Food Lipids: Chemistry, Nutrition, and Biotechnology

oleic acid. These results suggest that the substrate requirement for the hydrophilic
area of LOX is fairly broad, but in contrast, that for the hydrophobic area is strict
[124,125]. In addition to fatty acids, certain glycerides, fatty esters, and other fatty
acid derivatives also act as substrates for LOX [81,119,126,127]. However, the substrate
specificity obviously depends on LOX isozymes. Soybean LOX1 is relatively
more effective in the lipid-dependent O 2 uptake of free fatty acids compared to
esterified derivatives, whereas LOX2 or 3 is relatively more effective with esterified
18:2 than LOX1 [127].
One of the oldest but least understood characteristics of LOXs is their ability
to catalyze the bleaching of various pigments, such as carotene, xanthophylls, chlorophyll,
cholesterol, crocin, luein, and various dyes. This is a cooxidation reaction
that has been used as the basis for some LOX assays and in some industrial practices.
Pigment destruction during lipid peroxidation is usually hypothesized to stem
from reactive intermediates generated in the peroxidative reaction [128], and the
intermediates are thought to be free radicals. The observations that LOX is unable
to bleach when only preformed hydroperoxide is added to the reaction mixture and
that bleaching is able to occur anaerobically [126,129] indicate that pigment bleaching
is a radical-mediated pathway. The reports that the reaction conditions that can
retain conjugated diene production selectively inhibit bleaching and that the bleaching
is closely coupled with decreased carbonyl compound production (indicative of
peroxide breakdown) suggest that compounds produced before fatty hydroperoxide
formation are the ones active in bleaching.
Most models for cooxidation reaction involve the dissociation of the enzyme
radical complex. Both Weber et al. [130] and Grosch et al. [131] favor the lipid
peroxide (LOO�) as the reactive species in bleaching. However, Veldink et al. [119]
suggest that dissociation produces the unoxygenated fatty acid radical, which is then
free to attack other molecules, including pigments. Lack of stereospecificity of the
hydroperoxides formed and no requirement of aerobic conditions in bleaching implicate
the unoxygenated fatty acid pathway.
The bleaching activity of LOX depends on the enzyme sources and isozymes.
Enzymes from peas and beans (Phaseolus sp.) and the L-2 and L-3 from soybean
have a high cooxidation potential; potato LOX is intermediate, whereas the cooxidation
activity of wheat, flax, and soybean L-1 is poor [131,132]. The observed
differences in cooxidizing potentials among LOXs might result from the different
strengths of the associative bonds between enzyme and radical.
LOXs are inactivated by LOX substrate analogs and lipid antioxidants. The
antioxidants most commonly used as LOX inhibitors are �-tocopherol, nordihydroguaiaretic
acid (NDGA), propyl gallate, hydroquinone, and �-naphthol. These compounds
are generally thought to inhibit LOX by their ability to scavenge free radicals,
to reduce Fe(III), or both. Thus they are not truly specific LOX inhibitors. LOX
substrate mimics include competitive inhibitors (e.g., 18:1, PUFAs with E double
bonds) and suicide inhibitors, such as 5,8,11,4-eicosatetraynoic acid (ETYA), 12iodo-9Z-octadecenoic
acid, colneleic acid, and hexanal phenylhydrazone. So far, only
suicide inhibitors are considered to be specific LOX inhibitors and can be acceptably
used as one of the criteria for the identity of true LOXs. In addition, hydroxamic
acids, such as salicylhydroxamic acid (SHAM), phenylhydrazine/phenylhydrazones,
or nonsteroidal anti-inflammatory drugs, also cause inhibition of LOXs. The mech-
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.