Purification and characterization of human muscle glutathione S-transferases: Evidence that glutathione S-transferase ζ corresponds to a locus distinct from GST1, GST2, and GST3

Human muscle glutathione S-transferase isozyme, GSTζ (pI 5.2) has been purified by three different methods using immunoaffinity chromatography, DEAE cellulose chromatography, and isoelectric focusing. GSTζ prepared by any of the three methods does not recognize antibodies raised against the α, μ, or π class glutathione S-transferases of human tissues. GSTζ has a blocked N-terminus and its peptide fingerprints also indicate it to be distinct from the α, β, or π class isozymes. As compared to GSTs of α, μ, and π classes, GSTζ displays higher activities toward t-stilbene oxide and Leukotriene A₄ methyl ester. GSTζ also expresses GSH-peroxidase activity toward hydrogen peroxide. The K_ms of GSTζ for CDNB and GSH were comparable to those reported for other human GSTs but its V_max for CDNB, 7620 mol/mol/ min, was found to be considerably higher than that reported for other human GSTs. The kinetics of inhibition of GSTζ by hematin, bile acids, and other inhibitors also indicate that it was distinct from the three classes of GST isozymes. These studies suggest that GSTζ corresponds to a locus distinct from GST1, GST2, and GST3 and probably corresponds to the GST4 locus as suggested previously by Laisney et al. (1984, Human Genet. 68, 221-227). The results of peptide fingerprints and kinetic analysis indicate that as compared to the π and α class isozymes, GSTζ has more structural and functional similarities with the μ class isozymes. Besides GSTζ several other GST isozymes belonging to π and μ class have also been characterized in muscle. The π class GST isozymes of muscle have considerable charge heterogeneity among them despite identical N-terminal sequences.