The role of glutathione and glutathione S-transferases in the metabolism of chemical carcinogens and other electrophilic agents

Microsomal glutathione transferase. Purification in unactivated form and further characterization of the activation process, substrate specificity and amino acid composition

The procedure developed for purification of the N-ethylmaleimide-activated microsomal glutathione transferase was applied successfully to isolation of this same enzyme in unactivated form. The microsomal glutathione transferases, the unactivated and activated forms, were shown to be identical in terms of molecular weight, immunochemical properties, and amino acid composition. In addition the microsomal glutathione transferase purified in unactivated form could be activated 15-fold with N-ethylmaleimide to give the same specific activity with 1-chloro-2,4-dinitrobenzene as that observed for the enzyme isolated in activated form. This activation involved the binding of one molecule N-ethylmaleimide to the single cysteine residue present in each polypeptide chain of the enzyme, as shown by amino acid analysis, determination of sulfhydryl groups by 2,2'-dithiopyridyl and binding of radioactive N-ethylmaleimide. Except for the presence of only a single cysteine residue and the total absence of tryptophan, the amino acid composition of the microsomal glutathione transferase is not remarkable. The contents of aspartic acid/asparagine + glutamic acid/glutamine, of basic amino acids, and of hydrophobic amino acids are 15%, 12% and 54% respectively. The isoelectric point of the enzyme is 10.1. Microsomal glutathione transferase conjugates a wide range of substrates with glutathione and also demonstrates glutathione peroxidase activity with cumene hydroperoxide, suggesting that it may be involved in preventing lipid peroxidation. Of the nine substrates identified here, the enzymatic activity towards only two, 1-chloro-2,4-dinitrobenzene and cumene hydroperoxide, could be increased by treatment with N-ethylmaleimide. This treatment results in increases in both the apparent Km values and V values for 1-chloro-2,4-dinitrobenzene and cumene hydroperoxide. Thus, although clearly distinct from the cytosolic glutathione transferases, the microsomal enzyme shares certain properties with these soluble enzymes, including a relative abundance, a high isoelectric point and a broad substrate specificity. The exact role of the microsomal glutathione transferase in drug metabolism, as well as other possible functions, remains to be established.

Trace element inhibition of carcinogenesis and angiogenesis

In separate experimental models the effects of selenium, zinc, potassium, and copper on angiogenesis and carcinogenesis are compared. Inhibitory effects of Se as Na2SeO3 and of chloride salts of K and Zn, but not of Cu, on vascularization induced by amelanotic tumor implants (A Mel-4B32) in the Syrian hamster cheek pouch membrane are reported. In an earlier study the induction of new vasculature, angiogenesis, in control chambers implanted with A Mel-4B32 was initially observed on day 4 after implant. Addition of 5, 10, and 50 μg Se at the time of tumor implant delayed the initial capillary proliferation to days 7, 9, 10, respectively (Cancer Lett. 9, 353, 1980). Similarly, in this report addition of 50 μg Zn or 50 μg K delayed angiogenesis to days 7.and 6, respectively. By contrast, addition of 50 μg Cu caused severe inflammation and necrosis of the membrane before capillary proliferation could be observed and the animals had to be sacrificed.In an earlier study we reported the colon tumor incidence in 1,2-dimethylhydrazine (20 mg DMG/kg body weight for 20 weeks) -treated Sprague Dawley rats was reduced from 87 to 40% by a 4 ppm Se (as Na2SeO3) supplement in the drinking water (Cancer Lett. 2, 133, 1977). In this study rats were treated with the same dose of carcinogen, but were sacrificed 14 weeks following the last DMH injection. Drinking water supplements with 0.02% Zn (as ZnCl2), 0.5% K (as KCl), or 0.01% Cu (as CuCl2) were provided concurrently with the carcinogen and were continued until death or sacrifice. The colon tumor incidence was 18/20 in the DMH control and was reduced to 16/20 by supplemental Zn, reduced to 12/20 by supplemental K, and was unchanged (18/20) by Cu. The tumor incidences in the small intestine and Zymbal gland were all reduced by Zn, K, and Cu supplements compared with the DMH control. The hamster cheek pouch technique may provide a prescreen for potential inhibitors of chemical carcinogenesis in other experimental models.

Dietary antioxidant effects on the hepatic mixed-function oxidase system of rainbow trout (Salmo Gairdneri)

Rainbow trout (Salmo gairdneri) were fed a control diet with or without an antioxidant--3,5-di-tert-butyl-4-hydroxytoluene (BHT), 2(3)-tert-butyl-4-hydroxyanisole (BHA), mono-tert-butylhydroquinone (TBHQ) or ethoxyquin (EQ)--at a level of 5.56 mmol in 100 g oil/kg diet for 6 wk. The treated trout had reduced liver weight/body weight ratios. In comparison with trout fed control diet, microsomal protein content was lowered by 13% in TBQH-fed trout and elevated by 28% in EQ-fed trout, cytochrome P-450 content was 21% lower in BHA- and TBHQ-fed and 18% lower in EQ-fed trout and cytochrome b5 content was 46% lower in EQ-fed trout. Activities of benzo[a]pyrene hydroxylase, epoxide hydratase and ethoxycoumarin-O-deethylase were, respectively, 3.2-4.8, 1.2-1.7 and 1.3-5.5 times higher in antioxidant-fed trout. NADPH-cytochrome c reductase was elevated 1.2-1.3 times over the control value with dietary BHA, TBHQ and BHT, but was lowered with EQ. p-Nitroanisole-O-demethylase activity was completely suppressed in antioxidant-fed trout. The content of post-mitochondrial acid-soluble sulphydryl groups was 42% lower in BHA- and BHT-fed trout. Alterations in the enzyme activities of the mixed-function oxidase system, changes in the ethyl isocyanide binding ratio and decreases in cytochrome P-450 content suggest that dietary antioxidants could alter carcinogen activation and/or detoxification mechanisms in the hepatic microsomes of rainbow trout.

