The toxicity of acetaminophen and N-acetyl-p-benzoquinone imine in isolated hepatocytes is associated with thiol depletion and increased cytosolic Ca2+

The mechanism of thioacetamide-induced apoptosis in the L37 albumin-SV40 T-antigen transgenic rat hepatocyte-derived cell line occurs without DNA fragmentation

The hepatotoxicant thioacetamide (TH) has classically been used as a model to study hepatic necrosis; however, recent studies have shown that TH can also induce apoptosis. In this report we demonstrate that 2.68+/-0.54% of the albumin-SV40 T-antigen transgenic rat hepatocytes undergo TH-induced apoptosis, a level comparable to other in vivo models of liver apoptosis. In addition, TH could induce apoptosis and necrosis in the L37 albumin-SV40 T-antigen transgenic rat liver-derived cell line. Examination of dying L37 cells treated with 100 mM TH by electron microscopy revealed distinct morphological characteristics that could be attributed to apoptosis. Quantitation of apoptosis by FACS analysis 24 h after treatment with 100 mM TH revealed that 81.3+/-1.6% of the cells were undergoing apoptosis. In contrast, when L37 cells were treated with 250 mM TH, cells exhibited characteristics consistent with necrotic cell death. DNA fragmentation ladders were produced by growth factor withdrawal-induced apoptosis; however, in 100 mM TH-induced apoptosis, DNA fragmentation ladders were not observed. Analysis of endonuclease activity in L37 cells revealed that the enzymes were not inactivated in the presence of 100 mM TH. The data presented in this report indicate that the L37 cell line could be used to study the mechanism of TH-induced apoptosis that was not mediated through a mechanism requiring DNA fragmentation.

Mechanisms of protection by S-allylmercaptocysteine against acetaminophen-induced liver injury in mice

S-Allylmercaptocysteine (SAMC), one of the water-soluble organosulfur compounds in ethanol extracts of garlic (Allium sativum L.), has been shown to protect mice against acetaminophen (APAP)-induced liver injury. In this study, we examined the mechanisms underlying this hepatoprotection. SAMC (100 mg/kg, p.o.) given 2 and 24 hr before APAP administration (500 mg/kg, p.o.) suppressed the plasma alanine aminotransferase activity increases 3 to 12 hr after APAP administration significantly. The hepatic reduced glutathione levels of vehicle-pretreated mice decreased 1 to 6 hr after APAP administration, but SAMC pretreatment suppressed the reductions 1 to 6 hr after APAP administration significantly. These inhibitory effects of SAMC were dose-dependent (50-200 mg/kg) 6 hr after APAP administration. As SAMC pretreatment (50-200 mg/kg) suppressed hepatic cytochrome P450 2E1-dependent N-nitrosodimethylamine demethylase activity significantly in a dose-dependent manner, we suggest that one of its protective mechanisms is inhibition of cytochrome P450 2E1 activity. SAMC pretreatment also suppressed the increase in hepatic lipid peroxidation and the decrease in hepatic reduced coenzyme Q9 (CoQ9H2) levels 6 hr after APAP administration. The hepatic CoQ9H2 content of the SAMC pretreatment group was maintained at the normal level. Therefore, we suggest that another hepatoprotective mechanism of SAMC may be attributable to its antioxidant activity.

Temporal variation in hepatotoxicity and metabolism of acetaminophen in mice

Temporal variation in metabolism and hepatotoxicity of acetaminophen (APAP) was examined using male ICR mice. Animals were injected with a single dose of APAP (400 mg/kg, i.p.) at 08:00, 14:00 or 20:00 h. APAP at this dose was markedly hepatotoxic to mice when administered at 20:00 h as determined by increases in serum alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities, and by decreases in hepatic glucose-6-phosphatase (G-6-Pase) activity. However, mice appeared to be entirely insensitive to an identical dose of APAP given either at 08:00 or 14:00 h. Hepatic glutathione (GSH) level was significantly higher at 08:00, but no difference in GSH levels between 14:00 and 20:00 h was observed in normal mice. APAP and its metabolites in blood were monitored using HPLC for 3 h following the treatment. There were no significant differences in the plasma concentrations of APAP, APAP-glucuronide, APAP-sulfate, or APAP-mercapturate among the mice treated with this drug at 08:00, 14:00 or 20:00 h. However, the APAP-cysteine and APAP-GSH levels measured at 1 h following the APAP treatment were significantly lower in mice treated with this analgesic either at 14:00 or 20:00 h. In vitro hepatic microsomal p-nitrophenol hydroxylase activities were not different between 08:00, 14:00 and 20:00 h. But ethoxyresorufin O-deethylase and aminopyrine N-demethylase activities measured at 14:00 h were significantly lower than those of 08:00 or 20:00 h. Thus, the greater hepatotoxicity of APAP administered at 20:00 h appears to be related to the marked decrease in hepatic GSH at this time period, whereas the simultaneous reduction in APAP activation may be responsible for the lack of hepatotoxicity in mice treated with this analgesic at 14:00 h. These results suggest that the temporal variation in hepatotoxicity and metabolism of APAP is determined by interactions of multiple factors including the hepatic GSH level and drug metabolizing activities.

Hypothyroidism minimizes liver damage and improves survival in rats with thioacetamide induced fulminant hepatic failure

Recent data from animal studies suggest that induced hypothyroidism prevents the hyperdynamic circulation in portal vein ligated rats, liver cirrhosis in rats chronically treated with thioacetamide (TAA), and immune-mediated acute liver injury induced in mice by concanavalin A. Therefore, the aim of this present study is to determine whether hypothyroidism would likewise prevent fulminant hepatic failure (FHF) in rats. FHF was induced by 3 consecutive ip injections of TAA (400 mg/kg) at 24-hour intervals. Hypothyroidism was induced in rats by either methimazole (MMI) or propylthiouracil (PTU) and surgical thyroidectomy and was confirmed by elevated serum thyroid stimulating hormone levels. Serum levels of liver enzymes, blood ammonia, and prothrombin time were significantly lower in all 3 groups of hypothyroid rats. The stage of hepatic encephalopathy (HE) and the survival rates were significantly improved in the hypothyroid rats (P < .01); the histologic examination of their livers showed less necrosis and inflammation (P < .01). In the hypothyroid rats, the serum levels of malondialdehyde 48 hours after thioacetamide (TAA) administration were lower than in control rats (P < .01). Exogenous supplementation of hypothyroid rats with L-thyroxine started 48 hours before TAA administration abrogated the protective effects of hypothyroidism. The serum levels of tumor necrosis factor alfa (TNF-alpha), interleukin (IL) 2 and IL-6 after 24 hours were slightly lower in the hypothyroid rats, but the administration of soluble receptor of TNF (10-1,000 microg/rat) did not prevent the induction of fulminant liver failure by TAA. Oxygen extraction, studied in isolated perfused liver preparation, was significantly lower in livers of hypothyroid rats (P < .01). These results suggest that induced hypothyroidism decreases the development of liver injury in a rat model of FHF. The mechanism may involve diminished oxidative cell injury caused by decreased oxygen utilization and hypometabolism associated with hypothyroidism.

