The spontaneous and enzymatic reaction of N-acetyl-p-benzoquinonimine with glutathione: a stopped-flow kinetic study

Protective effect of Moutan Cortex extract on acetaminophen-induced hepatotoxicity in mice

Previously, we demonstrated that Moutan Cortex prevents acetaminophen (AAP)-induced cytotoxicity in vitro. The present study examined the protective effect of Moutan Cortex on AAP induced hepatotoxicity and the possible mechanisms underlying this effect in mice. When Montan Cortex was administered to ICR mice, followed by hepatotoxic dose of AAP (400 mg/kg, i.p.), Moutan Cortex pre-exposure prevented liver injury as indicated by the decrease of serum alanine aminotransferase level. Moutan Cortex also protected AAP-induced hepatic glutathione depletion. Cytochrome P450 2E1-dependent aniline and p-nitrophenol hydroxylases activities in microsomal incubations were significantly inhibited by Moutan Cortex. Abrogation of toxicity was also mirrored in DNA fragmentation. These observations demonstrate that Moutan Cortex pre-exposure may attenuate AAP-induced GSH depletion, cytochrome P450 2E1 activity, and hepatic DNA damage in vivo.

ACETAMINOPHEN-INDUCED HEPATOTOXICITY

The analgesic acetaminophen causes a potentially fatal, hepatic centrilobular necrosis when taken in overdose. The initial phases of toxicity were described in Dr. Gillette's laboratory in the 1970s. These findings indicated that acetaminophen was metabolically activated by cytochrome P450 enzymes to a reactive metabolite that depleted glutathione (GSH) and covalently bound to protein. It was shown that repletion of GSH prevented the toxicity. This finding led to the development of the currently used antidote N-acetylcysteine. The reactive metabolite was subsequently identified to be N-acetyl-p-benzoquinone imine (NAPQI). Although covalent binding has been shown to be an excellent correlate of toxicity, a number of other events have been shown to occur and are likely important in the initiation and repair of toxicity. Recent data have shown that nitrated tyrosine residues as well as acetaminophen adducts occur in the necrotic cells following toxic doses of acetaminophen. Nitrotyrosine was postulated to be mediated by peroxynitrite, a reactive nitrogen species formed by the very rapid reaction of superoxide and nitric oxide (NO). Peroxynitrite is normally detoxified by GSH, which is depleted in acetaminophen toxicity. NO synthesis (serum nitrate plus nitrite) was dramatically increased following acetaminophen. In inducible nitric oxide synthase (iNOS) knockout mice, acetaminophen did not increase NO synthesis or tyrosine nitration; however, histological evidence indicated no difference in toxicity. Acetaminophen did not cause hepatic lipid peroxidation in wild-type mice but did cause lipid peroxidation in iNOS knockout mice. These data suggest that NO may play a role in controlling lipid peroxidation and that reactive nitrogen/oxygen species may be important in toxicity. The source of the superoxide has not been identified, but our recent finding that NADPH oxidase knockout mice were equally sensitive to acetaminophen and had equal nitration of tyrosine suggests that the superoxide is not from the activation of Kupffer cells. It was postulated that NAPQI-mediated mitochondrial injury may be the source of the superoxide. In addition, the significance of cytokines and chemokines in the development of toxicity and repair processes has been demonstrated by several recent studies. IL-1beta is increased early in acetaminophen toxicity and may be important in iNOS induction. Other cytokines, such as IL-10, macrophage inhibitory protein-2 (MIP-2), and monocyte chemoattractant protein-1 (MCP-1), appear to be involved in hepatocyte repair and the regulation of proinflammatory cytokines.

Inactivation of the hepatic cytochrome P450 system by conditional deletion of hepatic cytochrome P450 reductase

Cytochrome P450 (CYP) monooxygenases catalyze the oxidation of a large number of endogenous compounds and the majority of ingested environmental chemicals, leading to their elimination and often to their metabolic activation to toxic products. This enzyme system therefore provides our primary defense against xenobiotics and is a major determinant in the therapeutic efficacy of pharmacological agents. To evaluate the importance of hepatic P450s in normal homeostasis, drug pharmacology, and chemical toxicity, we have conditionally deleted the essential electron transfer protein, NADH:ferrihemoprotein reductase (EC, cytochrome P450 reductase, CPR) in the liver, resulting in essentially complete ablation of hepatic microsomal P450 activity. Hepatic CPR-null mice could no longer break down cholesterol because of their inability to produce bile acids, and whereas hepatic lipid levels were significantly increased, circulating levels of cholesterol and triglycerides were severely reduced. Loss of hepatic P450 activity resulted in a 5-fold increase in P450 protein, indicating the existence of a negative feedback pathway regulating P450 expression. Profound changes in the in vivo metabolism of pentobarbital and acetaminophen indicated that extrahepatic metabolism does not play a major role in the disposition of these compounds. Hepatic CPR-null mice developed normally and were able to breed, indicating that hepatic microsomal P450-mediated steroid hormone metabolism is not essential for fertility, demonstrating that a major evolutionary role for hepatic P450s is to protect mammals from their environment.

Glutathione, glutathione-dependent enzymes and antioxidant status in erythrocytes from children treated with high-dose paracetamol

AIM

To investigate glutathione and antioxidant status changes in erythrocytes from febrile children receiving repeated supratherapeutic paracetamol doses.

METHODS

Fifty-one children aged 2 months to 10 years participated in the study. Three groups were studied: group 1 (n = 24) included afebrile children who did not receive paracetamol; and groups 2 (n = 13) and 3 (n = 14) included children who had fever above 38.5 degrees C for more than 72 h. Patients in group 2 received paracetamol at a dose of 50 +/- 15 (30-75) mg kg(-1) day(-1) and those in group 3 received paracetamol above the recommended therapeutic dose, ie 107 28 (80-180) mg kg(-1) day(-1). A blood sample was taken for the measurement of liver transaminases, gammaglutamil transferase (GGT), reduced glutathione (GSH), glutathione reductase (GR), glutathione peroxidase (GPX), glutathione S-transferase (GST), superoxide dismutase (SOD) and antioxidant status.