Interaction of benzo- and naphthoquinones with soluble glutathione S-transferases from rat liver

The in vitro interaction of 1,4-benzoquinone, 1,2- and 1,4-naphthoquinone, and 2-methyl-1,4-naphthoquinone with rat liver glutathione S-transferases (GST) was studied, using reduced glutathione and 1-chloro-2,4-dinitrobenzene (CDNB) as substrates. The inhibition of the GST activity by quinones in crude extracts was dose dependent. While most of the dihydroxynaphtalenes investigated also inhibited the GST activity, dihydroxybenzenes and catecholamines did not. The quinones inhibited all the GST isoenzymes, albeit at different degrees. Kinetic studies revealed mixed type function inhibition towards glutathione and competitive inhibition towards CDNB, implicating that quinones are GST substrates. This was further confirmed by titration of remaining glutathione in appropriate incubation mixtures. These results indicate that GST could have a protective function against quinones, and that catecholamines are conjugated with glutathione via a reactive quinone intermediate.

The effect of t-butylated hydroxytoluene on glutathione linked detoxification mechanisms in rat

When rats were fed a diet containing 0.4% (w/w) butylated hydroxytoluene (BHT), a three-fold increase in total glutathione (GSH) S-transferase activity towards 1-chloro-2,4-dinitrobenzene (CDNB) was observed in liver but not in lung or kidney. Hepatic GSH S-transferase activities towards styrene oxide (SO) and 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) were also increased, but to a lesser extent. Isoelectric focusing studies indicated that the activities of most of the rat liver GSH S-transferase isoenzymes were induced. Immunoprecipitation studies of the native and induced enzymes suggested that de novo synthesis of these proteins caused the increase in GSH S-transferase activity in liver. A two-fold increase in glutathione reductase activity in liver upon dietary administration of BHT was observed. Kinetic and physical properties of the native and induced enzymes were similar which may indicate that the induction is due to the synthesis of this enzyme. A significant increase in reduced glutathione (GSH) content in liver and lung was also seen in rats treated with BHT.

Radiometric assays for mammalian epoxide hydrolases and glutathione S-transferase

A number of epoxides, including cis- and trans-stilbene oxides, were assayed as substrates for epoxide hydrolases (EHs) by gas-liquid chromatography. Radiolabeled stilbene oxides were prepared by sodium borotritide reduction of desyl chloride followed by ring closure with base treatment. Rapid radiometric assays for EHs were performed by differential partitioning of the epoxide into dodecane, while the product diol remained in the aqueous phase. Glutathione (GSH) transferase was similarly assayed by partitioning the epoxide and diol, if formed metabolically, into 1-hexanol, while the GSH conjugate was retained in the aqueous phase. The cytosolic EH rapidly hydrates the trans isomer while the cis is very poorly hydrated. In contrast, the cis is a better substrate for the microsomal EH than the trans. GSH transferase utilized both epoxides as substrates, but conjugation is faster with the cis isomer. Cytosolic EH activity is high in mouse but very low in rat and guinea pig. Microsomal EH activity, in contrast, is highest in guinea pig, intermediate in rat, and the lowest in mouse. GSH transferase activity, which is high in all three species, can be inhibited by chalcone, with an I50 of 3.1 X 10(-5) M. These assays facilitate the rapid evaluation and direct comparison of epoxide-metabolizing systems in cell homogenates used in short-term mutagenicity assays, cell or organ culture, and possibly in vivo.

Properties of the increased glutathione S-transferase A form in rat preneoplastic hepatic lesions induced by chemical carcinogens

Glutathione S-transferase A form (GST-A) is increased markedly in rat preneoplastic hepatic lesions such as hyperplastic nodules induced by diethylnitrosamine followed by administration of N-2-fluorenylacetamide. GST-A was also significantly increased in livers of rats after short-term administration of some drugs. The increased activity and protein content of GST-A were demonstrated by CM-Sephadex C-50 column chromatography as well as by two-dimensional polyacrylamide gel electrophoresis following immuno-affinity column chromatography using antibody against GST-A. Immunologically, GST-A crossreacted strongly with GST-C, weakly with GST-C2, but not with ligandin, GST-B, or GST-AA. It was confirmed by subunit recombination that GST-C is a heterodimer composed of the subunits of homodimers, GST-A and GST-C2.

