The glutathione S-transferases: a group of multifunctional detoxification proteins

Cloning and sequence analysis of a cDNA for a rat liver glutathione S-transferase Yb subunit

We have isolated a Yb-subunit cDNA clone from a GSH S-transferase (GST) cDNA library made from rat liver polysomal poly(A) RNAs. Sequence analysis of one of these cDNA, pGTR200, revealed an open reading frame of 218 amino acids of Mr = 25,915. The deduced sequence is in agreement with the 19 NH2-terminal residues for GST-A. The sequence of pGTR200 differs from another Yb cDNA, pGTA/C44 by four nucleotides and two amino acids in the coding region, thus revealing sequence microheterogeneity. The cDNA insert in pGTR200 also contains 36 nucleotides in the 5' noncoding region and a complete 3' noncoding region. The Yb subunit cDNA shares very limited homology with those of the Ya or Yc cDNAs, but has relatively higher sequence homology to the placental subunit Yp clone pGP5. The mRNA of pGTR200 is not expressed abundantly in rat hearts and seminal vesicles. Therefore, the GST subunit sequence of pGTR200 probably represents a basic Yb subunit. Genomic DNA hybridization patterns showed a complexity consistent with having a multigene family for Yb subunits. Comparison of the amino acid sequences of the Ya, Yb, Yc, and Yp subunits revealed significant conservation of amino acids (approximately 29%) throughout the coding sequences. These results indicate that the rat GSTs are products of at least four different genes that may constitute a supergene family.

A subclass of glutathione S-transferases as intracellular high-capacity and high-affinity steroid-binding proteins

The distribution of glucocorticoids incubated with rat liver cytosol preparations or administered in vivo to adrenalectomized rats was analysed by chromatographic procedures. Corticosterone or dexamethasone was co-eluted with Yb-type GSH S-transferases in anion-exchange and gel-permeation chromatography systems, and these glucocorticoids also were bound to Yb forms in analyses by immunoadsorbent and lysyl-GSH affinity matrices. Pretreatment of cytosol with lysyl-GSH to extract GSH S-transferases or incubation with excess bilirubin, which is expected to compete with steroids for binding to the protein, yielded preparations that were devoid of this major steroid-binding component. In mixtures of the multiple rat GSH S-transferases, corticosterone preferentially interacted with Yb forms rather than Ya and Yc subgroups. All of the multiple Yb forms resolved by chromatofocusing procedures retained the steroid-binding capacity. It is suggested that these abundant proteins can account for a considerable share of intracellular glucocorticoid binding and represent a high-affinity non-saturable binding component with potential to function in steroid-hormone metabolism and action.

A comparative study of the regulation of cytochrome P-450 and glutathione transferase gene expression in rat liver

A cDNA clone for the Ya subunit of glutathione transferase from rat liver was constructed in E. coli. The clone hybridized to Ya and Yc subunit messenger RNAs. On the basis of experiments involving cell-free translation and hybridization to the cloned probe, it was shown that prototype inducers of cytochrome P-450 such as phenobarbitone and 3-methylcholanthrene as well as inhibitors such as CoCl2 and 3-amino-1,2,4-triazole enhanced the glutathione transferase (Ya+Yc) messenger RNA contents in rat liver. A comparative study with the induction of cytochrome P-450 (b+e) by phenobarbitone revealed that the drug manifested a striking increase in the nuclear pre-messenger RNAs for the cytochrome at 12 hr, but did not significantly affect the same in the case of glutathione transferase (Ya+Yc). 3-Amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone mediated increase in cytochrome P-450 (b+e) nuclear pre-messenger RNAs. These compounds did not significantly affect the glutathione transferase (Ya+Yc) nuclear pre-messenger RNA levels. The polysomal, poly (A)- containing messenger RNAs for cytochrome P-450 (b+e) increased by 12-15 fold after phenobarbitone administration, reached a maximum around 16 hr and then decreased sharply. In comparison, the increase in the case a glutathione transferase (Ya+Yc) messenger RNAs was sluggish and steady and a value of 3-4 fold was reached around 24 hr. Run-off transcription rates for cytochrome P-450 (b+e) increased by nearly 15 fold in 4 hr after phenobarbitone administration, whereas the increase for glutathione transferase (Ya+Yc) was only 2.0 fold. At 12 hr after the drug administration, the glutathione transferase (Ya+Yc) transcription rates were near normal. Administration of 3-amino-1,2,4-triazole and CoCl2 blocked the phenobarbitone-mediated increase in the transcription of cytochrome P-450 (b+e) messenger RNAs. These compounds at best had only marginal effects on the transcription of glutathione transferase (Ya+Yc) messenger RNAs. The half-life of cytochrome P-450 (b+e) messenger RNA was estimated to be 3-4 hr, whereas that for glutathione transferase (Ya+Yc) was found to be 8-9 hr. Administration of phenobarbitone enhanced the half-life of glutathione transferase (Ya+Yc) messenger RNA by nearly two fold. It is suggested that while transcription activation may play a primary role in the induction of cytochrome P-450 (b+e), the induction of glutathione transferase (Ya+Yc) may essentially involve stabilization of the messenger RNAs.

