Lipoxygenase and lipid oxidation in foods - Oxidation in Foods and Beverages and Antioxidant Applications - page 106

showed that soybean LOX-2 and LOX-3 (a number place after the enzyme name
is used to designate various isozymes) exhibited a strong preference for the free
acid form of 18:3 as a substrate over the free acid form of 18:2. LOX-1 showed a
higher activity toward the 18:2 fatty acid.
The soybean plant produces at least eight identified LOXs (Brash, 1999), and
its LOXs are the best characterized among the plant LOXs. However, mature
seed cotyledons contain primarily three LOX isozymes designated LOX-1,
LOX-2, LOX-3 (Axelrod et al., 1981) and a minor isoenzyme, LOX-4 (Brash,
1999). The LOX-4, 5 and 6 are produced or start to appear during seed
germination (Wang et al., 1999). The first three isoenzymes differ in their
substrate specificity and pH optimum. LOX-1 catalyzes oxygen addition on C13
of 18:2 and 18:3. LOX-1 prefers free fatty acids over esters, and it has an
optimum activity at pH 9. LOX-2, or a mixture of LOX-2 and LOX-3, have an
optimum activity at pH 7 and does not exhibit a strong positional specificity,
oxygenating both the C9 and C13 positions. Both LOX-2 and LOX-3 are said to
be equally activity toward 18:2 and 18:3 (Axelrod et al., 1981), and attack both
the free fatty acid and esterified forms quite well (Brash, 1999). However,
Fukushige and co-workers (2005) reported that LOX-2 gave hydroperoxides at
positions 13 and 9 in a ratio of 4:1. LOX-3 produced 65% 13-ROOH and 35% 9-
ROOH, but the proportion is said to be highly dependent on the reaction
conditions (Christopher et al., 1972). The type and quantity of the ROOHs will
affect the composition of their decomposition products and the flavor profile of
the oxidized foods may be different with the different LOXs or different reaction
conditions. A most recent study by Iassonova and co-workers (2009) showed
evidence of another possible isoenzyme in a LOX-null soybean which has
substrate specificity toward phosphatidylcholin, as discussed later in the chapter.
LOX isoenzymes are localized in the cytoplasm of the cotyledon cells (Wang
et al., 1999) and their physiological roles are not yet fully understood. No
harmful consequences have been observed in soybean mutants lacking specific
LOX isozymes (Siedow, 1991). LOXs are believed to play a role in the defense
of the plant, and presumably they might play such a role during seed
germination and maturation. During the germination of commodity soybeans,
LOX-1, 2, and 3 activities decreased (Song et al., 1990; Kato et al., 1992; Wang
et al., 1999) and isoenzymes LOX-4, 5, and 6 started to appear. This may
explain why triple-null soybeans (seeds with the three main LOX isoenzymes
removed) do not suffer from poor agronomic performance. The effect of seed
LOX elimination (triple-null) on the resistance to plant pathogens, such as stem
canker, frogeye leaf spot, and powdery mildew was studied, and in general LOX
removal did not affect the resistance of soybeans to these diseases (Martins et
al., 2002).
LOXs are also involved in lipid oxidation in animal tissues and they play an
important role in controlling cell functions. The 18:2 and 18:3 fatty acids in
animal tissues can be elongated and oxidized to produce the essential
eicosanoids, which are a class of physiologically important fatty acid derivatives
that include prostaglandins, thromboxanes, and leukotrienes. Two pathways or