The role of liver glutathione in the acute toxicity of retrorsine to rats

CYP2C and CYP2B Mediated Metabolic Activation of Retrorsine in Cyp3a Knockout Mice

Background:

Retrorsine is one of the hepatotoxic pyrrolizidine alkaloids, which could be converted into a highly reactive metabolite, dehydroretrorsine, by CYP3A, and to a lesser extent by CYP2C and CYP2B.

Objective:

We employed Cyp3a knockout (3AKO) mice to investigate whether the absence of CYP3A could attenuate dehydroretrorsine formation and the role of CYP2C and CYP2B in the formation.

Methods:

Blood and liver samples were collected after intragastrical administration of 35 mg/kg retrorsine or saline for seven days in wild-type (WT) and 3AKO mice. Blood pyrrole-protein adducts were semi quantified by high-performance liquid chromatography/quadrupole time-of-flight mass spectrometry. The formations of glutathionyl-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (GSH-DHP) and the activities of CYP3A, CYP2B and CYP2C were evaluated in the liver microsomes of WT and 3AKO mice before and after treatment. The metabolic phenotype of retrorsine was determined in human liver microsomes. The gene and protein expression of retrorsine metabolism-related CYP450s in the liver was measured by quantitative real-time PCR method and western blotting method. The serum cytokine level was detected by the ELISA method to reveal the potential mechanism of Cyp3a, Cyp2b and Cyp2c downregulation.

Results:

After an oral administration of 35 mg/kg retrorsine for seven days, the blood exposures of DHP adducts between WT and 3AKO mice were similar, consistent with the comparable formation of GSH-DHP in their liver microsomes. The chemical inhibitor experiment in liver microsomes indicated the predominant role of CYP3A and CYP2C in GSH-DHP formation in WT and 3AKO mice, respectively. Real-time qPCR analysis showed that the expressions of Cyp2b10 and Cyp2cs increased 2.3-161-fold in 3AKO mice, which was consistent with protein changes. The increased CYP2B activity in 3AKO mice supported the potential role of CYP2B in GSH-DHP formation. After a seven-day treatment of retrorsine, the yields of GSH-DHP were lower than the untreated ones in both alleles, accompanied by the decreased mRNA of Cyp3a, Cyp2b and Cyp2c. The increased serum IL6 might mediate the retrorsine-induced downregulation of Cyp450s.

Conclusion:

These data demonstrated the increased transcription of Cyp2c and Cyp2b caused by Cyp3a ablation, which played a vital role in the metabolic activation of retrorsine, and long-term exposure of retrorsine can reduce the CYP450 activities.

Effects of glutathione and cysteine on pyrrolizidine alkaloid-induced hepatotoxicity and DNA adduct formation in rat primary hepatocytes

Pyrrolizidine alkaloids (PAs) are hepatotoxic, genotoxic, and carcinogenic phytochemicals. Upon metabolic activation, PAs produce dehydropyrrolizidine alkaloids (dehydro-PAs) as reactive primary pyrrolic metabolites. Dehydro-PAs are unstable, facilely hydrolyzed to (±)-6,7-dihydro-7-hydroxy-1-hydroxymethyl-5H-pyrrolizine (DHP). Both dehydro-PAs and DHP are capable of binding to cellular DNA and proteins to form DHP-DNA and DHP-protein adducts leading to tumorigenicity and cytotoxicity. We recently determined that the reaction of dehydro-PAs with glutathione and cysteine generated 7-glutathione-DHP (7-GS-DHP) and 7-cysteine-DHP, respectively which can also bind to DNA to produce DHP-DNA adducts. In this study, we determined the effects of glutathione and cysteine on the induction of hepatocytotoxicity and the formation of DHP-DNA adducts in primary hepatocytes cultured with riddelliine and monocrotaline. We found that both glutathione and cysteine can drastically reduce hepatotoxicity while the levels of DHP-DNA adduct formation are slightly affected.

