Lipoxygenase plant tissues

Galliard and Phillips 144) have found that homogenates of potato tubers contain an enzyme which converts the primary lipoxygenase product of linoleic acid, the 9-D-hydroperoxyoctadecadienoic acid, into an 18-carbon ether (Figure 3). Here too, cleavage of the Gig chain must occur. The formation and role of possible cleavage products from linoleic and linolenic acids in aroma and physiology of plant tissues have already been mentioned. 

The formation of Cg 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 en2ymes, lipoxygenases and fatty acid hydroperoxide lyase, which cleave the fatty acid moiety to Cg... 

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 enzymic formation of aldehydes, ketones, alcohols, and oxoacids (from linoleic and linolenic acids) on disruption of plant tissues is an important biosynthetic pathway by which fruit and vegetable volatiles are formed. Some examples are (E)-2-hexenal ("leaf aldehyde") and ( )-3-hexenol ("leaf alcohol") in tea (E)-2-hexenal in apples (E,Z)-2,6-nonadienal ("violet Teaf aldehyde") and (E)-2-nonenal in cucumber ( Z)-5-nonenal in musk melon (Z,Z) -3,6-nonadienol in water melon, and 1-octen-3-ol ("mushroom alcohol") in certain edible mushrooms and Fungi. The enzyme system is highly substrate specific to a (Z,Z)-1,4-pentadiene system (like lipoxygenase) splitting the >C = C< double bond at the W - 6 and/or W - 9 position. Therefore linoleic-, linolenic-, and arachidonic acids are natural substrates. It seems to be a common principle in leaves, fruits, vegetables, and basidiomycetes. 

Table 11.4 Plant tissues containing high levels of lipoxygenase activity (from Galliard and Chan, 1980, with permission)...

Plant Tissues Containing High Levels of Lipoxygenase Activity"... 

The first route in the lipoxygenase pathway (Scheme 5) involves the cleavage of 13-hydroperoxides into 12-oxo-(9Z)-dodccenoic acid and C -aldehydes by hydroperoxide lyase, an enzyme that has been found in a large number of plants [176,184,185]. The 12-oxo-(9Z)-dodecenoic acid isomerizes readily into the more stable 12-oxo-(10 )-dodecenoic acid, which is also known as traumatin [186]. Traumatin has been reported to mimic the physiological effects seen upon wounding of plant tissue and can be converted into traumatic acid, which has... 

Lipoxygenase enzymes are quite common to plant tissues. The legumes (especially soybeans) contain substantial quantities of lipoxygenase enzymes. The beany flavor of soybeans is believed to be due to lipoxygenase activity in the bean once tissue damage has occurred [102-104], The reversion flavor of soybean oil is also believed to be due to lipoxygenase activity in the oil. Hsieh et al. [155] have shown that 2-pentylfuran is the primary oxidation product responsible for this OF. 

Thus, good substrates for the tea chloroplast enzyme system consisting of lipoxygenase and hydroperoxide lyase were llnolelc acid and linolenlc acid which are commonly occurring fatty acid In plant tissues particularly after acyl hydrolysis of lipids on aging, injury and processing etc. The resultant products are volatiles Including hexanal, (3Z)-hexenal and (2E)-hexenal which are responsible for characteristic odor of plant products. In addition 12-oxo-(9Z)-dodecenolc acid and 12-oxo-(10E)-dodecenolc acid reported as plant wound hormone are formed as a counterpart of volatile C -aldehydes. 

Recent work has revealed that lipoxygenases from some plant tissues oxidize a-ketols of a-linolenic acid to hydroperoxides of a-ketols [3]. This paper deals with the formation of similar products by the other way by hydroperoxide dehydration of double dioxygenation products. Incubation of 9-hydroxy-16-hydroperoxy-10( ),12(Z),14( )-[l-l C]octadecatrienoic acid with enzyme preparation from com seeds led to the formation of three polar metabolites. After RP- and SP-HPLC purifications two of these polar metabolites were identified by UV spectroscopy and electron-impact mass spectrometry as a- and y-ketols of 9-hydroxy derivative of a-linolenic acid 9,16-dihydroxy-15-oxo-I0(, 12(Z)-octadecadienoic acid and 9,12-dihydroxy-15-oxo-I0(jE, 13( )-octadecadienoic acid, respectively. Stmcture of the most polar metabolite is discussed. 

Soybean lipoxygenase-1 (SLO) is often used as the prototype for studying the family of lipoxygenases from tissues of different species. They are non-haem iron dioxygenases that catalyse the conversion of unsaturated fatty acids to hydroperoxides. Plant hpoxygenases... 

Although we do not know the physiological function of lipoxygenase, the effect of the enzyme in plant tissues is to yield volatile products with characteristic flavours and aromas - either desirable or undesirable (section 3.3.4). The substrates for lipoxygenase are the free fatty acids released from storage lipids by lipases (section 4.5) or from membrane lipids (Chapter 7) by non-specific lipolytic acylhydrolases . 

For over 40 years it has been known that plant tissues contain lipoxygenases which catalyse the introduction of oxygen into polyunsaturated fatty acids. In 1974 Hamberg and Samuelsson found that platelets contained a 12-lipoxygenase and since that time 5- and 15-lipoxygenases have also been discovered. The immediate products are hydroperoxy fatty acids which for arachidonate substrate are hydroperoxy eicosatetraenoic acids (HPETEs). 

Lipoxygenases catalyze oxidation of polyunsaturated fatty acids in plant lipids. Within animal tissues the lipoxygenase-catalyzed reaction of arachidonic acid with 02 is the first step in formation of Ieukotrienes and other mediators of inflammation. These reactions are discussed in Chapter 21. 

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