| 1. |
- Danielson, U Helena, et al.
(författare)
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Kinetic independence of the subunits of cytosolic glutathione transferase from the rat
- 1985
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Ingår i: Biochemical Journal. - 0264-6021 .- 1470-8728. ; 231:2, s. 263-267
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Tidskriftsartikel (refereegranskat)abstract
- The steady-state kinetics of the dimeric glutathione transferases deviate from Michaelis-Menten kinetics, but have hyperbolic binding isotherms for substrates and products of the enzymic reaction. The possibility of subunit interactions during catalysis as an explanation for the rate behaviour was investigated by use of rat isoenzymes composed of subunits 1, 2, 3 and 4, which have distinct substrate specificities. The kinetic parameter kcat./Km was determined with 1-chloro-2,4-dinitrobenzene, 4-hydroxyalk-2-enals, ethacrynic acid and trans-4-phenylbut-3-en-2-one as electrophilic substrates for six isoenzymes: rat glutathione transferases 1-1, 1-2, 2-2, 3-3, 3-4 and 4-4. It was found that the kcat./Km values for the heterodimeric transferases 1-2 and 3-4 could be predicted from the kcat./Km values of the corresponding homodimers. Likewise, the initial velocities determined with transferases 3-3, 3-4 and 4-4 at different degrees of saturation with glutathione and 1-chloro-2,4-dinitrobenzene demonstrated that the kinetic properties of the subunits are additive. These results show that the subunits of glutathione transferase are kinetically independent.
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| 2. |
- Danielson, U Helena, et al.
(författare)
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Paradoxical inhibition of rat glutathione transferase 4-4 by indomethacin explained by substrate-inhibitor-enzyme complexes in a random-order sequential mechanism
- 1988
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Ingår i: Biochemical Journal. - 0264-6021 .- 1470-8728. ; 250:3, s. 705-711
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Tidskriftsartikel (refereegranskat)abstract
- Under standard assay conditions, with 1-chloro-2,4-dinitrobenzene (CDNB) as electrophilic substrate, rat glutathione transferase 4-4 is strongly inhibited (I50 = 1 microM) by indomethacin. No other glutathione transferase investigated is significantly inhibited by micromolar concentrations of indomethacin. Paradoxically, the strong inhibition of glutathione transferase 4-4 was dependent on high (millimolar) concentrations of CDNB; at low concentrations of this substrate or with other substrates the effect of indomethacin on the enzyme was similar to the moderate inhibition noted for other glutathione transferases. In general, the inhibition of glutathione transferases can be explained by a random-order sequential mechanism, in which indomethacin acts as a competitive inhibitor with respect to the electrophilic substrate. In the specific case of glutathione transferase 4-4 with CDNB as substrate, indomethacin binds to enzyme-CDNB and enzyme-CDNB-GSH complexes with an even greater affinity than to the corresponding complexes lacking CDNB. Under presumed physiological conditions with low concentrations of electrophilic substrates, indomethacin is not specific for glutathione transferase 4-4 and may inhibit all forms of glutathione transferase.
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| 3. |
- Danielson, U Helena, et al.
(författare)
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Structure-activity relationships of 4-hydroxyalkenals in the conjugation catalysed by mammalian glutathione transferases
- 1987
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Ingår i: Biochemical Journal. - 0264-6021 .- 1470-8728. ; 247:3, s. 707-713
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Tidskriftsartikel (refereegranskat)abstract
- The substrate specificities of 15 cytosolic glutathione transferases from rat, mouse and man have been explored by use of a homologous series of 4-hydroxyalkenals, extending from 4-hydroxypentenal to 4-hydroxypentadecenal. Rat glutathione transferase 8-8 is exceptionally active with the whole range of 4-hydroxyalkenals, from C5 to C15. Rat transferase 1-1, although more than 10-fold less efficient than transferase 8-8, is the second most active transferase with the longest chain length substrates. Other enzyme forms showing high activities with these substrates are rat transferase 4-4 and human transferase mu. The specificity constants, kcat./Km, for the various enzymes have been determined with the 4-hydroxyalkenals. From these constants the incremental Gibbs free energy of binding to the enzyme has been calculated for the homologous substrates. The enzymes responded differently to changes in the length of the hydrocarbon side chain and could be divided into three groups. All glutathione transferases displayed increased binding energy in response to increased hydrophobicity of the substrate. For some of the enzymes, steric limitations of the active site appear to counteract the increase in binding strength afforded by increased chain length of the substrate. Comparison of the activities with 4-hydroxyalkenals and other activated alkenes provides information about the active-site properties of certain glutathione transferases. The results show that the ensemble of glutathione transferases in a given species may serve an important physiological role in the conjugation of the whole range of 4-hydroxyalkenals. In view of its high catalytic efficiency with all the homologues, rat glutathione transferase 8-8 appears to have evolved specifically to serve in the detoxication of these reactive compounds of oxidative metabolism.