RESULTS

Aspartate aminotransferase activity in group 3 was higher than in the other groups (P = 0.027). GSH, SOD and antioxidant status were significantly lower in group 3 compared with groups 1 and 2 (mean differences: for GSH 3.41 micromol gHb(-1), 95% confidence interval (CI) 2.10-4.72, and 2.15 micromol gHb(-1), 95% CI 0.65-3.65, respectively; for SOD 856 U min(-1) gHb(-1), 95% CI 397-1316, and 556 U min(-1) gHb(-1), 95% CI 30-1082, respectively; and for antioxidant status 0.83 mmol l(-1) plasma, 95% CI 0.30-1.36, and 0.63 mmol l(-1) plasma, 95% CI 0.02-1.24, respectively). GR activity was significantly lower in groups 3 and 2 in comparison with group 1 (mean differences 3.44 U min(-1) gHb(-1), 95% CI 0.63-6.25, and 5.64 U min(-1) gHb(-1), 95% CI 2.90-8.38, respectively). Using multiple regression analysis, paracetamol dose was found to be the only independent variable affecting GR, GST and SOD activities (P = 0.007, 0.003 and 0.008, respectively).

CONCLUSIONS

In febrile children, treatment with repeated supratherapeutic doses of paracetamol is associated with reduced antioxidant status and erythrocyte glutathione concentrations. These significant changes may indicate an increased risk for hepatotoxicity and liver damage.

Acetaminophen-glutathione conjugate formation in a coupled cytochrome P-450-glutathione S-transferase assay system mediated by subcellular preparations from adult and weanling rat tissues

Previous studies from this laboratory indicated that glutathione (GSH) conjugate formation with acetaminophen (APAP) is remarkably induced in liver of weanling rats in response to a single overdose of the drug administered intraperitoneally (ip). Increased APAP-GSH conjugation has been attributed to inducible glutathione S-transferases (GSTs) in dividing hepatocytes. In order to verify this finding, an in vitro reconstitution assay containing liver microsomes (source of cytochrome P-450) and cytosolic fractions (source of GST) from livers and kidneys of adult and weanling rats has been established. In vitro incubation of the reaction mixture was followed by solvent extraction, enzymatic digestion and HPLC analysis of the conjugate. Under controlled conditions, in vitro, the rate of APAP-GSH conjugation reflected the GST activity of cytosolic sample added to incubation system. The activity of cytosolic GST in catalyzing this reaction was measured using cytosols prepared from various tissue sources, particularly from animals pretreated with dietary butylated hydroxylanisole (BHA). The extent of APAP-GSH conjugate formation mediated by cytosols varied in this order: BHA-treated adult liver>BHA-treated weanling liver>control adult liver>control weanling liver>BHA-adult kidney>control adult kidney>BHA weanling kidney>control weanling kidney. In contrast to findings obtained from in vivo experiments, the rate of GST-dependent APAP conjugate formation with GSH in vitro is not induced in the presence of exogenous drug.

Modulation of acetaminophen-induced hepatotoxicity by the xenobiotic receptor CAR

We have identified the xenobiotic receptor CAR (constitutive androstane receptor) as a key regulator of acetaminophen metabolism and hepatotoxicity. Known CAR activators as well as high doses of acetaminophen induced expression of three acetaminophen-metabolizing enzymes in wild-type but not in CAR null mice, and the CAR null mice were resistant to acetaminophen toxicity. Inhibition of CAR activity by administration of the inverse agonist ligand androstanol 1 hour after acetaminophen treatment blocked hepatotoxicity in wild type but not in CAR null mice. These results suggest an innovative therapeutic approach for treating the adverse effects of acetaminophen and potentially other hepatotoxic agents.

Potentiation of the immunotoxicity of ethanol by acetaminophen in mice

To investigate the influences of acetaminophen (APP) on the immunotoxicity of ethanol (EtOH) in ICR mice, APP at a dose of 100 mg/kg was orally administered to mice daily for 28 consecutive days. Mice treated with EtOH were given freely with 20% w/v EtOH during the experimental period, and normal mice were given vehicle. The results of this study are summarized as follows: the combination of APP and EtOH significantly decreased the circulating leukocytes and the relative weights of liver, spleen and thymus, compared with the treatment of EtOH alone. In mice receiving the combination of AAP and EtOH when compared with the treatment of EtOH alone, there were also significant reductions in the splenic plaque forming cells (PFC) and hemagglutination (HA) titers to sheep red blood cells (SRBC), and the secondary IgG antibody response to bovine serum albumin (BSA). A tendency toward suppression of delayed-type hypersensitivity (DTH) reaction and phagocytic activity was also observed in the combination of AAP and EtOH. In addition, the combination of AAP and EtOH greatly increased serum alanine aminotransaminase (ALT) and total protein levels, compared with the treatment of EtOH alone. Significant decreases in serum albumin and A/G ratio were observed in EtOH alone-fed mice compared with those in normal animals, and their reductions were further induced in mice treated with AAP and EtOH. These findings indicate that EtOH-induced immunotoxicity is aggravated by the combination of APP and EtOH.

Endotelin ETA and ETB receptor antagonism during cold preservation in renal transplantation

BACKGROUND

In this study, we investigated the effects of selective endothelin (ET) receptor antagonism during different periods of cold ischemia on glomerular and tubular function and long-term survival in renal transplantation.

METHODS

Left renal transplantation was performed in Lewis rats after 2 hr of cold ischemia without (n=8) and with (n=6) ETA receptor antagonism and after 16 hr of cold ischemia without treatment (n=6), with ETA receptor antagonism (n=8) and with ETB receptor antagonism (n=6). A control group (n=8) underwent right nephrectomy and left renal denervation. The ETA and ETB receptor antagonists (BQ-610 and A-192621, respectively) were added to the preservation solution (EuroCollins). After transplantation, renal glomerular and tubular functions were monitored for up to 60 days or death.

