Front Matter

Figure 3B) by interacting with a basic residue at the bottom of the substrate-binding

cleft (see Figure 3B, residue B).

15.4 Formation of oxylipins with plant enzymes

In plants, LOX-derived hydroperoxy fatty acids are further metabolized via several

pathways (Figure 11), leading to a diversity of fatty acid derivatives functionalized

by oxygen (Hamberg, 1995; Blee, 1998b). These metabolites are collectively called

oxylipins (Blee, 1998b). The different pathways of oxylipin formation are named

according to the major enzyme involved, and some of these metabolic routes

have already been well characterized:

* The hydroperoxide lyase pathway (HPL), which involves an oxidative cleavage of

hydroperoxy fatty acids to short-chain aldehydes and keto fatty acids (C 12 -orC 9 -

x-keto compounds) (Matsui, 1998).

* The allene oxide synthase pathway (AOS), leading to the formation of an unstable

allene oxide intermediate, which subsequently is either hydrolyzed nonenzyma-

tically to yielda-orc-ketols, or is metabolized to the plant hormone jasmonic acid

(Wasternack and Parthier, 1997).

* During the peroxygenase pathway (POX), hydroperoxy fatty acids are converted

to epoxy-, epoxyhydroxy-, dihydroxy-, or polyhydroxy fatty acids. This conver-

sion of fatty acid hydroperoxides is paralleled by a co-oxidation of unsaturated

fatty acids (Blee, 1998a).

In addition to these well-characterized pathways, there are other metabolic routes

which include the formation of divinyl ethers (DES) (Grechkin, 1998), the forma-

tion of epoxy hydroxy fatty acids by an epoxy alcohol synthase (Hamberg, 1999),

and the reduction of hydroperoxy fatty acids to corresponding hydroxy fatty acids

(Feussner et al., 1997b).

Unfortunately, our knowledge on the biochemical characteristics of the enzymes

involved in these pathways is rather limited. Only some of them (AOS and HPL)

have been purified and/or cloned to date (Song and Brash, 1991; Song et al.,

1993; Laudert et al., 1996; Matsui, 1998). On the other hand, much experimental

data are available as to the chemistry of the reactions involved, and some of

them have recently been reviewed (epoxy-, dihydroxy-, epoxyhydroxy- or polyhy-

droxy fatty acids) (Gardner, 1997). In addition, a few reports described the use of

POX for formation of epoxy fatty acids in nonaqueous media (Piazza et al., 1999)

and immobilized HPL for the formationx-keto fatty acids and dicarboxylic fatty

acids of odd and even chain length (Nunez et al., 1997a,b; 1999).

332 15 Application of Lipoxygenases and Related Enzymes