Electrophysiological effects of a novel antiarrhythmic drug, EO-122, on guinea pig ventricular muscle and isolated myocytes

EO-122, a newly developed structural analog of lidocaine, has recently been shown to suppress ventricular arrhythmias in a few clinical studies in patients and in experimental animals. In the present study, we investigated the effects of EO-122 on the electrophysiological properties of guinea pig papillary muscle and ventricular myocytes by means of standard microelectrode and whole-cell recording techniques, respectively, At the concentration range of 10(-7)-10(-4) M (cycle length, 2000 ms), resting potential and action potential duration (APD90) were not altered by the drug. Action potential amplitude and APD50 were reduced (p less than 0.01) by 10(-4) M, and Vmax was reduced (p less than 0.01) by EO-122 greater than or equal to 10(-5) M. The effect of EO-122 on Vmax was use-dependent. At 10(-6) and 10(-5) M (cycle length, 2000 ms), the time constant for onset of block (tau on) was 37.0 +/- 13.2 and 26.0 +/- 3.4 s, respectively. The recovery kinetics from use-dependent block was not monoexponential, and the estimated "time constant" for recovery was 76.5 s. We examined the effects of EO-122, 10(-5) M, on the membrane currents in ventricular myocytes and found that the drug attenuated the slow inward current (Isi). EO-122 reduced peak Isi by 68.6 +/- 5.2% (p less than 0.005), whereas the outward current was unchanged. The present study demonstrates that EO-122 blocks both the fast inward (Na+) and the slow inward (Ca2+) channels, and these effects are probably responsible for the antiarrhythmic effects of the drug.     

Inhibition of coal fly ash polycyclic aromatic hydrocarbons and metals induced mixed-function oxidase activity in rat lung and liver by vitamin A and citrate

Administration of benzene-soluble fraction (FAE) and benzene-insoluble fraction (FAR) of fly ash for 3 consecutive days to rats significantly raised cytochrome P-450 levels, aryl hydrocarbon hydroxylase (AHH) activity, and glutathione S-transferase activity in liver. This treatment also significantly increased pulmonary AHH and glutathione S-transferase activity. Intratracheal administration of FAR (5 mg/100 g body weight) alone for 6 consecutive days also significantly increased hepatic cytochrome P-450 levels and the activity of glutathione S-transferase. Intragastric administration of retinyl palmitate (5000 IU/100 g body weight), along with intratracheal FAE and FAR administration, significantly reduced P-450 levels, activity of glutathione S-transferase in liver, and activity of AHH and glutathione S-transferase in lung of rats. Intraperitoneal administration of citrate (40 mg/100 g body weight) along with FAR significantly reduced FAR-induced increase in hepatic cytochrome P-450 levels and glutathione S-transferase activity. The activity of AHH was not affected by these treatments.     

Modulatory influence of noscapine on the ethanol-altered hepatic biotransformation system enzymes, glutathione content and lipid peroxidation in vivo in rats.

The modulatory potential of noscapine, an opium alkaloid was assessed on the ethanol-induced changes in hepatic drug metabolizing enzyme systems, glutathione content and microsomal lipid peroxidation. Noscapine was administered orally to male Wistar rats at a dose level of 200 mg/kg bw alone as well as in combination with 50% ethanol (v/v) for 5 days. Noscapine administration was associated with a approximately 91% decrease in hepatic microsomal cytochrome P-450 content. A decline of approximately 36% was observed in the NADPH-cytochrome c reductase activity on noscapine administration. The lowering of cytochrome P-450 levels on noscapine administration was accompanied by a concomitant increase in heme oxygenase activity as well as serum bilirubin levels. Our results indicate that the combination dosage of noscapine and ethanol antagonised the ethanol-induced elevation of cytochrome P-450 levels. Noscapine fed rats had decreased glutathione (GSH) content and enhanced lipid peroxidation compared to control rats as indexed by MDA method. Further, noscapine and ethanol coexposure produced a more pronounced elevation in lipid peroxidation and the glutathione levels also decreased significantly. We speculate on the basis of our results that the significant enhancement of lipid peroxidation on combination dosage of noscapine and ethanol is a consequence of depletion of glutathione to certain critical levels. The inhibition of glutathione-S-transferase (GST) as well as lowering of cytochrome P-450 suggests that the biotransformation of noscapine and ethanol is significantly altered following acute coexposures.     

