Inhibition of glutathione S-transferase by bile acids

Evaluation of the non-catalytic binding function of Ts26GST a glutathione transferase isoform of Taenia solium

Taenia solium glutathione transferase isoform of 26.5 kDa (Ts26GST) was observed to bind non-catalytically to porphyrins, trans-trans-dienals, bile acids and fatty acids, as assessed by inhibition kinetics, fluorescence spectroscopy and competitive fluorescence assays with 8-anilino-1-naphthalene sulfonate (ANS). The quenching of Ts26GST intrinsic fluorescence allowed for the determination of the dissociation constants (KD) for all ligands. Obtained data indicate that Ts26GST binds to all ligands but with different affinity. Porphyrins and lipid peroxide products inhibited Ts26GST catalytic activity up to 100% in contrast with only 20-30% inhibition observed for bile acids and two saturated fatty acids. Non-competitive type inhibition was observed for all enzyme inhibitor ligands except for trans-trans-2,4-decadienal, which exhibited uncompetitive type inhibition. The dissociation constant value KD = 0.7 μM for the hematin ligand, determined by competitive fluorescence assays with ANS, was in good agreement with its inhibition kinetic value Ki = 0.3 μM and its intrinsic fluorescence quenching KD = 0.7 μM. The remaining ligands did not displace ANS from the enzyme suggesting the existence of different binding sites. In addition to the catalytic activity of Ts26GST the results obtained suggest that the enzyme exhibits a ligandin function with broad specificity towards nonsubstrate ligands.

Structures of class pi glutathione S-transferase from human placenta in complex with substrate, transition-state analogue and inhibitor

BACKGROUND

Glutathione S-transferases (GSTs) are detoxification enzymes, found in all aerobic organisms, which catalyse the conjugation of glutathione with a wide range of hydrophobic electrophilic substrates, thereby protecting the cell from serious damage caused by electrophilic compounds. GSTs are classified into five distinct classes (alpha, mu, pi, sigma and theta) by their substrate specificity and primary structure. Human GSTs are of interest because tumour cells show increased levels of expression of single classes of GSTs, which leads to drug resistance. Structural differences between classes of GST can therefore be utilised to develop new anti-cancer drugs. Many mutational and structural studies have been carried out on the mu and alpha classes of GST to elucidate the reaction mechanism, whereas knowledge about the pi class is still limited.

RESULTS

We have solved the structures of the pi class GST hP1-1 in complex with its substrate, glutathione, a transition-state complex, the Meisenheimer complex, and an inhibitor, S-(rho-bromobenzyl)-glutathione, and refined them to resolutions of 1.8 A, 2.0 A and 1.9 A, respectively. All ligand molecules are well-defined in the electron density. In all three structures, an additionally bound N-morpholino-ethansulfonic acid molecule from the buffer solution was found.

CONCLUSIONS

In the structure of the GST-glutathione complex, two conserved water molecules are observed, one of which hydrogen bonds directly to the sulphur atom of glutathione and the other forms hydrogen bonds with residues around the glutathione-binding site. These water molecules are absent from the structure of the Meisenheimer complex bound to GST, implicating that deprotonation of the cysteine occurs during formation of the ternary complex which involves expulsion of the inner bound water molecule. The comparison of our structures with known mu class GST structures show differences in the location of the electrophile-binding site (H-site), explaining the different substrate specificities of the two classes. Fluorescence measurements are in agreement with the position of the N-morpholino-ethansulfonic acid, close to Trp28, identifying a possible ligandin-substrate binding site.

Is intestinal cytosolic glutathione S-transferase an alternative detoxification pathway in two-thirds hepatectomized rats?

