Enzymes lipoxygenases

PGH synthase and the related enzyme lipoxygenase occupy a position at the interface of peroxidase chemistry and free radical chemistry and can clearly trigger metabolic activation by both mechanisms. The peroxidase pathway activates compounds such as diethylstilbestrol and aromatic amines whereas the free radical pathway activates polycyclic hydrocarbons (59). Both pathways require synthesis of hydroperoxide in order to trigger oxidation. 

Molecular oxygen, as distinct from reactions involving radicals or singlet oxygen, is directly inserted into free fatty acids by lipoxygenase (LOX) enzymes. Lipoxygenases, both regio- and stereospecific enzymes, react on the 1,4-pentadienyl moieties such as those of linoleic and a-linolenic acids. 

Each of the other families of eicosanoids—thromboxanes and leukotrienes—has interesting biosynthetic pathways too, but we will mention only one small detail. A completely different oxidation enzyme, lipoxygenase, initiates a separate pathway leading to the leukotrienes, but the first steps are very similar. They just occur elsewhere in the arachidonic acid molecule. 

JAs are derived from linolenic acid via an octadecanoid pathway consisting of several enzymatic steps (Figure 36). Multiple compartments in plant cells participate in JA synthesis. The early steps of this pathway occur in chloroplasts, where linolenic acid is converted to OPDA by means of the three enzymes lipoxygenase (LOX), allene oxide synthase (AOS), and allene oxide cyclase (AOC).867-869 Linolenic acid is oxygenated by 13-LOX producing a peroxidized fatty acid 13-hydroperoxylinolenic acid. The product is subsequently metabolized by AOS to an unstable compound allene oxide. Allene oxide is sequentially converted by AOC to produce OPDA. An alternative pathway from another trienoic fatty acid, hexadecatrienoic acid (16 3), is present in chloroplasts.870 In this pathway, dinor OPDA is produced instead of OPDA. OPDA and dinor OPDA are transported into the peroxisome. An ABC transporter involved in this transport was identified in... 

Reactions which generate unstable intermediates with a potential to chemiluminesce are well-known in living cells. Such examples include haem enzymes NADPH oxidase (Allen et al., 1972), cytochrome P-450 (Cadenas et al., 1980), cyclooxygenase (Marnett et al., 1974) and the non-haem iron-containing enzyme lipoxygenases (Veldink et al., 1977 Boveris et al., 1980). 

In 1886 the use of malted barley flour was mentioned for use in breadmaking. The enzymes in the malt converted starch into sugars which could serve as yeast substrate during fermentation. Almost 50 years later, the use of a purified enzyme (lipoxygenase) was described for bleaching the cmmb of bread. Since then enzyme usage has become widespread in food applications. 

Among these enzymes, lipoxygenase and hydroperoxide lyase seem to be of the most importance, since product specificity of lipoxyge-... 

We have applied this theoretical formulation [26-28] to a series of PCET reactions. The systems were chosen based on the availability of experimental data that had not yet been fully explained. The systems that will be discussed in this section are iron bi-imidazoline complexes, ruthenium polypyridyl complexes, amidinium-car-boxylate interfaces, DNA-acrylamide complexes, tyrosine oxidation, and the enzyme lipoxygenase. In all cases, the solvent was treated as a dielectric continuum [58, 59]. 

GROSCH w, WEBER F and FISCHER K H (1977), Bleaching of carotenoid by the enzyme lipoxygenase , Hun Technol Agric, 26, 133-137. 

Soymilk. In the traditional process, soybeans are soaked in water, ground into a slurry, cooked, and filtered to remove the insoluble cell wall and hull fractions. A number of modifications have been made in the process since the 1960s, including heat treatment before or during grinding to inactivate the enzyme lipoxygenase and thus prevent formation of grassy and beany flavors. The soymilks are available in plain and flavored, eg, vanilla and chocolate, forms (90,91). 

Sophorastilbene A (637) and (+)-a-viniferin (635) isolated from Sophora moorcroftiana BENTH ex BAKER inhibited Cu -induced protein oxidative modification (ICsos 1.3 and 3.5 pM, respectively), lipid peroxidation and generation of superoxide anion [505,506]. Paeoninol (661) from Paeonia emodi displayed potent inhibitoiy potential against the enzyme lipoxygenase in a concentration-dependent fashion with the ICsoof0.77 M [316]. 

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... 

The basic chemistry of enzyme catalyzed oxidation of food lipids such as in cereal products, or in many fruits, and vegetables is the same as for autoxidation, but the enzyme lipoxygenase (LPX) is very specific for the substrate and for the method of oxidation." Lipoxygenases are globulins with molecular weights ranging from 0.6-1 x 10 Da, containing one iron atom per molecule at the active site. 

Another example is the production of green note compounds using lipoxygenase pathway enzymes. Lipoxygenase and hydroperoxide lyase are the determinant enzymes for the conversion of fatty acids into natural food flavor components. However, these compounds present in natural sources at very low levels and because of their instable nature it has been difficult to purify these compounds. As a result, considerable efforts have been made to clone these two enzymes for commercial uses in the production of natural flavor components. Many plant lipoxygenase from different plants have been cloned and expressed in E. coli or yeast [5],... 

As housekeeping enzymes, lipoxygenases are found constitutively and ubiquitously in various cell types such as haematopoietic cells, epithelial cells and endocrine cells. When the tissue distribution of lipoxygenases is discussed, we should note that the enzyme activity may be attributed to contaminant blood cells or resident macrophages and granulocytes [113]. 

Preparation and storage of products from both oilseeds is often inhibited by rancidity and bitter aroma defects caused mostly by volatile aroma active carbonyl compounds, e. g., (Z)-3-hexenal, (Z)-l,5-octadien-3-one and 3-methyl-2,4-nonan-dione. The rancidity-causing compounds are formed through peroxidation of linolenic acid, accelerated by the enzyme lipoxygenase and/or by hem(in) proteins (cf. 3.T.2.2). Furan fatty acids are the precursors in the case of the dione (cf. 14.3.2.2.5). Lipid peroxidation is also involved in the formation of another very potent odorant, 2-pentylpyridine, which produces grassy aroma defects in soybean products. Defatted soybean protein isolates contained 60-510 pg/kg of this compound, which with an odor threshold... 

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