Endogenous induction of epoxide hydrolase, benzo(a)pyrene hydroxylase and glutathione-S-transferase in "responsive" C57Bl/6 mice and in "nonresponsive" DBA/2 mice during pregnancy

Gestational changes in activity for three enzymes associated with different hepatic and pulmonary drug metabolizing systems were investigated in C57Bl/6 and DBA/2 mice: benzo(a)pyrene hydroxylase, epoxide hydrolase and glutathione-S-transferase. The gestational profiles of hepatic and pulmonary benzo(a)pyrene hydrolase, and epoxide hydrolase were similar in both strains. In addition, we demonstrated higher endogenous stimulation of the three studied enzymic activities in C57Bl/6 mice. During the second moiety of pregnancy, temporal variations were observed: a peak of activity occurred between days 17 and 18 for lung and liver benzo(a)pyrene hydroxylase and at day 20 for epoxide hydrolase in both strains. Hepatic glutathione-S-transferase variations were similar in both strains. However, pulmonary glutathione-S-transferase increased gradually throughout pregnancy in C57Bl/6 mice, while a peak of glutathione-S-transferase activity occurred on day 18 of gestation in DBA/2 mice.

Acetaminophen and its toxicity

Acetaminophen (APAP) is considered one of the safest of all minor analgesics, but when taken in large doses (greater than 10 g) toxicity occurs. Severely poisoned patients experience hepatic and/or renal failure. The major metabolic pathway of APAP is formation of glucuronide and sulfate conjugates. A minor pathway is formation of a reactive metabolite that conjugates with glutathione (GSH). When GSH is depleted, the reactive metabolite causes necrosis of hepatic and other tissues. Treatment of APAP toxicity involves supplying alternate sulfhydryl donors or inhibiting oxidative formation of the reactive metabolite. Estimation of plasma APAP levels is necessary for effective treatment.

Glutathione peroxidase, glutathione reductase, glutathione S-transferase, and gamma-glutamyltranspeptidase activities in the human early pregnancy placenta

GSH peroxidase, GSSG reductase, GSH S-transferase, and gamma-glutamyltranspeptidase activities were measured in the supernatant of 13 human early pregnancy placenta homogenates. From measurements of GSH peroxidase activity with both H2O2 and cumene hydroperoxide as second substrate it was deduced that immature placenta contains only the Se-dependent form. All the specimens investigated exhibited GSSG reductase and gamma-glutamyltranspeptidase activities. GSH S-transferase activity was noted only using 1-chloro-2,4-dinitrobenzene as electrophilic substrate, while no detectable activity was found with 1,2-dichloro-4-nitrobenzene, 1,2-epoxy-3-(p-nitrophenoxy) propane, and p-nitrobenzylchloride. It is concluded that human placenta is equipped, from early pregnancy, with the enzymatic systems which are involved in GSH-mediated cellular detoxication and in preserving the integrity of the sulfhydryl status of the cells.

Effects of dietary broccoli and butylated hydroxyanisole on liver-mediated metabolism of benzo[a]pyrene

Groups of male Wistar rats were given either a basal diet or diets supplemented with 10 or 25% broccoli or 0.8% BHA. Liver fractions were assayed for cytochrome P-450, for aryl hydrocarbon hydroxylase (AHH), glutathione-S-transferase and epoxide hydrolase activities and for benzo[a]pyrene (BaP) metabolism. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis of liver microsomes was also performed. Mean relative liver weight in the BHA group was significantly higher than that of the control and 10% broccoli groups but not significantly higher than that of the 25% broccoli group. Gel electrophoresis of liver microsomes indicated a diet-dependent variation in intensity in a band that corresponded in mol wt to those of certain cytochrome P-450s. Diet-dependent increases (20-90% over control) in cytochrome P-450 and in the activities of AHH, glutathione-S-transferase and epoxide hydrolase were observed in livers from rats given broccoli-supplemented diets. Except for AHH activity, such increases also occurred in the group fed BHA. Analysis of BaP metabolites revealed that the proportion of 4,5-diol formed relative to the major diols identified was unchanged in the broccoli- or BHA-treated groups relative to the control group. The proportion of 9,10-diol formed was unchanged in the broccoli-fed groups but was significantly higher in the BHA group than in the control group. The proportion of cis and trans-7,8-diol formed was unchanged in both broccoli-fed groups but was significantly lower in the BHA group. In comparison with the control group, the ratio of phenol I (comprising primarily 9-OH-BaP) to total phenols (primarily 9-OH and 3-OH) was significantly decreased by about 30% in the 25%-broccoli group and by about 70% in the BHA group. Qualitative differences in the phenol-II peak (comprising 3-OH and 7-OH phenols) were also observed between samples from the controls and those of 25%-broccoli- and BHA-fed rats. The implications of these findings are discussed with respect to the effects of broccoli and BHA on benzo[a]pyrene toxicity.

Isolation and characterization of an anionic glutathione S-transferase from rat liver cytosol

An anionic glutathione S-transferase representing approximately 20% of the total glutathione S-transferase protein and 10% of the total transferase activity toward 1-chloro 2,4-dinitrobenzene has been purified to homogeneity from the 105,000 x g supernatant of rat liver homogenate. The SDS gel electrophoretic data on subunit composition revealed that the anionic isozyme is composed of two subunits with an identical Mr of 26,000. The Km values for 1-chloro 2,4-dinitrobenzene and reduced glutathione were determined to be 0.94 mM and 0.23 mM respectively. A significant amount of glutathione peroxidase activity toward cumene hydroperoxide is associated with the new isozyme.