The present study was designed to investigate the effect of partial (two-thirds) hepatectomy (PH) on hepatic and intestinal glutathione S-transferases (GSTs) activities. A significant decrease of cytosolic hepatic GSTs activity was observed after the PH. The lowest value of hepatic GSTs was obtained 48 h after the surgery. On the other hand, intestinal GSTs activities increased after PH, reaching the highest values 48 h after the hepatic lobes resection. The hepatic GSTs activities diminution was attributed, in part, to the high accumulation of bile acids in the liver tissue of hepatectomized rats, also demonstrated by a higher retention of [14C] taurocholate. The kinetic analysis performed with 1-chloro-2,4-dinitrobenzene (CDNB) as substrate showed two sets of parameters, indicating the presence of isozymes of high and low affinities. Vmax1 and Vmax2 were lower in PH rats suggesting a non competitive inhibition mechanism. The inhibitory effect of bile acids decreased during liver regeneration process of hepatectomized rats disappearing at 7 days after PH. Conversely, in non regenerating rats (GABA treated) the inhibitory mechanism was still observed at 7 days after the surgery. The increase of intestinal GSTs activities (isozymes of high and low affinities) was attributed to the presence of polyamines, mainly putrescine, produced during the hepatic regeneration process. In this regard, it was showed that GABA treatment, which inhibits polyamine synthesis, completely abolished the increase on intestinal GSTs activities. Finally, the treatment with exogenous putrescine showed that in hepatectomized and sham-operated rats, the polyamine induced GSTs activities in both tissues. In PH rats, the putrescine dependent increase of hepatic GSTs was masked by the inhibitory effect of bile acids. In addition, a summation effect of endogenous and exogenous putrescine was probably the reason of the induction of intestinal GSTs after PH. The GSH/GSSG ratio did not change during the treatments, as well as the microsomal GST activity of both tissues. The work points out the hypothetical detoxification power of the intestine during the hepatocellular insufficiency which follows a two-thirds hepatectomy.

Influences of dietary deoxycholic acid on progression of hepatocellular neoplasms and expression of glutathione S-transferases in rats

Serial magnetic resonance imaging (MRI) was used to evaluate the influences of dietary deoxycholic acid (DCA) on the rate of progression of chemically induced hepatocellular neoplasms in rats. Male Fischer-344 rats with established persistent hepatocellular nodules generated by the Solt-Farber protocol were exposed to dietary DCA (0.3%) between 6 and 12 mo of age. Growth of nodules and carcinomas in vivo was measured by morphometric quantification of tumor images obtained every 6 wk. The final stages of neoplastic progression were determined by terminal histopathological examination and by expression and functional evaluation of glutathione S-transferase (GST) isoenzyme phenotypes. Dietary DCA increased the number of hepatocellular neoplasms per rat, accelerated the rate of growth of persistent nodules, and increased the histological progression of liver tumors. Expression of immunoreactive GST subunits Yf, Ya, and Yb1 was induced in early persistent nodules, a pattern that was maintained throughout the study in both basal diet and DCA-fed groups. However, 5% of early nodules and about 75% of advanced neoplasms were partially or completely deficient in GST Yb2 expression in both groups. DCA did not alter the cytosolic activity for the GST substrates 1-chloro-2,4-dinitrobenzene (CDNB) or trans-4-phenyl-3-buten-2-one (tPBO) in tumors or surrounding liver. However, in both groups, CDNB activity was increased in the tumors relative to the surrounding nonneoplastic tissue, whereas activity for tPBO, a substrate more specific for the Yb2 subunit, was reduced in the tumors. All advanced neoplasms were similarly more resistant than surrounding liver to DNA-binding metabolites of aflatoxin B1 or benzo[a]pyrene. These data demonstrate that DCA can increase the progression of established hepatocellular nodules to larger, more advanced neoplasms but does not preferentially select for a specific GST phenotype. Preferential loss of constitutively expressed GST Yb2 in both basal diet and DCA-fed groups may be an important aspect of progression from resistant nodules to advanced cancers in this model. These studies also demonstrate that serial MRI is a useful tool for measuring the rates of enlargement and patterns of growth in established hepatocellular neoplasms.