Comparative toxicity of eugenol and its quinone methide metabolite in cultured liver cells using kinetic fluorescence bioassays

Comparative kinetic analyses of the mechanisms of toxicity of the alkylphenol eugenol and its putative toxic metabolite (quinone methide, EQM) were carried out in cultured rat liver cells (Clone 9, ATCC) using a variety of vital fluorescence bioassays with a Meridian Ultima laser cytometer. Parameters monitored included intracellular GSH and calcium levels ([Ca2+]i), mitochondrial and plasma membrane potentials (MMP and PMP), intracellular pH, reactive oxygen species (ROS) generation, and gap junction-mediated intercellular communication (GJIC). Cells were exposed to various concentrations of test compounds (1 to 1000 microM) and all parameters monitored directly after addition at 15 s intervals for at least 10 min. Eugenol depleted intracellular GSH, inhibited GJIC and generation of ROS, and had a modest effect on MMP at concentrations of 10 to 100 microM. At high concentrations (1000 microM), eugenol also affected [Ca2+]i, PMP, and pH. Effects of EQM were seen at lower concentrations (1 to 10 microM). The earliest and most potent effects of either eugenol or EQM were seen on GSH levels and GJIC. Coadministration of glutathione ethyl ester enhanced intracellular GSH levels by almost 100% and completely protected cells from cell death caused by eugenol and EQM. These results suggest that eugenol mediates its hepatotoxic effects primarily through depletion of cytoprotective thiols and interference in thiol-dependent processes such as GJIC. Furthermore, our results support the hypothesis that the toxic effects of eugenol are mediated through its quinone methide metabolite.

Diaminotoluenes induce intrachromosomal recombination and free radicals in Saccharomyces cerevisiae

The carcinogenicity of aniline-based aromatic amines is poorly reflected by their activity in short-term mutagenicity assays such as the Salmonella typhimurium reverse mutation (Ames) assay. More information about the mechanism of action of such carcinogens is needed. Here we report the effects on DEL recombination in Saccharomyces cerevisiae of the carcinogen 2,4-diaminotoluene and its structural isomer 2,6-diaminotoluene, which is reported to be non-carcinogenic. Both compounds are detected as equally mutagenic in the Salmonella assay. In the absence of any external metabolizing system both compounds were recombinagenic in the DEL assay with the carcinogen being a more potent inducer of deletions than the non-carcinogen. In the presence of Aroclor-induced rat liver S9, however, the carcinogen 2,4-diaminotoluene became a 2-fold more potent inducer of deletions, and the non-carcinogen 2,6-diaminotoluene was rendered less toxic and no induced recombination was observed. 2,4-Diaminotoluene is distinguished from its non-carcinogen analog in the DEL assay, therefore, on the basis of a preferential activation of the carcinogen in the presence of a rat liver microsomal metabolizing system. Free radical species are produced by several carcinogens and have been implicated in carcinogenesis. We further investigated whether exposure of yeast to either 2,4-diaminotoluene or 2,6-diaminotoluene resulted in a rise in intracellular free radical species. The effects of the free radical scavenger N-acetylcysteine on toxicity and recombination induced by the two compounds and intracellular oxidation of the free radical-sensitive reporter compound dichlorofluorescin diacetate were studied. Both 2,4- and 2,6-diaminotoluene produced tree radical species in yeast, indicating that the reason for the differential activity of the compounds for induced deletions is not reflected in any difference in the production of free radical species.

Role of glutathione and nitric oxide in the energy metabolism of rat liver mitochondria

Previous studies have suggested that nitric oxide (NO) inhibited mitochondrial respiration. NO and/or its intermediate(s) react with various molecules, such as hemeproteins and free SH groups. The inhibitory effect of NO on mitochondrial respiration was decreased by exogenously added glutathione (GSH). However, a decrease of intramitochondrial GSH by pretreating animals with L-buthionine sulfoximine had no appreciable effect on the inhibitory effect of isolated mitochondria. Furthermore, the effect of NO was not affected by depleting free SH residues in mitochondria by N-ethylmaleimide. These results suggest that cytosolic but not intramitochondrial GSH might be an important factor that determines the NO-dependent regulation of mitochondrial energy metabolism.

Inhibition of carbamyl phosphate synthetase-I and glutamine synthetase by hepatotoxic doses of acetaminophen in mice

The primary mechanisms proposed for acetaminophen-induced hepatic necrosis should deplete protein thiols, either by covalent binding and thioether formation or by oxidative reactions such as S-thiolations. However, in previous studies we did not detect significant losses of protein thiol contents in response to administration of hepatotoxic doses of acetaminophen in vivo. In the present study we employed derivatization with the thiol-specific agent monobromobimane and separation of proteins by SDS-PAGE to investigate the possible loss of specific protein thiols during the course of acetaminophen-induced hepatic necrosis. Fasted adult male mice were given acetaminophen, and protein thiol status was examined subsequently in subcellular fractions isolated by differential centrifugation. No decreases in protein thiol contents were indicated, with the exception of a marked decrease in the fluorescent intensity, but not of protein content, as indicated by staining with Coomassie blue, of a single band of approximately 130 kDa in the mitochondrial fractions of acetaminophen-treated mice. This protein was identified by isolation and N-terminal sequence analysis as carbamyl phosphate synthetase-I (CPS-I) (EC 6.3.4.16). Hepatic CPS-I activities were decreased in mice given hepatotoxic doses of acetaminophen. In addition, hepatic glutamine synthetase activities were lower, and plasma ammonia levels were elevated in mice given hepatotoxic doses of acetaminophen. The observed hyperammonemia may contribute to the adverse effects of toxic doses of acetaminophen, and elucidation of the specific mechanisms responsible for the hyperammonemia may prove to be useful clinically. However, the preferential depletion of protein thiol content of a mitochondrial protein by chemically reactive metabolites generated in the endoplasmic reticulum presents a challenging and potentially informative mechanistic question.

Role of macrophages in acetaminophen (paracetamol)-induced hepatotoxicity

Research into the pathogenesis of acetaminophen (paracetamol)-induced hepatotoxicity has concentrated on the generation of toxic metabolites by the hepatocytes. It has, however, recently been shown that human macrophages cultured with acetaminophen secrete increased quantities of tumour necrosis factor (TNF). This study examines whether macrophages have a direct role in acetaminophen toxicity, using a mouse model in which it is possible to eliminate more that 99 per cent of hepatic macrophages by previously injecting liposomes containing dichloromethylene disphosphonate (DMDP). Acetaminophen-induced liver damage was assessed biochemically and histologically. It was shown that the liver damage occurring 0.5, 1, and 2 h after an intraperitoneal injection of acetaminophen was significantly less in mice previously injected with liposomes containing DMDP than in previously untreated mice, or mice previously injected with empty liposomes. By 4 h there was no difference between the groups. We conclude that macrophages play an early and probably a direct role in mediating the liver damage due to acetaminophen. This is consistent with the role that macrophages have been shown to play in the pathogenesis of alcohol-induced liver damage.