Use of immuno-blot techniques to discriminate between the glutathione S-transferase Yf, Yk, Ya, Yn/Yb and Yc subunits and to study their distribution in extrahepatic tissues. Evidence for three immunochemically distinct groups of transferase in the rat

The glutathione S-transferases are dimeric enzymes whose subunits can be defined by their mobility during sodium dodecyl sulphate/polyacrylamide-gel electrophoresis as Yf (Mr 24,500), Yk (Mr 25,000), Ya (Mr 25,500), Yn (Mr 26,500), Yb1 (Mr 27,000), Yb2 (Mr 27,000) and Yc (Mr 28,500) [Hayes (1986) Biochem. J. 233, 789-798]. Antisera were raised against each of these subunits and their specificities assessed by immuno-blotting. The transferases in extrahepatic tissues were purified by using, sequentially, S-hexylglutathione and glutathione affinity chromatography. Immune-blotting was employed to identify individual transferase polypeptides in the enzyme pools from various organs. The immuno-blots showed marked tissue-specific expression of transferase subunits. In contrast with other subunits, the Yk subunit showed poor affinity for S-hexylglutathione-Sepharose 6B in all tissues examined, and subsequent use of glutathione and glutathione affinity chromatography. Immuno-blotting was employed to identify a new cytosolic polypeptide, or polypeptides, immunochemically related to the Yk subunit but with an electrophoretic mobility similar to that of the Yc subunit; high concentrations of the new polypeptide(s) are present in colon, an organ that lacks Yc.

Suppression of high-affinity ligand binding to the major glutathione S-transferase from Galleria mellonella by physiological concentrations of glutathione

The major glutathione S-transferase from larvae of Galleria mellonella binds a number of synthetic triphenylmethane dyes with dissociation constants of the order of 10(-6) M or less. The organ distribution of the enzyme activity does not parallel the uptake of such dyes by the insect's organs in vivo. The affinity of the protein for such dyes is decreased by about an order of magnitude by the presence of glutathione in normal physiological concentration. This appears to be the cause of this protein's lack of efficacy as a 'ligandin' in vivo. The dyes appear to be acting as ineffective substrate analogues, binding at the catalytic site and impeding, in a reciprocal fashion, the binding of glutathione. Fluorescence-quenching titration and kinetic experiments together indicate the existence of a single ligand-binding and catalytic site per dimeric enzyme molecule.

Stimulation of mouse liver glutathione S-transferase activity in propylthiouracil-treated mice in vivo by tri-iodothyronine

Female C57Bl/6J mice were given drinking water containing 0.05% propylthiouracil to induce a hypothyroid condition. Mitochondrial glycerol-3-phosphate dehydrogenase activity, used as an index of hypothyroidism, was 57.1 +/- 4.5 and 29.4 +/- 3.8 nmol/min per mg of protein for control and propylthiouracil-treated animals respectively. Administration of tri-iodothyronine resulted in an approx. 4.5-fold increase in dehydrogenase activity in propylthiouracil-treated animals. A dose-dependent increase in hepatic GSH S-transferase activity in propylthiouracil-treated animals was observed at tri-iodothyronine concentrations ranging from 2 to 200 micrograms/100 g body wt. This increase in transferase activity was seen only when 1,2-epoxy-3-(p-nitrophenoxy)propane was used as substrate for the transferase. Transferase activity with 1-chloro-2,4-dinitrobenzene and 1,2-dichloro-4-nitrobenzene as substrate was decreased by tri-iodothyronine. Administration of actinomycin D (75 micrograms/100 g body wt.) inhibited the tri-iodothyronine induction of transferase activity. Results of these studies strongly suggest that tri-iodothyronine administration markedly affected the activities of GSH S-transferase by inducing a specific isoenzyme of GSH S-transferase and suppressing other isoenzymic activities.