Studies on glutathione metabolism in ventral prostate and chemically induced prostatic carcinoma in rats

Glutathione content and the activity of glutathione reductase were examined in ventral prostate and chemically induced 11095 squamous-cell prostatic carcinoma in rats. Castration produced a significant reduction in the levels of reduced (GSH) and oxidized (GSSG) glutathione and glutathione reductase activity in the prostate. Replacement of testosterone (50 mg/kg) daily for 7 days to castrated animals elevated the reduced glutathione level and the activity of glutathione reductase almost to normal limits. Squamous-cell carcinoma was implanted in castrated and intact animals. Tumor growth in normal rats produced a decrease of almost 30% in the weight of the ventral prostate at 21 days post-implantation, although the glutathione levels remained unaffected. Much greater activity of glutathione reductase was detected in the tumor in comparison to the values noted for the normal tissue. The tumor also showed significantly higher values for the GSH/GSSG ratio. No apparent difference could be found in the rate of the growth of tumors whether implanted in normal or castrated animals. The levels of reduced and oxidized glutathione and glutathione reductase activity also seemed identical in tumors obtained from both groups of animals. Administration of testosterone (50 mg/kg) or beta-estradiol (2 mg/kg) daily for 11 days to tumor-bearing castrated animals did not alter the levels of glutathione and glutathione reductase activity. A significantly higher level of blood reduced glutathione was found in tumor-bearing rats in comparison to that seen for the normal subjects. Our results demonstrate that androgen depletion and replacement therapy influence the metabolism of glutathione in rat ventral prostate. Squamous-cell carcinoma of the prostate appears to differ from the normal tissue with respect to the observed androgen effects. There is dissimilarity in the metabolism of glutathione in the two tissues since greater activity of glutathione reductase and lower values of reduced glutathione were seen in the tumor as compared to those of the ventral prostate. Treatment with beta-estradiol, an antiprostatic agent, does not seem to influence the growth or glutathione metabolism of squamous-cell carcinoma of the prostate. The observed changes in blood glutathione levels might prove to be useful as an index of rapid growth of the neoplastic tissue.

The role of glutathione in detoxication

Glutathione (GSH) is a strong nucleophile which reacts well with soft electrophiles, but poorly with both weak and strong electrophiles. Weak electrophiles have low reactivity with all nucleophiles while strong electrophiles react well with weak nucleophiles including superabundant H(2)O. There are enzymes, the GSH transferases, which catalyze GSH conjugation with all the types of electrophiles described above. In order to deal with the wide variety of potential substrates, a multiplicity of GSH transferases exists-each tissue having its own collection and each enzyme having a different substrate specificity. These enzymes are often very abundant, e.g., in the rat liver cytosol, their concentration is 0.2 mM. THE FOLLOWING SUBSTRATES ARE CONSIDERED IN SOME DETAIL: 1-chloro-2,4-dinitrobenzene, the electrophile derived metabolically from paracetamol N-acetyliminoquinone?), benzo(a)pyrene-4-5-oxide, cholesterol-5alpha,6alpha-oxide, benzo(a)pyrene-7,8-diol-9,10-oxide and the electrophiles derived metabolically from aflatoxin B(1) (the 2,3-oxide?). According to the substrate, optimal enzyme rates vary over seven orders of magnitude from 10(-5) to 10(-12) mole/min/mg. Despite the wide embrace of the GSH transferases, not all metabolically produced electrophiles are substrates. We know of the following examples: N-methylol-4-aminoazobenzene and its 4'-hydroxy derivative (these are soft electrophiles and react well with GSH noncatalytically), N-sulfonyloxy-N-methyl-4-aminoazobenzene, N-sulfonyloxy-N-acetyl-2-aminofluorene (these are strong electrophiles which do not react selectively with GSH) and N-hydroxy-2-aminofluorene which appears to react only slowly with GSH. It is of interest in the present context that all these compounds are derived from either arylamine or arylamide carcinogens. Whether the reaction be enzymic or nonenzymic, conjugation with GSH is a very important means of detoxication accounting in some cases for up to 60% of the biliary metabolites. As seen in the example of aflatoxin B(1), very low enzymic rates observed in vitro are sufficient to account for apparently high rates of biliary excretion of GSH conjugates.GSH transferases have evolved other functions apart from the catalysis of GSH conjugation. GSH transferase B participates in the hepatic uptake of bilirubin and the intracellular distribution of the heme prosthetic group. It also has GSH peroxidase activity which suggests that it might participate in the detoxication of by-products of oxygen utilization including those produced by the action of cytochrome P-450. It is shown that GSH transferase B inhibits lipid peroxidation in vitro.

Mechanism of formation of mercapturic acids from aromatic aldehydes in vivo

Male adult Wistar rats dosed i.p. with o-substituted benzaldehydes (o-F, o-Cl, and o-Br = V, VI, and VII) excreted mercapturic acids in urine. These acids were identified as N-acetyl-S-(ortho-substituted benzyl)cysteines (I, II, III). The total mercapturic acid excretion as % dose (2.0 mmol/kg, n = 4) was 1.2 +/- 0.4, 6.8 +/- 0.9, and 10.4 +/- 2.0 for V, VI, and VII. p-Cl-benzaldehyde administered in the same dose showed a non-significant urinary thioether excretion. The aim of the investigation was to prove in vivo a postulated metabolic pathway of substituted benzaldehydes via sulphate esters to mercapturic acids. After a single administration of the sodium salts of o- and p-Cl-benzylsulfuric acid a significant increase in mercapturic acid excretion of 21.2 +/- 1.8% and 14.5 +/- 1.2% of dose (2.0 mmol/kg, n = 4) was found. By pretreatment with pyrazole the mercapturic acid excretion increased after administration of o-Cl-benzyl alcohol (IX) whereas a significant decrease was found after administration of o-Cl-benzaldehyde (VI). After simultaneous administration of ethanol with IX and VI an increase in mercapturic acid excretion was observed. After previous administration of pentachlorophenol a significant decrease in urinary mercapturic acid excretion for IX and VI was found. These findings are in accordance with a metabolic pathway of substituted benzaldehydes via benzyl alcohols, subsequently sulphate esters to the corresponding benzylmercapturic acids.

