Enzymatically-derived PGE2 is an optically pure compound whereas PGE2 derived from the free radical-catalyzed peroxidation of arachidonate is a racemic mixture. Ent-PGE2 is the opposite enantiomer of PGE2. Significant amounts of racemic PGE2 (rac-PGE2) are generated in vitro and in vivo in settings of oxidative stress via the isoprostane pathway. A proposed mechanism for the formation of rac-PGE2 involves the base catalyzed equilibration from 15-E2t-isoprostane (8-iso-PGE2), generated from the 15-H2t-isoprostane endoperoxide.