Inhibition of hepatic and extrahepatic glutathione S-transferases by primary and secondary bile acids

Glutathione S-transferases are a complex family of dimeric proteins that play a dual role in cellular detoxification; they catalyse the first step in the synthesis of mercapturic acids, and they bind potentially harmful non-substrate ligands. Bile acids are quantitatively the major group of ligands encountered by the glutathione S-transferases. The enzymes from rat liver comprise Yk (Mr 25 000), Ya (Mr 25 500), Yn (Mr 26 500), Yb1, Yb2 (both Mr 27 000) and Yc (Mr 28 500) monomers. Although bile acids inhibited the catalytic activity of all transferases studied, the concentration of a particular bile acid required to produce 50% inhibition (I50) varies considerably. A comparison of the I50 values obtained with lithocholate (monohydroxylated), chenodeoxycholate (dihydroxylated) and cholate (trihydroxylated) showed that, in contrast with all other transferase monomers, the Ya subunit possesses a relatively hydrophobic bile-acid-binding site. The I50 values obtained with lithocholate and lithocholate 3-sulphate showed that only the Ya subunit is inhibited more effectively by lithocholate than by its sulphate ester. Other subunits (Yk, Yn, Yb1 and Yb2) were inhibited more by lithocholate 3-sulphate than by lithocholate, indicating the existence of a significant ionic interaction, in the bile-acid-binding domain, between (an) amino acid residue(s) and the steroid ring A. By contrast, increasing the assay pH from 6.0 to 7.5 decreased the inhibitory effect of all bile acids studied, suggesting that there is little significant ionic interaction between transferase subunits and the carboxy group of bile acids. Under alkaline conditions, low concentrations (sub-micellar) of nonsulphated bile acids activated Yb1, Yb2 and Yc subunits but not Yk, Ya and Yn subunits. The diverse effects of the various bile acids studied on transferase activity enables these ligands to be used to help establish the quaternary structure of individual enzymes. Since these inhibitors can discriminate between transferases that appear to be immunochemically identical (e.g. transferases F and L), bile acids can provide information about the subunit composition of forms that cannot otherwise be distinguished.

Isolation and characterization of leukotriene C4 synthetase of rat basophilic leukemia cells

When leukotriene (LT) A4 was incubated with subcellular fractions of sonicated rat basophilic leukemia (RBL) cells in the presence of glutathione, the enzyme producing LTC4, designated LTC4 synthetase, was found in the 105,000 X g pellet (microsomes) with a 3-fold enrichment in specific activity over that of the sonicate. The identification of the reaction product as LTC4 was confirmed by its identical retention time on reverse-phase HPLC to that of synthetic LTC4, the incorporation of [3H]glutathione into the product, its reactivity in a radioimmunoassay, and its UV absorption spectrum. In contrast, glutathione S-transferase activity, measured spectrophotometrically with 1-chloro-2,4-dinitrobenzene, was detected predominantly in the 105,000 X g supernatant (89%) and also in the microsomes (7%). The microsomal glutathione S-transferase and LTC4 synthetase were solubilized with 0.4% Triton X-102 and separated by DEAE-Sephacel chromatography; the former appeared in the effluent and the latter in the eluate after the addition of 0.16 M NaCl to the equilibration buffer. Solubilized, microsomal glutathione S-transferase was inhibited by S-hexylglutathione with an IC50 of 36 microM and was stable at 40 degrees C for 5 min, whereas LTC4 synthetase was only slightly inhibited (IC50, 2.3 mM) by S-hexylglutathione and retained no activity after incubation at 40 degrees C for 5 min. The partially purified LTC4 synthetase showed a specific activity of 1.34 +/- 0.51 nmol of LTC4 per 10 min per mg of protein (mean +/- SD, n = 9), representing a 10-fold purification from the sonicate and catalyzed the dose- and time-dependent production of LTC4 from LTA4 and glutathione. The apparent Km values for LTA4 and glutathione were estimated by Lineweaver-Burk plots to be 5-10 microM and 3-6 mM, respectively. These results indicate that the conjugation of LTA4 with glutathione to form LTC4 is catalyzed by a unique microsomal enzyme.

