Catalysis of the reaction: 12S-5Z,8Z,10E,14Z-12-hydro(pero)xy-eicosa-5,8,10,14-tetraenoic acid = (5Z,9E,14Z)-(8,11R,12S)-11,12-epoxy-8-hydroxyicosa-5,9,14-trienoate. 12S-5Z,8Z,10E,14Z-12-hydro(pero)xy-eicosa-5,8,10,14-tetraenoic acid is also known as 12S-HpETE, and (5Z,9E,14Z)-(8,11R,12S)-11,12-epoxy-8-hydroxyicosa-5,9,14-trienoate as hepoxilin A3.
hepoxilin A3 synthase activity