Peroxidase-catalyzed oxidation of 3,5-dimethyl acetaminophen causes cell death by selective protein thiol modification in isolated rat hepatocytes

In this study we used a peroxidase model system (glucose/glucose oxidase and horseradish peroxidase) to investigate the effect of extracellularly generated reactive metabolites of 3,5-Me2-acetaminophen on cell viability and on cellular thiol levels. Incubation of hepatocytes with 3,5-Me2-acetaminophen in the presence of glucose/glucose oxidase and horseradish peroxidase caused a concentration-dependent loss of cell viability. Loss of viability was associated with decreased protein thiol levels. Addition of the reducing agent DTT, but not catalase, during the incubation restored cellular protein thiol levels and arrested the cell killing. Protein thiol depletion occurred selectively to the mitochondrial and microsomal fractions and was specific for a very limited number of protein bands. The data suggest that the oxidative modification of individual protein cysteine residues within the latter two organelle fractions is critically involved in the mechanism of toxicity.

Biotransformation and membrane transport in nephrotoxicity

The kidney is a frequent target organ for toxic effects of xenobiotics. In recent years, the molecular mechanisms responsible for the selective renal toxicity of many nephrotoxic xenobiotics have been elucidated. Accumulation by renal transport mechanisms, and thus aspects of renal physiology, plays an important role in the renal toxicity of some antibiotics, metals, and agents binding to low molecular weight proteins such as alpha(2u)-globulin. The accumulation by active transport of metabolites formed in other organs is involved in the kidney-specific toxicity of certain polyhaloalkanes, polyhaloalkenes, hydroquinones, and aminophenols. Other xenobiotics are selectively metabolized to reactive electrophiles by enzymes expressed in the kidney. This review summarizes the present knowledge on the mechanistic basis of target organ selectivity of these compounds.

Intracellular calcium concentration impairment in hepatocytes from thioacetamide-treated rats. Implications for the activity of Ca(2+)-dependent enzymes

METHODS/RESULTS

Thioacetamide induced a severe perivenous necrosis followed by a hepatocellular regenerative response, when administered in a single dose of 6.6 mmol/kg to rats. As (Ca2+)i plays an important role in both toxic cell killing and cell proliferation, the disturbances in the basal cytosolic calcium as well as the levels of Ca2+ sequestered in the endoplasmic reticulum were determined in hepatocytes isolated at 0, 12, 24, 48 and 72 h after thioacetamide administration. The basal Ca2+ increased progressively, reaching a maximum at 24 h of the intoxication (205%, p < 0.001), while the microsomal sequestered Ca2+ decreased at 24 h to 16% (p < 0.001) when compared with untreated controls. Changes in the activity of glycogen phosphorylase alpha paralleled those of basal free calcium and showed the maximum value also at 24 h (291%; p < 0.001). Moreover, there was a close association in time between the basal concentration of Ca2+ and the inhibition of microsomal Ca(2+)-dependent ATPase activity.

CONCLUSIONS

The significant decrease in the levels of GSH and protein thiols indicates that oxidative stress is involved in thioacetamide-induced cell injury, but these decreases did not precede changes in cytosolic Ca2+ level. In the sequence of events leading to hepatic cell injury and regeneration, thioacetamide mobilized hepatic (Ca2+)i via inhibition of microsomal Ca(2+)-ATPase which may have activated Ca(2+)-dependent mechanisms involved both in cell death and in acute mitogen response.

Effects of dicoumarol on cytotoxicity caused by tert-butylhydroquinone in isolated rat hepatocytes

The effects of dicoumarol, an inhibitor of DT-diaphorase, on the cytotoxicity of tert-butylhydroquinone (tBHQ) were studied in freshly isolated rat hepatocytes. Addition of tBHQ (0.5 mM) to hepatocytes resulted in a time-dependent cell death accompanied by depletion of intracellular ATP, glutathione (GSH), and protein thiols. Pretreatment of hepatocytes with dicoumarol (30 microM) did not affect cell viability or cellular levels of ATP, GSH, or protein thiols during the incubation period; however, dicoumarol did promote the appearance of cell blebs and the depletion of ATP and protein thiols induced by tBHQ and ultimately enhanced the cytotoxicity of tBHQ.

Biochemical basis of hepatocellular injury

The hepatotoxic response elicited by a chemical agent depends on the concentration of the toxicant (parent compound or metabolite) delivered to the hepatocytes across the liver acinus via blood flow. Hepatotoxicants produce characteristic patterns of cytolethality in specific zones of the acinus due to the differential expression of enzymes and the concentration gradients of cofactors and toxicant in blood across the acinus. Most hepatotoxic chemicals produce necrosis, characterized by swelling in contiguous tracts of cells and inflammation. This process has been contrasted with apoptosis, where cells and organelles condense in an orderly manner under genetic control. Biotransformation can activate a chemical to a toxic metabolite or decrease toxicity. Quantitative or qualitative species differences in biotransformation pathways can lead to significant species differences in hepatotoxicity. Fasted rodents are more susceptible to the hepatotoxic effects of many chemicals due to glutathione depletion and cytochrome P-450 induction. Freshly isolated hepatocytes are the most widely used in vitro system to study mechanisms of cell death. Hepatotoxicants can interact directly with cell macromolecules or via a reactive metabolite. The reactive metabolite can alkylate critical cellular macromolecules or induce oxidative stress. These interactions generally lead to a loss of calcium homeostasis prior to plasma membrane lysis. Mitochondria have been shown to be important cellular targets for many hepatotoxicants. Decreasing hepatocellular adenosine triphosphate concentrations compromise the plasma membrane calcium pump, leading to increased cellular calcium concentrations. Calcium-dependent endonucleases produce double-strand breaks in DNA before cell lysis. These biochemical pathways induced by necrosis-causing toxicants are similar to the biochemical pathways involved in apoptosis, suggesting that apoptosis and necrosis differ in intracellular and extracellular control points rather than in the biochemistry involved in cell death.