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.

Stereoselectivity of isozyme C of glutathione S-transferase toward arene and azaarene oxides

Three of the isozymes of glutathione S-transferase (EC 2.5.1.18) from rat liver (isozymes A, B, and C) catalyze the addition of glutathione to phenanthrene 9, 10-oxide with varying degrees of efficiency and stereoselectivity. Isozyme C is 2-fold and 35-fold more efficient toward this substrate than are isozymes A and B, respectively, and gives a 20 to 1 ratio of the two possible diastereomeric products. The stereoselectivities of isozymes A (approximately 1 to 1) and B (3 to 1) are considerably lower. The major product diastereomer from isozyme C is deduced to have the 9S, 10S absolute configuration by circular dichroism spectroscopy, implying attack of glutathione on the oxirane carbon on R absolute configuration. Isozyme C shows little kinetic discrimination between other K-region arene oxides such as pyrene 4,5-oxide and the enantiomers of benz[a]anthracene 5,6-oxide and benzo[a]pyrene 4,5-oxide. However, the stereoselectivity toward all the substrates is conserved with predominant (greater than 95%) attack at the oxirane carbon of R absolute configuration to give the S,S product. The stereoselectivity of isozyme C is very sensitive to the introduction and location of nitrogen substitution in the phenyl rings of phenanthrene 9,10-oxide. As a result isozyme C shows little or no stereoselectivity toward 4,5-diaza- and 4-azaphenanthrene 9,10-oxide. In contrast, 1-azaphenanthrene 9,10-oxide is attacked preferentially at the R carbon of the oxirane. The results suggest that hydrophobic interactions between the enzyme surface and the substrate distal to the oxirane ring are important in determining the stereoselectivity of the enzyme toward arene oxides.

Abnormal radiosensitizing and cytotoxic properties of ortho-substituted nitroimidazoles

Various 5-substituted 4-nitroimidazoles have been shown to be much more efficient radiosensitizers and much more toxic than would have been predicted from their electron affinities, as measured by values of one-electron reduction potential, E17. Using Chinese hamster V79 cells in vitro, a comparison has been made with some isomeric 4-substituted 5-nitroimidazoles. These compounds have E17 values some 64mV greater than the 4-nitroimidazoles, yet show much lower sensitizing efficiency and also lower toxicity. Neither series of compounds shows the greater toxicity towards hypoxic cells usually associated with nitroaromatic and nitroheterocyclic compounds. The second-order rate constants, k2, for reaction of these isomeric nitroimidazoles with glutathione and dithiothreitol were determined. Within each series the value of k2 increased with increasing electron affinity, however, the 4-nitroimidazoles were always more reactive than their corresponding 5-nitro isomers. The sensitizing and toxic properties of these compounds may involve depletion of intracellular thiols; this possibility is discussed.

Urinary excretion of thioethers among low-tar and medium-tar cigarette smokers

The urinary excretion of thioethers was studied among smokers of medium-tar cigarettes (16.3 mg/cig.) and low-tar cigarettes (5.4 mg/cig.). The test persons (26 smokers and 11 nonsmokers) were healthy young men in the military service. The smokers had an increased excretion of thioethers into the urine (P = 0.002 with Mann-Whitney U-test) when compared with the nonsmokers. No differences in the amount of urinary thioethers were detected between the low-tar and medium-tar cigarette smokers. The smokers (varying from 2.0 to 8.5 mmol/mol creatinine) had more individual variation than the nonsmokers (varying from 1.5 to 4.5 mmol/mol creatinine). Furthermore, the individual variation in the group of low-tar cigarette smokers did not markedly differ from the individual variation of medium-tar cigarette smokers.

Glutathione and hepatic mixed-function oxidase activity

It is apparent that hepatic GSH may function in drug metabolism not only as a substrate for conjugation but also in regulation of cytochrome P-450 activity. The remarkable aspect of the latter activity is its specificity. Loss of hepatic GSH depresses N-demethylation of DAB while ring hydroxylation is unaffected. On the other hand, the effect is to some degree nonspecific in that control as well as PB- or MC-induced N-demethylation is inhibited. Thus the response may not simply be specific to one isozyme of cytochrome P-450 but may be associated with one aspect of the enzymic activity of several cytochrome P-450 isozymes (i.e., N-demethylation). We have postulated that sensitivity of this activity to lipid peroxidation underlies the relationship to GSH since the tripeptide serves as a major protection against hepatic lipid peroxidation and its consequences. It is as yet not clear as to how or why this particular aspect of P-450 activity is more sensitive to lipid peroxidation than are other activities such as ring hydroxylation. Ongoing investigations include attempts to identify the cytochrome P-450 isozyme(s) which inhibit this response to GSH depletion. GSH-lipid peroxidation relationships have already been reported with isolated hepatocytes, and there may be a possible connection between this and the relative instability of cytochrome P-450 in cultured hepatocytes. Another factor which may be involved is heme oxygenase activity, which is markedly induced in the liver after GSH depletion, after cobalt administration (which also depresses cytochrome P-450 activity), and during incubation of isolated hepatocytes. This enzyme catalyzes the rate-limiting step in heme breakdown and may contribute to the loss of cytochrome P-450 activity associated with GSH depletion.