RESULTS

All animals in the control and 2-hr groups survived the follow-up protocol, with early postoperative recovery of glomerular and tubular function while the entire untreated 16-hr group died between day 3-6 postoperatively. BQ-610 treatment had no measurable effect on the renal function in the 2-hr group, however, it improved glomerular and tubular functions and led to 50% long-term survival (60 days) in the 16-hr group. A-192621 treatment had no effect on long-term survival or renal parameters.

CONCLUSION

ETA receptor antagonism had protective renal effects after prolonged ischemic preservation in renal transplantation while ETB receptor antagonism had not.

Paracetamol (acetaminophen)-induced toxicity: molecular and biochemical mechanisms, analogues and protective approaches

An overview is presented on the molecular aspects of toxicity due to paracetamol (acetaminophen) and structural analogues. The emphasis is on four main topics, that is, bioactivation, detoxication, chemoprevention, and chemoprotection. In addition, some pharmacological and clinical aspects are discussed briefly. A general introduction is presented on the biokinetics, biotransformation, and structural modification of paracetamol. Phase II biotransformation in relation to marked species differences and interorgan transport of metabolites are described in detail, as are bioactivation by cytochrome P450 and peroxidases, two important phase I enzyme families. Hepatotoxicity is described in depth, as it is the most frequent clinical observation after paracetamol-intoxication. In this context, covalent protein binding and oxidative stress are two important initial (Stage I) events highlighted. In addition, the more recently reported nuclear effects are discussed as well as secondary events (Stage II) that spread over the whole liver and may be relevant targets for clinical treatment. The second most frequent clinical observation, renal toxicity, is described with respect to the involvement of prostaglandin synthase, N-deacetylase, cytochrome P450 and glutathione S-transferase. Lastly, mechanism-based developments of chemoprotective agents and progress in the development of structural analogues with an improved therapeutic index are outlined.

Increased resistance to acetaminophen hepatotoxicity in mice lacking glutathione S-transferase Pi

Overdose of acetaminophen, a widely used analgesic drug, can result in severe hepatotoxicity and is often fatal. This toxic reaction is associated with metabolic activation by the P450 system to form a quinoneimine metabolite, N-acetyl-p-benzoquinoneimine (NAPQI), which covalently binds to proteins and other macromolecules to cause cellular damage. At low doses, NAPQI is efficiently detoxified, principally by conjugation with glutathione, a reaction catalyzed in part by the glutathione S-transferases (GST), such as GST Pi. To assess the role of GST in acetaminophen hepatotoxicity, we examined acetaminophen metabolism and liver damage in mice nulled for GstP (GstP1/P2((-/-))). Contrary to our expectations, instead of being more sensitive, GstP null mice were highly resistant to the hepatotoxic effects of this compound. No significant differences between wild-type (GstP1/P2((+/+))) mice and GstP1/P2((-/-)) nulls in either the rate or route of metabolism, particularly to glutathione conjugates, or in the levels of covalent binding of acetaminophen-reactive metabolites to cellular protein were observed. However, although a similar rapid depletion of hepatic reduced glutathione (GSH) was found in both GstP1/P2((+/+)) and GstP1/P2((-/-)) mice, GSH levels only recovered in the GstP1/P2((-/-)) mice. These data demonstrate that GstP does not contribute in vivo to the formation of glutathione conjugates of acetaminophen but plays a novel and unexpected role in the toxicity of this compound. This study identifies new ways in which GST can modulate cellular sensitivity to toxic effects and suggests that the level of GST Pi may be an important and contributing factor in the sensitivity of patients with acetaminophen-induced hepatotoxicity.

The contributions of various chloroquine salts to the biliary and urinary execretion of hepatic paracetamol conjugation metabolites in the rat

As an approach to explain the possible in vivo interaction of paracetamol (acetaminophen) with various chloroquine salts that are often administered during malaria tropica, the effects of these salts (chloroquine sulphate, chloroquine phosphate, chloroquine hydrochloride and ferrous sulphate) were examined in male rats. The coadministration of chloroquine salts with paracetamol for 7 days showed varied effects on urinary and biliary excretion of paracetamol sulphate and paracetamol glucuronide conjugates--the major metabolites of paracetamol metabolism. These findings suggest that chloroquine sulphate and ferrous sulphate may enhance the sulphation pathway in paracetamol metabolism and influence detoxification of paracetamol in the liver and thus protect the liver. Chloroquine sulphate is therefore a better choice compared to other chloroquine salts in the treatment of malaria with paracetamol as an antipyretic and analgesic.

Hepatocellular response to chemical stress in CD-1 mice: induction of early genes and gamma-glutamylcysteine synthetase

Exposure of cells to toxic chemical species can result in reduced glutathione (GSH) depletion, generation of free radicals, and/or binding to critical cell determinants. Chemical stress is usually followed by a concerted cellular response aimed at restoring homeostasis, although the precise initial stimulus for the response is unclear. We have focused on one component of this stress response, the up-regulation of gamma-glutamylcysteine synthetase (gamma-GCS) and the preceding molecular events involved in its regulation in an in vivo mouse model. Male CD-1 mice received buthionine sulphoximine (BSO; 7.2 mmol/kg), diethyl maleate (DEM; 4.2 mmol/kg), paracetamol (APAP; 3.5 and 1.0 mmol/kg), or carbon tetrachloride (CCl(4); 1.0 and 0.2 mmol/kg). Biochemical (serum transaminase and hepatic GSH levels) and molecular (c-jun and c-fos messenger RNA [mRNA] levels and activator protein 1 [AP-1] DNA binding activity) parameters were measured, as well as the consequent effects on gamma-GCS levels and activity. All compounds produced GSH depletion, but only the higher doses of APAP and CCl(4) caused liver damage. DEM, APAP, and CCl(4) increased c-jun and c-fos mRNA levels, together with an increase in AP-1 binding; BSO failed to induce AP-1 despite an increase in c-fos. Interestingly, the effects on gamma-GCS varied markedly according to the compound: BSO and DEM increased gamma-GCS enzyme activity, although only DEM, but not BSO, resulted in an increase in gamma-GCS(h) mRNA and protein. In contrast, APAP and CCl(4) both increased gamma-GCS(h) mRNA and protein; however, there was a marked dose-dependent decrease in gamma-GCS activity. These data indicate that the effect of chemical stress on the liver is compound specific and is not merely dependent on depletion of GSH.