Mechanism of protection of lobenzarit against paracetamol-induced toxicity in rat hepatocytes

The protective effects of lobenzarit, an antioxidative agent and antirheumatic drug, on the cytotoxicity of paracetamol in rat hepatocytes were studied, as well as the inhibitory effects of lobenzarit on cytochrome P-450s and glutathione S-transferases (GSTs) in rat liver. Paracetamol was selected as a model toxin, since it is known to be bioactivated by specific cytochrome P-450s presumably to N-acetyl-p-benzoquinoneimine, a reactive metabolite which upon overdosage of paracetamol causes protein and non-protein thiol depletion, lipid peroxidation and cytotoxicity measurable as LDH leakage. At concentrations of lobenzarit of 0.2 and 0.3 mM, added 30 min before paracetamol, the drug prevented paracetamol-induced leakage of lactate dehydrogenase (LDH) almost completely and lipid peroxidation (LPO) and depletion of glutathione (GSH) substantially and also the formation of the 3-glutathionyl conjugate of paracetamol. However, at a concentration of 0.05 mM Lobenzarit did not protect anymore against the paracetamol toxicity, When added to the hepatocytes 1 h and 2 h before paracetamol, 0.05 and 0.2 and 0.3 mM concentrations of lobenzarit did not protect against the cytotoxicity induced by paracetamol either. Lobenzarit did not inhibit cytochromes P-450 1A1/1A2, 2B1/2B2 and 2E1 which were measured as ethoxyresorufin O-deethylation (EROD) activity in beta-naphthoflavone-induced rat liver microsomes, as pentoxyresorufin de-pentylation (PROD) activity in phenobarbital-induced microsomes and as p-nitrophenol hydroxylation (PNPH) activity in pyrazol-induced microsomes. Lobenzarit did not show inhibition of glutathione S-transferase (GST) activity towards 1-chloro-2,4-dinitrobenzene (CDNB) in cytosol from liver of rats treated with phenobarbital, pyrazol and beta-naphthoflavone either. It is concluded that the cytoprotective effect of lobenzarit is most likely due to its antioxidant effects and/or to its ability to stimulate GSH reductase.

Effect of nifedipine, verapamil, diltiazem and trifluoperazine on acetaminophen toxicity in mice

The hepatotoxicity of acetaminophen overdose depends on the metabolic activation to a toxic reactive metabolite by the hepatic mixed function oxidases. There is evidence that an increase in cytosolic Ca2+ is involved in acetaminophen hepatotoxicity. The effects of the Ca2+-antagonists nifedipine (NF), verapamil (V), diltiazem (DL) and of the calmodulin antagonist trifluoperazine (TFP) on the activity of some drug-metabolizing enzyme systems, lipid peroxidation and acute acetaminophen toxicity were studied in male albino mice. No changes in the drug-metabolizing enzyme activities studied and in the cytochrome P-450 and b5 contents were observed 1 h after oral administration of V (20 mg/kg). DL (70 mg/kg) and TFP (3 mg/kg). NF (50 mg/kg) increased cytochrome P-450 content, NADPH-cytochrome c reductase and ethylmorphine-N-demethylase activities. DL and TFP significantly decreased lipid peroxidation. NF, V, DL and TFP administered 1 h before acetaminophen (700 mg/kg orally) increased the mean survival time of animals. A large increase of serum aspartate aminotransferase(AST), and liver weight and depletion of liver reduced glutathione (GSH) occurred in animals receiving toxic acetaminophen dose. NF, V and DL prevented and TFP decreased the acetaminophen-induced hepatic damage measured both by plasma AST and by liver weight. NF, V, DL and TFP changed neither the hepatic GSH level nor the GSH depletion provoked by the toxic dose of acetaminophen. This suggests that V, DL and TFP do not influence the amount of the acetaminophen toxic metabolite formed in the liver. The possible mechanism of the protective effect of NF, V, DL and TFP on the acetaminophen-induced toxicity is discussed.

Effects of acetaminophen on the ultrastructure of isolated rat hepatocytes

The effects of acetaminophen (AA) on the ultrastructure of isolated hepatocytes (IHC) of rat following incubation of IHC suspensions with AA were examined by electron microscopy. The effect of N-acetyl-p-benzoquinone imine (NAPQI), a putative toxic metabolite of AA, were also observed. IHC were prepared from livers obtained from phenobarbital-treated rats by collagenase perfusion method. With 5 and 20 mM AA, surface blebs mainly containing smooth endoplasmic reticulum (ER) occurred in IHC. Dilatation of Golgi apparatus, partial degranulation of rough ER and enlargement of mitochondria were also observed. The altered mitochondria showed a low electron-dense matrix with loss of mitochondrial granules. With 500 microM NAPQI, surface blebs containing various organelles occurred in IHC. Disorderly distributions of cytoplasmic organelles, mild dilatation of rough and smooth ER and cytoplasmic myeloid bodies were observed. The characteristic myeloid bodies were seemingly derived from degranulated rough ER.

Protective effects of Osbeckia octandra against paracetamol-induced liver injury

1. Osbeckia octandra is a plant used in traditional medicine to treat jaundice and other liver disorders. In this study, the effects of Osbeckia leaf extract on paracetamol-induced liver injury were investigated both in vivo in mice and in rat hepatocytes in vitro. 2. Oral administration of Osbeckia extract (330 mg/kg) at the same time as paracetamol (450 mg/kg) to mice, resulted in a significant protection (p < 0.05) against liver damage, as assessed by improvements in the blood Normotest (39.1 +/- 1.9 versus 46.3 +/- 2.0 s), total liver glutathione (730 +/- 39 versus 574 +/- 27 micrograms/250 mg liver), plasma aspartate aminotransferase level (916 +/- 225 versus 1965 +/- 291 iu/l), and liver histopathology at 24 h after paracetamol administration. 3. In experiments to assess the direct effects of Osbeckia extract, significant protection was also found in freshly isolated rat hepatocytes against damage induced by 185 microM 2,6-dimethyl N-acetyl p-quinoneimine (2,6-diMeNAPQI, an analogue of NAPQI, the toxic metabolite of paracetamol) in vitro. When Osbeckia extract (500 micrograms/ml) was added to the incubation medium at the same time as 2,6-diMeNAPQI significant changes in cell viability (78.4 +/- 3.3 versus 47.2 +/- 5.8% of control, p < 0.001), cell reduced glutathione (GSH) level (35.0 +/- 3.1 versus 23.8 +/- 1.5%, p = 0.009), and reduced release of lactate dehydrogenase (129.9 +/- 6.6 versus 224.6 +/- 12.1%, p < 0.001) were demonstrated after 1 h incubation as compared with 2,6-diMeNAPQI alone. 4. Significant protection was still obtained against 2,6-diMeNAPQI in vitro when addition of Osbeckia extract was delayed by 20 min. These results indicate that Osbeckia extract can protect against paracetamol-induced liver injury.

Failure of calcium antagonistic agents to prevent hepatotoxicity induced by diclofenac

Diclofenac (0.5-2 mM) dose- and time-dependently reduces the viability of isolated hepatocytes. This effect cannot be counteracted by the calcium channel blockers diltiazem (0.05-0.1 mM) and verapamil (0.05-0.5 mM), the calmodulin antagonist calmidazolium (0.01 mM) or Quin 2-AM (0.1 mM), an intracellular calcium chelating agent. On the contrary, verapamil even accentuates the toxic effects of diclofenac. It is concluded from these results, that diclofenac causes cell damage by other mechanisms than calcium overload.