5 alpha,6 alpha-Epoxy-cholestan-3 beta-ol (cholesterol alpha-oxide): A specific substrate for rat liver glutathione transferase B

A semi-micro assay was developed for the conjugation of 5 alpha,6 alpha-epoxy-cholestan-3 beta-ol (cholesterol alpha-oxide) with glutathione. The soluble supernatant of rat liver homogenate catalysed the reaction at a rate of 0.2-0.5 pmol . min-1 . mg protein-1 with 4 microM cholesterol alpha-oxide, while the reaction in the presence of GSH alone was barely detectable. Enzymic activity in the soluble supernatant was due equally to the two forms of glutathione transferase B (approximately 100 pmol . min-1 . mg protein-1), glutathione transferases AA, A, C and E being unreactive. The activity of purified glutathione transferase B was about 5-times that expected from the activity of the soluble supernatant. Complex enzyme kinetics were obtained suggestive of substrate inhibition.

Non-mutagenicity of 2-methyl-2,3-epoxybutane and factors influencing the mutagenicity of 2,3-epoxybutane

The compound 2,3-epoxybutane (2,3-EB) is a direct mutagen for the base-pair-substitution-sensitive strains TA 1530, TA 1535 and TA 100, in the absence and in the presence of metabolic activation by liver S9-mix. Under the same experimental conditions, the 2-methyl derivative of 2,3-EB was not mutagenic, up to 10 mg per plate. The parent compound (2,3-EB) was more active on strain TA 1530 in the presence of liver S9-mix. This was observed at every epoxide concentration tested. The increase of mutagenicity depended on the S9 concentration, the mix composition and to a small extent on the method utilized.

Enzymes of mercapturic acid production in rat mammary gland

1. The enzymes glutathione S-transferase, gamma-glutamyl peptidyltransferase and dipeptidase, which participate in the detoxification pathway through mercapturic acid production, were measured in rat mammary gland during pregnancy and lactation. 2. Mammary-gland concentration of reduced glutathione showed, concomitantly with the enzyme activities, a significant increase during lactation. 3. The mammary-gland glutathione S-transferase exhibits characteristics quite similar to those described for the liver and kidney enzymes with respect to substrates, isoenzymes, molecular weight and probenecid and bilirubin inhibition. 4. In view of these similarities, mammary-gland glutathione S-transferase may play the same role as a cytoplasmic organic-anion receptor proposed for the hepatic enzyme. It may also represent a detoxification pathway for protecting the mammary tissue during the lactogenic cycle.

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.

Detoxification Enzyme Differences Between a Herbivorous and Predatory Mite

The detoxification capabilities of the predatory mite Amblyseius fallacis and its herbivorous prey Tetranychus urticae are fundamentally different. The activities of mixed-function oxidase and trans-epoxide hydrolase are higher in the prey than in the predator; those of cis-epoxide hydrolase and glutathione transferase are lower; and esterase activity is similar. Dissimilarities may be related both to differing adaptations to plant allelochemicals and to the higher respiration rate of the predator. Hydrolytic and conjugating reactions appear more important than oxidative pathways in imparting organophosphate resistance to these acarines. These resistances provide insecticide selectivity favorable to the predator and improved integrated pest control.

High-performance liquid chromatographic assay of cytosolic glutathione S-transferase activity with styrene oxide

A high-performance liquid chromatographic (HPLC) assay for measuring cytosolic glutathione S-transferase activity with styrene oxide is described. After incubating lung or liver cytosol with reduced glutathione and styrene oxide, unreacted styrene oxide is extracted into ethyl acetate. An aliquot of the aqueous phase is evaporated to dryness and reconstituted in the mobile phase for HPLC analysis. The two glutathione conjugates of styrene oxide [S-(1-phenyl-2-hydroxyethyl)glutathione and S-(2-phenyl-2-hydroxyethyl)glutathione] are separated in less than 10 min; quantitation of transferase activity is based on the comparison of the UV absorbance of the two conjugates at 254 nm with synthetic conjugate standards. As little as 1 nmole of either conjugate can be quantitated with good precision. This assay has advantages over previously published methods for measuring styrene oxide glutathione S-transferase activity as it does not depend on the use of relatively unstable and expensive radiolabelled substrates.

Gene for glutathione S-transferase-1 (GST1) is on human chromosome 11

The glutathione S-transferases (GST) are a group of related enzymes that can detoxify potentially carcinogenic electrophiles by conjugating them with reduced glutathione (GSH). The chromosomal location of one of the enzyme forms, GST1, reported recently to be polymorphic, was determined utilizing man-mouse somatic cell hybrids segregating human chromosomes. The expression of GST1 by hybrid clones was compared with that of 34 enzyme markers representing 23 chromosomes, and karyotypes of selected cell hybrids were analyzed. The evidence indicated that GST1 is assigned to chromosome 11 in humans. Utilizing and X/11 translocation segregating in hybrids, GST1 was localized to the p13 leads to qter region of chromosome 11.