Stimulation of biliary glutathione secretion by sulfonylureas

In isolated perfused rat livers, infusion of the sulfonylureas, glyburide (2.5 microM) and tolbutamide (0.5 mM), stimulated by 2-fold the rate of biliary glutathione secretion. This increase was mainly the result of an apparent increase in the rate of reduced glutathione release by the liver since oxidized glutathione levels in the bile remained unchanged. Sulfonylurea infusion into perfused livers did not alter the rate of glutathione release in the perfusate, indicating that sinusoidal release was not perturbed. N-Benzylimidazole (0.2 mM), an inhibitor of cytochrome P-450, blocked the tolbutamide-mediated increase in biliary release of glutathione. However, the cytochrome P-450 inhibitor did not alter the glyburide-induced increase in biliary glutathione secretion. Glyburide infusion into perfused livers also decreased tissue oxidized glutathione content without altering the total tissue levels of glutathione. The stimulation of biliary glutathione release by sulfonylureas is probably the result of excretion of labile conjugates of glutathione and sulfonylurea metabolites. Although the precise identity of these metabolites is presently unknown, formyltolbutamide and hydroxyglyburide formed during metabolism of tolbutamide and glyburide, respectively, may be the prime candidates for forming labile glutathione conjugates.     

Interaction between 1,2-dichloroethane and tetraethylthiuram disulfide (disulfiram). II. Hepatotoxic manifestations with possible mechanism of action

The synergistic hepatotoxicity of dietary disulfiram (DSF) with 1,2-dichloroethane (DCE) subchronically administered by inhalation at three concentration levels (150, 300, and 450 ppm) was studied. The criteria for hepatotoxicity were treatment-related increases in serum activities of sorbitol dehydrogenase, 5'-nucleotidase, and alkaline phosphatase, and in liver-to-body weight ratios. DSF alone did not elicit these responses while DCE at the highest concentration level increased liver-to-body weight ratios and the activity of 5'-nucleotidase. Exposure to DSF alone decreased cytochrome P450 levels, but in combination with DCE, the decrement of cytochrome P450 was additive in a DCE concentration-dependent manner. However, depression of cytochrome P450 by DCE alone was not concentration dependent. Although DSF and DSF/DCE combination increased the activity of glutathione S-transferases (GSTs), both DSF and DCE singly and in combination increased the tissue levels of reduced glutathione (GSH). Evidence is presented showing that the potentiation of the hepatotoxicity of DCE observed in the presence of DSF may be due to an inhibition of microsomal mixed-function oxidase-mediated metabolism of DCE and to a compensatory increase in DCE metabolism to reactive metabolites generated by GST-mediated conjugation of DCE with GSH.     

Cylindrospermopsin genotoxicity and cytotoxicity: role of cytochrome P-450 and oxidative stress

Cylindrospermopsin (CYN) is a cyanobacterial toxin found in drinking-water sources world wide. It was the likely cause of human poisonings in Australia and possibly Brazil. Although CYN itself is a potent protein synthesis inhibitor, its acute toxicity appears to be mediated by cytochrome p-450 (CYP450)-generated metabolites. CYN also induces genotoxic effects both in vitro and in vivo, and preliminary evidence suggests that tumors are generated by oral exposure to CYN. To understand the role of CYP450-activated CYN metabolites on in vitro genotoxicity, this study quantified the process in primary mouse hepatocytes using the COMET assay in both the presence and absence of CYP450 inhibitors known to block acute CYN cytotoxicity. CYN was cytotoxic at concentrations above 0.1 microM (EC50 = 0.5 microM) but produced significant increases in Comet tail length, area, and tail moment at 0.05 microM and above; hence genotoxicity is unlikely to be secondary to metabolic disruption due to toxicity. The CYP450 inhibitors omeprazole (100 microM) and SKF525A (50 microM) completely inhibited the genotoxicity induced by CYN. The toxin also inhibits production of glutathione (GSH), a finding confirmed in this study. This could potentiate cytotoxicity, and by implication genotoxicity, via reduced reactive oxygen species (ROS) quenching. The lipid peroxidation marker, malondialdehyde (MDA) was quantified in CYN-treated cells, and the effect of the reduced glutathione (GSSG) reductase (GSSG-rd.) inhibitor 1,3-bis(chloroethyl)-l-nitrosourea (BCNU) on both MDA production and lactate dehydrogenase (LDH) leakage was examined. MDA levels were not elevated by CYN treatment, and block of GSH regeneration by BCNU did not affect lipid peroxidation or cytotoxicity. It therefore seems likely that CYP450-derived metabolites are responsible for both the acute cytotoxicity and genotoxicity induced by CYN.     