Hepatic vectorial transport of xenobiotics

The vectorial transport of xenobiotics across the hepatocyte is mediated by various transport and transfer proteins that differ in ligand specificity and function. The influx of xenobiotics from the blood across the sinusoidal membrane of the hepatocyte can occur through passive or active transport processes. Once in the cell, xenobiotics can be sequestered by intracellular transfer proteins that prevent refluxing of the chemical back through the sinusoidal membrane. Transfer proteins may also facilitate the localization of the xenobiotics within the cell to sites of metabolism (i.e., the endoplasmic reticulum) or elimination (i.e., the canalicular membrane). Intracellular transfer proteins include glutathione S-transferases, fatty acid-binding proteins, and 3 alpha-hydroxysteroid dehydrogenase. Intracellular nuclear transfer proteins have also been identified that facilitate the transfer of chemical carcinogens from the cytoplasm into the cell nucleus. Several active transport proteins exist on the canalicular membrane of the hepatocyte that mediate the efflux of chemicals from the cell into the biliary canaliculus. Xenobiotic efflux proteins include the multispecific organic anion transporter, that eliminates xenobiotics that have undergone conjugation with glutathione, glucuronic acid, and possibly sulfate; and, P-glycoprotein, an active transporter that actively effluxes a variety of structurally diverse xenobiotics. Induction of P-glycoprotein by the amplification of its gene has been identified as a major cause of resistance of tumor cells to the toxicity of a variety of anti-cancer drugs. The hepatic induction of P-glycoprotein may also contribute to acquired resistance of organisms to environmental toxicants. Continued elucidation of xenobiotic transport and transfer processes at the cellular levels will significantly advance our understanding of processes involved in xenobiotic toxicity and acquired resistance to chemical toxicity.

Chemical modification of bile acid: CoA ligase

The effect of chemical modification of bile acid:CoA ligase on its enzymatic activity was examined. Reagents which modify tyrosine and carboxyl groups did not affect the activity of either the purified enzyme or the enzyme in its native microsomal environment. The modification of arginine residues with either diacetyl or phenylglyoxal resulted in a loss of activity for both the purified and microsomal forms of the enzyme. ATP was able to protect the enzyme from inactivation. Neither cholate nor CoA were able to alter the time-course of inactivation. The sulfhydryl reagent N-ethylmaleimide (MalNEt) produced a biphasic effect on both the purified and microsomal forms of the enzyme. At short reaction times ligase activity increased, but further reaction lead to nearly complete inactivation. With the purified enzyme, ATP increased the extent of activation by MalNEt and decreased the rate of inactivation. With microsomes, ATP did not affect the extent of activation by MalNEt, but did slow the rate of inactivation. For both the purified and microsomal forms cholate provided no protection. Treatment of both forms of the enzyme with the sulfhydryl reagent iodoacetic acid produced a similar biphasic activation/inactivation of the ligase. It was hypothesized that modification of a fast-reacting cysteine leads to activation while a slower-reacting cysteine leads to inactivation. This latter cysteine appeared to be in the ATP-binding site on the enzyme.

Photoaffinity labelling of steroid-hormone-binding glutathione S-transferases with [3H]methyltrienolone. Inhibition of steroid-binding activity by the anticarcinogen indole-3-carbinol