The hepatoprotective effects of Solanum alatum Moench. on acetaminophen-induced hepatotoxicity in mice

Solanum alatum Moench. has been shown to have a protective effect against carbon tetrachloride (CCl4)-induced liver injury. Solanum alatum treatment (100 mg/kg, p.o.) decreased the elevation of serum alanine aminotransferase (ALT; GPT) and aspartate aminotransferase (AST; GOT) induced by acetaminophen (paracetamol) (600 mg/kg, i.p.) administration. It also decreased the extent of visible necrosis in liver tissue. In addition, Solanum alatum treatment restored hepatic glutathione (GSH) depletion induced by acetaminophen (600 mg/kg, i.p.) administration. Microsomal enzyme levels such as P-450, reductase, and aniline hydroxylation enzyme were also restored to normal levels after Solanum alatum administration. The hepatoprotective mechanism may function through direct binding with acetaminophen toxic metabolites, decreasing the attraction of acetaminophen metabolites for other cellular GSH or thiol protein. Additionally, Solanum alatum treatment increased the concentration of hepatic GSH and maintained a high level activity of GSTase, which led to acceleration of the excretion of toxic acetaminophen metabolites.

Protein and nonprotein cysteinyl thiol modification by N-acetyl-p-benzoquinone imine via a novel ipso adduct

N-acetyl-p-benzoquinone imine (NAPQI), a reactive metabolite of acetaminophen (APAP), can arylate and oxidize protein and nonprotein thiols in the pathogenesis of APAP-induced hepatotoxicity. We report the first direct evidence for the formation of a labile ipso adduct between glutathione (GSH) and NAPQI using a combination of techniques including liquid chromatography/tandem mass spectrometry and liquid chromatography/NMR spectroscopy. Decomposition kinetics of the GSH-NAPQI ipso adduct and product ratios suggested that the ipso adduct was readily reversible back to NAPQI under neutral and basic conditions. The significance of the ipso adduct is that it may migrate from its site of formation to other cell compartments where it can either oxidize protein thiols or covalently modify them. Ipso adduct formation with protein thiols was demonstrated with a cysteine protease, papain, whose catalytic activity relies on the presence of an active site cysteinyl thiol. The formation and reactions of cysteinyl thiol ipso adducts of NAPQI provides significant new insights into possible reactions of quinone imines with cellular peptides and proteins.

Use of transgenic animals in understanding molecular mechanisms of toxicity

Understanding molecular mechanisms of chemical toxicity and the potential risks of drugs to man is a pivotal part of the drug development process. With the dramatic increase in the number of new chemical entities arising from high throughput screening, there is an urgent need to develop systems for the rapid evaluation of potential drugs so that those agents which are most likely to be free of adverse effects can be identified at the earliest possible stage in drug development. The complex mechanisms of action of chemical toxins has made it extremely difficult to evaluate the precise toxic mechanism and also the relative role of specific genes in either potentiating or ameliorating the toxic effect. This problem can be addressed by the application of genetic strategies. Such strategies can exploit strain differences in susceptibility to specific toxic agents or, with the rapidly developing technologies, can exploit the use of transgenic animals where specific genes can be manipulated and subsequent effects on chemical toxicity evaluated. Transgenic animals can be exploited in a variety of ways to understand mechanisms of chemical toxicity. For example, a human gene encoding a drug metabolizing enzyme can be directly introduced and the effects on toxic response evaluated. Alternatively, specific genes can be deleted from the mouse genome and the consequences on toxicological response determined. Many toxic chemical agents modulate patterns of gene expression within target cells. This can be used to screen for responses to different types of toxic insult. In such experiments the promotor of a stress-regulated gene can be ligated to a suitable reporter gene, such as lacZ, or green fluorescent protein, and inserted into the genome of an appropriate test species. On administration of a chemical agent, cells which are sensitive to the toxic effects of that chemical will express the reporter, which can then be identified using an appropriate assay system. This latter strategy provides the potential for screening a large number of compounds rapidly for their potential toxic effects and also provides information on tissue and cellular specificity. Experiments using transgenic animals can be complex, and care must be taken to ensure that the results are not affected by background activities within the species being used. For example, the introduction of a specific human cytochrome P450 gene may have no effect on the metabolic disposition of a drug or toxin because of the background activity within the mouse. As the toxicity of a chemical agent is determined by a wide range of different factors including drug uptake, metabolism, detoxification and repair, differences between man and the species being used could potentially generate a toxic response in the animal model whereas no toxicity may be observed in man. In spite of these confounding factors, the application of transgenic animals to toxicological issues has enormous potential for speeding up the drug discovery process and will undoubtedly become part of this process in the future.

Oxidation of acetaminophen to its toxic quinone imine and nontoxic catechol metabolites by baculovirus-expressed and purified human cytochromes P450 2E1 and 2A6

Acetaminophen (APAP), a widely used analgesic and antipyretic agent, is bioactivated by cytochromes P450 to cause severe hepatotoxicity. APAP is oxidized by two pathways to form a toxic intermediate, N-acetyl-p-benzoquinone imine (NAPQI), and a nontoxic catechol metabolite, 3-hydroxy-APAP (3-OH-APAP). We investigated the role of P450 2E1 and 2A6 in APAP oxidation by using baculovirus-expressed and highly purified forms of human P450 2E1 and 2A6. An electrochemical HPLC assay was developed to quantify both oxidative metabolites simultaneously. For the first time, it was demonstrated that human P450 2E1 selectively oxidized APAP to NAPQI (assayed as its glutathione conjugate, GS-APAP), whereas human P450 2A6 selectively oxidized APAP to 3-OH-APAP. At 1 mM APAP, the relative ratio for the formation of GS-APAP vs 3-OH-APAP with human P450 2E1 was approximately 6:1, whereas the ratio with human P450 2A6 was 1:3. Apparent Km and Vmax values for the formation of GS-APAP by human P450 2E1 were 1.3 mM and 6.9 nmol/min/nmol of P450, respectively, whereas they were 4.6 mM and 7.9 nmol/min/nmol of P450 for P450 2A6. Apparent Km and Vmax values for the formation of 3-OH-APAP by human P450 2E1 were 4.0 mM and 2.5 nmol/min/nmol of P450, respectively, whereas they were 2.2 mM and 14.2 nmol/min/nmol of P450, respectively, for P450 2A6. Thus, although at toxic doses of APAP P450 2E1 is the more efficient catalyst for the formation of the toxic metabolite NAPQI, P450 2A6 also can contribute significantly to NAPQI production.