Identification of Yb-glutathione-S-transferase as a major rat liver protein labeled with dexamethasone 21-methanesulfonate

Dexamethasone 21-methanesulfonate, an affinity label for glucocorticoid-binding proteins, was incubated with rat liver cytosol preparations. The predominant covalently labeled component was identified as Yb-glutathione-S-transferase on the basis of chromatographic properties, electrophoretic mobility, and specific retention by an anti-Yb-immunoadsorbent. Affinity labeling of this protein was blocked by excess dexamethasone. Preferential reactivity of dexamethasone 21-methanesulfonate with the Yb subclass of glutathione-S-transferase (glutathione transferase, EC 2.5.1.18) was also evident with mixtures containing the multiple forms of the enzyme. Yb-glutathione-S-transferase, the nonsaturable glucocorticoid-binding component of rat liver cytosol should, therefore, be reclassified; because of its high concentration and selective interaction with steroids, this enzyme may be an intracellular glucocorticoid-binding protein and, thereby, influence transport, metabolism, and action of the steroids.

Cloning and the nucleotide sequence of rat glutathione S-transferase P cDNA

A cDNA library prepared from poly(A)+ RNA of 2-acetylaminofluorene (AAF) induced rat hepatocellular carcinoma was screened by synthetic DNA probes deduced from a partial amino acid sequence of glutathione S-transferase P subunit that had been isolated from the tumor by two-dimensional gel electrophoresis. One of the four clones analyzed contained an mRNA region encoding the total amino acid sequence of this enzyme subunit and the complete 3'-noncoding region. The nucleotide sequence indicates that this enzyme subunit has 209 amino acids (calculated Mr=23,307) distinct from other glutathione S-transferase subunits such as Ya and Yc. Comparison of the amino acid sequences between these proteins indicates that glutathione S-transferase P subunit gene has been evolved from the ancestral gene at an earlier stage than the separation of Ya and Yc and that there are at least three domains having a considerable homology with each other in these enzymes. The very large increase of this mRNA in chemically induced hepatocellular carcinoma suggests a characteristic derepression of this gene during hepatocarcinogenesis.

Acidic glutathione S-transferases of rat testis

In most organs of the rat the predominant forms of glutathione S-transferase have alkaline (greater than 7.0) pI values. In contrast, in the cytosol from rat testes almost 50% of the transferase activity is due to isoenzymes with acidic (less than 7.0) pI values. We have purified three acidic forms of glutathione S-transferase from rat testis cytosol. One form accounted for more than 90% of the enzymic activity in the acidic fraction. This major form was a homodimer of a new subunit, termed Yt. This subunit had an electrophoretic mobility that was different from the subunits that form the alkaline transferases. In addition, functional and immunological studies were consistent with the unique nature of the Yt subunit. The two minor acidic enzymes of rat testis appeared to be heterodimers of the Yt subunit and a subunit with an electrophoretic mobility identical with that of the Yb subunit present in some alkaline enzymes.

Purification and characterization of glutathione S-transferases P, S and N. Isolation from rat liver of Yb1 Yn protein, the existence of which was predicted by subunit hybridization in vitro

The glutathione S-transferases are dimeric proteins and comprise subunits of Mr 25 500 (Ya), 26 500 (Yn), 27 000 (Yb1 and Yb2) and 28 500 (Yc). Enzymes containing Ya and/or Yc subunits have been isolated as have forms containing binary combinations of Yn, Yb1 and Yb2 subunits. To date only one enzyme, transferase S, has been described that is a YbYn heterodimer [Hayes & Chalmers (1983) Biochem. J. 215, 581-588]; the identity of the Yb monomer found in transferase S has not been reported previously. The identification and isolation of a YnYn dimer (transferase N) from rat testis is now described. This has enabled structural and functional comparisons to be made between Yb1, Yb2 and Yn monomers. Reversible dissociation experiments between the YnYn and Yb1Yb1 homodimers and between the YnYn and Yb2Yb2 homodimers demonstrated that Yn monomers can hybridize with both Yb1 and Yb2 monomers. Reversible dissociation of transferases N and C (Yb1Yb2) showed that both Yb1 and Yb2 monomers can hybridize with Yn monomers under competitive conditions. The hydridization data suggest that transferase S represents the Yb2Yn subunit combination. A knowledge of the elution position from chromatofocusing columns of the Yb1Yn hybrid that was formed in vitro enabled a purification scheme to be devised for an enzyme from rat liver (transferase P) believed to consist of Yb1Yn subunits. A comparison of the chromatographic behaviour of the YnYn, Yb1Yb1 and Yb2Yb2 dimers on chromatofocusing and hydroxyapatite columns with the behaviour of transferases P and S on the same matrices suggests these two enzymes may be identified as the Yb1Yn and Yb2Yn dimers respectively. The catalytic activities and the inhibitory effects of non-substrate ligands on transferases P and S are significantly different and again suggest they comprise Yb1 and Yn subunits and Yb2 and Yn subunits respectively; transferase P exhibits a 6-fold higher specific activity for 1,2-dichloro-4-nitrobenzene than does transferase S, whereas, conversely, transferase S possesses a 9-fold higher specific activity for trans-4-phenylbut-3-en-2-one than does transferase P. The quaternary structure of transferases P and S was verified by using peptide mapping and 'Western blotting' techniques.