Effect of Aegle marmelos on biotransformation enzyme systems and protection against free-radical-mediated damage in mice

The effect of hydroalcoholic (80% ethanol, 20% water) extract of leaves of Aegle marmelos was examined on carcinogen-metabolizing phase-I and phase-II enzymes, antioxidant enzymes, glutathione content, lactate dehydrogenase and lipid peroxidation, using two doses of dried extract (50 and 100 mg kg(-1) daily for 14 days), in the liver of mice. The modulatory effect of the extract was also examined on extrahepatic organs (lung, kidney and fore-stomach) for effects on the activity of glutathione S-transferase, DT-diaphorase, superoxide dismutase and catalase. Extract treatment significantly increased the basal levels of acid-soluble sulphydryl (-SH) content, cytochrome P450, NADPH-cytochrome P450 reductase, cytochrome b5, NADH-cytochrome b5 reductase, glutathione S-transferase, DT-diaphorase, superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase in the liver. Aegle acted as a bifunctional inducer since it induced both phase-I and phase-II enzyme systems. Both doses significantly decreased the activity of lactate dehydrogenase and formation of malondialdehyde in liver, suggesting a role in cytoprotection as well as protection against pro-oxidant-induced membrane damage. Butylated hydroxyanisole (positive control) induced almost all the antioxidative parameters measured in this study. The extract was effective in inducing glutathione S-transferase, DT-diaphorase, superoxide dismutase and catalase in lung, glutathione S-transferase, DT-diaphorase and superoxide dismutase in fore-stomach, and DT-diaphorase and superoxide dismutase in lung. These significant changes in the levels of drug-metabolizing enzymes and antioxidative profiles are strongly indicative of the chemopreventive potential of this plant, especially against chemical carcinogenesis.     

In vitro studies on the metabolism and covalent binding of [14C]1,1-dichloroethylene by mouse liver, kidney and lung

The metabolism and covalent binding of 1,1-dichloro[1,2-14C]ethylene (DCE) to subcellular fractions of liver, kidney and lung of C57BL/6N mice have been investigated in vitro. Covalent binding was NADPH- and cytochrome P-450-dependent. The microsomal fraction bound more radiolabel than any other subcellular fraction, and the levels of covalent binding in cell fractions correlated well with their cytochrome P-450 content. Covalent binding by mouse liver and lung microsomes also reflected their cytochrome P-450 content. However, although mouse kidney microsomes contained twice as much total cytochrome P-450 as the lung, no detectable covalent binding of DCE-derived radioactivity occurred in kidney. Omission of NADPH, heat inactivation of microsomes, carbon monoxide, addition of SKF-525A, piperonyl butoxide or reduced glutathione (GSH), all inhibited (40-90%) covalent binding of radiolabel to liver and lung microsomes. The absence of O2 (incubation under N2) did not greatly affect the metabolism and covalent binding. Pretreatment of mice with various inducers, phenobarbital (PB), beta-naphthoflavone (beta-NF), pregnenolone 16 alpha-carbonitrile (PCN) and 3-methylcholanthrene (3-MC), evoked increases in total liver microsomal cytochrome P-450 content (2-fold) and corresponding increases in covalent binding (3-fold). However, microsomes from PCN-treated mice showed only a 50% increase in DCE binding. Kidney microsomes from control, PB-, and beta-NF-pretreated mice were incapable of covalent binding of radiolabel but those from PCN- and 3-MC-pretreated mice showed levels of binding similar to untreated mouse lung microsomes. It is proposed that the nephrotoxicity of DCE may be due to translocation of reactive metabolites from the liver to the kidney.     

Effect of glutathione depletion on the in vivo inhibition of drug metabolism by agents forming an inactive cytochrome P-450 Fe(II):metabolite complex. Studies with amiodarone and troleandomycin

The relative contribution of competitive inhibition versus formation of a P-450:metabolite complex to the in vivo inhibition of drug metabolism for several agents is unclear. The present investigation examined the contribution of these two mechanisms to the in vivo inhibition of drug metabolism by amiodarone through manipulation of glutathione turnover. In vivo P-450-dependent metabolism in rats was assessed by determining antipyrine clearance. Pretreatment with amiodarone (50 mg/kg, iv) decreased antipyrine clearance with or without prior glutathione depletion. Depletion of glutathione by buthionine sulfoximine (1.6 g/kg, ip) did not enhance the magnitude of inhibition of antipyrine clearance by amiodarone. Moreover, administration of a normally subinhibitory dose of amiodarone after buthionine sulfoximine pretreatment did not influence antipyrine clearance. Similarly, depletion of glutathione via buthionine sulfoximine or diethylmaleate (1 mL/kg, po) did not influence the magnitude of inhibition caused by a single po dose of troleandomycin (500 or 350 mg/kg, respectively). These data indicate that glutathione content may not be a critical determinant for the in vivo inhibition of drug metabolism by agents which form a P-450:metabolite complex.     