Inactivation of glyceraldehyde-3-phosphate dehydrogenase by a reactive metabolite of acetaminophen and mass spectral characterization of an arylated active site peptide

Acetaminophen (4'-hydroxyacetanilide, APAP) is a widely used analgesic and antipyretic drug that can cause hepatic necrosis under some circumstances via cytochrome P450-mediated oxidation to a reactive metabolite, N-acetyl-p-benzoquinone imine (NAPQI). Although the mechanism of hepatocellular injury caused by APAP is not fully understood, it is known that NAPQI forms covalent adducts with several hepatocellular proteins. Reported here is the identification of one of these proteins as glyceraldehyde-3-phosphate dehydrogenase [GAPDH, D-glyceraldehyde-3-phosphate: NAD+ oxidoreductase (phosphorylating), EC 1.2.1.12]. Two hours after the administration of hepatotoxic doses of [14C]APAP to mice, at a time prior to overt cell damage, hepatocellular GAPDH activity was significantly decreased concurrent with the formation of a 14C-labeled GAPDH adduct. A nonhepatotoxic regioisomer of APAP, 3'-hydroxyacetanilide (AMAP), was found to decrease GAPDH activity to a lesser extent than APAP, and radiolabel from [14C]AMAP bound to a lesser extent to GAPDH at a time when its overall binding to hepatocellular proteins was almost equivalent to that of APAP. In order to determine the nature of the covalent adduct between GAPDH and APAP, its major reactive and toxic metabolite, NAPQI, was incubated with purified porcine muscle GAPDH. Microsequencing analysis and fast atom bombardment mass spectrometry (FAB-MS) with collision-induced dissociation (CID) were used to characterize one of the adducts as APAP bound to the cysteinyl sulfhydryl group of Cys-149 in the active site peptide of GAPDH.

Synthesis, isolation and identification of glucuronides and mercapturic acids of a novel antiparasitic agent, licochalcone A

1. Four glucuronic acid conjugates of licochalcone A (Lica), and their metabolites, have been synthesized using rabbit and pig liver microsomes and purified by preparative hplc. 2. The glucuronides were identified as E-Lica 4'-O-beta-glucuronide, E and Z-Lica 4-O-beta-glucuronide and a mono-glucuronide conjugate of a beta-hydroxylated Lica metabolite. The metabolites were identified by hplc-nmr (one and two-dimensional nmr) as well as hplc-ms. 3. At pH 8.5 Lica reacted with N-acetyl-L-cysteine giving the two epimeric conjugates, which were then isolated by preparative hplc and identified by one and two-dimensional nmr spectroscopic methods. 4. Only two glucuronic acid conjugates (E- and Z-Lica 4-O-beta-glucuronide) were found in the urine of rat after i.p. administration of a single dose of Lica.

Acetaminophen binds to mouse hepatic and renal DNA at human therapeutic doses

Alkylation of DNA by acetaminophen metabolites has been reported previously, but has received little attention, and the biological impact of this alkylation is essentially unknown. In the present study, apparent covalent binding of acetaminophen metabolites to DNA in male ICR mice was observed at levels of 2.0 +/- 0.4 to 18.5 +/- 5.5 pmol of acetaminophen/mg of DNA in liver and 0.6 +/- 0.1 to 26.9 +/- 2.6 pmol of acetaminophen/mg of DNA in kidney with doses ranging from 10 to 400 mg/kg. Investigations of the reaction of [3H]-N-acetyl-p-benzoquinone imine (NAPQI) or [ring-14C]NAPQI with DNA in vitro yielded low levels of DNA alkylation. Greater apparent binding of [3H]NAPQI to DNA occurred in reactions containing nuclear proteins, such as by using chromatin or whole nuclei. The binding of NAPQI to purified DNA also was enhanced by the presence of 0.1 mM cysteine, but not by 1.0 mM cysteine. Increased binding of NAPQI to DNA in the presence of cysteine or nuclear protein is in contrast to the effects of alternate sulfhydryls on the binding of NAPQI to proteins, which implies that the mechanisms responsible for binding to DNA may be different than the mechanisms that mediate alkylation of protein. The alkylation of DNA by [ring-14C]NAPQI was enhanced markedly at buffer pH < 4.0, suggesting participation of a protonated form of NAPQI in binding to DNA under these conditions. Acetaminophen binding to DNA also was assessed in metabolic activation systems, including microsomes with cumene hydroperoxide or NADPH, and with horseradish peroxidase (HRP) and H2O2. Measurable binding was obtained in all systems, but HRP and H2O2 produced binding levels 200-fold greater than was observed with the microsomal systems. The 32P-postlabeling of DNA from acetaminophen-treated mice, and of DNA reacted with acetaminophen, HRP, and H2O2, produced unique spots that were not identical. The present data further support the hypothesis that acetaminophen metabolites bind covalently to DNA and demonstrate that this apparent binding is observed in experimental animals in vivo at doses that mimic therapeutic doses in humans.

