Food Lipids: Chemistry, Nutrition, and Biotechnology

Another type of HPLS (the so-called homolytic HPLS pathway), affording
cleavage fragments different from those discussed above, has been reported in fungi,
algae, and a species of grass [86]. In algae, the enzyme cleaves the 13-hydroperoxide
of 18:2 or 18:3 to produce 5- and 13-carbon fragments, rather than 6- and 12-carbon
fragments as observed in higher plants. The freshwater alga Chlorella pyrenoidosa
possesses an HPLS that converts the 13-HPOD or HPOT to pentane or pentene and
13-oxo-9Z,11E-tridecadienoic acid [213]. HPLS from blue-green alga Oscillatoria
sp. produces pentanol rather than pentane [214]. In mushroom (Psalliota bispora
Lange), a LOX may exist that is specific for oxidizing the 10S position of fatty acid.
Mushroom HPLS cleaved the 10S-HPOD into 1-octen-3-ol and 10-oxo-8E-decenoic
acid [215]. 1-Octen-3-ol is an important flavor component of mushrooms.
The existence of HPLS was first predicted from studies on the biogenesis of
volatiles by bananas and other fruits [216]. Since then, the enzyme activity has
widely detected among the higher plants and variety of plant organs [93]. Most
HPLSs appear to be membrane-bound. In the flesh of cucumber fruit, HPLS is associated
with three membrane fractions: plasma membrane, Golgi apparatus, and
endoplasmic reticulum [83,217]. The enzyme in leaves is typically found in chloroplasts,
where it is bound to the thylakoid membrane [96,218].
HPLSs have been partially purified and characterized from several plant
sources, mostly dicotyledons [124,218–221]. Native soybean HPLS, estimated by
gel filtration, is approximately 250 kDa [202], but sodium dodecyl sulfate gel electrophoresis
of the purified enzyme reveals a protein band with an M r value of only
62 kDa, suggesting that soybean HPLS exists as a tetramer in its native state. However,
the enzyme from tea leaves is probably monomeric in its native state (M r 63
kDa) [93]. In immature fruits of green bell pepper [222], HPLS is a homotrimer of
55-kDa subunits. The activity is considerably inhibited by lipophilic antioxidants.
The enzyme shows strict substrate specificity, and 13-HPOT seems to be a sole
substrate in vivo. Tissue-print immunoblot analysis indicated that HPLS was most
abundant in the outer parenchymal cells of the pericarp of the fruits. Spectrophotometric
analysis of HPLS indicated that it is a heme protein. Although CO treatment
of the enzyme causes no appearance of a peak at 450 nm, the spectroscopic feature
highly resembled that of HPDS. Further analysis of amino acid sequence showed
highest homology with flax HPDS, suggesting that the HPLS gene is a member of
a novel family of cytochrome P450 that acts on fatty acid hydroperoxides.
Both 13- and 9-hydroperoxides of C 18-PUFAs serve as substrates for the enzyme,
but HPLS specificities depend on the enzyme isoforms and 18:2 or 18:3 [83].
The enzyme in nonphotosynthetic plant tissues is typically more active with HPOD,
whereas in photosynthetic organs HPLS usually shows higher activity with the hydroperoxides
of linolenic acid [93,95,127,223]. In the majority of plants, HPLS is
specific for 13-hydroperoxides. However, the enzyme from cucumber and kidney
bean leaves utilizes both 9- and 13-hydroperoxide isomers. In pear fruit, only the 9hydroperoxide
isomer is a suitable substrate for HPLS [83].
Studies of substrate specificity of tea leaf HPLS using an entire series of �-6S
hydroperoxy-C 14–C 24 dienoic and trienoic acids as substrates show that the 22-carbon
hydroperoxides but not the natural substrates (i.e., those of 18 C atoms) have the
highest reactivities. The reactivities of the hydroperoxides of trienoic acids are always
four to ten times higher than those of the dienoic acids [125]. In the soybean seeds,
C 6-aldehydes are formed in the presence of free PUFAs rather than the PUFA glyc-
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