Protective effect of glutathione on the in vitro inhibition of hepatic cytochrome P-450 by captan.

The in vitro effect of various concentrations of captan on hepatic microsomal cytochrome P-450 from pehnobarbital-pretreated rats was studied. The I-50 value, namely the concentration of the inhibitor necessary to produce 50% loss of cytochrome P-450 was determined from theplotted inhibition curve. The presence of ethylenediaminetetraacetic acid (EDTA) in microsomal incubations prior to the addition of captan failed to prevent the loss of cytochrome P-450 by captan. In contrast, reduced glutathione (0.5 mM) added to microsomal incubations before captan (0.1 mM) afforded almost complete protection of cytochrome P-450 from captan inhibition. These data indicate that the inhibitory effect of captan on vitally important drug-metabolizing enzyme system, of which cytochrome P-450 is a major component, can be prevented by prior presence of reduced glutathione (GSH) but not of EDTA.     

Studies on the inhibition of human cytochromes P450 by selenocysteine Se-conjugates

1. To investigate whether cytochrome P450 (P450) inhibition can contribute to the chemopreventive activity of selenocysteine Se-conjugates (SeCys conjugates), 21 SeCys conjugates were screened for their inhibitory potency towards seven of the most important human P450s. 2. The majority of the SeCys conjugates produced near complete inhibition of CYP1A1 at a concentration of 250 microm. The most potent inhibitor, Se-benzyl-L-selenocysteine, displayed an IC50 of 12.8 +/- 1.2 microm. CYP2C9, -2C19 and -2D6 were moderately (50-60%) inhibited by the SeCys conjugates. CYP1A2, -2E1 and -3A4 were least inhibited. 3. Studies on the susceptibility of CYP1A1 to SeCys conjugates implicated a thiol-reactive intermediate, as evidenced by reduced inhibition levels in the presence of glutathione and N-acetyl cysteine. Uncoupling of the P450-catalytic cycle was of no importance as ROS scavengers did not influence inhibition levels. 4. P450 inhibition by two physiologically relevant metabolite classes of SeCys conjugates was also studied. N-acetylation of SeCys conjugates consistently increased the inhibitory potency towards CYP1A2, -2C19, -2E1 and -3A4. Beta-lyase catalysed bioactivation of alkyl-substituted SeCys conjugates or Se-benzyl-L-selenocysteine produced little or no additional inhibition of P450 activity. For Se-phenyl-L-selenocysteine, however, significant increases in P450 inhibition were obtained by beta-lyase pre-incubation. 5. It is concluded that the potent and relatively selective CYP1A1 inhibition exerted by SeCys conjugates may contribute to their chemopreventive activity.     

Protective Effect of Glutathione on the In Vitro Inhibition of Hepatic Cytochrome P-450 by Captan

ABSTRACT

The in vitro effect of various concentrations of captan on hepatic microsomal cytochrome P-450 from phenobarbital-pretreated rats was studied. The 1–50 value, namely the concentration of the inhibitor necessary to produce 50% loss of cytochrome P-450 was determined from the plotted inhibition curve. The presence of ethylenediaminetetraacetic acid (EDTA) in microsomal incubations prior to the addition of captan failed to prevent the loss of cytochrome P-450 by captan. In contrast, reduced glutathione (0.5 mM) added to microsomal incubations before captan (0.1 mM) afforded almost complete protection of cytochrome P-450 from captan inhibition. These data indicate that the inhibitory effect of captan on vitally important drug-metabolizing enzyme system, of which cytoehrome P-450 is a major component, can be prevented by prior presence of reduced glutathione (GSH) but not of EDTA.     

Brain lipid peroxidation and antioxidant status after acute methacrylonitrile intoxication

The status of brain antioxidant enzymes and glutathione in methacrylonitrile (MeAN)-intoxicated Wistar rats was correlated with the levels of lipid peroxidation products. Optimum changes were observed 30 min and 60 min after oral administration of MeAN at dosages of 50 mg/kg body weight per day (0.25 LD50) and 100 mg/kg body weight per day (0.5 LD50). An increase in lipid peroxidation products, decrease in the activity of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione reductase (GR), glutathione S-transferase (GST), and decrease in reduced glutathione (GSH) were observed. These studies suggest that the membrane lipid peroxidation observed in MeAN intoxication is related, in part, to a compromised antioxidant defense system.     