Elimination of glyceryl trinitrate: effects of sex, age, species and route of administration

1 Orally administered glyceryl trinitrate to rats undergoes extensive first pass metabolism leading to low bioavailability. 2 Sex differences in the plasma elimination of glyceryl trinitrate were seen in the rat, the female exhibiting the longer plasma half-life. No sex differences in this respect were detected in the rabbit. 3 The plasma half-life of glyceryl trinitrate was longer and the volume of distribution larger, in older animals. 4 The plasma elimination of glyceryl trinitrate was different in various animal species. There was a good correlation between plasma half-life and animal bodyweight.

Glutathione, lipid peroxidation and regulation of cytochrome P-450 activity

Depletion of hepatic glutathione leads to an increase in lipid peroxidation and depression of cytochrome P-450-catalyzed metabolism of the azo dye carcinogen, N,N-dimethyl-4-aminoazobenzene. This contributes to the marked decrease in biliary excretion of N-demethylated metabolites of the dye. Parallel time courses are seen for decreased hepatic glutathione, enhanced lipid peroxidation and depressed excretion of dye metabolites. In vitro metabolism of DAB by hepatic 10,000 g supernatant fractions is depressed by iron only after glutathione depletion. In view of the iron requirement for microsomal lipid peroxidation, it is proposed that glutathione depletion leads to an increase in the intracellular iron available for activation of lipid peroxidation. In this way, glutathione may contribute to the regulation of cytochrome P-450 activity.

Determination of formaldehyde in biological tissues by gas chromatography/mass spectrometry

A quantitative method is described for the determination of formaldehyde in biological tissues by stable isotope dilution using gas chromatography/mass spectrometry. (13C2H2)Formaldehyde is used as the isotopic diluent. After tissue homogenization, derivatization is carried out in situ with pentafluorophenylhydrazine, followed by extraction and analysis using selected ion monitoring. The sensitivity of the technique is higher than that of conventional methods of formaldehyde analysis, enabling endogenous formaldehyde to be quantitatively analyzed in tissues, even in samples as small as 20 mg wet weight. The effects of exposure to airborne formaldehyde or to airborne methyl chloride on the formaldehyde concentrations of several tissues of Fischer-344 rats are reported.

Effects of phenobarbital and sodium salicylate on cytochrome P-450 mixed function oxygenase and glutathione S-transferase activities in rat brain

The effects of acute and therapeutic doses of phenobarbital and sodium salicylate on cytochrome P-450 mixed function oxygenase (EC 1.14.14.1) and glutathione S-transferase (EC 2.5.1.18) activities have been studied in rat brain and compared with those of rat liver. P-450 enzymic activity was assayed by N-demethylation of p-chloro-N-methylaniline and 1-chloro-2,4-dinitrobenzene was used as substrate for glutathione S-transferase activity. The acute effects of a single daily dose of phenobarbital (75 mg/kg/day; i.p.) and sodium salicylate (500 mg/kg/day; i.p.) for 3 days increased cytochrome P-450 as well as glutathione S-transferase in rat liver. But the same doses of both drugs decreased glutathione S-transferase levels in rat brain and increased cytochrome P-450 dependent N-demethylation of p-chloro-N-methylaniline. The therapeutic doses of sodium salicylate (50 mg/kg/day; i.p.) and phenobarbital (10 mg/kg/day; i.p.) daily for 21 days increased cytochrome P-450 in rat liver as well as in brain. The increase in brain glutathione S-transferase by prolonged treatment of phenobarbital was significant compared to the control values.

Effect of the triazolobenzodiazepine estazolam on hepatic drug-metabolizing enzyme activity in rats

Oral doses of the sedative/hypnotic estazolam (500 mg kg-1 day-1) to rats for 21 days caused statistically significant increases in liver weight, ascorbate excretion, cytochrome P-450 concentrations, and in aniline hydroxylase, ethylmorphine N-demethylase and glutathione S-transferase activities, as did approximately equivalent doses of flurazepam hydrochloride. Histologically, the centrilobular hepatocytes were enlarged. Some of these parameters were also increased after doses of estazolam of 100 mg kg-1 day-1, but not after 5 mg kg-1 day-1, which is about 50-fold greater than a clinical dose. Estazolam was a much less potent enzyme inducer than phenobarbitone under the conditions of these studies.

Spectrophotometric assay for mammalian cytosolic epoxide hydrolase using trans-stilbene oxide as the substrate

A continuous spectrophotometric assay based on the differences in the ultraviolet spectra of trans-stilbene oxide (TSO) and its reaction product 1,2-diphenyl-1,2-ethanediol is described for the measurement of mammalian cytosolic epoxide hydrolase activity. Rates of TSO hydration determined by this method were similar to those obtained by independent analytical methods, and the assay is rapid, reproducible and relatively free from interference.

Dose-related effects of m-xylene inhalation on the xenobiotic metabolism of the rat

1. Male Wistar rats were exposed for two weeks (6 h/d, 5 d/week) to 0, 50, 400 or 750 p.p.m. of m-xylene vapour in the air. 2. Microsomal enzyme activities were significantly increased in the liver as follows: NADPH-cytochrome c reductase (less than or equal to 1.2-fold), O-deethylation reactions (less than or equal to 1.8-fold) and UDP-glucuronosyltransferase activity (less than or equal to 2.9-fold). Cytochrome P-450 concentrations also increased with dose. 3. Microsomal affinity to m-xylene increased with dose of m-xylene in vivo, similar to the increased affinity seen in type I spectra390-420nm formation with cytochrome P-450 in vitro. 4. Reduced glutathione concn. in liver declined significantly, while in the kidneys the glutathione remained unchanged. 5. In the kidneys, the concn. of cytochrome P-450 (less than or equal to 1.3-fold), O-deethylation activity (less than or equal to 5.3-fold) and the rate of glucuronidation (less than or equal to 1.8-fold) increased with the dose of xylene. 6. Microsomal protein contents in liver and kidneys increased with the two highest doses of xylene. 7. The hepatic and renal effects were dose-dependent and closely related to the accumulation of m-xylene in perirenal fat. 8. Despite stimulation of xenobiotic metabolism by m-xylene, inhalation did not alter the serum transaminase activity or cause morphological changes in the liver.