Enzymic- and thiol-mediated activation of halogen-substituted diaziridinylbenzoquinones: redox transitions of the semiquinone and semiquinone-thioether species

Activation of 2,5-diaziridinyl-1,4-benzoquinones bearing halogen (Cl, Br, or F) substituents at C3 and C6 by NADPH-cytochrome P450 reductase and glutathione nucleophilic substitution was examined in terms of free radical production and DNA strand scission. A semiquinone species was observed by direct ESR in aerobic conditions during: (a) NADPH-cytochrome P450 reductase-catalyzed reduction of the above quinones. (b) The interaction of these quinones with GSH entailing primarily reactivity of halogen substituents toward sulfur substitution. (c) NADPH-cytochrome P450 reductase-catalyzed activation of products resulting from the quinone/GSH interaction. The semiquinone ESR signal observed during enzymic catalysis was suppressed by superoxide dismutase and was not affected by catalase. ESR studies in conjunction with the spin trapping technique on the autoxidation of the semiquinones formed by the above reaction pathways indicated the formation of superoxide radicals. In addition, thiyl radicals were formed during the reactions following glutathione necleophilic substitution of the above quinones. The ESR signals of both superoxide and thiyl radicals were abolished by superoxide dismutase. No hydroxyl radicals were formed in solution during the redox transitions of these halogen-containing diaziridinylbenzoquinones. Bioreductive activation of these compounds via NADPH-cytochrome P450 reductase or sulfur nucleophilic substitution was associated with the formation of DNA strand breaks. This process was substantially inhibited (74-86%) by superoxide dismutase and to a lesser extent (23-31%) by catalase. It is suggested that DNA strand breakage proceeds in a manner entailing a semiquinone-dependent reduction of metal-ligands bound at the DNA surface and leading to site-specific, hydroxyl radical production.

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.

The characterization of two biliary glutathione conjugates of amsacrine using liquid secondary ion mass spectrometry

An additional biliary glutathione (GSH) conjugate of the anilinoacridine anti-tumour agent amsacrine (4'-(9-acridinylamino)methanesulphon-m-anisidide, NSC 249992) has been identified in bile collected from male Wistar rats by cannulation of the common bile duct and from male BDF1 mice by removal of the gall bladder after treatment with amsacrine. The presence of this conjugate, at the 6'-position of the anilino ring, has been confirmed by liquid secondary ion (LSI) mass spectrometric analysis of selected biliary metabolites separated by high-performance liquid chromatography. The two major metabolites each gave a daughter ion spectrum which was diagnostic for either 5'- or 6'-GSH conjugation. This pattern was confirmed by comparison with LSI mass spectral data obtained from authentic chemical standards formed on reaction of the quinone diimine derivative of amsacrine with methanethiol or mercaptoethanol.

The role of leukocyte-generated reactive metabolites in the pathogenesis of idiosyncratic drug reactions

Evidence strongly suggests that many adverse drug reactions, including idiosyncratic drug reactions, involve reactive metabolites. Furthermore, certain functional groups, which are readily oxidized to reactive metabolites, are associated with a high incidence of adverse reactions. Most drugs can probably form reactive metabolites, but a simple comparison of covalent binding in vitro is unlikely to provide an accurate indication of the relative risk of a drug causing an idiosyncratic reaction because it does not provide an indication of how efficiently the metabolite is detoxified in vivo. In addition, the incidence and nature of adverse reactions associated with a given drug is probably determined in large measure by the location of reactive metabolite formation, as well as the chemical reactivity of the reactive metabolite. Such factors will determine which macromolecules the metabolites will bind to, and it is known that covalent binding to some proteins, such as those in the leukocyte membrane, is much more likely to lead to an immune-mediated reaction or other type of toxicity. Some reactive metabolites, such as acyl glucuronides, circulate freely and could lead to adverse reactions in almost any organ; however, most reactive metabolites have a short biological half-life, and although small amounts may escape the organ where they are formed, these metabolites are unlikely to reach sufficient concentrations to cause toxicity in other organs. Many idiosyncratic drug reactions involve leukocytes, especially agranulocytosis and drug-induced lupus. We and others have demonstrated that drugs can be metabolized by activated neutrophils and monocytes to reactive metabolites. The major reaction appears to be reaction with leukocyte-generated hypochlorous acid. Hypochlorous acid is quite reactive, and therefore it is likely that many other drugs will be found that are metabolized by activated leukocytes. Some neutrophil precursors contain myeloperoxidase and the NADPH oxidase system, and it is likely that these cells can also oxidize drugs. Therefore, although there is no direct evidence, it is reasonable to speculate that reactive metabolites generated by activated leukocytes, or neutrophil precursors in the bone marrow, could be responsible for drug-induced agranulocytosis and aplastic anemia. This could involve direct toxicity or an immune-mediated reaction. These mechanisms are not mutually exclusive, and it may be that both mechanisms contribute to the toxicity, even in the same patient. In the case of drug-induced lupus, a prevalent hypothesis for lupus involves modification of class II MHC antigens.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutathione metabolism and its role in hepatotoxicity

Glutathione (GSH) fulfills several essential functions: Detoxification of free radicals and toxic oxygen radicals, thiol-disulfide exchange and storage and transfer of cysteine. GSH is present in all mammalian cells, but may be especially important for organs with intense exposure to exogenous toxins such as the liver, kidney, lung and intestine. Within the cell mitochondrial GSH is the main defense against physiological oxidant stress generated by cellular respiration and may be a critical target for toxic oxygen and electrophilic metabolites. Glutathione homeostasis is a highly complex process, which is predominantly regulated by the liver, lung and kidney.