The identification and characterization of steroid-hormone-binding glutathione S-transferases (GST) were undertaken using photoaffinity-labelling techniques. Irradiation of mouse liver cytosol, in the presence of 50 nM-[3H]methyltrienolone, resulted in the specific affinity labelling of five proteins. One of these proteins, designated MBP27, had an approximate molecular mass of 27 kDa under denaturing conditions and was induced by treatment of mice with either 2(3)-t-butyl-4-hydroxyanisole (BHA) or phenobarbital (PB). An additional affinity-labelled protein, MBP25, which was not detected in untreated mouse cytosol, was induced in the liver cytosols from BHA- and PB-treated mice. The molecular masses of these proteins and their induction by BHA and PB suggested that they may be steroid-hormone-binding GST subunits. Irradiation of mouse liver cytosol in the presence of [3H]methyltrienolone, followed by immunoprecipitation using GST-specific antibodies established that both GST mu and GST alpha bind [3H]methyltrienolone and both contribute to the affinity-labelled protein designated MBP27. GST Ya1 Ya1, an alpha class GST that is not expressed in untreated mouse liver but is induced by BHA and PB, was also found to bind [3H]methyltrienolone and is identical with the affinity-labelled protein designated MBP25. Experiments were undertaken next to assess the effects of the anticarcinogenic plant compound indole-3-carbinol (I3C) on GST-mediated steroid hormone-binding using the photoaffinity labelling techniques. Treatment of mice with I3C resulted in the induction of immunoreactive GST mu and GST Ya1 Ya1. However, the steroid-binding activity of these proteins in vitro was severely inhibited by the acid-condensation products of I3C that are generated in the stomach after ingestion. These results suggest that I3C may inhibit GST-mediated steroid-binding activity which could contribute to the anticarcinogenic activity of this compound.

A factor in the increased risk of colorectal cancer due to ingestion of animal fat is inhibition of colon epithelial cell glutathione S-transferase, an enzyme that detoxifies mutagens

Dietary animal fat increases the risk of colorectal cancer. A factor in the increased risk is hypothesised to result from the inhibition of isoforms of a colonic epithelial cell enzyme that detoxifies genotoxins, glutathione S-transferase, by one of the major secondary bile acids produced in the colon by fat digestion, lithocholic acid. The inhibition allows mutagens to persist in colonic epithelial cells while proliferation is stimulated by secondary bile acids, with a concomitant greater frequency of neoplasia-associated mutations than when proliferation is stimulated in the absence of the mutagens. Elements in the hypothesis include the ability of relatively low concentrations of lithocholic acid to inhibit isoforms of glutathione S-transferase found in colon epithelial cells, entry of lithocholic acid into the epithelial cells, and the correlation of neoplasia-associated colon pathology with high levels of lithocholic acid in fecal water. Higher pH values in the colonic stream are identified as exacerbating the effects of lithocholic acid by increasing its solubility. Lithocholic acid is suggested to be more inhibitory to glutathione S-transferase than the other major colonic secondary bile acid, deoxycholic acid, on the basis of inhibition-structure relationships.

Inverse relationship between total glutathione S-transferase content and bile acid release in isolated hepatocytes from untreated, phenobarbital pretreated and hypothyroid rats

The role of cytosolic anion binding proteins (glutathione S-transferases) in the hepatic transport of bile acids remains controversial. To investigate whether increased levels of the hepatocyte total glutathione S-transferase content were associated with changes in the release of bile acids from the hepatocyte, we measured the rate of release of radioactive bile acids in isolated hepatocytes from thyroidectomized, phenobarbital pretreated and untreated rats. The isolated hepatocytes were preincubated with either 14C-cholic acid or 14C-taurocholic acid, and the release rate of radiolabeled bile acids was determined. Hepatocyte total glutathione S-transferase content was measured by rocket immunoelectrophoresis. The release rate of the radiolabeled bile acids was significantly (P less than 0.005) decreased in both hypothyroid and phenobarbital pretreated hepatocytes. The levels of total glutathione S-transferase content were significantly (P less than 0.001) increased in the hepatocytes from both hypothyroid and phenobarbital pretreated animals. Our findings reveal a striking inverse relationship between the total glutathione S-transferase content of the hepatocyte and the release rate of radiolabeled bile acids in isolated hepatocytes from two independent animal models. These observations support the hypothesis that cytosolic anion binding proteins (glutathione S-transferases) may influence the net flux across the hepatocyte plasma membrane largely by limiting efflux.

