12-Oxophytodienoic acid (OPDA) is isomerized in the gut of herbivorous insects to tetrahydrodicranenone B (iso-OPDA).The transformation is achieved by a glutathione S-transferase present in the gut epithelium. Experiments with 9-[ 2 H]-iso-OPDA demonstrated the complete retention of the deuterium atom in the product 11-[ 2 H]-OPDA consistent with an intramolecular 1,3-hydrogen shift. Homology modeling based on the x-ray structure of a glutathione S-transferase from Anopheles gambiae revealed that the co-factor glutathione does not covalently bind to the substrate but appears to be involved in the initial deprotonation and enolization of the OPDA. The transformation resembles that of a mammalian GST-catalyzed isomerization of ⌬ 5 -3-ketosteroids to ⌬ 4 -3-ketosteroids or the conversion of prostaglandin A 1 to the biologically inactive prostaglandin B 1 .