A NOVEL GLUTATHIONE-S-TRANSFERASE ISOZYME SIMILAR TO GST-8-8 OF RAT AND MGSTA4-4 (GST-5.7) OF MOUSE IS SELECTIVELY EXPRESSED IN HUMAN TISSUES

A mouse glutathione S-transferase (GST) isozyme designated as GST 5.7 or mGSTA4-4 belongs to a distinct subclass of the alpha-class isozymes of GST. It is characterized by kinetic properties intermediate betwee n the alpha- and pi-classes of GSTs. We have recently cloned and expre ssed this isozyme (rec-mGSTA4-4) in E. coli and have reported its comp lete primary sequence (Zimniak, P., et al. (1992) FEBS Lett., 313, 173 -176). Using antibodies raised against the homogenous rec-mGSTA4-4 exp ressed in E. coli, we now demonstrate that an ortholog of this isozyme was selectively expressed in various human tissues. The human ortholo g of mGST A4-4 purified from liver had a pi value of 5.8 and constitut ed approx. 1.7% of total GST protein of human liver. Similar to other cu-class GSTs, the N-terminus of this isozyme (GST 5.8) was also block ed. CNBr digestion of the enzyme yielded two major fragments with M(r) values of 12 kDa and 6 kDa. The sequences of these two fragments show ed identities in 16 out of 20 residues and 17 out of 20 residues with the corresponding sequences of its mouse ortholog (mGSTA4-4), and show ed significant homologies with the rat and chicken orthologs, GST 8-8 and GST CL3. Human liver GST 5.8 showed more than an order of magnitud e higher activity towards t-4-hydroxy-2-nonenal as compared to 1-chlor o-2,4-dinitrobenzene. This isozyme also expressed glutathione-peroxida se activity towards fatty acid, as well as phospholipid hydroperoxides suggesting its role in protection mechanisms against the toxicants ge nerated during lipid peroxidation. Western blot analysis of human tiss ues revealed that this GST isozyme was selectively expressed in human liver, pancreas, heart, brain and bladder tissues, but absent in lung, skeletal muscle, spleen and colon.