Characterization of the activation of rat liver glutathione S-transferases by nonsubstrate ligands

In previous work (D.A. Vessey and T.D. Boyer, 1986, Biochem. Pharmacol., 35, 289-295) the activity of glutathione S-transferase form YcYc from rat liver was found to be stimulated by the herbicide 2,4,5-T. We have extended that work and examined the effect of over 40 structural analogs on the activity of YcYc. Over half of these compounds stimulated by 10 to 232% when added to assays at a concentration of 1 mM. The best activators all contained the "2,4,5-trichlorophenyl-" structure. While 2,4,5-T gave the greatest activation at 1 mM (2.3-fold), 2,4,5-trichlorobenzene sulfonate gave the greatest maximum activation (6.0-fold). Compounds that had no effect on activity did not affect activation by 2,4,5-T suggesting that they have a poor affinity for the enzyme. Two of the analogs tested (chloramine-T and 6-hydroxydopamine) proved to be good inhibitors and ethacrynic acid was an extremely potent inhibitor. Indomethacin activated at low concentrations but inhibited above 2 mM. Activations were greater at low temperature (5 degrees C) and decreased with increasing temperature. The extent of activation was largely unaffected by the concentration of either substrate. Examination of the organic peroxidase activity of the enzyme revealed inhibition by 2,4,5-T and 2,4-D rather than activation.

Glutathione transferases--structure and catalytic activity

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.

The effects of in vitro lipid peroxidation on the activity of rat liver microsomal glutathione S-transferase from rats supplemented or deficient in antioxidants

Glutathione S-transferases are a group of multifunctional isozymes that play a central role in the detoxification of hydrophobic xenobiotics with electrophilic centers (1). In this study we investigated the effects of in vitro lipid peroxidation on the activity of liver microsomal glutathione S-transferases from rats either supplemented or deficient in both vitamin E and selenium. Increased formation of malondialdehyde (MDA), a by-product of lipid peroxidation, was associated with a decreased activity of rat liver microsomal glutathione S-transferase. The inhibition of glutathione S-transferase occurred rapidly in microsomes from rats fed a diet deficient in both vitamin E and selenium (the B diet) but was delayed for 15 minutes in microsomes from rats fed the same diet but supplemented with these micro-nutrients (B+E+Se diet). Lipid peroxidation inhibits microsomal glutathione S-transferase and this inhibition is modulated by dietary antioxidants.

Inhibition of human cationic glutathione S-transferase by nonsubstrate ligands

Inhibition of a major hepatic form of human cationic glutathione S-transferase by bilirubin, biliverdin, indocyanine green and chenodeoxycholic acid was investigated as a function of pH (range = 6.5 to 9.1). Changes in pH had little effect on the extent of inhibition by indocyanine green. However, inhibition by bilirubin, biliverdin and chenodeoxycholic acid was found to be pH-dependent, with markedly less inhibition at the high values of pH. The reduced inhibition at the high values of pH could not be ascribed to a failure of the enzyme to bind the nonsubstrate ligand. Instead, the complete inhibition observed at pH 6.5 became partial (hyperbolic) inhibition at pH 9.1. This behavior can be ascribed to the binding of the nonsubstrate ligands at a site other than the active site, i.e., at high values of pH there is formation of an enzyme-substrate-inhibitor complex which still retains considerable catalytic activity. At physiologic values of pH (7.0), the human transferase was completely inhibited by saturating concentrations of the tested nonsubstrate ligands. This is in contrast to our previous studies performed with the rat transferases where, although inhibition also was affected by buffer pH, some forms of the enzyme retained significant catalytic activity at pH 7.0 despite high concentrations of nonsubstrate ligands. We conclude that the ability of the human cationic glutathione S-transferases to serve as enzymes of detoxification in the presence of high intracellular concentrations of nonsubstrate ligands may be significantly reduced, and this may render the cholestatic liver unusually susceptible to injury by toxic electrophiles.