Glutathione S-transferases of human brain. Evidence for two immunologically distinct types of 26500-Mr subunits

Human brain contains one cationic (pI8.3) and two anionic (pI5.5 and 4.6) forms of glutathione S-transferase. The cationic form (pI8.3) and the less-anionic form (pI5.5) do not correspond to any of the glutathione S-transferases previously characterized in human tissues. Both of these forms are dimers of 26500-Mr subunits; however, immunological and catalytic properties indicate that these two enzyme forms are different from each other. The cationic form (pI8.3) cross-reacts with antibodies raised against cationic glutathione S-transferases of human liver, whereas the anionic form (pI5.5) does not. Additionally, only the cationic form expresses glutathione peroxidase activity. The other anionic form (pI4.6) is a dimer of 24500-Mr and 22500-Mr subunits. Two-dimensional gel electrophoresis demonstrates that there are three types of 26500-Mr subunits, two types of 24500-Mr subunits and two types of 22500-Mr subunits present in the glutathione S-transferases of human brain.

Studies on the glutathione S-transferase activity associated with rat liver mitochondria

The major proportion of rat liver glutathione S-transferase is cytosolic. Carefully washed mitochondria contain 0.25-0.47% of the cytosolic activity. Subfractionation of washed mitochondria using digitonin treatment revealed that glutathione S-transferase release did not parallel that of any of the mitochondrial marker enzymes. Glutathione S-transferase release paralleled that of lactate dehydrogenase, suggesting that these 'mitochondrial' activities are due to loosely bound cytoplasmic forms.

Glutathione S-transferase Ya subunit is coded by a multigene family located on a single mouse chromosome

A cloned DNA probe of Ya, the major glutathione S-transferase subunit in rat liver, was used to study the organization of Ya genes in the mouse genome. Southern blot analysis of mouse genomic DNA indicates that the Ya subunit is encoded by a multigene family. The chromosomal distribution of Ya genes was determined by analysis of DNA from a panel of mouse-Chinese hamster somatic cell hybrids. All detectable Ya genes were found to be located on chromosome 9. At least some of the Ya-specific DNA sequences are clustered since, by screening a mouse genomic library, two recombinant phages, each containing two different Ya DNA sequences in the same insert, have been isolated. The finding that Ya is encoded by a cluster of different genes raises the question of the specificity of the different Ya DNA sequences.

Evidence for two forms of ligandin (YaYa dimers of glutathione S-transferase) in rat liver and kidney

CM-cellulose chromatography of rat liver and kidney cytosol at pH6 reveals the presence of a second Ya-subunit dimer of glutathione S-transferase (GST-F) in addition to the recently described GST-YaYa (GST-L; our nomenclature) [Hayes & Clarkson (1982) Biochem. J. 207, 459-470]. The two forms are structurally similar (by the criteria of CNBr- and Staphylococcus-V8-proteinase-cleavage peptide maps), and both are sensitive to inhibition by haemin. However, their kinetic parameters with 1-chloro-2,4-dinitrobenzene are quite distinct, and they show differential inducibility by phenobarbitone. These results suggest a similar heterogeneity in Ya-subunits to that previously described for Yb-subunits of glutathione S-transferase and indicate that significant gene duplication may have occurred in these multifunctional intracellular binding proteins.

Purification and properties of glutathione S-transferases from larvae of Wiseana cervinata

The glutathione S-transferases from the porina moth, Wiseanna cervinata, were purified by affinity chromatography, cation-exchange chromatography and preparative isoelectrofocusing. The major transferase (IV) was purified to homogeneity by a factor of 530-fold with a yield of 83%. Other transferases present were purified to a smaller degree (approx. 50-fold) to a stage of near-homogeneity. The transferases examined all had Mr values about 45 000-50 000. They appeared to be homodimers of either of two types of subunit, of Mr 22 800 and 24 600. Enzymes consisting of the different types of subunit were not immunologically cross-reactive. The major enzyme fractions separated by cation-exchange chromatography were both active with 1-chloro-2,4-dinitrobenzene, 1,2-dichloro-4-nitrobenzene, ethacrynic acid and iodomethane, but were inactive with 4-nitropyridine N-oxide, 1,2-epoxy-3-(p-nitrophenoxy)propane, bromosulphophthalein and p-nitrobenzyl chloride. The kinetics of the enzyme-catalysed reaction with enzyme IV were non-Michaelean with respect to both substrates. Both products were inhibitory. The results appear to be compatible with a random steady-state mechanism. It is concluded that these enzymes are very similar, in their physical and chemical constitution, in their catalytic properties and in their relationships with each other, to those enzymes that have been isolated from vertebrate organisms.

