Sigma Class Glutathione Transferase of F. hepatica www.plosntds.org 50 mM NaCl, 0.5 mM tryptophan, 1 mM hematin, 1 U COX-1 enzyme, 100 mM arachidonic acid (All Sigma, UK. COX-1 [C0733]) and rFhGST-S1 at final concentration ranges of 0.1-100.0 mg/ml. Negative control reactions lacking either GST or COX-1 were also prepared. Reactions were incubated for 5-10 min in a water bath at 37uC. This was followed by 4 minutes incubation at 25uC in a shaking water bath. Prostaglandins were extracted by adding 860 mL of ice-cold ethyl acetate. Reactions were vortexed for 30 s then centrifuged briefly at 10,0006g at 4uC for 2 min. The upper ethyl acetate layer was retained and solvent was evaporated under a nitrogen stream at 45uC. The remaining residue was reconstituted in 50 ml of methanol/water/fomic acid (25:75:0,1) mix at pH 2.8 and stored at 280uC until ready for mass spectrometry analysis. Standards of prostaglandins D2, E2 and F2a (Cayman, Ltd) were also prepared in methanol/water/ formic acid mix for analysis.