Lipoxygenase Occurrence and Properties

Lipoxygenase is a metal-bound protein with an Fe-atom in its active center. The enzyme 

In the rate-limiting step of catalysis, the isoenzyme LOX 1 from soybeans abstracts the pro-(S)-hydrogen from the n-8 methylene 

Lipoxygenases from plants mostly exhibit 9- or 13-regiospecificity. A LOX with C-8 specificity has been found in a mushroom (Table 3.33). 

Non-specific LOX occur in legumes, e. g., LOX 1 in peas (Table 3.33) and LOX III in soybeans (pH optimum 6.5). These enzymes oxidize linoleic acid to mixtures of 9- and 13-hydroperoxides, which approach racemic proportions. In addition, oxo fatty acids and volatile compounds are formed, i. e., the product spectrum resembles that formed by the autoxi-dation of linoleic acid. Moreover, they also react with esterified substrate fatty acids. In contrast to specific LOX, they do not require prior release of fatty acids by a lipase enzyme for activity in food. 

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). 

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