Studies of the relationship between the catalytic activity and binding of non-substrate ligands by the glutathione S-transferases

The dimeric enzyme glutathione S-transferase B is composed of two dissimilar subunits, referred to as Ya and Yc. Transferase B (YaYc) and two other transferases that are homodimers of the individual Ya and Yc subunits were purified from rat liver. Inhibition of these three enzymes by Indocyanine Green, biliverdin and several bile acids was investigated at different values of pH (range 6.0-8.0). Indocyanine Green, biliverdin and chenodeoxycholate were found to be effective inhibitors of transferases YaYc and YcYc at low (pH 6.0) but not high (pH 8.0) values of pH. Between these extremes of pH intermediate degrees of inhibition were observed. Cholate and taurochenodeoxycholate, however, were ineffective inhibitors of transferase YcYc at all values of pH. The observed differences in bile acids appeared to be due, in part, to differences in their state of ionization. In contrast with the above results, transferase YaYa was inhibited by at least 80% by the non-substrate ligands at all values of pH. These effects of pH on the three transferases could not be accounted for by pH-induced changes in the enzyme's affinity for the inhibitor. Thus those glutathione S-transferases that contain the Yc subunit are able to act simultaneously as both enzymes and binding proteins. In addition to enzyme structure, the state of ionization of the non-substrate ligands may also influence whether the transferases can perform both functions simultaneously.

The purification of the hepatic glutathione S-transferases of rainbow trout by glutathione affinity chromatography alters their isoelectric behaviour

1. The basic glutathione S-transferases from rainbow-trout liver were more stable than the acidic ones. 2. The apparent pI values of these enzymes were lowered when they were eluted from a glutathione affinity column by reduced glutathione at pH 8.85. 3. The pI effect was not a function of the high pH alone, was diminished under conditions less favourable to glutathione oxidation, and did not occur when S-hexylglutathione affinity chromatography was used instead.

Glutathione S-transferase activities in the yellow-fever mosquito [Aedes aegypti (Louisville)] during growth and aging

Our previous findings [Hazelton & Lang (1978) Fed. Proc. Fed. Am. Soc. Exp. Biol. 37(6), 2378 (abstr.)] demonstrated aging-specific changes in glutathione concentrations in the yellow-fever mosquito [Aedes aegypti (Louisville)]. A possible mechanism could be increased utilization via glutathione S-transferase. Thus glutathione S-transferase activities were measured in mosquito samples from the entire life span, including growth, maturity and senescence. Methods were validated for the quantitative determination of transferase activities with 1,2-dichloro-4-nitrobenzene (DCNB) and 1-chloro-3,4-dinitrobenzene (CDNB) as second substrates. Marked changes occurred during the life span, and the profiles for both DCNB and CDNB activities were identical. The activities increased throughout larval development and reached a maximum in the metamorphosis stage. The activities decreased at the end of metamorphosis in the 5-day-old adult, reached a plateau during maturity (5-20 days), and then decreased 31% (P less than 0.007) during senescence (after 33 days). This senescence-specific decrease occurred in both sexes and was localized in the abdominal region. Further kinetic analyses indicated that the lower enzyme activities were most likely due to lower amounts of active enzyme rather than a change in kinetic properties. These findings indicate that the capacity for GSH utilization via glutathione S-transferase is diminished with aging. This does not explain our previously observed decreases in GSH, but the results suggest that GSH-linked detoxification would be impaired during senescence.