Reactions of the Wurster's blue radical cation with thiols, and some properties of the reaction products

Formation of 1-electron oxidation products of aromatic amines in biological systems have been ascertained. The mechanisms of the toxic actions of the aminyl radicals and their corresponding detoxication reactions are much less established. During the studies of reactions of GSH with the N,N,N',N'-tetramethyl-p-phenylenediamine radical cation (TMPD) (Wurster's blue) two pathways were detected: (1) a slow second order reaction (k = 5 M-1.s-1) which gave the parent amine and (ultimately) GSSG, and (2) a fast, complex reaction which yielded 2-(glutathione-S-yl)-N,N,N',N'-tetramethyl-p-phenylenediamine (2-GS-TMPD). From kinetic reasons, this reaction was suggested to be composed of a rapid disproportionation reaction followed by a reductive 1,4-Michael-addition. This reaction pathway prevailed at GSH concentrations below 1 mM. At higher GSH concentrations formation of the thioether was suppressed. This hypothesis was confirmed when the reaction of the highly labile N,N,N',N'-tetramethyl-p-quinonediiminium dication (TMQDI++) with GSH was followed: In this case, thioether formation outweighed clearly reductive mechanisms, the latter yielding ultimately the amine and GSSG. Similar to N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), 2-GS-TMPD was also capable of producing ferrihemoglobin in a catalytic reaction. Its rate, however, was only 3% that observed with the parent amine. During this reaction the thioether was apparently oxidized to the corresponding quinonediiminium dication, which gave the corresponding quinonemonoimine on acidification.

Molecular mechanisms of quinone cytotoxicity

Quinones are probably found in all respiring animal and plant cells. They are widely used as anticancer, antibacterial or antimalarial drugs and as fungicides. Toxicity can arise as a result of their use as well as by the metabolism of other drugs and various environmental toxins or dietary constituents. In rapidly dividing cells such as tumor cells, cytotoxicity has been attributed to DNA modification. However the molecular basis for the initiation of quinone cytotoxicity in resting or non-dividing cells has been attributed to the alkylation of essential protein thiol or amine groups and/or the oxidation of essential protein thiols by activated oxygen species and/or GSSG. Oxidative stress arises when the quinone is reduced by reductases to a semiquinone radical which reduces oxygen to superoxide radicals and reforms the quinone. This futile redox cycling and oxygen activation forms cytotoxic levels of hydrogen peroxide and GSSG is retained by the cell and causes cytotoxic mixed protein disulfide formation. Most quinones form GSH conjugates which also undergo futile redox cycling and oxygen activation. Prior depletion of cell GSH markedly increases the cell's susceptibility to alkylating quinones but can protect the cell against certain redox cycling quinones. Cytotoxicity induced by hydroquinones in isolated hepatocytes can be attributed to quinones formed by autoxidation. The higher redox potential benzoquinones and naphthoquinones are the most cytotoxic presumably because of their higher electrophilicty and thiol reactivity and/or because the quinones or GSH conjugates are more readily reduced to semiquinones which activate oxygen.

Immunoblot analysis of protein containing 3-(cystein-S-yl)acetaminophen adducts in serum and subcellular liver fractions from acetaminophen-treated mice

The hepatotoxicity of acetaminophen is believed to be mediated by the metabolic activation of acetaminophen to N-acetyl-p-benzoquinone imine which covalently binds to cysteinyl residues on proteins as 3-(cystein-S-yl)acetaminophen adducts. The formation of these adducts in hepatic protein correlates with the hepatotoxicity. In this study, the formation of 3-(cystein-S-yl)acetaminophen adducts in specific cellular proteins was investigated using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and detected using affinity-purified antisera specific for 3-(cystein-S-yl)acetaminophen adducts on immunoblots. These techniques were used to investigate the liver 10,000g supernatant and serum from B6C3F1 mice that received hepatotoxic doses of acetaminophen. More than 15 proteins containing 3-(cystein-S-yl)acetaminophen adducts were detected in the liver 10,000g supernatant. The most prominent protein containing 3-(cystein-S-yl)acetaminophen adducts in the hepatic 10,000g supernatant had a relative molecular mass of 55 kDa. Serum proteins containing 3-(cystein-S-yl)acetaminophen adducts had molecular masses similar to those found in the liver 10,000g supernatant (55, 87, and approximately 102 kDa). These data, combined with our previous findings describing the temporal relationship between the appearance of 3-(cystein-S-yl)acetaminophen adducts in protein in the serum and the decrease in the levels of 3-(cystein-S-yl)acetaminophen adducts in protein in the liver, suggested that liver adducts were released into the serum following lysis of hepatocytes. The temporal relationship between the formation of specific adducts and hepatotoxicity in mice following a hepatotoxic dose of acetaminophen was examined using immunoblots of mitochondria, microsomes, cytosol, and plasma membranes. Hepatotoxicity indicated by serum alanine aminotransferase levels was increased at 2 and 4 hr after dosing. The cytosolic fraction contained numerous proteins with 3-(cystein-S-yl)acetaminophen adducts, the most intensely stained of which was a 55-kDa protein. 3-(Cystein-S-yl)acetaminophen adducts were detected in the 55-kDa liver protein 30 min after dosing and prior to the development of significant toxicity. Examination of gels suggested that maximal levels of immunochemically detectable adducts in the 55-kDa protein occurred at 1-2 hr, with a decrease in intensity 4 hr after dosing. The presence of 3-(cystein-S-yl)acetaminophen adducts in proteins prior to hepatotoxicity suggests a threshold for adduct formation in the development of toxicity. Protein in microsomes which contained 3-(cystein-S-yl)acetaminophen adducts ranged in molecular weight from 38 to approximately 106 kDa. The major proteins containing 3-(cystein-S-yl)acetaminophen adducts in the mitochondria had molecular masses of 39, 50, 68, and 79 kDa.(ABSTRACT TRUNCATED AT 400 WORDS)

Acetaminophen-induced alterations in pancreatic beta cells and serum insulin concentrations in B6C3F1 mice

