PEROXYGENASE-CATALYZED FATTY-ACID EPOXIDATION IN CEREAL SEEDS - SEQUENTIAL OXIDATION OF LINOLEIC-ACID INTO 9(S),12(S),13(S)-TRIHYDROXY-10(E)-OCTADECENOIC ACID

Peroxygenase-catalyzed epoxidation of oleic acid in preparations of ce real seeds was investigated. The 105,000 g particle fraction of oat (A vena sativa) seed homogenate showed high peroxygenase activity, i.e. 3 034 +/- 288 and 2441 +/- 168 nmol (10 min)(-1) mg(-1) protein in two c ultivars, whereas the corresponding fraction obtained from barley (Hor deum vulgare and Hordeum distichum), rye (Secale cereale), and wheat ( Triticum aestivum) showed only weak activity, i.e. 13 to 138 nmol (10 min)(-1) mg(-1) protein. In subcellular fractions of oat seed homogena te, peroxygenase specific activity was highest in the 105,000 g partic le fraction, whereas lipoxygenase activity was more evenly distributed and highest in the 105,000 g supernatant fraction. Incubation of [1-C -14]linoleic acid with the 105,000 g supernatant of oat seed homogenat e led to the formation of several metabolites, i.e. in order of decrea sing abundance, 9(S)hydroxy-10(E),12(Z)-octadecadienoic acid, 9(S),12( S),13(S)-trihydroxy-10(E)-octadecenoic acid, cis-9,10-epoxy-12(Z)-octa decenoic acid [mainly the 9(R),10(S) enantiomer], cis-12,13-epoxy-9(Z) -octadecenoic acid [mainly the 12(R),13(S) enantiomer], threo-12,13-di hydroxy-9(Z)-octadecenoic acid, and 12(R),13(S)-epoxy-9(S)-hydroxy-10( E)-octadecenoic acid. Incubation of linoleic acid with the 105,000 g p article fraction gave a similar, but not identical, pattern of metabol ites. Conversion of linoleic acid into 9(S),12(S),13(S)trihydroxy-10(E )-octadecenoic acid, a naturally occurring oxylipin with antifungal pr operties, took place by a pathway involving sequential catalysis by li poxygenase, peroxygenase, and epoxide hydrolase.