Studies of endogenous inhibitors of microsomal glutathione S-transferase

Glutathione S-transferase is present in rat liver microsomal fraction, but its activity is low relative to the transferase activity present in the soluble fraction of the hepatocyte. We have found, however, that the activity of microsomal glutathione S-transferase is increased 5-fold after treatment with small unilamellar vesicles made from phosphatidylcholine. The increase in activity is due to the removal of an inhibitor of the enzyme from the microsomal membrane. The inhibitor is present in the organic layer of a washed Folch extract of the microsomal fraction. When this fraction of the microsomal extract is reconstituted in the form of small unilamellar vesicles, it inhibits microsomal glutathione S-transferase that had been activated by prior treatment with small unilamellar vesicles of pure phosphatidylcholine, but does not affect the activity of unactivated microsomal glutathione S-transferase. The inhibitor did not seem to be formed during the isolation of the microsomal fraction, and hence may be a physiological regulator of microsomal glutathione S-transferase. In this regard, both free fatty acid (palmitate) and lysophosphatidylcholine were shown to inhibit the enzyme reversibly. The results indicate that the activity of microsomal glutathione S-transferase is far greater than appreciated until now, and that this form of the enzyme may be an important factor in the hepatic metabolism of toxic electrophiles.

Cloning and sequence analysis of a cDNA plasmid for one of the rat liver glutathione S-transferase subunits

We describe the construction and characterization of a cDNA plasmid for one of the rat liver glutathione S-transferase subunits. Poly(A)-RNA isolated from rat livers was enriched for glutathione S-transferase mRNA activity and used as templates to synthesize double stranded cDNA. The double stranded cDNAs were annealed to pBR322 through terminal deoxynucleotidyl transferase generated GC-tails followed by transformation into E. coli. Several candidate clones were selected by colony hybridization using polynucleotide kinase labeled liver and testis poly(A)-RNA probes. These candidate clones were further characterized by hybrid-selected translation of mRNA followed by immunoprecipitation and SDS gel electrophoresis. The positive clone, pGTR112 was mapped with restriction endonuclease analysis and sequenced by the chemical method of Maxam and Gilbert. The largest upen reading frame contains 142 amino acids very rich in Arg and Lys residues. The C-terminal residue phenylalanine of this open reading frame is consistent with what was reported for one of the ligandin subunits by Bhargava et al., (J. Biol. Chem. 253, 4116-4119, 1978). Among the 352 nucleotides covered by both pGTR112 and pGST94 described by Kalinyak and Taylor (J. Biol. Chem. 257, 523-530, 1982), there are only 9 nucleotide differences resulting in four changes of amino acid sequences.

Glutathione S-transferases of the bovine retina. Evidence that glutathione peroxidase activity is the result of glutathione S-transferase

We have purified two isoenzymes of glutathione S-transferase from bovine retina to apparent homogeneity through a combination of gel-filtration chromatography, affinity chromatography and isoelectric focusing. The more anionic (pI = 6.34) and less anionic (pI = 6.87) isoenzymes were comparable with respect to kinetic and structural parameters. The Km for both substrates, reduced glutathione and 1-chloro-2,4-dinitrobenzene, bilirubin inhibition of glutathione conjugation to 1-chloro-2,4-dinitrobenzene, 1-chloro-2,4-dinitrobenzene inactivation of enzyme activity and molecular weight were similar. However, pH optimum and energy of activation were found to differ considerably. Retina was found to have no selenium-dependent glutathione peroxidase activity. The total glutathione peroxidase activity fractionated with the transferases in the gel-filtration range of mol.wt. 49000 and expressed activity with only organic hydroperoxides as substrate. Only the more anionic isoenzyme expressed both transferase and peroxidase activity.

Inhibition of glutathione S-transferase by bile acids

The effects of bile acids on the detoxification of compounds by glutathione conjugation have been investigated. Bile acids were found to inhibit the total soluble-fraction glutathione S-transferase activity from rat liver, as assayed with four different acceptor substrates. Dihydroxy bile acids were more inhibitory than trihydroxy bile acids, and conjugated bile acids were generally less inhibitory than the parent bile acid. At physiological concentrations of bile acid, the glutathione S-transferase activity in the soluble fraction was inhibited by nearly 50%. This indicates that the size of the hepatic pool of bile acids can influence the ability of the liver to detoxify electrophilic compounds. The A, B and C isoenzymes of glutathione S-transferase were isolated separately. Each was found to be inhibited by bile acids. Kinetic analysis of the inhibition revealed that the bile acids were not competitive inhibitors of either glutathione or acceptor substrate binding. The microsomal glutathione S-transferase from guinea-pig liver was also shown to be inhibited by bile acids. This inhibition, however, showed characteristics of a non-specific detergent-type inhibition.

Tissue distribution and subunit structures of the multiple forms of glutathione S-transferase in the rat

A study of the subunit structures of the multiple forms of glutathione S-transferase in rat kidney, testis, lung and spleen is shown to be consistent with a proteolytic model for the generation of the multiple forms.