Use of physiologically based kinetic modelling-facilitated reverse dosimetry to convert in vitro cytotoxicity data to predicted in vivo liver toxicity of lasiocarpine and riddelliine in rat

Lasiocarpine and riddelliine are pyrrolizidine alkaloids (PAs) present in food and able to cause liver toxicity. The aim of this study was to investigate whether physiologically based kinetic (PBK) modelling-facilitated reverse dosimetry can adequately translate in vitro concentration-response curves for toxicity of lasiocarpine and riddelliine to in vivo liver toxicity data for the rat. To this purpose, PBK models were developed for lasiocarpine and riddelliine, and predicted blood concentrations were compared to available literature data to evaluate the models. Concentration-response curves obtained from in vitro cytotoxicity assays in primary rat hepatocytes were converted to in vivo dose-response curves from which points of departure (PODs) were derived and that were compared to available literature data on in vivo liver toxicity. The results showed that the predicted PODs fall well within the range of PODs derived from available in vivo toxicity data. To conclude, this study shows the proof-of-principle for a method to predict in vivo liver toxicity for PAs by an alternative testing strategy integrating in vitro cytotoxicity assays with in silico PBK modelling-facilitated reverse dosimetry. The approach may facilitate prediction of acute liver toxicity for the large number of PAs for which in vivo toxicity data are lacking.

Disturbance of gene expression in primary human hepatocytes by hepatotoxic pyrrolizidine alkaloids: A whole genome transcriptome analysis

1,2-unsaturated pyrrolizidine alkaloids (PA) are plant metabolites predominantly occurring in the plant families Asteraceae and Boraginaceae. Acute and chronic PA poisoning causes severe hepatotoxicity. So far, the molecular mechanisms of PA toxicity are not well understood. To analyze its mode of action, primary human hepatocytes were exposed to a non-cytotoxic dose of 100 μM of four structurally different PA: echimidine, heliotrine, senecionine, senkirkine. Changes in mRNA expression were analyzed by a whole genome microarray. Employing cut-off values with a |fold change| of 2 and a q-value of 0.01, data analysis revealed numerous changes in gene expression. In total, 4556, 1806, 3406 and 8623 genes were regulated by echimidine, heliotrine, senecione and senkirkine, respectively. 1304 genes were identified as commonly regulated. PA affected pathways related to cell cycle regulation, cell death and cancer development. The transcription factors TP53, MYC, NFκB and NUPR1 were predicted to be activated upon PA treatment. Furthermore, gene expression data showed a considerable interference with lipid metabolism and bile acid flow. The associated transcription factors FXR, LXR, SREBF1/2, and PPARα/γ/δ were predicted to be inhibited. In conclusion, though structurally different, all four PA significantly regulated a great number of genes in common. This proposes similar molecular mechanisms, although the extent seems to differ between the analyzed PA as reflected by the potential hepatotoxicity and individual PA structure.

7-glutathione pyrrole adduct: a potential DNA reactive metabolite of pyrrolizidine alkaloids

Pyrrolizidine alkaloid (PA)-containing plants are the most common poisonous plants affecting livestock, wildlife, and humans. PAs require metabolic activation to form pyrrolic metabolites to exert cytotoxicity and tumorigenicity. We previously determined that metabolism of tumorigenic PAs produced four DNA adducts, designated as DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4, that are responsible for liver tumor initiation. 7-Glutathione-(±)-6,7-dihydro-1-hydroxymethyl-5H-pyrrolizine (7-GS-DHP), formed in vivo and in vitro, and 7,9-di-GS-DHP, formed in vitro, are both considered detoxified metabolites. However, in this study we determined that incubation of 7-GS-DHP with 2'-deoxyguanosine (dG) and 2'-deoxyadenosine (dA) yields DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4 adducts as well as the reactive metabolite DHP. Furthermore, reaction of 7-GS-DHP with calf thymus DNA in aqueous solution at 37 °C for 4, 8, 16, 24, 48, or 72 h, followed by enzymatic hydrolysis yielded DHP-dG-3, DHP-dG-4, DHP-dA-3, and DHP-dA-4 adducts. Under our current experimental conditions, DHP-dA-3 and DHP-dA-4 adducts were formed in a trace amount from the reaction of 7,9-di-GS-DHP with dA. No DHP-dG-3 or DHP-dG-4 adducts were detected from the reaction of 7,9-di-GS-DHP with dG. This study represents the first report that the 7-GS-DHP adduct can be a potential reactive metabolite of PAs leading to DNA adduct formation.

Pyrrolizidine Alkaloids—Genotoxicity, Metabolism Enzymes, Metabolic Activation, and Mechanisms

Pyrrolizidine alkaloid-containing plants are widely distributed in the world and are probably the most common poisonous plants affecting livestock, wildlife, and humans. Because of their abundance and potent toxicities, the mechanisms by which pyrrolizidine alkaloids induce genotoxicities, particularly carcinogenicity, were extensively studied for several decades but not exclusively elucidated until recently. To date, the pyrrolizidine alkaloid-induced genotoxicities were revealed to be elicited by the hepatic metabolism of these naturally occurring toxins. In this review, we present updated information on the metabolism, metabolizing enzymes, and the mechanisms by which pyrrolizidine alkaloids exert genotoxicity and tumorigenicity.