3 alpha-hydroxysteroid dehydrogenase activity of the Y' bile acid binders in rat liver cytosol. Identification, kinetics, and physiologic significance

Rat Y' bile acid binders (33 kD) have been previously recognized as cytosolic bile acid binding proteins (Sugiyama, Y., T. Yamada, and N. Kaplowitz, 1983, J. Biol. Chem., 258:3602-3607). We have now determined that these Y' binders are 3 alpha-hydroxysteroid dehydrogenases (3 alpha-HSD), bile acid-metabolizing enzymes. 3 alpha-HSD activity copurified with lithocholic acid-binding activity after sequential gel filtration, chromatofocusing, and affinity chromatography. Three peaks of 3 alpha-HSD activity (I, II, III) were observed in chromatofocusing and all were identified on Western blot by a specific Y' binder antiserum. 3 alpha-HSD-I, the predominant form, was purified and functioned best as a reductase at pH 7.0 with a marked preference for NADPH. Michaelis constant values for mono- and dihydroxy bile acids were 1-2 microM, and cholic acid competitively inhibited the reduction of 3-oxo-cholic acid. Under normal redox conditions, partially purified 3 alpha-HSD-I and freshly isolated hepatocytes catalyzed the rapid reduction of 3-oxo-cholic to cholic acid without formation of isocholic acid, whereas the reverse reaction was negligible. The Y' bile acid binders are therefore 3 alpha-HSD, which preferentially and stereospecifically catalyze the reduction of 3-oxo-bile acids to 3 alpha-hydroxy bile acids.

Glutathione transferase activity during Bufo bufo development

High levels of glutathione transferase activity were measured during the development of the embryos of Bufo bufo including unfertilized eggs. After stage 4 glutathione transferase activity gradually decreased until stage 25 when the minimum was reached. No change in the number of isozymes was noted during development according to isoelectric focusing analysis performed on the cytosolic fractions of selected stages.

The hepatic metabolism and biliary excretion of benzo[a]pyrene in guinea-pigs fed normal, high-fat or high-cholesterol diets

Approx. one-third of an i.v. dose of 14C-benzo[a]pyrene was excreted within four hours in the bile of guinea-pigs fed a normal diet. The extent of excretion was not altered by feeding high-fat or high-cholesterol diets. Hepatic cytochromes P-450 and b5, and benzo[a]pyrene hydroxylase activity were unaltered by the administration of high-fat and high-cholesterol diets. Pretreatment with low oral doses of benzo[a]pyrene (6 X 3 mg/kg) did not induce these parameters in animals given any of the diets. High-fat and high-cholesterol diets altered the pattern of benzo[a]pyrene metabolites in the bile, with significantly increased excretion of dihydrodiol glucuronides in both the high-fat and high-cholesterol groups. Hepatic epoxide hydrolase activity and glutathione content were unaltered by the high-fat or high-cholesterol diets, and therefore cannot explain the alteration in the profile of biliary metabolites of benzo[a]pyrene. The altered pattern of biliary excretion in animals fed high-fat or high-cholesterol diets would lead to an increase in the delivery to the colon of dihydrodiol metabolites of benzo[a]pyrene.

Inhibition of hepatic glutathione transferases by propylthiouracil and its metabolites

The effects of propylthiouracil (PTU) and its metabolites on the activity of GSH transferases were examined using rat liver cytosol. PTU inhibited the enzyme activity toward both CDNB and DCNB in a concentration-dependent manner. At the concentration of 10 mM, PTU caused 25% inhibition, which was the maximum effect. PTU derivatives such as propyluracil and thiouracil showed the same effect as the parent compound. On the other hand, S-oxides of PTU such as PTU-SO2 and PTU-SO3, which were chemically synthesized by the oxidation of PTU, were more potent inhibitors of GSH transferases than the parent PTU. A significant inhibition was observed at a concentration of 0.1 mM of PTU S-oxides. At a concentration of 10 mM the S-oxides caused an 80% inhibition of the enzyme activity. PTU inhibited the transferase activity by competing with GSH but the S-oxides of PTU acted by another mechanism. In contrast to the effect on GSH transferases, PTU-SO3 had a weak inhibitory effect on GSH peroxidase activity. Thus, oxidation of PTU leads to products which are potent inhibitors of GSH transferases.