Interaction of nefopam and orphenadrine with the cytochrome P-450 and the glutathione system in rat liver

Nefopam, a cyclic analogue of orphenadrine, exhibits a type I (substrate) and a type II (ligand) interaction with ferri-cytochrome P-450 in control and phenobarbitone induced rat hepatic microsomes respectively. In-vitro metabolism of nefopam in phenobarbitone-induced microsomes leads to the production of a reactive metabolite which complexes with cytochrome P-450. In contrast to the known complexation of orphenadrine, complexation by nefopam can be inhibited by glutathione (GSH, 0.1-1.0 mM). However, in-vivo administration of nefopam to rats does not diminish the GSH content of liver cytosol nor increase oxidized glutathione levels nor alter the activities of GSH transferase and GSH peroxidase. In-vivo administration does not lead to cytochrome P-450 induction nor cytochrome P-450 complexation as has been shown for orphenadrine. Finally, nefopam inhibits the NADPH dependent endogenous H2O2 production in both control and phenobarbitone-induced microsomes.     

Inhibition of human cytochrome P450 enzymes by 1,2-dithiole-3-thione, oltipraz and its derivatives, and sulforaphane

Recent studies have demonstrated that two chemoprotective agents, oltipraz (OPZ), a synthetic derivative of the natural compound 1, 2-dithiole-3-thione (D3T), and sulforaphane (SF), an isothiocyanate, are not only inducers of glutathione S-transferases but also inhibitors of some major cytochrome P450 enzymes (P450s) involved in xenobiotic metabolism. We examined P450 inhibition by the two compounds and compared two OPZ metabolites (OPZ M(3) and M(8)) and D3T using human P450s expressed in Escherichia coli membranes. OPZ was a more potent inhibitor than D3T or SF, in the following order of inhibition: P450 1A2 > 3A4 > 1A1 approximately 1B1 > 2E1. OPZ M(3) also inhibited P450s 1A2, 1A1, 1B1, and 3A4 but not more effectively than OPZ. OPZ M(8) was not inhibitory. OPZ behaved as a competitive inhibitor of P450 1A2, with a K(i) of 1.5 microM. Incubation of P450 1A2 with OPZ and NADPH led to a first-order loss of the P450 spectrum, and the loss was not blocked by glutathione. No such time-dependent loss of P450 was seen with P450 1A2 and D3T, P450 1A2 and OPZ M(3), P450 1A2 and SF, P450 3A4 and OPZ, P450 3A4 and D3T, P450 2E1 and OPZ, or P450 2E1 and D3T. The time- and concentration-dependent loss of P450 1A2 activity in the presence of OPZ was characterized with a K(i) of 9 microM and a k(inactivation) of 0.19 min(-)(1). The activation of 2-amino-3,5-dimethylimidazo[4, 5-f]quinoline (MeIQ) in an E. coli lac-based mutagenicity tester system containing functional human P450 1A2 was inhibited by OPZ (IC(50) < 1 microM) but not appreciably by 40 microM D3T. Our results indicate that OPZ is a competitive and mechanism-based inhibitor of P450 1A2, and the extent of this inhibition is significantly greater than that of other chemoprotective chemicals with P450 1A2 or other human P450s.     

The inhibitory effects of polychlorinated biphenyl Aroclor 1254 on Leydig cell LH receptors, steroidogenic enzymes and antioxidant enzymes in adult rats

Polychlorinated biphenyls (PCBs) are global pollutants of major concern to human and animal reproductive health. The present study has examined the impact of Aroclor 1254 exposure on oxidative stress and testicular Leydig cell function. Adult albino male rats of the Wistar strain were dosed for 30 days with daily intraperitoneal injections of 2 mg/kg Aroclor 1254 or vehicle (corn oil). One day after the last treatment, animals were euthanized and blood collected for the assay of serum testosterone and estradiol. Testes were removed and Leydig cells were isolated for the assay of luteinizing hormone (LH) receptors, steroidogenic enzymes cytochrome P450 side chain cleavage enzyme (P450 scc), 3beta-hydroxysteroid dehydrogenase (3beta-HSD) and 17beta-hydroxysteroid dehydrogenase (17beta-HSD). Cellular antioxidant enzymes superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPX), glutathione reductase (GR), and glutathione-S-transferase (GST) were also assayed. Lipid peroxidation (LPO) and reactive oxygen species (ROS) were quantified. Results showed that Aroclor 1254 exposure lowered serum testosterone and estradiol levels. Leydig cell LH receptor density, activities of the steroidogenic enzymes P450 scc, 3beta-HSD, 17beta-HSD, antioxidant enzymes SOD, CAT, GPX, GR, and GST were significantly diminished whereas, LPO and ROS significantly elevated. Taken together, these results suggest that inefficient LH receptors, steroidogenic enzymes and antioxidant enzymes are possible mechanisms by which Aroclor 1254 treatment disrupts Leydig cell steroidogenesis.     

