Lipoxygenase cleaving enzymes

The eicosanoids, so called because of their derivation from a 20-carbon unsaturated fatty acid, arachidonic acid (eicosatetraenoic acid), are obtained from membrane phospholipids and synthesized de novo at the time of cellular stimulation. Arachidonic acid is cleaved from membrane-bound phosphatidylcholine by the enzyme phospholipase A2. Alternatively, arachidonic acid may be derived by the sequential actions of phospholipase C and diacylglyceryl lipase. Arachidonic acid can then follow either of two enzymatic pathways that result in the production of inflammatory mediators. The pathway initiated by cyclooxygenase (COX) produces prostaglandins the lipoxygenase pathway generates leukotrienes (Fig. 36.2). 

The unsaturated Cg aldehydes and alcohols probably arise in wheat from cleavage of the 9—10 double bond of unsaturated C g fatty acids, linoleic and linolenic acids (Figure 2). Galliard and coworkers (12,13) have partially purified enzyme systems capable of catalyzing these transformations. Lipoxygenase initially converts linoleic and linolenic acids to 9-hydroperoxides which are subsequently cleaved by hydroperoxide lyase to volatile Cg unsaturated aldehydes and 9-oxo-nonanoic acid. The 3-enals are the primary volatile cleavage products from the fatty acids and these are transformed by an isomerase to the more stable 2—enals (14). The 3—enals are rather unstable but Hatanaka et al. (15) have confirmed their presence in plant tissue with authentic samples. The Cg... 

The formation of C6 aldehyde and alcohols in plant tissues is related to cell destruction. Disruption of intact cells during crushing and milling results in the release of lipid-degrading enzymes, lipoxygenases and fatty acid hydroperoxide lyase, which cleave the fatty acid moiety to C6... 

Figure 5 shows a reaction scheme which may explain the formation of Cg- and C-] 2-components in leaves and fruits. Hatanaka et al. (12) proposed a cleaving system operative in Thea sinensis leaves and other plants which is located in the chloroplasts. E2 shows a similar substrate specifity as lipoxygenase E-. The enzymic breakdown products from linolenic acid are ( )-3-hexenal (I) and 12-oxo-( )-9-dodecenoic acid (V). Both constituents are transformed into the corresponding ( )-2-enals by E3 and/or by chemical reactions. During these transformation the carbonyls may be reduced to alcohols by alcohol oxidoreductase E4. In ripe fruits the right pathway E-j, E2f E3 seems to become operative. 

The enzymic formation of Cs- and Cg-components in vegetables is determined by the cleaving-system E2. Tomatoes possess a lipoxygenase-system which forms 9-LOOH and 13-LOOH in a ratio of 95 5 (23). According to Matthew et al. (2 ) only the 13-LOOH is decomposed into Cg- and C-j2 components by the lyase-system E2 This is consistent with the results of Kazeniac et Hall (2 ) who demonstrated the formation of ( ) -2-hexenal, ( )-3-hexenal and ( )-3-hexen-1-ol from linolenic acid in tomato homogenates. 

Sulphur was found through spark source mass spectrometry to be abundant in SRS-A [93], Also, incorporation of S into SRS has been reported [64,94-96], The observation that several thiols enhance SRS formation in different systems [97-100] and that arylsulfatase (an enzyme that cleaves sulfate monoesters of phenolic and other unsaturated hydroxylated systems) inactivates SRS-A [101,102] opened the view that a thiol is a constituent of the active compound. However, it has recently been shown that the SRS-inactivating action of commercially available arylsulfatase is not due to sulfate ester cleavage, but to a dipeptidase contaminant [103,104]. The destruction of different SRS compounds by hpoxygenase [67,105,106] shows that SRS contains a cij,c/s-1,4-pentadiene structure, since this is a prerequisite for a lipoxygenase substrate [107], For a review of the earlier structural work on SRS, see ref. 108. 

In barley grains, lipoxygenase has been well characterized and two isoenzymes have been purified [1-3] but the characterization of hydroperoxide-metabolizing enzymes has received less attention. Hydroperoxide-consuming enzymes can be divided into two types of enzymes. Hydroperoxide lyase cleaves fatty acid hydroperoxides into aldehydes and oxoacids, and hydroperoxide dehydrase (also named hydroperoxide isomerase) catalyzes the formation of a-and 7-ketols. 

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