Studies on protective effect of Cyperus scariosus extract on acetaminophen and CCl4-induced hepatotoxicity

1. The hepatoprotective activity of aqueous-methanolic extract of Cyperus scariosus (Cyperaceae) was investigated against acetaminophen and CCl4-induced hepatic damage. 2. Acetaminophen produced 100% mortality at a dose of 1 g/kg in mice while pretreatment of animals with plant extract (500 mg/kg) reduced the death rate to 30%. 3. Acetaminophen at a dose of 640 mg/kg produced liver damage in rats as manifested by the rise in serum levels of alkaline phosphatase (ALP), glutamate oxaloacetate transaminase (GOT) and glutamate pyruvate transaminase (GPT) to 430 +/- 68, 867 +/- 305 and 732 +/- 212 IU/l (n = 10) respectively, compared to respective control values of 202 +/- 36, 59 +/- 14 and 38 +/- 7. 4. Pretreatment of rats with plant extract (500 mg/kg) significantly lowered (P < 0.05) the respective serum ALP; GOT and GPT levels to 192 +/- 31, 63 +/- 9 and 35 +/- 8. 5. The hepatotoxic dose of CCl4 (1.5 ml/kg; orally) raised serum ALP, GOT and GPT levels to 328 +/- 30, 493 +/- 102 and 357 +/- 109 IU/l (n = 10) respectively, compared to respective control values of 177 +/- 21, 106 +/- 15 and 47 +/- 12. 6. The same dose of plant extract (500 mg/kg) was able to significantly prevent (P < 0.05) CCl4-induced rise in serum enzymes and the estimated values of ALP, GOT and GPT were 220 +/- 30, 207 +/- 95 and 75 +/- 38, respectively. 7. The plant extract also prevented CCl4-induced prolongation in pentobarbital sleeping time confirming hepatoprotectivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Protective effect of nifedipine against cytotoxicity and intracellular calcium alterations induced by acetaminophen in rat hepatocyte cultures

Alteration of calcium homeostasis has been proposed to play a major role in cell necrosis induced by a variety of chemical agents such as acetaminophen (APAP). In this study, a potential protective effect of the dihydropyridine calcium channel blocking agent, nifedipine, was investigated in vitro on acetaminophen-induced hepatocyte damage. Rat hepatocytes were exposed during 20 hours to various concentrations of APAP (0.50 to 4.00 mM). The following metabolic and functional parameters were investigated: -lactate dehydrogenase (LDH) release as an indicator of plasma membrane integrity, -cell viability evaluated by the colorimetric MTT assay, and intracellular calcium concentration as evaluated by two fluorimetric methods: a scanning laser cytometer using indo-1-AM as fluorescent probe and a fluorescence plate reader using fluo-3-AM as calcium indicator. Incubation of hepatocytes with APAP alone in the range 0.50 to 4.00mM resulted in a dose-response relationship with regard to LDH release (243% to 750% of control) and to the loss of cell viability (0 to 67% of control). Moreover these results were correlated with a significant increase in cytosolic calcium content (189 to 406 nM). Nifedipine treatment prior to APAP exposure, partially prevented LDH release, the plasma membrane blebbing, and thereby the loss of viability. In addition, intracellular calcium level progressively returned within the limits of the control values with increasing concentrations of nifedipine. It can be concluded that, in vitro conditions, nifedipine pretreatment exhibits a preventive effect against acetaminophen hepatocyte injury.

High resolution NMR spectroscopic studies on the metabolism and futile deacetylation of 4-hydroxyacetanilide (paracetamol) in the rat

Paracetamol (4-hydroxyacetanilide, acetaminophen) was synthesized with the acetyl group labelled with C2H3 (paracetamol-C2H3), and dosed to rats i.p. at 25 mg/kg (N = 5) and 40 mg/kg (N = 3) body weight. Paracetamol, with a 13CH3 in the acetyl group (paracetamol-13CH3) was also synthesized and dosed to rats i.p. at 40 mg/kg (N = 3). The metabolism and excretion of the 2H-labelled compound was followed in the rat using 600 MHz 1H and 92.1 MHz 2H NMR spectroscopy of urine collected 0-8, 8-24, 24-32 and 32-48 hr post-dosing. The metabolism of paracetamol-13CH3 was also monitored using 600 MHz 1H NMR spectroscopy of urine collected 0-8, 8-24 and 24-48 hr post-dosing. For paracetamol-C2H3 the total recovery of the sulphate, glucuronide and N-acetyl cysteinyl metabolites via the urine accounted for 61.2 +/- 14.1% of the 25 mg/kg dose and 61.4 +/- 8.8% of the 40 mg/kg dose. For paracetamol-13CH3 the recovery was 102.7 +/- 3.7% indicating that the low % urinary recovery with the C2H3-labelled drug is the result of isotope effects on the disposition of paracetamol. In the case of the paracetamol-C2H3, quantitative 1H NMR analysis of urine showed that 13.3 +/- 0.5 and 10.0 +/- 1.2 mole % (25 and 40 mg/kg, respectively) of the urinary paracetamol sulphate recovered following dosing of the deuterium labelled drug had the C2H3 acetyl groups replaced by C1H3 acetyl groups from endogenous sources. In the case of the paracetamol-13CH3 8.9 +/- 0.7 mole % of the sulphate conjugate had also been transacetylated to paracetamol-12CH3. There was no significant difference between the level of futile deacetylation observed for the deuterated and 13C-labelled drug. Overall these data indicate a high level of deacetylation followed by reacetylation (i.e. futile deacetylation) prior to excretion of paracetamol via the nephrotoxic intermediate 4-aminophenol. The level of deacetylation is much higher than has previously been thought which may cast new light on the role of 4-aminophenol in the development of paracetamol induced nephrotoxicity.

Preventive and curative effects of Artemisia absinthium on acetaminophen and CCl4-induced hepatotoxicity

1. Effect of aqueous-methanolic extract of Artemisia absinthium (Compositae) was investigated against acetaminophen- and CCl4-induced hepatic damage. 2. Acetaminophen produced 100% mortality at the dose of 1 g/kg in mice while pretreatment of animals with plant extract (500 mg/kg) reduced the death rate to 20%. 3. Pretreatment of rats with plant extract (500 mg/kg, orally twice daily for two days) prevented (P < 0.01) the acetaminophen (640 mg/kg) as well as CCl4 (1.5 ml/kg)-induced rise in serum transaminases (GOT and GPT). 4. Post-treatment with three successive doses of extract (500 mg/kg, 6 hr) restricted the hepatic damage induced by acetaminophen (P < 0.01) but CCl4-induced hepatotoxicity was not altered (P > 0.05). 5. Plant extract (500 mg/kg) caused significant prolongation (P < 0.05) in pentobarbital (75 mg/kg)-induced sleep as well as increased strychnine-induced lethality in mice suggestive of inhibitory effect on microsomal drug metabolizing enzymes (MDME). 6. These results indicate that the crude extract of Artemisia absinthium exhibits hepatoprotective action partly through MDME inhibitory action and validates the traditional use of plant in hepatic damage.

The evaluation of hepatoprotective effects of Taiwan folk medicine 'teng-khia-u'

'Teng-khia-u' is a folk medicine of Taiwan, derived from the entire plants of Elephantopus scaber L., E. mollis H.B.K. and Pseudoelephantopus spicatus (Juss.) Rohr. The hepatoprotective effects of water extracts of these three plants against beta-D-galactosamine (D-GalN)- and acetaminophen (APAP)-induced acute hepatic damage were determined in rats. The results indicated that the serum glutamate-oxalate-transaminase (sGOT) and the serum glutamate-pyruvate-transaminase (sGPT) levels caused by D-GalN and APAP decreased after treatment with crude extracts of 'teng-khia-u' (P < 0.005). The pathological changes of hepatic lesions, caused by D-GalN and APAP, improved following treatment with the drug extracts mentioned above.