The effects of pretreatment with glycyrrhizin and glycyrrhetinic acid on the retrorsine-induced hepatotoxicity in rats

A wide variety of medicinal herbs contain hepatotoxic pyrrolizidine alkaloids (PAs), and often cause acute and chronic liver damages in man. Liquorice, a known antihepatitis, is commonly used with PA-containing herbs concurrently, and hepatotoxicity induced by such combined uses was not pronounced. The present study is to investigate effects of glycyrrhizin (GL) and 18beta-glycyrrhetinic acid (GA), the major biologically active ingredients of liquorice, against PA-induced hepatotoxicity in rats. Single dose (35 mg/kg, i.p.) of retrorsine (RET), a typical potent hepatotoxic PA, was given to rats to induce liver injury. A single dose pretreatment with GL or GA prior to retrorsine challenge did not show hepatoprotection. However, when rats were pretreated with either GL (200 mg/kg/day, i.p.) or GA (10 mg/kg/day, i.p.) for three consecutive days prior to retrorsine exposure, the elevated serum GOT and GPT levels induced by retrorsine were significantly reduced. Serum levels of transaminases almost returned to normal (GOT: 56+/-2 (control), 104+/-5 (RET), 64+/-3 (GL + RET) and 59+/-3 (GA + RET). GPT: 40+/-2 (control), 90+/-7 (RET), 45+/-2 (GL + RET) and 45+/-4 (GA + RET) SF units/ml). Furthermore, no extensive hepatocellular damages were observed. The results demonstrated that a three-day pretreatment with either GL or GA exhibited protective effect on retrorsine-induced liver damage in rats.

The effect of the pyrrolizidine alkaloids, monocrotaline and trichodesmine, on tissue pyrrole binding and glutathione metabolism in the rat

One day after in vivo administration of equitoxic doses of the hepatotoxic and pneumotoxic pyrrolizidine alkaloid, monocrotaline (65 mg/kg, i. p.) or the related hepatotoxic and neurotoxic alkaloid trichodesmine (15 mg/kg, i. p.) hepatic GSH levels are increased by more than 50%. These doses of alkaloids represent 60% of the LD50 values. Accompanying these changes in GSH levels is an increase in the overall rate of GSH synthesis in supernatants of alkaloid-exposed livers. The ability of the rat to metabolize the two alkaloids was shown by the appearance of tissuebound pyrrolic metabolites of pyrrolizidines in various organs. The levels of these metabolites appear to correlate with organ toxicity. For the hepatic and pneumotoxic alkaloid, monocrotaline, higher levels are found in liver (17 nmoles/g tissue) and lung (10 nmoles/g) than for trichodesmine (7 nmoles/g and 8 nmoles/g, respectively). For the neurotoxic alkaloid, trichodesmine, higher levels are found in brain (3.8 nmoles/g tissue) than for monocrotaline (1.7 nmoles/g tissue).

Perturbation of glutathione status and generation of oxidative stress in mouse skin following application of contact allergenic sesquiterpene lactones and isothiocyanates

1. The sensitizing or non-sensitizing status of selected sesquiterpene lactones and isothiocyanates was confirmed in mouse by open epicutaneous application. 2. Glutathione status of mouse skin was determined 12 h after lactone/isothiocyanate application; glutathione S-transferase activity also was determined 12 h after lactone application. 3. NAD(P)H utilization by rat liver microsomal preparations exposed to the sesquiterpene lactones and isothiocyanates was measured. 4. A correlation was observed between sensitizing status and the ability to perturb glutathione status, to induce glutathione S-transferase activity, and to stimulate NAD(P)H utilization. 5. It was concluded that sensitizing sesquiterpene lactones and isothiocyanates could induce oxidative stress in mouse skin, possibly as a result of their reductive metabolism.

