Biosynthesis and isomerization of 11-hydroperoxylinoleates by manganese- and iron-dependent lipoxygenases.

Abstract

Manganese lipoxygenase (Mn-LO) oxygenates linoleic acid (LA) to a mixture of the hydroperoxides--11 (S)-hydroperoxy-9Z,12Z-octadecadienoic acid [11(S)-HPODE] and 13(R)-hydroperoxy-9Z,11 E-octadecadienoic acid [13(R)-HPODE]-- and also catalyzes the conversion of 11 (S)-HPODE to 13(R)-HPODE via oxygen-centered (LOO-) and carbon-centered (L.) radicals [Hamberg, M., Su, C., and Oliw, E. (1998) Manganese Lipoxygenase. Discovery of a Bis-allylic Hydroperoxide as Product and Intermediate in a Lipoxygenase Reaction, J. Biol. Chem. 273, 13080-13088]. The aims of the present work were to investigate whether 11-HPODE can also be produced by iron-dependent lipoxygenases and to determine the enzymatic transformations of stereoisomers of 11-HPODE by lipoxygenases. Rice leaf pathogen-inducible lipoxygenase, but not soybean lipoxygenase-1 (sLO-1), generated a low level of 11-HPODE (0.4%) besides its main hydroperoxide, 13(S)-HPODE, on incubation with LA. Steric analysis revealed that 11-HPODE was enriched with respect to the R enantiomer [74% 11(R)]. In agreement with previous results, 11 (S)-HPODE incubated with Mn-LO provided 13(R)-HPODE, and the same conversion also took place with the methyl ester of 11(S)-HPODE. 11(R,S)-HPODE was metabolized biphasically in the presence of Mn-LO, i.e., by a rapid phase during which the 11(S)-enantiomer was converted into 13(R)-HPODE and a slow phase during which the 11(R)-enantiomer was converted into 9(R)-HPODE. sLO-1 catalyzed a slow conversion of 11 (S)-HPODE into a mixture of 13(R)-HPODE (75%), 9(S)-HPODE (10%), and 13(S)-HPODE (10%), whereas 11(R,S)-HPODE produced a mixture of nearly racemic 13-HPODE (approximately 70%) and 9-HPODE (approximately 30%). The results showed that 11HPODE can also be produced by an iron-dependent LO and suggested that the previously established mechanism of isomerization of 11(S)-HPODE involving suprafacial migration of O2 is valid also for the isomerizations of 11(R)-HPODE by Mn-LO and of 11(S)-HPODE by sLO-1.