Effect of thimerosal and other sulfhydryl reagents on calcium permeability in thymus lymphocytes

We have studied the effects of thimerosal, a mercurial compound extensively used as a preservative, as well as other sulfhydryl reagents (e.g. p-hydroxymercurybenzoate, hydrogen peroxide, bromophenacyl bromide, and mercuric chloride) on Ca2+ homeostasis and the redox status of sulfhydryl groups in thymus lymphocytes. They all induced an increase in [Ca2+]i which was blocked with dithiothreitol, suggesting that they act via the oxidation or blockade of sulfhydryl groups. [Ca2+]i increase could be directly related to the effect of the different reagents on cellular protein sulfhydryl content. Experiments with ethidium bromide indicate that the observed rise in [Ca2+]i was not due to a non-specific increase in membrane permeability. Thimerosal differs from the other agents studied in its oxidative properties, which is probably linked to the production of a potent reductor molecule, thiosalicylic acid, which may modulate its oxidative capacity.

Mechanisms of the formation and disposition of reactive metabolites that can cause acute liver injury

Acetaminophen and pulegone are just two examples for many agents that can form reactive metabolites that can cause acute liver injury. Two other classic organic compounds that have been extensively studied are carbon tetrachloride (for a recent review see Ref. 159, and for other discussions see Refs. 8 and 9) and bromobenzene (for review see Ref. 160). Different kinds of protein adducts of reactive metabolites of bromobenzene have been partially characterized [161], and specific antibodies to these adducts are now being used to isolate and identify the proteins that are modified (162). In contrast, carbon tetrachloride and other agents, such as the herbicide diquat, may form radicals that bind to and/or oxidize lipids and proteins in causing liver injury (163, 164). Therefore, the recent development [165] of antibodies to detect oxidative damage to proteins will be important in the identification and characterization of macromolecules that do not form adducts with reactive metabolites but are damaged oxidatively. Thus, some major challenges in the coming years are to identify hepatocellular macromolecules that are modified by reactive metabolites, and then approach the more difficult task of integrating this information into a time course and sequence of events leading to lethal hepatocellular injury.

Effects of N-acetylcysteine and dithiothreitol on glutathione and protein thiol replenishment during acetaminophen-induced toxicity in isolated mouse hepatocytes

Isolated mouse hepatocytes were incubated with 1.0 mM acetaminophen (AA) for 1.5 h to initiate glutathione (GSH) and protein thiol (PSH) depletion and cell injury. Cells were subsequently washed to remove non-covalently bound AA and resuspended in medium containing N-acetylcysteine (NAC, 2.0 mM) or dithiothreitol (DTT, 1.5 mM). The effects of these agents on the replenishment of GSH and total PSH content were related to the development of cytotoxicity. When cells exposed to AA were resuspended in medium containing NAC or DTT, both agents replenished GSH and total PSH content to levels observed in untreated cells but only DTT was able to attenuate cytotoxicity. Addition of the GSH synthesis inhibitor, buthionine sulfoximine (BSO, 1.0 mM, 1.5 h), to cells in incubation medium containing AA, enhanced GSH and total PSH depletion and potentiated cytotoxicity. Resuspension of these cells in medium containing NAC did not alter the potentiating effects of BSO; GSH and PSH levels were not replenished and no cytoprotective effects were observed. However, when cells exposed to AA and BSO were resuspended in medium containing DTT, PSH content was replenished but GSH levels were not restored. In addition, DTT was able to delay the development of cytotoxicity. It appears that DTT, unlike NAC, has a GSH-independent mechanism of PSH replenishment. These observations suggest that while replenishment of GSH and total PSH content does not result in cytoprotection, the regeneration of critical PSH by DTT may play an important role in the maintenance of proper cell structure and/or function.

The consequences of nitrofurantoin-induced oxidative stress in isolated rat hepatocytes: evaluation of pathobiochemical alterations

Oxidative stress was induced in isolated rat hepatocytes by incubation with nitrofurantoin in the absence and presence of the GSSG reductase inhibitor BCNU. In both cases nitrofurantoin markedly reduced glutathione but exerted cytotoxicity as measured by LDH release and loss of intracellular potassium only in BCNU pretreated cells. The onset of cytotoxicity was accompanied by an increase of lipid peroxidation. Oxidation of protein thiols, however, could not be detected in the early phase of cell damage. The cytoprotective activity of N-acetyl-cysteine > dithiothreitol = deferoxamine revealed the substantial importance of glutathione for cellular defence and the sensitivity of not yet identified thiol-dependent targets of oxidative stress.

Allyl alcohol cytotoxicity in isolated rat hepatocytes: mechanism of cell death does not involve an early rise in cytosolic free calcium

We examined the effect of a toxic concentration of allyl alcohol (0.5 mM) on intracellular calcium concentrations in isolated rat hepatocytes. An increase in phosphorylase a activity was evident in the hepatocytes after 30 min of incubation with allyl alcohol, suggesting that the toxicant may produce an early rise in cytosolic free calcium. The increase in phosphorylase a activity was not reversed by the addition of dithiothreitol (DTT), a sulfhydryl compound that reverses the events that initiate cell killing by allyl alcohol. When intracellular calcium concentrations were measured directly, using fura-2 as the calcium indicator, there was no effect of allyl alcohol on cytosolic free calcium during the first 60 min of exposure, a critical period for development of irreversible damage. Incubation with allyl alcohol did not interfere with the measurement of intracellular calcium. The increases in cytosolic free calcium produced by phenylephrine or ATP were similar to those reported by others and not affected by the presence of allyl alcohol. The results from this study demonstrate that increased cytosolic free calcium is not essential for allyl alcohol-induced cytotoxicity to isolated rat hepatocytes.

Mechanism of protection of ebselen against paracetamol-induced toxicity in rat hepatocytes

The protective effect of ebselen (PZ 51), an anti-inflammatory agent, on paracetamol-induced (1 mM) cytotoxicity in hepatocytes freshly isolated from beta-naphthoflavone-pretreated rats was studied. At a concentration of 50 microM added simultaneously with paracetamol, ebselen prevented paracetamol-induced leakage of lactate dehydrogenase (LDH) almost completely and lipid peroxidation (LPO) and depletion of glutathione (GSH) substantially. These protective effects were even more pronounced at 100 microM concentration of ebselen. When added to the hepatocytes 1 hr before paracetamol, 50 microM of ebselen also prevented LDH leakage, LPO and GSH depletion. Reverse addition of paracetamol and ebselen did not result in protection. Simultaneous incubation of 100 microM ebselen and paracetamol inhibited GSH conjugation of paracetamol by more than 50%, however, without any effect on glucuronidation and sulfation of paracetamol. Ebselen was shown not to react directly with paracetamol nor to inhibit cytochrome P450 activity measured as 7-ethoxycoumarin O-deethylase (ECD) activity in the hepatocytes. At mixing, synthetic ebselen selenol and synthetic N-acetyl-p-benzoquinone imine (NAPQI) were shown to form paracetamol and ebselen diselenide. No indication was found for the formation of an ebselen-paracetamol conjugate upon reacting synthetic NAPQI and synthetic ebselen selenol. Reduction of NAPQI, the reactive metabolite of paracetamol, by ebselen selenol is discussed in terms of the mechanism of cytoprotection.