Biochemical responses of skin to allergenic and non-allergenic nitrohalobenzenes. Evidence that an NADPH-dependent reductase in skin may act as a prohapten-activating enzyme

Using a selection of 'classic' haptens (dinitrohalobenzenes and picryl chloride) and related non-sensitizing analogous, we examined changes in levels of glutathione (GSH) and glutathione disulphide (GSSG) in mouse skin 12 h after their epicutaneous application. We observed that elevation of GSSG levels and/or depletion of GSH levels correlated well with contact allergenic potential. Non-sensitizing analogous failed to perturb GSH/GSSG status. In vitro assays using mouse skin and rat liver microsomal preparations indicated that only the allergenic nitrohalobenzenes initiated NADPH-dependent oxygen utilization, with the activity falling off in the order picryl chloride >> DNIB > DNBB > DNCB > DNFB. In addition, an examination of the colour of mouse skin homogenates ex vivo after application of the dinitrohalobenzenes showed significant yellowing (consistent with aromatic nucleophilic substitution) only with DNFB. Our results indicate that, while an aromatic nucleophilic substitution reaction with skin protein can possibly account for the allergenicity of DNFB, it does not seem to occur with DNCB, DNBB or DNIB. These may instead behave mainly as prohaptens which are activated enzymically by NADPH-dependent reductase(s) within the skin, with the concomitant generation of superoxide and hydrogen peroxide, to form potentially protein-reactive free radical and other metabolites. Picryl chloride appears capable of both conjugating directly with proteins by aromatic nucleophilic substitution and undergoing NADPH-dependent metabolism to other potentially protein-reactive metabolites.

Hepatic glutathione concentrations and the release of pyrrolic metabolites of the pyrrolizidine alkaloid, monocrotaline, from the isolated perfused liver

We have examined the relationship between the metabolism of the pyrrolizidine alkaloid, monocrotaline, and glutathione concentration in the isolated, perfused rat liver. On perfusion of monocrotaline (300 microM) through the isolated liver, high concentrations (1.1 mM) of its metabolite glutathionyldehydroretronecine are released into bile, while much lower amounts (4.86 microM; 0.05 mumol/g liver) accumulate in the perfusate over a 1 hr perfusion period. Metabolite concentration in both the bile and perfusate increase when the level of monocrotaline perfused is increased to 900 microM. Metabolite release is also elevated in livers pretreated with phenobarbital. Monocrotaline perfusion lowered glutathione concentrations in the liver from 30 min onwards. Livers from animals treated with buthionine sulfoximine or chloroethanol showed much lower glutathione levels after 60 min perfusion. Livers from chloroethanol-treated (but not buthionine sulfoximine-treated) animals showed significantly lower release of pyrroles into the bile on perfusion with monocrotaline, but there is no effect on the rate of build-up of pyrrolic metabolites in the perfusate. We conclude that hepatic glutathione concentrations and the release of pyrrolic metabolites of monocrotaline mutually interact. Exposure of the liver to monocrotaline reduces glutathione concentrations, while marked depletion of liver glutathione concentration leads to a decrease in the release of monocrotaline metabolites.

Fluorimetric determination of oxidised and reduced glutathione in cells and tissues by high-performance liquid chromatography following derivatization with dansyl chloride

A high-performance liquid chromatographic method utilising fluorimetric detection of oxidised and reduced glutathione, following derivatization with dansyl chloride is described. Dansyl derivatives are separated on an aminopropyl silica column with a methanol-sodium acetate gradient system giving detection limits (signal-to-noise ratio = 2) of 1 pmol. This is in the order of 100-fold more sensitive than established methods based on the ultraviolet detection of dinitrophenylglutathione derivatives. The present procedures have been used to determine oxidised and reduced glutathione in rat lung tissues and in alveolar macrophages.

Identity of a biliary metabolite formed from monocrotaline in isolated, perfused rat liver

A pneumotoxic pyrrolic metabolite, previously isolated from the bile when rat liver was perfused with the pyrrolizidine alkaloid, monocrotaline, has been identified as 7-glutathionyl-dehydroretronecine. The metabolite showed a TLC spot and HPLC peak corresponding with the latter compound, and a procedure for replacing the thioether group with an ethoxy group converted the metabolite to dehydroretronecine 7-ethyl ether, confirming that the glutathionyl moiety was attached to the 7-position of dehydroretronecine. The same metabolite was detected in bile from rat liver perfused with retrorsine, which is a diester alkaloid similar to monocrotaline, whereas it was not formed from heliotrine, an alkaloid lacking the 7-ester function.