Glutathione and gamma-glutamyl cycle enzymes in rat mammary gland

The main enzymes of the gamma-glutamyl cycle during the lactogenic cycle in rat mammary gland were studied. A significant increase was found in all of them with the onset of lactogenesis. The effect of methionine sulfoximine on reduced glutathione concentration was studied in tissue slices of lactating mammary gland. The findings suggest that this compound effects glutathione synthesis by inhibiting gamma-glutamylcysteine synthetase.

Protective role of endogenous pulmonary glutathione and other sulfhydryl compounds against lung damage by alkylating agents. Investigations with 4-ipomeanol in the rat

Because endogenous glutathione is known to participate in the detoxification of highly reactive, hepatotoxic drug metabolites, we studied the role of this substance in the pulmonary toxicity of 4-ipomeanol [1-(3-furyl)-4-hydroxypentanone] in rats. 4-Ipomeanol was an appropriate model for these studies since previous investigations have indicated that an alkylating metabolite, formed in situ, is responsible for selective lung damage by 4-ipomeanol. Toxic doses of 4-ipomeanol preferentially depleted rat lung glutathione. Pretreatment of animals with piperonyl butoxide, an inhibitor of the metabolic activation of 4-ipomeanol, prevented both the depletion of lung glutathione and the pulmonary toxicity of 4-ipomeanol. Prior depletion of lung glutathione by diethylmaleate increased both the pulmonary covalent binding and the toxicity of 4-ipomeanol, whereas administration of cysteine and cysteamine decreased both the covalent binding and the toxicity. These in vivo studies, in conjunction with previous in vitro studies which showed inhibitory effects of sulfhydryl compounds on the covalent binding of 4-ipomeanol, are consistent with the view that pulmonary glutathione plays a protective role against pulmonary alkylation and lung toxicity by 4-ipomeanol, probably by reacting with the toxic metabolite(s) to form nontoxic conjugate(s). Pulmonary glutathione may similarly provide protection against other electrophilic drugs or metabolites that can damage the lungs.

Metabolic activation of saturated aldehydes to cause destruction of cytochrome P-450 in vitro

The results of this paper demonstrate for the first time that a number of saturated aliphatic aldehydes, e.g. octanal, when incubated with rat liver microsomes and NADPH in vitro cause the time dependent destruction of cytochrome P-450 and the formation of green pigments. No loss of this cytochrome occurred when microsomes were incubated with aldehydes in the absence of NADPH. The activating enzymes had some characteristics typical of the microsomal mixed-function oxidases. Formaldehyde and acetaldehyde did not appear to be activated by this system. Octanal had no effect on hepatic microsomal cytochrome P-450 when administered to rats (1 mmol/kg, i.p.) in vivo. Under similar conditions, the ethynyl-substituted 1-octyne caused a significant destruction of this cytochrome. In order to prelabel liver haem with 14C, rats were pretreated with 5-amino[4-14C]laevulinic acid. Microsomal fractions prepared from these animals were incubated with octanal. Destruction of cytochrome P-450 was accompanied by the appearance of 14C-labelled green pigments separated by HPLC. The properties of the green pigments produced by incubating microsomes with either octanal or 1-ocytne have been compared. From their chromatographic behaviour and spectrophotometrically they appear to be similar though not identical. The results suggest that octanal induced green pigments may contain chelated iron which is not readily removed. The possibility that aldehyde could be metabolically activated via a ketene intermediate was investigated. Although attempts to prepare the ketene analogue of octanal were not successful, ketene gas, prepared chemically, reacted with haem in chloroform and to a limited extent with aqueous microsomal suspensions. However, the reaction products were brown in colour and gave an absorption spectrum indistinguishable from that of haem. There was no evidence of green pigment formation.

The use of drugs which deplete intracellular glutathione in hypoxic cell radiosensitization

Diethylmaleate (DEM) is a thiol-binding reagent with specificity toward glutathione. Treatment of Chinese hamster ovary (CHO) cells in vitro with 2 x 10(-4) M DEM for one hour results in a decrease in glutathione content to less than 5% of control, without cytotoxicity. This treatment results in dose-modifying sensitization to radiation under hypoxic conditions, with no effect on the shoulder of the radiation survival curve. No effect on the radiation sensitivity of oxygenated cells was seen. DEM pretreatment enhances the radiosensitization of hypoxic cells by misonidazole, as well. Similar results were obtained in vivo with EMT6 tumors in BALB/c mice. Analysis of DNA damage by the alkaline elution assay indicates that DEM enhances radiation-induced single-strand breaks, but does not significantly affect repair, while diamide and N-ethylmaleimide inhibit repair, in addition to enhancing radiation-induced single-strand breaks.