Two fatty acid ethyl ester (FAEE) synthase isoenzymes purified from human myocardium were reported to be glutathione S-transferases (GST) [(1989) Proc. Natl. Acad. Sci. USA 86, 4470-4473; and (1989) J. Clin. Invest, 84, 1942-19461. In the present study, the FAEE synthase activity of several purified and well characterized human GSTs were examined with ethanol and [14C]oleic acid as substrates. Three isoenzymes, GSTl, GST2 and GST3 which are members of the evolutionary classes /i, CI and II, respectively, were studied and failed to show any significant synthesis of FAEE after 45 min incubation at 37°C. FAEE synthase activity and GS'T3 activity in human placental extracts can be readily separated by ion exchange chromatography on DEAE cellulose, Thus the results show that FAEE synthase activity is not a feature of the major GSTS found in human tissues, The two FAEE synthase isoenzymes isolated by Bora et al. may have been co-purified with GST isoenzymes or these FAEE synthases may be members of the GST super family that have low specific activity in conventional GST assays and have not been previously described.Glutathione S-transferase; Fatty acid ethylester synthase INTRODUCTION In order to determine if FAEE synthase activity is a common feature of all human GSTs or if it is confinedIt has recently been reported [l] that a fatty acid ethyl to particular classes or isoenzymes, we have determined ester (FAEE) synthase isoenzyme purified from human the FAEE synthase activity of several highly purified myocardium had a very similar N-terminal sequence to human GST isoenzymes that represent examples of the that of an acidic human glutathione S-transferase three main evolutionary classes of cytosolic GST. In ad-(GST). The FAEE synthase also had GST activity and dition, we have followed the FAEE synthase activity of VVZ~ able to catalyse the conjugation of GSH and human placental extracts during the purification of 1 -chloro-2,4_dinitrobenzene.In addition, another GST3, the major GST isoenzyme expressed in placenta, FAEE synthase isoenzyme purified from human and the GST isoenzyme with the closest N-terminal'se-