On the mechanisms of 3-tert-butyl-4-hydroxyanisole- and its metabolites-induced cytotoxicities in isolated rat hepatocytes

The cytotoxic effects of 3-tert-butyl-4-hydroxyanisole (BHA) and its metabolites, 3-tert-butylhydroquinone (tBHQ) and 3-tert-butyl-4,5-dihydroxyanisole (BHA-OH), were investigated in freshly isolated rat hepatocytes. These compounds caused a time-dependent cell death accompanied by loss of intracellular ATP, glutathione (GSH) and protein thiols at concentration of 0.5 mM. Supplementation of the hepatocyte suspension with 5 mM N-acetylcysteine, a precursor of intracellular GSH, significantly delayed the onset of cytotoxicity induced by BHA-OH and tBHQ; the loss of intracellular ATP, GSH and protein thiols was also prevented. Although N-acetylcysteine did not affect BHA disappearance in the cell suspension, disappearance of tBHQ and formation of tBHQ-GSH conjugate were stimulated by N-acetylcysteine. In addition, N-acetylcysteine prevented BHA-OH disappearance and 3-tert-butyl-5-methoxy-1,2-benzoquinone (BHA-Q) formation. In isolated hepatic mitochondria, BHA, tBHQ and BHA-OH impaired respiration related to oxidative phosphorylation; tert-butylquinone (tBQ) and BHA-Q, quinones derived from tBHQ and BHA-OH, resulted in the significant inhibition of mitochondrial respiration. These results indicate that BHA-OH is the most cytotoxic followed by tBHQ and BHA and that protein thiols and mitochondrial respiratory system are important targets for BHA and its intermediates.

Protective effect of Artemisia scoparia extract against acetaminophen-induced hepatotoxicity

1. Hepatoprotective activity of hydro-methanolic extract of Artemisia scoparia (Compositae) was investigated against acetaminophen-induced hepatic damage. 2. Acetaminophen at a dose of 1 g/kg produced 100% mortality in mice while pretreatment of animals with plant extract (150 mg/kg) reduced the death rate to 20%. 3. Acetaminophen at a dose of 640 mg/kg produced liver damage in rats as manifested by the rise in serum levels of GOT and GPT to 1528 +/- 310 and 904 +/- 261 IU/l (n = 10) respectively, compared to respective control values of 80 +/- 11 and 38 +/- 09. 4. Pretreatment of rats with plant extract (150 mg/kg) lowered significantly the respective serum GOT and GPT levels to 85 +/- 11 and 23 +/- 06. 5. These results indicate that Artemisia scoparia contains hepatoprotective constituents and this study rationalizes the traditional use of this plant in hepatobiliary disorders.

Paracetamol (acetaminophen) poisoning

Paracetamol poisoning caused by intentional overdose remains a common cause of morbidity. In this article the mechanism of toxicity and the clinical effects and treatment of poisoning, including specific antidotal therapy, are reviewed. Areas for further research directed at reducing morbidity and mortality from paracetamol poisoning are considered.

Relationship between genomic DNA ploidy and parameters of liver damage during necrosis and regeneration induced by thioacetamide

Thioacetamide proved to be a potent necrogenic agent when a single dose of 6.6 mmol/kg was administered intraperitoneally to rats. Its necrogenic ability was assessed on the basis of morphological and biochemical changes. The injury of centrilobular hepatocytes showed a peak of cell death 24 hr after thioacetamide administration; it was followed immediately by the regenerative response. Parallel increases of serum aminotransferases, isocitrate dehydrogenase and gamma-glutamyl transferase activities were observed. Severe liver damage was also evident at 24 hr on the basis of glutathione depletion (29% of control), malondialdehyde production (169%), cytochrome P-450 level decrease (26%) and increased activity of glutathione S-transferase (160%). We checked the regenerative response by determining nuclear DNA content in isolated hepatocytes 0, 6, 12, 18, 24, 36, 48 and 72 hr after thioacetamide administration. Changes in DNA cell distribution between G0-G1, S and G2 + M phases of the cell cycle were observed. The sharp decrease in the percentage of the tetraploid cell population (G2 + M phases) and the abrupt increase of the S-phase cells at 36 and 48 hr suggest transition from adult to fetal in hepatocyte populations obtained 24 and 36 hr after thioacetamide treatment. At 72 hr of treatment, hepatocyte populations showed recovery to adult state. In the shift from the adult to fetal, registered at 24, 36 and 48 hr after thioacetamide administration, mitosis seemed to precede the synthesis of DNA.

The effect of fluorine substitution on the hepatotoxicity and metabolism of paracetamol in the mouse

The widely used analgesic paracetamol (P) produces fulminant hepatocellular necrosis in humans when taken in overdose. The toxicity is mediated by drug oxidation and depletion of hepatic glutathione. We have, therefore, explored the effects of fluorine substitution on the hepatotoxicity of P in female CD1 mice. 3-Fluoro-4-hydroxyacetanilide (1FPO), 3,5-difluoro-4-hydroxyacetanilide (2FPO), 2,6-difluoro-4-hydroxyacetanilide (2FPN) and 2,3,5,6-tetrafluoro-4-hydroxyacetanilide (4FP) were synthesized, characterized and investigated for their potential to cause hepatotoxicity in the mouse. Introduction of fluorine into P increases the oxidation potential of the drug. The oxidation potentials of paracetamol and its fluorinated analogues were measured by cyclic voltametry and found to increase in the order P < 1FPO < 2FPO < 2FPN < 4FP. Serum transaminase (ALT) and hepatic glutathione were measured 24 and 6 hr, respectively, after administration of a single dose (2.65 mmol/kg) of each compound to female CD1 mice. There was significant elevation of ALT in mice given P, 1FPO and 2FPO, but not in those which received either 2FPN or 4FP. Hepatic glutathione was reduced significantly by administration of P and IFP, but not after administration of 2FPO, 2FPN or 4FP. Accordingly, glucuronide and sulphate conjugates, but not thioether metabolites, were detected in urine after administration of 14C-labelled 2FPO, 2FPN and 4FP. These data indicate that introduction of fluorine into the 2 and 6 positions increases the oxidation potential of paracetamol which in turn reduces the propensity of the molecule to undergo oxidative bioactivation, and thereby reduces the in vivo toxicity of the molecule.