Recovery of the pyrrolic nucleus of pyrrolizidine alkaloid metabolites from sulphur conjugates in tissues and body fluids

Reactive pyrrolic metabolites formed when pyrrolizidine alkaloids are given to rats can alkylate soluble and tissue-bound thiol groups. Pyrrolic thioethers thus formed are relatively stable, and may persist in tissues for long periods. A simple procedure has been developed for recovering the nucleus of the pyrrolic metabolite from such S-binding, whether in solution or attached to solid tissues, in an easily identifiable form. The thioether bond was broken by silver nitrate and the pyrrolic moiety allowed to react with ethanol or methanol to form an ethoxy or methoxy derivative. The chemical basis of the procedure was established by model experiments on a preparative scale, but for small scale recovery from tissues, pyrrolic ethers were extracted and identified by TLC, HPLC, capillary GC and mass spectrometry. Because the pyrrolic derivatives thus formed were easily recognised and unrelated to any physiological compound, the recovery method described, especially when applied to blood samples, provided a way to monitor animals for previous exposure to toxic pyrrolizidine alkaloids.

Model systems for detecting the hepatic toxicity of pyrrolizidine alkaloids and pyrrolizidine alkaloid N-oxides

Indicine N-oxide (INO) is a pyrrolizidine alkaloid (PA) with antitumor activity in animals and humans. Prior studies showed that despite the known hepatic toxicity of the PAs, INO did not produce hepatic toxicity in animals but caused unpredictable lethal hepatic toxicity in humans. In this study we have attempted to find a model system for predicting the hepatotoxic potential of antitumor PAs. Primary cultures of rat hepatocytes showed toxicity only with the most hepatotoxic PAs such as lasiocarpine, but did not detect toxicity with other PAs. Subchronic intraperitoneal administration of PAs to weanling rats and adult mice produced, in surviving animals, hepatic megalocytosis and centrilobular necrosis with heliotrine (H) and 9-O-(R(-)-2-(4'-chlorophenyl)-2-hydroxybutyryl)retronecine N-oxide (RC1NO) but only megalocytosis with INO. Thus, despite previous reports, weanling rats offered no advantage over adult mice for detecting significant hepatic toxicity with PAs. Phenobarbital pretreatment of the mice did not increase the hepatic toxicity of any of the PAs. Subchronic oral administration of PAs to adult mice produced hepatic megalocytosis and centrilobular necrosis in surviving animals with H and RC1NO and megalocytosis with INO. Animals that died acutely following oral administration of INO showed hepatic centrilobular necrosis. Administration of several courses of INO intravenously to dogs produced histological evidence of centrilobular hemorrhagic necrosis. It is concluded that there is no single animal model that will predict hepatic toxicity of the type seen in humans with the antitumor PAs. A combination of studies using adult mice and dogs and lethal doses of the PAs offers the best way of detecting potential hepatic toxicity.

Oxygen dependence of glutathione synthesis in hepatocytes

The O2 dependence of glutathione (GSH) synthesis was studied in freshly isolated hepatocytes of white male rats. The rate of synthesis with methionine as the sulfur-containing amino acid precursor was decreased at hypoxic O2 concentrations and was half-maximal at 5 microM O2. ATP-dependent formation of S-adenosylmethionine was the rate-limiting step in GSH synthesis under these hypoxic conditions as shown by studies of S-adenosylmethionine concentrations and effects of compounds that inhibit mitochondrial ATP production. GSH synthesis with cysteine as the sulfur-containing precursor amino acid was relatively resistant to O2 deficiency. The rate under anoxia was 48% of the aerobic rate and the O2-dependent rate was half-maximal at 0.9 microM O2. These results indicate that GSH synthesis from methionine is likely to be impaired under physiological and pathological conditions involving hypoxia, but synthesis from cysteine is not likely to be greatly affected except during anoxia. In addition, the sensitivity of the cystathionine pathway to hypoxia suggests that other products of the pathway, such as choline, creatine, epinephrine, and methylated tRNA's, may also be decreased by hypoxia.

Dose-related inhibition of the drug-metabolizing enzymes of rat liver by the pyrrolizidine alkaloid, monocrotaline

Adult, male Sprague-Dawley rats were given 0, 10, 20, 40 or 80 mg kg-1 of monocrotaline intraperitoneally and the following toxicity parameters determined 24 h post treatment. Compared with the control none of the doses caused significant change in either the relative liver weight or the hepatic microsomal protein concentration. Microsomal cytochrome P450 content and activities of benzphetamine N-demethylase and aniline hydroxylase did not differ from the control at 10 or 20 mg kg-1 dosage. But, there was a significant loss of cytochrome P450 at 40 and 80 mg kg-1 dosages and decrease in the activity of the two enzymes only at the highest dose. Similarly, the highest dose caused a marked elevation of serum sorbitol dehydrogenase and glutamic pyruvic transaminase activity suggestive of severe liver damage.

