Involved in isoprene degradation (PubMed:9687433, PubMed:10094686, PubMed:10715003). Catalyzes the glutathione-dependent ring opening of various epoxides (PubMed:9687433, PubMed:10094686). The highest conversion rate is observed with the physiological substrate, 3,4-epoxy-3-methyl-1-butene, which is the primary oxidation product of isoprene (PubMed:9687433, PubMed:10094686). It can also use other epoxides, including epoxyethane, epoxypropane, epithiopropane, epichlorohydrin, epifluorohydrin, epibromohydrin, 1,2-epoxybutane, 1,2-epoxyhexane, cis-2,3-epoxybutane, cis-1,2-dichloroepoxyethane and trans-1,2-dichloroepoxyethane (PubMed:9687433, PubMed:10094686). 2-glutathionyl-2-methylbut-3-en-1-ol = (3R)-3,4-epoxy-3-methylbut-1-ene + glutathione Activity is inhibited by 1,2-epoxyhexane. Optimum pH is 8.5-9.0. Homodimer. Belongs to the GST superfamily.