Administration of acetaminophen (500 mg/kg) to male B6C3F1 mice resulted in alterations of pancreatic beta cell ultrastructure. These alterations were characterized by pronounced intercellular spaces, cytoplasmic vacuolization, damaged membranes of cytoplasm, secretory granules, and other organelles, and pyknotic nuclei with disrupted membranes. Concomitant with these changes, acetaminophen also caused increaes in serum insulin concentrations from 24 microU/ml at 0 time to 160 microU/ml at 8 hr and increases in serum alanine aminotransferase (ALT) concentrations from 42 to 13,279 U/liter, which indicated hepatic damage. Quantitation of 3-(cystein-S-yl)acetaminophen adducts in hepatic 10,000g supernatant protein using a particle concentration fluorescence immunochemical assay indicated a positive correlation between binding and the occurrence of the hepatotoxicity consistent with what has been previously reported; however, 3-(cystein-S-yl)acetaminophen protein adducts were not detected in pancreatic 10,000g supernatant. Immunohistochemical analysis of the liver and pancreas from acetaminophen-treated mice revealed acetaminophen-protein adducts in the centrilobular regions of the liver but not in the pancreatic islets. Doses of 100 and 200 mg/kg produced no evidence of hepatotoxicity and no increase in serum insulin; 300 mg/kg and higher doses produced both hepatotoxicity and increased serum insulin concentrations. A comparison of the time course for the increase in serum levels of ALT and insulin following a toxic dose of acetaminophen indicated that the increase in ALT preceded the increase in insulin. Thus the hepatotoxicity of acetaminophen correlates with the formation of 3-(cystein-S-yl)acetaminophen protein adducts in liver, which supports the concept that this toxicity is mediated by the reactive metabolite N-acetyl-p-benzoquinone imine; however, the toxicity of acetaminophen to beta cells in the pancreas is apparently not mediated by this mechanism.

Bioreductive activation of quinones: redox properties and thiol reactivity

Redox properties and thiol reactivity are central to the therapeutic and toxicological properties of quinones. The use of other physicochemical parameters to establish predictive relationships for redox properties of quinones is discussed, and attention drawn to situations where such relationships may be unreliable. The rates of reaction of semiquinone radicals with oxygen, including those of chemotherapeutic agents such as mitomycin and the anthracyclines, can be predicted with reasonable confidence from the redox properties. The reactions of quinones with thiols such as glutathione produces reduced quinones and radicals, but the reactions are complex and all the features are not well understood.

Utility of solution electrochemistry mass spectrometry for investigating the formation and detection of biologically important conjugates of acetaminophen

On-line formation and detection of glutathione and cysteine conjugates of acetaminophen were accomplished by the interfacing of a coulometric electrochemical cell with a thermospray mass spectrometer in a flow-injection experiment using a liquid chromatographic pump. Formation of the conjugates occurred only after acetaminophen was oxidized electrochemically by a two-electron transfer to N-acetyl-p-benzoquinoneimine and reacted in a mixing tee with either glutathione or cysteine. The newly formed conjugate was detected by thermospray mass spectrometry by observing the [M + H]+ ion for the acetaminophen-glutathione conjugate at m/z 457, or the [M + H]+ ion for the acetaminophen cysteine conjugate at m/z 271. Both the glutathione and cysteine conjugates produced a common fragment ion at m/z 184. The on-line reaction of glutathione and electrochemically generated N-acetyl-p-benzoquinoneimine was monitored at varying pH. At pH 8.5 the ion intensity for the acetaminophen-glutathione conjugate was greater than at lower pH, indicating that lower proton concentration enhanced the reaction of glutathione with N-acetyl-p-benzoquinoneimine. This on-line electrochemical-thermospray mass spectrometric method demonstrated that acetaminophen conjugates may be formed and detected in the time frame of 1 s.

Purification and characterization of a labile rat glutathione transferase of the Mu class

A labile GSH transferase homodimer termed 11-11 was purified from rat testis by GSH-agarose affinity chromatography followed by anion-exchange f.p.l.c. The enzyme is unstable in the absence of thiol(s) and has relatively low affinity for both 1-chloro-2,4-dinitrobenzene (Km 4.4 mM) and GSH (Km(app.) 4.4mM). Its mobility on SDS/polyacrylamide-gel electrophoresis is slightly less than that of subunits 3 and 4 and its pI is 5.2. Subunit 11 has a blocked N-terminal amino acid residue, but after CNBr cleavage fragments accounting for 113 amino acid residues were sequenced and showed 65% homology with corresponding sequences in subunit 4, indicating that it is a member of the Mu family. GSH transferase 11 is a major isoenzyme in testis, epididymis, prostate and brain and present at lower concentrations in other tissues.

Protective role of glutathione and glutathione transferases in mutagenesis and carcinogenesis

Glutathione (GSH) alone detoxifies electrophiles with an effectiveness which depends on the rate of the reaction and the concentration of GSH. If electrophiles are substrates for GSH transferase isoenzymes, the effectiveness of detoxication is much enhanced due to the increased rate of reaction and it is also independent of GSH concentration to low levels of GSH depletion, since the Km for GSH is approximately 0.1 mM. In this paper detoxication of electrophilic metabolites of the hepatocarcinogen N-methyl-4-aminoazobenzene which are not substrates for GSH transferases and the carcinogenic electrophile derived from the hepatocarcinogen aflatoxin B1 which is a poor substrate is compared with detoxication of electrophiles which are good substrates and which although bacterial mutagens are not carcinogenic in organs containing the appropriate GSH transferases. GSH transferases detoxify not only electrophiles derived from xenobiotics, but also endogenous electrophiles which are usually the consequence of free radical damage in the presence of oxygen to lipids and DNA and include lipid and DNA hydroperoxides and alkenals arising from the decomposition of lipid hydroperoxides. Studies in the rat and other mammals show the GSH transferases to be dimers in which the subunits are members of a gene super-family. There are three, perhaps four multigene families namely, alpha containing subunits 1, 2, 8 and 10; mu containing subunits 3, 4, 6 and 9; pi containing subunit 7 and subunits 5 and 5* which are so far unassigned. Subunit 5* is apparently restricted to the nucleus and is noteworthy for its activity towards DNA hydroperoxides. Studies in the human are not as advanced as in the rat but so far reveal close similarities. The ability of GSH transferases to detoxify electrophiles is important in carcinogenesis at a number of points. They may inhibit initiation and tumour proportion, but they may be advantageous to the developing tumour cell, and may be acquired in increased amounts during malignant progression. In many tumour cells the development of lines resistant to anticancer drugs is associated with an increased expression of GSH transferases, particularly GSH transferase pi in human cells.