Lipoxygenase cooxidation

In addition lipohydroperoxides formed by C-15 lipoxygenase after its homolysis can give rise to lipid alkoxyl radicals which induce cooxidation of other unsaturated lipids such as p-carotene [8] (Figure 4). 

Figure 4, The cooxidation of P-carotene (X.=450 nm) by secondary lipid free radicals which formed during arachidonic acid peroxidation (A,=233 nm) by animal (rabbit reticulocyte) C-15 lipoxygenase in the water...

The destruction of p-carotene during lipid peroxidation is readily observed by bleaching of the carotene color (44). Presumably, p-carotene oxidation is initiated by H-abstraction, and such a mechanism has been proposed for the cooxidation of carotenoids during the lipoxygenase catalyzed oxidation of polyunsaturated fatty acids (45), as shown by Reaction Q. 

Lipoxygenases will also catalyse co-oxidation reactions. This is used both in assay methods for the enzyme and in commercial applications. An example of the latter is the addition of soybean or broad bean flours (both rich in lipoxygenase activity) to wheat flour in order to bleach pigments for white bread production. Enzymes from different sources differ in their co-oxidation ability, e.g. soybean type-I enzyme has poor activity in this regard while soybean type-II enzyme has high co-oxidation activity. The reaction probably proceeds by a free-radical process (Veldink et al., 1977) and requires the presence of a substrate (e.g. linoleic acid) as well as the co-substrate. The extent of the cooxidation may depend on the lifetime of the radical intermediates and the relative efficiency of the lipoxygenase-mediated radical reduction (Weber and Grosch, 1976). 

Lipoxygenases are particularly relevant to food plants because their action destroys the essential polyunsaturated fatty acids, producing hydroperoxides that can decompose to form derivatives with characteristic tastes and odors (both desirable and undesirable). Moreover, LOXs catalyze cooxidation reactions, resulting in the bleaching of natural pigments (chloro-... 

Lipoxygenases from different sources differ in their cooxidation activity. Enzymes from peas and beans (Phaseolus sp.) and the LOX-2 from soybean have a high cooxidation potential potato LOX is intermediate, whereas wheat, flax, and soybean LOX-1 have poor cooxidation activity (Grosch et al., 1976, 1977). Thus, the carotene oxidase activity of soybeans is associated with the LOX-2 isoenzyme, and this explains the observed selective loss of carotenoid bleaching activity during the purification and heat treatment of the classic (i.e., LOX-1) enzyme from soybean (Kies et al., 1969). 

Grosch, W., Laskawy, G. and Weber, F. Formation of volatile carbonyl compounds and cooxidation of j8-carotene by lipoxygenase from wheat, potato, flax, and beans. J. Agric. Food Chem. 24, 456-459 (1976). 

The non-specific lipoxygenases can cooxidize carotenoids and chlorophyll and thus can degrade these pigments to colorless products. This property is utilized in flour bleaching (cf. 15.4.1.4.3). The involvement of LOX in cooxidation reactions can be explained by the possibility that the peroxy radicals are not as rapidly and fully converted to their hydroperoxides as in the case of specifically reacting enzymes. Thus, a fraction of the free peroxy radicals are released by the enzyme. It can abstract an H-atom either from the unsaturated fatty acid present (pathway 2a in Fig. 3.30) or from a polyene (pathway 2b in Fig. 3.30). 

Fig. 3.30. Reactions of non-specific lipoxygenase (according to Weber and Grosch, 1976). (1) Main catalysis pathway (2a) and (2b) cooxidation pathways. LH linoleic acid Car-H carotenoid LOOH linoleic acid hydroperoxide...

Lipoxygenases (E.C. 1.13.11.12) catalyze the peroxidation of compounds that possess a cis cis-1,4-pentadiene structure. Their principal substrates in higher plants are the di- and tri-unsaturated fatty acids, linoleic (CIS 2) and linolenic (Cl 8 3) acids. Lipoxygenases (LOXs) are also known to catalyze the cooxidation of chlorophylls and carotenoids (Axelrod et al, 1987 Vick and Zimmermann, 1987). 

In many cases, the products formed are colorless, thus revealing the bleaching effect of many oxidants on carotenoids. Further oxidation of the carotenoid or carotenoid radical can occur as in studies of soybean (Glycine max) and recombinant pea (Pisum sativum) lipoxygenase-mediated cooxidation of carotenoids... 

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