A new acidic form of glutathione S-transferase (GST, pI 6.2) was purified from rat brain by S-hexylglutathione affinity chromatography followed by chromatofocusing. This form occupied 20-25% of the total activity bound to the affinity column. It had a molecular mass (subunit 26 kDa) similar to that of a major GST form of rat testis (MT or 6-6) on sodium dodecyl sulfate/polyacrylamide gel electrophoresis. However, it differed from the MT in isoelectric point, activity towards 1,2-dichloro-4-nitrobenzene and immunological properties. On two-dimensional gel electrophoresis the brain form gave a spot which was identical in molecular mass, isoelectric point and immunological properties to a less acidic one (Yn1) of two spots (Yn1 and Yn2) of the testis GST-MT. Therefore, the brain acidic form is a homodimer, and named GST-Yn1Yn1. The activity was inhibited by sulfasalazine, an inhibitor of leukotriene-C4 synthase. This form (GST-Yn1Yn1) showed the highest leukotriene-C4 synthase activity, 496 nmol/mg protein in 5 min, among nine cytosolic GST isoenzymes from the rat. The Km values for leukotriene A4 and glutathione were 26 microM and 3.5 mM respectively. A major GST form of rat brain, occupying about 40% of the total activity, was identical with GST-P (7-7) purified from rat liver bearing preneoplastic hyperplastic nodules and localized at astroglias. GST-P also showed the significant leukotriene-C4 synthase activity, 67.2 nmol/mg protein in 5 min, but the Km for leukotriene A4 was 100 microM, fourfold higher than that of GST-Yn1 Yn1. These results suggest that mainly GST-Yn1 Yn1 may be involved in leukotriene-C4 synthesis in rat brain.