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.

Differential activation and inhibition of different forms of rat liver glutathione S-transferase by the herbicides 2,4-dichlorophenoxyacetate (2,4-D) and 2,4,5-trichlorophenoxyacetate (2,4,5-T)

The predominant forms of the dimeric enzyme glutathione S-transferase were purified from rat liver. Forms YbY'b and YbYb (also known as forms C and A, respectively) could be almost completely inhibited by 2,4-dichlorophenoxyacetate (2,4-D). Half-maximal inhibition was obtained at 0.5 mM 2,4-D. Inhibition was seen even at extrapolated infinite concentrations of both substrates for YbYb but not YbY'b. These same forms could also be inhibited 70 to 80% by 2,4,5-trichlorophenoxyacetate (2,4,5-T) with half maximal inhibition occurring at 0.2 mM. Glutathione S-transferase form YaYa was maximally inhibited by 72 and 30%, respectively, by 2,4-D and 2,4,5-T. The 30% inhibition of YaYa caused by 2,4,5-T was shown to reduce the nearly complete inhibition caused by a previously characterized inhibitor, namely bile acids. This suggests competition for a common binding site on the enzyme. In contrast to the above results, it was found that form YcYc (also termed AA) was activated 2.7-fold by 2,4,5-T and 1.4-fold by 2,4-D. This activation could be blocked by chenodeoxycholate which, by itself, did not affect the activity of the enzyme. The effects of 2,4,5-T and 2,4-D on the heterodimer YaYc (also termed form B) were intermediate between their effects on YaYa and YcYc, suggesting that each subunit contributes its unique property to the heterodimer. The microsomal membrane-bound form of glutathione S-transferase was also examined and found to be inhibited by both 2,4-D and 2,4,5-T. However, unlike the inhibitions of soluble forms, 2,4,5-T caused more extensive inhibition than 2,4-D.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Bile acid inhibition of basic and neutral glutathione S-transferases in rat liver

A purification scheme is described for the neutral glutathione S-transferases of rat liver. Discontinuous sodium dodecyl sulphate/polyacrylamide-gel electrophoresis revealed that one of these enzymes contains a previously unidentified subunit, which has a molecular mass of 23 000 Da and has been designated Yn. Bile acids inhibited the activity of all the basic and neutral transferases investigated, but marked differences in the effects of bile acids on individual enzymes were observed. The activity of each transferase was inhibited more by lithocholate 3-sulphate than by chenodeoxycholate, which in turn was more inhibitory than cholate. The enzymes that were most sensitive to cholate inhibition were not found to be as readily inhibited as other transferases by chenodeoxycholate or lithocholate 3-sulphate. Conversely, the activity of transferase AA was more resistant to cholate, chenodeoxycholate and lithocholate 3-sulphate inhibition than was any of the other enzymes studied.

Effect of lantana toxicity on lysosomal and cytosol enzymes in guinea pig liver

Oral administration of lantana leaf powder to guinea pigs caused an increase in the hepatic postmitochondrial fraction:homogenate ratios of activities of lysosomal enzymes--acid phosphatase, cathepsin B and DNase II. Enzyme activities of glucokinase, aldolase, lactate dehydrogenase and glucose-6-phosphate dehydrogenase were elevated whereas activity of glutathione-S-transferase decreased. Alterations in the activities of lysosomal and cytosol enzymes appear to constitute an important biochemical lesion in the pathogenesis of guinea pig liver in lantana toxicity.

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.