Genotoxicity and cytotoxicity of selected pyrrolizidine alkaloids, a possible alkenal metabolite of the alkaloids, and related alkenals

Recently our laboratory isolated trans-4-OH-2-hexenal from the hepatic microsomal metabolism of the macrocyclic pyrrolizidine alkaloid (PA) senecionine and demonstrated in vivo that hepatic necrosis occurred following injection into the hepatic portal vein. To demonstrate similarities in the toxic effects of these compounds, as well as additional macrocyclic PAs and alkenals, genotoxicity and cytotoxicity were examined in primary cultures of rat hepatocytes. A positive cytotoxic response was exhibited by senecionine, retrorsine, seneciphylline, 19-OH-senecionine, trans-4-OH-2-hexenal, trans-4-OH-2-nonenal, and nonenal as measured by the release of LDH. A weaker response was elicited by hexenal. Dosages used of each of these compounds ranged from 30 to 600 nmol/10(6) cells, with each compound exhibiting a linear dose response within this range. All eight compounds exhibited a positive, dose-related genotoxic response as measured by autoradiographic detection of unscheduled DNA synthesis. These results would predict a carcinogenic role for both the PAs and the alkenals. This would suggest similarities in the mechanisms of action of the PAs and alkenals, lending support to the proposed role of trans-4-OH-2-hexenal as an important toxic metabolite of the PAs.

trans-4-Hydroxy-2-hexenal: a reactive metabolite from the macrocyclic pyrrolizidine alkaloid senecionine

The toxicity of macrocyclic pyrrolizidine alkaloids in the livers of man and animals has been attributed to the formation of reactive pyrroles from dihydropyrrolizines. Now a novel metabolite, trans-4-hydroxy-2-hexenal, has been isolated from the macrocyclic pyrrolizidine alkaloid senecionine, in an in vitro hepatic microsomal system. Other alkenals such as trans-4-hydroxy-2-nonenal have previously been isolated from microsomal systems when treated with halogenated hydrocarbons or subjected to lipid peroxidation. The in vivo pathology caused by trans-4-hydroxy-2-hexenal appears to be identical to that previously attributed to reactive pyrroles. There are similarities between the toxic effects of this alkenal and those of centrilobular hepatotoxins such as CCl4 and other alkenals formed during lipid peroxidation.

Covalent binding and glutathione depletion in the rat following niridazole (ambilhar) pretreatment

In vivo and in vitro studies with rats have shown that (14C) niridazole (Ambilhar) binds covalently to tissue proteins, but not to nucleic acids. In the in vitro experiments, binding required the presence of NADPH in the incubation medium, suggesting the production of an active metabolite via a cytochrome P-450-mediated reaction. Niridazole also caused significant dose-dependent decreases in liver and kidney glutathione levels, even though it had no apparent effect on blood glutathione. Alteration of tissue glutathione availability by pretreatment with chloracetamide or cysteine respectively either increased or decreased the NADPH-dependent covalent binding. Pretreatment with phenobarbital, 3-methylcholanthrene or cobaltous chloride, which change the rate of metabolism of (14C) niridazole, similarly altered the extent of protein binding. It is shown that the decrease in tissue glutathione concentration is not due to an effect of the drug on the activities of either glucose-6-phosphate dehydrogenase or glutathione-S-transferases. However, there is a significant reduction in glutathione reductase activity in all the tissues studied. The possible relationships between the results obtained and the cytotoxic effects of niridazole have been discussed.

Pyrrolic and N-oxide metabolites formed from pyrrolizidine alkaloids by hepatic microsomes in vitro: relevance to in vivo hepatotoxicity

