Purification and Properties of Prostaglandin H‐E Isomerase from the Cytosol of Human Brain: Identification as Anionic Forms of Glutathione S‐Transferase

: Prostaglandin H‐E isomerase (EC 5.3.99.3) was purified from human brain cytosol. Purification was by ammonium sulfate fractionation, diethylaminoethyl‐Sephar‐ose chromatography, gel filtration on a BioGel P‐100 column, GSH‐agarose chromatography, and MonoQ chromatography. The activity was eluted in two peaks from the MonoQ column, which were designated peaks 1 and 2. The molecular weights of peaks 1 and 2, determined by gel filtration, were 42,000 and 44,000, respectively. On sodium dodecyl sulfate‐polyacrylamide gel electrophoresis, peak 1 showed two bands at the molecular weights of 24,500 and 25,000, and peak 2 showed a single band at the molecular weight of 25,000, results suggesting that both were dimeric proteins. The pI values of both enzymes were ∼5.4. The enzymes catalyzed selective conversion of prostaglandin H2 to prostaglandin E2. The Km values for prostaglandin H2 of peaks 1 and 2 were 147 and 308 μM, respectively, and the Vmax values were 380 and 720 nmol/min/mg of protein, respectively. GSH was required for the catalysis of both enzymes, and no other sulfhydryl compounds could support the reaction. A part of glutathione S‐transferase (EC 2.5.1.18) was copurified with peaks 1 and 2 of prostaglandin H‐E isomerase. Prostaglandin H‐E isomerase activity of peak 2 enzyme was competitively inhibited by 1‐chloro‐2,4‐dinitrobenzene, a substrate of glutathione S‐transferase. These results suggested that prostaglandin H‐E isomerases in human brain cytosol were identical with anionic forms of glutathione S‐transferase.