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

The absolute stereochemistry of the hydroxyl group in the previously reported red algal metabolite, 13-hydroxyarachidonic acid (13-HAA), was established as 13R by degradation and derivatization to chiral fragments. A cell-free homogenate of this alga was catalytically active in transforming exogenous arachidonic acid to 13R-HAA, 11R-hydroxyeicosatetraenoic acid (11R-HETE), 15S-HETE and eicosa-5Z,8Z,10E,12E,14Z-pentaenoic acid. While the hydroxyl groups in the HETE products contained a high, but less than quantitative level of label when the incubation was performed with O-18(2), the hydroxyl group of 13R-HAA was essentially unlabelled. Rather, its hydroxyl function derives from water as shown by (H2O)-O-18 labelling. Furthermore, facile chemical transformation of 13R-HAA into 11-HETE and 15-HETE was observed with concurrent incorporation of O-18 from (H2O)-O-18.