An analytical method of improved sensitivity has enabled measurements to be made of N-oxide as well as pyrrolic metabolites formed from a range of unsaturated pyrrolizidine alkaloids in hepatic microsome preparations. Using microsomes from livers of phenobarbitone-pretreated male Fischer rats, all 13 alkaloids tested were metabolised to both N-oxides and pyrroles. The most lipophilic alkaloids gave enhanced rates of metabolism. No consistent relationship existed between rates of N-oxide and of pyrrole formation. The two pathways appeared to be independent. The ratio of N-oxide to pyrrolic metabolites varied, depending on the type of ester: it was highest for 'open' diester alkaloids, lowest for 12 membered macrocyclic diesters and for monoesters. Steric hindrance by the acid moiety could account for these differences, by affecting the balance between microsomal oxidation of the amino alcohol moiety at the nitrogen and C8 positions respectively and could explain the high pyrrole yields given by some macrocyclic diesters. The levels of pyrrolic metabolites bound to liver tissues and responsible for hepatotoxicity in rats given pyrrolizidine alkaloids, did not necessarily reflect the rates of formation of such metabolites measured in vitro. In the animal additional factors could influence the formation and tissue binding of pyrrolic metabolites, including the detoxication of alkaloids by hydrolysis and the chemical reactivity and stability of the toxic metabolites. A comparison of heliotridine esters with retronecine esters showed that the 7-hydroxyl or -ester configuration had a relatively small influence on the balance between formation of pyrrolic metabolites and detoxication by N-oxidation. The results did not support any hypothesis that heliotridine esters should generally be more hepatotoxic than analogous retronecine esters. The structure of the acid moiety was likely to have at least as much influence on toxicity as the base configuration.

Protective action of zinc against pyrrolizidine alkaloid-induced hepatotoxicity in rats

The influence of Zn on the acute hepatotoxicity of pyrrolizidine alkaloids (PAs) was determined in male rats. Zinc, 72 mumol/kg as ZnCl2, was administered ip for 3 consecutive days, followed 16 h after the last dose by a single ip injection of purified mixed PAs (80, 120, or 160 mg/kg) obtained from tansy ragwort (Senecio jacobaea). Hepatotoxicity of the PAs was assessed by measuring the activities of plasma glutamic-oxaloacetic transaminase (GOT) and glutamic-pyruvic transaminase (GPT) and by histological examination of the liver. There was a dose-dependent increase in plasma GOT and GTP 24 h after PA administration, whereas no significant increase of these enzymes was seen after administering Zn alone. The 7-fold increase in plasma GOT and 12-fold increase in GPT after PA (120 mg/kg) were reduced to 2.4- and 2.1-fold, respectively, by Zn pretreatment. The PA-induced liver necrosis was either reduced in severity or abolished by Zn when the PA dose was 80 or 120 mg/kg. These results suggest a protective effect of Zn against PA hepatotoxicity. The protective effect was associated with a marked increase in liver metallothionein and a significant decrease in hepatic cytochrome P-450 content, aminopyrine N-demethylase activity, and in vitro microsomal conversion of the PAs to pyrroles. Liver nonprotein sulfhydryls were unchanged. The possible role of metallothionein in the sequestration of pyrrole metabolites merits further investigation.

Excretion of aflatoxin B1 as a glutathione conjugate

Intraperitoneal injection of 3H - GSH (15.4 mg, 0,5 microCi) 15 minutes before a similar i.p. treatment with Aflatoxin B1 (AFB1) (2mg/kg) showed less than 40% of the radioactivity in the 24 h urine and 30-35% in the bile. Analysis of the urine and bile samples by TLC and separation with XAD-2, showed only traces of the AFB1 - GSH adduct present in the urine and 14% in the bile, AFB1 metabolites isolated from the urine of the 3H - GSH treated rats identical with those of rats treated with AFB1 only. While GSH Conjugate appears therefore to be a major component of biliary excretion of AFB1, it does not seem to be significant in the urinary excretion of the toxin.

Cytotoxic effects of norethindrone-4 beta,5 beta-epoxide to Walker cells in culture and to rat liver in vivo

1. Norethindrone-4 beta,5 beta-epoxide was toxic to Walker cells in culture. The concentration required to produce a 50% reduction in the increase in cell numbers 72 h after exposure (ID50) was 0.05 mM. In this assay, the parent contraceptive steroid, norethindrone, was at least four times less toxic than the epoxide. 2. Norethandrolone-4 beta,5 beta-epoxide and norethynodrel-5 beta,10 beta-epoxide were as toxic as norethindrone epoxide to the Walker cells. 3. The cytotoxicity of norethindrome epoxide was dependent on the time of exposure of the cells to this compound if excess unreacted epoxide was removed by washing the cells with cysteine. The results are consistant with norethindrone epoxide causing cell death by reacting with sulphydryl groups of cellular proteins. 4. No metabolites toxic to Walker cells could be detected when the cells were incubated with norethindrone, rat liver microsomes and a NADPH generating system. 5. Cells treated with an ID50 of norethindrone epoxide for 1 h showed marked cytoplasmic vacuolation 3 hr after exposure. This vacuolation was much less marked in cells treated with an ID50 of norethindrone or in the controls. Neither group showed any nuclear abnormalities. 6. Norethindrone epoxide when given to rats in large doses (50 mg/kg) by lateral tail vein injection also caused cytoplasmic vacuolar degeneration of the liver hepatocytes, especially in the perilobular areas 3 days after dosing. When this compound was administered at a similar dose level via the hepatic portal vein massive haemorrhagic necrosis of the liver resulted. No damage to either lungs or kidneys was evident, irrespective of the route of administration.

