摘要：

Glutathione transferases with basic isoelectric points are present in the cytosol of all human livers investigated. Adults as well as fetuses contain such basic proteins, referred to as transferases α-ε. Some adult (about 60%), but no fetal, livers have a different enzyme, glutathione transferase μ, with an isoelectric point at pH 6.6 [Warholm, M., Guthenberg, C., Mannervik, B., & von Bahr, C. (1981) Biochem. Biophys. Res. Commun. 98, 512-519]. This enzyme has also been found in adult adrenal glands. Chemical and physical properties distinguish three major types of human transferases: transferase μ, transferases α-ε, and transferase π from human placenta. Transferase μ has two subunits of Mr 26 300; Mr 53000 and Stokes radius = 3.0 nm were determined independently for the native dimeric protein. Analyses of the amino acid compositions show that the three types of transferases are not interconvertible by posttranslational modifications. Antibodies against any of the human transferases did not cross-react with the other proteins or with rat liver glutathione transferases. Circular dichroism spectra in the near-ultraviolet region are clearly different for the three types of transferases. Estimation of the secondary protein structure from the circular dichroism in the far-ultraviolet region gave 23% α-helix and 25% β-structure for transferase μ. Noteworthy kinetic properties of transferase μ are high specific activities with trans-4-phenyl-3-buten-2-one, benzo[a]pyrene 4,5-oxide, and styrene 7,8-oxide; pH optimum at 7.5. A random order sequential reaction scheme could explain the steady-state kinetics; experimental data were evaluated by means of nonlinear regression analysis. Transferase μ is highly efficient with benzo[a]pyrene 4,5-oxide as substrate: Km = 0.9 μM, and kcat/Km = 3.2 × 107 min-1 M-1. Glutathione derivatives with hydrophobic S-substituents were strong reversible inhibitors: Ki = 0.75 μM (competitive with glutathione) for S-n-hexylglutathione. Deoxycholate (Ki = 21 μM) and cholate (Ki = 40 μM) were both competitive with the second substrate, 1-chloro-2,4-dinitrobenzene, whereas bromosulfophthalein (Ki = 0.8 μM) and bilirubin (nonlinear inhibition) were noncompetitive with both substrates. Equilibrium binding measured by the quenching of the intrinsic protein fluorescence indicated hyperbolic saturation with bromosulfophthalein (Kd = 1 μM) and bilirubin (Kd 10 μM). Transferase μ was irreversibly inactivated by Hg2+, N-ethylmaleimide, N-phenylmaleimide, 2,4,6-trinitrobenzenesulfonate, and 1-fluoro-2,4-dinitrobenzene. The results indicate strongly that three genetically distinct types of human glutathione transferases exist. The high activity of transferase μ with epoxides may provide better protection against some chemical mutagens and carcinogens to those individuals having transferase μ in their tissues.

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