Food Lipids: Chemistry, Nutrition, and Biotechnology

Other biological effects of methyl jasmonate/jasmonic acid are summarized in
Table 1 [88–91,199–202].
b. Hydroperoxide Lyase Pathway. HPLS is an enzyme that cleaves fatty acid
hydroperoxides into two fragments at either side of hydroperoxy group to form an
oxo-acid and alkane/alkene or their derivatives.
In the higher plants, HPLS cleaves fatty acid hydroperoxides at the position
between the hydroperoxide-bearing carbon and the double bond (the so-called heterolytic
HPLS pathway). This cleavage results in aldehydic functions on both sides
of the site of cleavage, producing both an alkanal/alkenal and an oxo-acid (Fig. 11).
When 13-HPOD and HPOT are used as substrates, HPLS produces hexanal
and 3Z-hexenal, respectively, as well as 12-oxo-Z-9-dodecenoic acid. The 3Z-hexenal
and 12-oxo-9Z-dodecenoic acid are often isomerized enzymically or nonenzymically
into 2E-hexenal and 12-oxo-10E-dodecenoic acid, respectively. The aldehydes
could be reduced by alcohol dehydrogenases into the corresponding C 6-alcohols
[93,203,204]. Recently, the pathway has been expanded to include the conversion of
3Z-hexenal into 4-hydroxy-2E-hexenal [205,206]. Pulse-chase labeling of carnation
petals revealed that C 6-aldehydes and their derivatives were formed within a hydrophobic
subcompartment of lipid-protein particles within the cytosol and then presumedly
are released to the cytosol by blebbing of lipid-protein particles [206]. It is
now well established that the C 6-aldehydes produced by HPLS are important components
of the characteristic aromas and flavors of many fruits and vegetables. Hexanal
has a rancid green odor, whereas 3Z-hexenal and 2E-hexenal have intense grassy
and spicy-green odors, respectively. The corresponding alcohols lead to similar,
more subdued odors. The C 6-aldehydes and their derivatives have been demonstrated
to be responsible for ‘‘fresh note’’ of tomato, apple [207] and banana (Musa spp.);
however, higher levels of hexanal result in the rancid off-flavors associated with raw
legumes and deteriorated vegetable oils and nut products [81]. We found that the
control of C 6-aldehyde synthesis is very complicated in plant tissues. In soybean
seeds, LOX isomer 2 stimulated, but L-3 inhibited, C 6-aldehyde production compared
to L-1 [127,208]. In Arabidopsis leaves, changes of C 6-aldehyde generation paralleled
with alteration of only C 18 PUFA composition of chloroplastic lipids [209].
However, no such relationship was found in the soybean leaves and fruit during
development [95,210]. An alcohol dehydrogenase (ADH) mutant of Arabidopsis
ecotype, which was deficient in ADH activity, resulted in significantly quantitative
and qualitative changes in the accumulation of C 6 volatiles compared to the wildtype
plants. The total quantity of LOX-derived volatiles was greater on a fresh weight
basis in the ADH mutant. Qualitatively, hexanol and 3-hexenol levels were approximately
62% and 51% lower in the mutant, respectively, whereas levels of hexenal
were approximately equal to 10 times higher. Hexanal accumulation, however, was
unaffected [211].
Cleavage of 9-hydroperoxides proceeds by the process described above, except
the products are either 3Z-nonenal or 3Z,6Z-nonadienal from the HPOD or HPOT,
respectively, as well as 9-oxo-nonanoic acid (Fig. 11). The 9-hydroperoxide-specific
HPLS has been separated from the 13-hydroperoxide-specific lyase, indicating that
they are individual isozymes in cucumber [212]. After the C 9-aldehydes have formed,
other transformations occur similar to those discussed above for the aldehydes derived
from 13-hydroperoxides. The cucumber odor is principally due to 3Z-nonenal
and 3Z,6Z-nonadienal.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.