BIOSYNTHESIS OF 13R-HYDROXYARACHIDONIC ACID, AN UNUSUAL OXYLIPIN FROMTHE RED ALGA LITHOTHAMNION-CORALLIOIDES

The absolute stereochemistry of the hydroxyl group in the previously r eported red algal metabolite, 13-hydroxyarachidonic acid (13-HAA), was established as 13R by degradation and derivatization to chiral fragme nts. A cell-free homogenate of this alga was catalytically active in t ransforming exogenous arachidonic acid to 13R-HAA, 11R-hydroxyeicosate traenoic acid (11R-HETE), 15S-HETE and eicosa-5Z,8Z,10E,12E,14Z-pentae noic acid. While the hydroxyl groups in the HETE products contained a high, but less than quantitative level of label when the incubation wa s performed with O-18(2), the hydroxyl group of 13R-HAA was essentiall y unlabelled. Rather, its hydroxyl function derives from water as show n by (H2O)-O-18 labelling. Furthermore, facile chemical transformation of 13R-HAA into 11-HETE and 15-HETE was observed with concurrent inco rporation of O-18 from (H2O)-O-18.