathway, as well as with E-2-hexenal and E-2-Z-6-nonadienal, wich are alternative intermediates in the degradation of linoleate through the same pathway. The... 

Saturated and unsaturated hydrocarbons with odd and even numbers of carbon atoms in the molecule (about C11-C35) are present as the primary substances in all vegetable oils and animal fats. Alkanes, alkenes, alkadienes and alkatrienes also arise as oxidation products of unsaturated fatty acids, catalysed by lipoxygenases or by autoxidation of fatty acids during food storage and processing. Only the lower hydrocarbons can play a role as odour-active substances. The main hydrocarbons resulting from oxidation of unsaturated fatty acids are ethane from Hnolenic acid, pentane and butane from Hnoleic acid and hexane and octane from oleic acid. The immediate precursors of hydrocarbons are the fatty acid hydroperoxides (Table 8.4). The unsaturated hydrocarbons are predominantly (Z)-isomers. Numerous other hydrocarbons, including ahcycHc hydrocarbons, appear as secondary hpid oxidation products. 

Enzymatically active materials (fruits, vegetables and some fats) contain a large number of other aldehydes that are produced from essential fatty acids, mainly linoleic and Knolenic acids (Table 8.9) by oxidation reactions catalysed by lipoxygenases. In some vegetables (e.g. in cucumbers), aldehydes also result from a-oxidation of fatty acids (Figure 8.15). The primary oxidation products of essential fatty acids are hydroperoxides, which break down to aldehydes and other products under the action of lyases and can be... 

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