Food Lipids: Chemistry, Nutrition, and Biotechnology

Plant oxylipin pathway, also named as LOX pathway, starts with the LOXcatalyzed
peroxygenation of 18:2 or 18:3 to both enantiomeric and stereospecific
hydroperoxides of fatty acids. The hydroperoxides further serve as substrates for
several pathways: the hydroperoxide lyase (HPLS) pathway, the hydroperoxide dehydratase
(HPDS) pathway, the hydroperoxide isomerase pathway (HPIS), and the
hydroperoxide-dependent peroxygenase (HPPR)/epoxygenase (HPEP) pathway. Conversion
of the hydroperoxides by these pathways results in the formation of various
oxylipins, including regulatory molecules and some other substances of importance
in plant defense reactions.
Discovery of the plant oxylipin pathway can be traced to the early 1930s [80].
Food processing led to the first references to LOX concerning the bleaching of wheat
(Triticum aestivum L.) flour and dough following the addition of small amounts of
soybean [Glycine soya (L.) Merr.] flour. Since then, the impact of oxylipin pathway
on foods has been of great interest to food scientists [81–84]. We now know that
the oxylipin pathway influences aroma, tastes, and maybe deterioration of fresh plant
products [81,85]. However, its significance in plant physiology was very incompletely
understood until about 60 years later. In the last few years, novel possible
physiologic roles for the oxylipin pathway have been accumulating quite rapidly,
especially after the demonstration of jasmonins as a chemical messenger in plant
stress responses. These discoveries have resulted in new applications of LOX pathway
products to the preservation of fresh plant produce during postharvest storage.
For example, C6 aldehydes produced by the LOX/HPLS pathway are demonstrated
to be effective fungicides, bactericides, and insecticides, and are used by plants in
pest defense [86,87]. Several research groups are now testing the application of C6 aldehydes to control microbial growth on fruits after harvest. Since C6 aldehydes are
natural compounds in plant tissues, this treatment is considered to be much more
acceptible than the use of synthetic biocides. Jasmonic acid, a product of the LOX/
HPDS (also known as ADS) pathway, is found to function in plants as a chemical
message that triggers protective responses to stresses. Jasmonin treatment of chillingsensitive
fruits and vegetables reduces chilling injury and is suggested for storage of
these commodities [88–91].
Thus, in addition to introducing the basic concepts of the oxylipin pathway,
this section also focuses on some recent advances into our understanding of its
physiological significance in plants. Study of this mechanism should result in new
insights into food aspects of the oxylipin pathway product functions. Since the
oxylipin pathway greatly interests plant physiologists and biochemists, the next several
years should be fascinating times for research in this field.
1. Lipoxygenase (LOX)
The term ‘‘lipoxygenase’’ (linoleate: oxygen oxidoreductase, EC 1.13.11.12) refers
to a group of enzymes that catalyze the dioxygenation, by molecular oxygen, of
PUFAs containing Z,Z-pentadiene moieties; the resulting enantiomeric hydroperoxy
fatty acids have the Z,E-diene conjugation. Enzymes of this type were formerly
known as fat oxidase, carotene oxidase, and lipoxidase and have also been listed
under EC 1.99.2.1 and EC 1.13.1.13.
LOXs are widespread throughout the plant kingdom [92], being found among
the algae, bryophytes, and vascular plants. Considerable LOX activity is detected in
various plant organs, such as seeds, stems, leaves, fruits, tuber, and florets [92,93].
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.