Effects of TBEP on the induction of oxidative stress and endocrine disruption in Tm3 Leydig cells

The flame retardant tris (2‐butoxyethyl) phosphate (TBEP) is a frequently detected contaminant in the environment. In the cultured TM3 cells (originated from ATCC), effects of TBEP on the induction of oxidative stress and endocrine disruption were evaluated. It was observed that exposure to 100 μg/mL TBEP for 24 h significantly reduced the viability of TM3 cells. The mRNA levels of genes related to oxidative stress including Sod, Gpx1, Cat, and Gsta1 were changed in a dose‐dependent and/or time‐dependent manner after exposure to 30 and 100 μg/mL TBEP for 6, 12, or 24 h. Moreover, notable decrease in glutathione (GSH) contents and increases in oxidized glutathione (GSSG) contents as well as the antioxidant enzyme activities like superoxide dismutase, catalase, glutathione peroxidase, and glutathione S‐transferase were found in the group treated with 100 μg/mL TBEP for 24 h, indicating that TBEP induced oxidative stress in TM3 Leydig cells. In addition, the expression of genes related to testosterone (T) synthesis including cytochrome P450 cholesterol side‐chain cleavage enzyme (P450scc), cytochrome P450 17α‐hydroxysteroid dehydrogenase (P450‐17α), and 17β‐hydroxysteroid dehydrogenase (17β‐HSD) and T levels in medium were remarkably declined by the treatment of 100 μg/mL TBEP for 24 h. And TBEP could inhibit the expression of P450‐17α and 17β‐HSD and T levels up‐regulated by hCG in TM3 cells. Taken together, these findings indicated that TBEP can induce oxidative stress and alter steroidogenesis in TM3 cells. © 2015 Wiley Periodicals, Inc. Environ Toxicol 31: 1276–1286, 2016.     

The curative effects of cysteamine,cysteine, and dithiocarb in experimental paracetamol poisoning

In a curative test (i.p. injection 1 hr after administration of paracetamol) cysteamine (100 mg/kg), cysteine (200 mg/kg), and dithiocarb (100 mg/kg) reduced the death rate from paracetamol poisoning (1.5g/kg p.o.) in male mice from 67 to 10, 15 or 10%, respectively. A reduction of mortality rate to 30 or 35% was induced by glutathione (100 mg/kg) and thiazolidine carboxylic acid (50 mg per kg), respectively, whereas penicillamine, thioctic acid, silymarin, and dimercaprol were ineffective. When given 2 or 4 hrs after paracetamol, cysteine, unlike cysteamine, had a curative effect on paracetamol toxicity.Hepatotoxic activity of paracetamol (0.5 g/kg p.o.) was evident by high increases in the levels of serum enzymes (GOT, GPT, GLDH, SDH). Paracetamol-induced enzyme elevations were prevented completely by treatment with cysteamine (50 mg/kg) or cysteine (100 mg/kg) and partially by treatment with dithiocarb (100 mg/kg) 1 hr after paracetamol. LD₅₀ values (i.p. injection) were 450 mg per kg for cysteamine, 660 mg/kg for cysteine, and 1800 mg/kg for dithiocarb. With regard to the therapeutic index cysteamine, cysteine, and dithiocarb can be recommended as antidotes for paracetamol poisoning.A nearly total depletion of hepatic glutathione occurred after paracetamol (0.5 g/kg p.o.). Administration of cysteine (100 mg/kg), one hr after paracetamol, induced a complete repletion of liver glutathione whereas cysteamine (100 or 200 mg/kg) and dithiocarb (100 mg/kg) induced a partial repletion. Glutathione repletion probably accounts for the antidote efficiency of cysteine, cysteamine, and dithiocarb. Its mechanism, however, remains obscure.