The detoxication of aflatoxin B1 with glutathione in the rat

1. The deactivation of aflatoxin B1 by glutathione (GSH) has been investigated in rat. Binding of metabolites of aflatoxin B1 to [3H]glutathione in vitro with rat liver microsomes is insignificant. Incubation with rat liver 10 000 g supernatant results in increased binding. Under identical conditions, benzo(a)pyrene metabolites are bound to [3H]glutathione much more than is aflatoxin B1. 2. Pre-treatment of rats with aflatoxin 1 (2 mg/kg) caused depletion in GSH of rat liver with a minimum at 6 h but returning to above normal at 24 h. GSH S-transferase activity was marginally increased at 6 h also and returned to normal at 24 h. 3. Kidney GSH was not significantly decreased, but kidney GSH S-transferase activity showed a sudden increase in 6 h, returning to almost normal at 24 h. 4. Pre-treatment with benzo(a)pyrene (2 mg/kg) caused greater depletion of hepatic GSH than occurred with aflatoxin B1 but did not show any effect on kidney GSH. 5. Hepatic and kidney GSH S-transferase in benzo(a)pyrene-treated rats showed greatest activity at 2 h followed by a gradual fall through 24 h. 6. GSH was therefore a less efficient nucleophile for aflatoxin B1 metabolites than for benzo(a)pyrene metabolites.

Excretion of pyrrolic metabolites in the bile of rats given the pyrrolizidine alkaloid retrorsine or the bis-N-ethylcarbamate of synthanecine A

(1) A comparison has been made in male rats between the biliary excretion of pyrrolic metabolites from the pyrrolizidine alkaloid retrorsine and a synthetic analogue 1-methyl-2,3-pyrroline-bis-N-ethyl-carbamate (Synthanecine A bis-N-ethylcarbamate). (2) When bile duct-cannulated rats were given retrorsine (40 mg/kg) and the bile collected at 1 h intervals, the relative concentration of pyrrolic metabolites was greatest in the first 1 h sample and was negligibly small by 7 h after dosing. By 7 h, about 25% of the dose had been excreted as pyrrolic metabolites. (3) In rats given [3H]synthanecine A bis-N-ethylcarbamate (40 mg/kg), the amounts of radioactivity and the relative levels of pyrrolic metabolites in the bile were greatest in the first 0.5 h sample and became negligibly small by 4 h after dosing. Within this time, about 25% of the dose had been excreted as [3H]radioactivity in the bile only about 5% as pyrrolic metabolites. (4) When rats were given [3H]2,3-bishydroxymethyl-1-methyl pyrrole (10 mg/kg), a proposed pyrrolic metabolite of synthanecine A bis-N-ethylcarbamate, about 17% of the dose was excreted for as [3H]radioactivity in the bile but only about 3% could be accounted for as pyrrolic metabolites. (5) Thin-layer chromatography of bile from rats given [3H]2,3-bishydroxy-methyl-1-methylpyrrole or [3H]synthanecine A bis-N-ethylcarbamate showed little redioactivity or Ehrlich-positive pyrrolic metabolites with an RF value corresponding to that of 2,3-bishydroxymethyl-1-methylpyrrole. A large proportion of the radioactivity label and most of the Ehrlich positive pyrrolic metabolites were associated with highly polar derivatives at the origin of the TLC plate. (6) It was concluded that biliary excretion plays an important role in the disposal of metabolites from both retrorsine and synthanecine A bis-N-ethyl-carbamate though not for the parent compounds. The expected hydroxy-methyl pyrrolic metabolites undergo further modification to more polar derivatives. In the case of retrorsine, these retained their abilities to react with Ehrlich reagent, while with pyrrolic metabolites from synthanecine A bis-N-ethylcarbamate, substatial conversion to Ehrlich negative derivatives occurred. (7) When bile of rats given [3H]synthanecine A bis-N-ethylcarbamate was injected intraduodenally into recipient animals, reabsorbtion of the radioactive label accounted for less than 4% of the dose given to the donor animals, indicating the enterohepatic circulation is probably only of minor importance influencing the elimination of these compounds.