Food Lipids: Chemistry, Nutrition, and Biotechnology

and Feng [171] later found a ‘‘new’’ enzyme that catalyzes the conversion of 13S-
HPOT into a cyclic fatty acid, 12-oxo-PDA, and consequently the term ‘‘hydroperoxide
cyclase’’ was used for this reaction. In 1987 Hamberg [165] showed that the
�- and �-ketols are derived from hydrolysis of the actual enzymatic product, an
allene oxide fatty acid. He renamed the enzyme as hydroperoxide dehydrase. Researchers
soon recognized that the allene oxide fatty acid was the intermediate involved
in cyclization of 13S-HPOT into 12-oxo-PDA [172]. Based on these findings,
in 1991 the nomenclature was changed again to allene oxide synthase [173]. However,
the International Union of Biochemistry and Molecular Biology gives this enzyme
the name hydroperoxide dehydratase (HPDS) [174].
HPDS occurs in most cereal seeds or seedlings, flaxseed, lettuce (Lactuca sativa
L.), sunflower, spinach, cotton seedlings, Vicia faba, eggplant (Solanum melongena
L.), and Chlorella pyrenoiclosa [175]. HPDS is a membrane-bound enzyme and
routinely isolated as a microsomal pellet. In spinach leaves, both HPDS and HPLS
are associated with whole and broken chloroplasts [176].
Study of the molecular biology of jasmonic acid biosynthesis implies that linolenic
acid is converted to jasmonic acid by a process that is likely to begin in
chloroplasts and end in peroxisomes, at least in leaves. Conversion of linolenic acid
to 12-oxophytodienoic acid in chloroplasts is accomplished through a multistep enzymatic
process involving lipoxygenase, allene oxide synthase, and allene oxide
cyclase. Jasmonic acid synthesis proceeds with the action of a cytoplasmic 12-oxophytodienoic
acid reductase and three rounds of �-oxidation that take place in peroxisomes.
Most of the enzymes and the corresponding coding genes involved have
been isolated and characterized in different plant species. The majority of these genes
are transcriptionally activated by wounding and some of them are also activated by
JA, allowing feedback regulation of the biosynthetic pathway [177]. Many woundinducible
genes are induced by JA but only some insect-inducible genes are induced
by JA [178]. A mutant of Arabidopsis, which lacks a functional 3-ketolacyl-CoA
thiolase protein and is defective in glyoxysomal fatty acid �-oxidation, has been
extensively characterized [179]. A comparison of thiolase genes between the wild
type and mutant revealed that the ATT codon for Ile100 at the fourth exon in the
wild type is changed to ATGG in the mutant. This nucleotide substitution of T to
GG causes a frameshift and produces a stop codon within the fourth exon. Therefore,
the mutant thiolase gene encodes a smaller protein, which is unstable and is degraded
rapidly [179]. Experiments by Meshack Afitlhile (personal communication) indicate
that this thiolase mutant has reduced biosynthesis of JA but not nearly to the extent
of the decrease in normal fatty acid �-oxidation, suggesting that there are both similarities
and differences between standard peroxisomal fatty acid �-oxidation and JA
biosynthesis.
The specificity of HPDS toward the 13- or 9-HPOD or HPOT is species-dependent.
In flaxseed and cotton, there is a strong preference for the 13-hydroperoxide
isomer [180,181], whereas the enzyme from corn germ shows no preference. The
HPDS gene from flaxseed was cloned and sequenced in 1993 [182]. The purification
and characterization of the enzyme showed that it is a cytochrome P450 with an M r
of 55 kDa [173]. HPDS from other plant sources frequently, however, has an M r
between 220 and 250 kDa [96,181], suggesting that it may exist as a tetramer.
An intriguing role proposed for jasmonic acid that has received wide attention
is its function as a signal (i.e., a chemical messenger) in response to certain stressors.
Copyright 2002 by Marcel Dekker, Inc. All Rights Reserved.