Food Lipids: Chemistry, Nutrition, and Biotechnology

In general, young and developing plant tissues or organs have higher LOX activity
than the mature ones [94,95]. In cells, LOX is primarily a soluble, cytoplasmic
enzyme. LOX is also associated with the chloroplasts in photosynthetic tissues [96],
vacuoles in paraveinal mesophyll cells [97,98], and lipid bodies in oilseeds [99].
Many plants contain multiple LOX isozymes encoded by different genes. It is
estimated that there are up to 12 different LOX genes expressed as different developmental
stages in the life cycle of the soybean [100,101]. Peas have at least seven
isozymes, five of which are minor [102]. The isozymes show different properties and
likely play different roles throughout the life cycle of plants [99]. During soyseed
germination, the seed LOX isomers disappear, with increases in synthesis of the
seeding iso-LOXs [103,104].
Soybean seed LOX was one of the first enzymes to be purified and crystallized
[105] and is the best characterized of all plant LOXs. It is composed of three major
isozymes, L-1, L-2, and L-3, and one minor isozyme, L-4. L-1, L-2, and L-3 show
differences in regiospecificity, pH optimum, and enzymatic properties. The type 1
LOX (L-1) catalyzes the incorporation of oxygen predominantly at C-13 of 18:2 and
18:3 and has its optimum activity at pH 9. Its preferred substrate is free fatty acid.
Type 2, or mixture LOXs (L-2 and L-3), which have their maximum activities at
neutral pH values, do not show positional selectivity for oxygen incorporation. Indeed,
they catalyze the formation of approximately equal proportions of 9- and 13hydroperoxide
isomers. Preliminary studies with purified isozymes indicate that L-2
mostly produces the 13-hydroperoxide and L-3 the 9-hydroperoxide (unpublished
results). L-2 and L-3 are about equally active with 18:2, methyl 18:2, and linoleoyl
glycerol [106,127].
All three major soyseed LOXs are composed of single polypeptide chains of
similar lengths (about 96 kDa and 850 amino acids). The complete sequence of the
LOX isozymes [108–110] shows that L-1, L-2, and L-3 share high homology in the
carboxyl-terminal half of their polypeptide chains. Within this section, there is a
region known as the ‘‘histidine region,’’ which is particularly highly conserved in
both plant and animal LOXs [111–113]. Missense mutation substituting glutamine
for histidine—532 result in the loss of LOX2 activity and protein from mature
soybean seeds [113a]. Sequences of LOXs from a variety of plants have led to
classification of LOXs into two major groups, LOX1 and LOX2, based on general
homologies [114]. Most plant LOXs are of the LOX1 class, having DNA sequence
like soybean L-1. The LOX2 class has putative plastid transit sequences, indicating
that they are chloroplastic enzymes.
LOXs contain one atom of nonheme iron. The iron atom in lipoxygenase alternates
between the Fe(II) and the Fe(III) states during catalysis. The native-resting
LOX, a relatively inactive form, is in the high-spin Fe(II) state. Activation of the
native E-Fe(II) requires oxidation of the iron atom from Fe(II) to Fe(III) by the
reaction product, fatty acid hydroperoxides or H 2O 2 [86]. Because of the product
activation requirement, the oxygenation reaction exhibits a characteristic initial lag
period.
The LOX-catalyzed reaction is initiated by the stereospecific removal of hydrogen
from the C-11 methylene group when 18:2 is used as a substrate (Fig. 9).
This occurs possibly by a basic amino acid, followed by the transfer of an electron
back to the iron atom to reduce it to E-Fe(II).
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.