Effect of colchicine on acetaminophen-induced liver damage

The effect of colchicine on liver damage induced by acetaminophen (APAP) intoxication was studied. Wistar male rats pretreated (72 h) with 3-methylcholanthrene (3-MC) (20 mg/kg i.p.) were divided into six groups: animals in group 1 were treated with acetaminophen (APAP) (500 mg/kg p.o.); group 2 consisted of animals that received colchicine (65 micrograms/kg/day p.o.) for 7 days before APAP intoxication; group 3 was treated like group 2, but the dose of colchicine was 300 micrograms/kg/day; animals in groups 4 and 5 received the same doses of colchicine as groups 2 and 3, respectively, but received the vehicle instead of APAP; and rats in group 6 (control) received the equivalent amount of the vehicles. Animals were sacrificed at different times after APAP administration. Reduced glutathione (GSH), lipid peroxidation and glycogen were measured in liver and, gamma-glutamyl transpeptidase (gamma-GTP), and glutamic pyruvic transaminase (GPT) activities were measured in serum. After APAP intoxication, GSH and glycogen decreased very fast (1 h) and remained low for 6 h. Lipid peroxidation increased three times over control 4 h after APAP treatment. Enzyme activities increased at 18 h after intoxication. Pretreatment with 65 micrograms/kg of colchicine failed to prevent liver damage induced by APAP. However, when a dose of 300 micrograms/kg of colchicine was given, levels of lipid peroxidation and serum gamma-GTP activity remained within the control values, while GPT activity and glycogen content were only partially attenuated. It was concluded that colchicine protects against APAP intoxication, probably through its antioxidant properties, possibly acting as a free radical scavenger.

An investigation into the formation of stable, protein-reactive and cytotoxic metabolites from tacrine in vitro. Studies with human and rat liver microsomes

Tacrine (1,2,3,4-tetrahydro-9-aminoacridine hydrochloride; THA) is known to undergo extensive oxidative metabolism to a variety of mono- and dihydroxylated metabolites in animals and humans. The potential for tacrine to undergo metabolism to stable, protein-reactive and cytotoxic metabolites has been investigated in incubations with human and rat liver microsomes. Using lymphocytes as sensitive markers to quantify cytotoxicity, THA (50 microM) underwent NADPH-dependent bioactivation to a cytotoxic metabolite(s). NADPH-dependent cytotoxicity in the presence of rat and human microsomes was 9.8 +/- 3.1% (P < 0.05 cf. -NADPH control) and 6.2 +/- 2.0% (P < 0.05 cf. -NADPH control), respectively. Stable and protein-reactive metabolites were also formed in microsomes from both species. These accounted for 28.2 +/- 12.7% and 1.22 +/- 0.79% of incubated radioactivity in human microsomes and 6.4 +/- 2.2% and 0.4 +/- 0.1% of incubated radioactivity in rat microsomes. In microsomes pooled from six human livers the NADPH-dependent cytotoxicity was 9.4 +/- 1.1%. Formation of stable and protein-reactive metabolites accounted for 29.2 +/- 2.3% and 1.2 +/- 1.0% of incubated radioactivity. Reduced glutathione (500 microM) completely blocked NADPH-dependent cytotoxicity and inhibited protein-reactive metabolite formation by 60% (P < 0.05). Ascorbic acid (500 microM) inhibited the generation of cytotoxic and protein-reactive metabolites by 75% (P < 0.05) and 35% (P < 0.05), respectively. Cyclohexene oxide was without effect. Human serum albumin was found to protect the lymphocytes against toxicity. In microsomes prepared from the livers of four donors known to have been smokers there were no significant differences in the generation of metabolites from THA compared with microsomes prepared from livers of non-smokers. Enoxacin, a specific inhibitor of cytochrome P450 1A2 significantly inhibited all routes of THA metabolism. We have therefore demonstrated that THA may be oxidatively metabolized to stable, protein-reactive and cytotoxic metabolites in human and rat liver microsomes. A number of inhibitors may affect these process, whilst inhibition by enoxacin indicates a role for cytochrome P450 1A2 in THA metabolism.

Oxidative stress in cultured hepatocytes exposed to acetaminophen

The effect of acetaminophen (APAP) exposure on the formation of oxidized glutathione (GSSG) was investigated in cultured mouse hepatocytes to determine if oxidative damage is involved in the toxicity of this drug. Incubations of hepatocytes for 24 hr with 1 mM APAP produced a time-dependent loss of cell viability which was preceded by depletion of reduced glutathione (GSH) and an increase in GSSG formation. Pretreatment with 1,3-bis(chloroethyl)-1-nitrosourea (BCNU) (0.1 mM) for 30 min, which irreversibly inhibited glutathione reductase (GSSG-Rd) activity, increased the extent of GSSG formation produced by APAP exposure and potentiated its cell killing. Pretreatment of hepatocytes with 20 mM deferoxamine (DFO) for 1 hr to chelate ferric iron decreased GSSG formation and cell killing produced by APAP. Pretreatment with BCNU or DFO did not affect APAP oxidation as determined by the formation of the APAP-GSH conjugate or the covalent binding of APAP metabolites to cellular protein. Hence, increasing the susceptibility of hepatocytes to an oxidative stress with BCNU increased both the formation of GSSG and cell killing produced by APAP. Conversely, decreasing their susceptibility to an oxidative stress by chelating iron with DFO decreased GSSG formation and cell injury. It follows that APAP toxicity involves oxidative processes that occur early in the poisoning process and are a major factor contributing to injury in these cells.

Generalized cytochrome P450-mediated oxidation and oxygenation reactions in aromatic substrates with activated N-H, O-H, C-H, or S-H substituents

1. The general mechanism of metabolic oxidation of substrates by cytochromes P450 (P450s) appears to consist of sequential one-electron oxidation steps rather than of a single concerted transfer of activated oxygen species from P450 to substrates. 2. In case of the acetanilides paracetamol (PAR), phenacetin (PHEN), and 4-chloro-acetanilide (4-CLAA), the first one-electron oxidation step consists of a hydrogen abstraction from the acetylamino nitrogen and/or from the other side-chain substituent on the aromatic ring. The substrate radicals thus formed delocalize their spin and the respective reactive centres of the substrate radical recombine with a P450 iron-bound hydroxyl radical to either yield oxygenated metabolites, or undergo a second hydrogen abstraction forming dehydrogenated products. By this mechanism, the formation of all known oxidative metabolites of PAR, PHEN, and 4-ClAA can be explained. Furthermore, this mechanism is consistent with all available experimental data on [18O]PAR/PHEN, [2H]PAR, and [14C]PHEN. 3. The oxidative metabolic reactions proposed for the acetanilides PAR, PHEN, and 4-ClAA are used to generalize P450-mediated oxidations of these and other acetanilides, such as analogues of PAR and 2-N-acetyl-aminofluorene. 4. A further generalization of the hydrogen abstraction, spin delocalization, radical recombination concept is derived for other aromatic substrates with abstractable hydrogen atoms, notably those with activated N-H, O-H, C-H, or S-H bonds directly attached to the aromatic nucleus.