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| 4. |
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| 5. |
- Mannervik, Bengt, et al.
(författare)
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Glutathione transferases - structure and catalytic activity
- 1988
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Ingår i: Crc Critical Reviews in Biochemistry. - 0045-6411. ; 23:3, s. 283-337
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Tidskriftsartikel (refereegranskat)abstract
- The glutathione transferases are recognized as important catalysts in the biotransformation of xenobiotics, including drugs as well as environmental pollutants. Multiple forms exist, and numerous transferases from mammalian tissues, insects, and plants have been isolated and characterized. Enzymatic properties, reactions with antibodies, and structural characteristics have been used for classification of the glutathione transferases. The cytosolic mammalian enzymes could be grouped into three distinct classes--Alpha, Mu, and Pi; the microsomal glutathione transferase differs greatly from all the cytosolic enzymes. Members of each enzyme class have been identified in human, rat, and mouse tissues. Comparison of known primary structures of representatives of each class suggests a divergent evolution of the enzyme proteins from a common precursor. Products of oxidative metabolism such as organic hydroperoxides, epoxides, quinones, and activated alkenes are possible "natural" substrates for the glutathione transferases. Particularly noteworthy are 4-hydroxyalkenals, which are among the best substrates found. Homologous series of substrates give information about the properties of the corresponding binding site. The catalytic mechanism and the active-site topology have been probed also by use of chiral substrates. Steady-state kinetics have provided evidence for a "sequential" mechanism.
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| 6. |
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| 7. |
- Principato, Giovanni B, et al.
(författare)
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Relaxed thiol substrate specificity of glutathione transferase effected by a non-substrate glutathione derivative
- 1988
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Ingår i: FEBS Letters. - : Wiley. - 0014-5793 .- 1873-3468. ; 231:1, s. 155-158
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Tidskriftsartikel (refereegranskat)abstract
- Rat glutathione transferase 4-4 catalysed the conjugation of 2-mercaptoethanol with 1-chloro-2,4-dinitrobenzene in the presence of S-methyl-glutathione. The reaction was linearly dependent on enzyme concentration and saturation was seen with respect to both 2-mercaptoethanol and S-methyl-glutathione concentration. High concentrations of S-methyl-gluta-thione were inhibitory. The results suggest that the natural substrate glutathione has two distinct functions in the normal catalytic reaction, (i) induction of a catalytically competent conformation of the enzyme and (ii) provision of the substrate sulfhydryl group in the reaction catalyzed.
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| 8. |
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| 9. |
- Ålin, Per, et al.
(författare)
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Purification of major basic glutathione transferase isoenzymes from rat liver by use of affinity chromatography and fast protein liquid chromatofocusing
- 1985
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Ingår i: Analytical Biochemistry. - : Elsevier BV. - 0003-2697 .- 1096-0309. ; 146:2, s. 313-320
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Tidskriftsartikel (refereegranskat)abstract
- Seven major isoenzymes of glutahione transferase with isoelectric points ranging from pH 6.9 to 10 were isolated from rat liver cytosol. The purification procedure included affinity chromatography on immobilized S-hexylglutathione followed by high-performance liquid chromatofocusing. Characteristics, such as physical properties, reactions with antibodies, specific activities with various substrates, kinetic constants, and sensivities to a set of inhibitors, are given for discrimination and identification of the different isoenzymes. The multiple forms of the enzyme correspond to glutathione transferases 1-1, 1-2, 2-2, 3-3, 3-4, and 4-4 in the recently introduced nomenclature [W. B. Jakoby et al. (1984) Biochem. Pharmacol. 33, 2539–2540]. A seventh form appears to be a heterodimeric protein composed of subunit 3 and an as yet unidentified subunit.
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