Metabolic activation of drugs in mutagenicity tests in vitro

The metabolic pathways of chemical compounds of pharmaceutical interest are reviewed in relation to the role of activation and detoxification in the process of mutagenicity. The properties and subcellular localization of the enzymes involved are given together with the main reactions they catalyze. The role of metabolism in mutagenicity testing in vitro is discussed with special emphasis on the choice of the enzymatic system. Parameters such as species, strain, sex, diet, and induction are considered. The effect of various enzymatic effectors added in vitro is also discussed. It is concluded that the metabolism of drugs is very complex, involving both activation and detoxification processes catalyzed by a large variety of enzymes. Production of mutations in vitro in prokaryotic or eukaryotic cells is the results of a balance between all those pathways. Metabolic activation thus merits special attention.

Interrelationship between cationic and anionic forms of glutathione S-transferases of bovine ocular lens

Since the eye is constantly exposed to potentially damaging chemical compounds present in the atmosphere and vascular system, we investigated the physiological role of glutathione S-transferase (GSH S-transferase) in detoxification mechanisms operative in the ocular lens. We have purified an anionic and a cationic GSH S-transferase from the bovine lens to homogeneity through a combination of gel filtration, ion-exchange and affinity chromatography. The anionic (pI 5.6) and cationic (pI 7.4) S-transferases were found to have distinct kinetic parameters (apparent Km and Vmax. pH optimum and energy of activation). However, both species were demonstrated to have similar molecular weights and amino acid compositions. Double-immunodiffusion and immunotitration studies showed that both lens S-transferases were immunologically similar. The very close similarity in amino acid compositions and immunological properties strongly indicates that these two transferases either originate from the same gene or at least share common antigenic determinants and originate from similar genes. The bovine lens GSH S-transferases had no glutathione peroxidase activity with either t-butyl hydroperoxide or cumene hydroperoxide as substrate. However, the antibody raised against the homogeneous anionic glutathione S-transferase from the bovine lens was found to precipitate both glutathione S-transferase and glutathione peroxidase activities out of solution in the supernatant of a crude bovine liver homogenate.

Interrelationship between anionic and cationic forms of glutathione S-transferases of human liver

Human liver glutathione S-transferases (GSH S-transferases) were fractionated into cationic and anionic proteins. During fractionation with (NH4)2SO4 the anionic GSH S-transferases are concentrated in the 65%-saturated-(NH4)2SO4 fraction, whereas the cationic GSH S-transferases separate in the 80%-saturated-(NH4)2SO4 fraction. From the 65%-saturated-(NH4)2SO4 fraction two new anionic GSH S-transferases, omega and psi, were purified to homogeneity by using ion-exchange chromatography on DEAE-cellulose, Sephadex G-200 gel filtration, affinity chromatography on GSH bound to epoxy-activated Sepharose and isoelectric focusing. By a similar procedure, cationic GSH S-transferases were purified from the 80%-saturated-(NH4)2SO4 fraction. Isoelectric points of GSH S-transferases omega and psi are 4.6 and 5.4 respectively. GSH S-transferase omega is the major anionic GSH S-transferase of human liver, whereas GSH S-transferase psi is present only in traces. The subunit mol.wt. of GSH S-transferase omega is about 22500, whereas that of cationic GSH S-transferases is about 24500. Kinetic and structural properties as well as the amino acid composition of GSH S-transferase omega are described. The antibodies raised against cationic GSH S-transferases cross-react with GSH S-transferase omega. There are significant differences between the catalytic properties of GSH S-transferase omega and the cationic GSH S-transferases. GSH peroxidase II activity is displayed by all five cationic GSH S-transferases, whereas both anionic GSH S-transferases do not display this activity.

A comparison of erythrocyte glutathione S-transferase activity from human foetuses and adults

Glutathione S-transferase activity was measured in partially purified haemolysates of erythrocytes from human foetuses and adults. Enzyme activity was present in erythrocytes obtained between 12 and 40 weeks of gestation. The catalytic properties of the enzyme from foetal cells were similar to those of the enzyme from adult erythrocytes, indicating that probably only one form of the erythrocytes enzyme exists throughout foetal and adult life.

Cholic acid binding by glutathione S-transferases from rat liver cytosol

Cholic acid-binding activity in cytosol from rat livers appears to be mainly associated with enzymes having glutathione S-transferase activity; at least four of the enzymes in this group can bind the bile acid. Examination of the subunit compositions of different glutathione S-transferases indicated that cholic acid binding and the ability to conjugate reduced glutathione with 1,2-dichloro-4-nitrobenzene may be ascribed to different subunits.