Reactive intermediates and their toxicological significance

Enzymology of reactive intermediate protection: kinetic analysis and temperature dependence of the mesophilic membrane protein catalyst MGST1

Glutathione transferases (GSTs) are a class of phase II detoxifying enzymes catalysing the conjugation of glutathione (GSH) to endogenous and exogenous electrophilic molecules, with microsomal glutathione transferase 1 (MGST1) being one of its key members. MGST1 forms a homotrimer displaying third-of-the-sites-reactivity and up to 30-fold activation through modification of its Cys-49 residue. It has been shown that the steady-state behaviour of the enzyme at 5 °C can be accounted for by its pre-steady-state behaviour if the presence of a natively activated subpopulation (~ 10%) is assumed. Low temperature was used as the ligand-free enzyme is unstable at higher temperatures. Here, we overcame enzyme lability through stop-flow limited turnover analysis, whereby kinetic parameters at 30 °C were obtained. The acquired data are more physiologically relevant and enable confirmation of the previously established enzyme mechanism (at 5 °C), yielding parameters relevant for in vivo modelling. Interestingly, the kinetic parameter defining toxicant metabolism, k_cat /K_M , is strongly dependent on substrate reactivity (Hammett value 4.2), underscoring that glutathione transferases function as efficient and responsive interception catalysts. The temperature behaviour of the enzyme was also analysed. Both the K_M and K_D values decreased with increasing temperature, while the chemical step k₃ displayed modest temperature dependence (Q₁₀ : 1.1-1.2), mirrored in that of the nonenzymatic reaction (Q₁₀ : 1.1-1.7). Unusually high Q₁₀ values for GSH thiolate anion formation (k₂ : 3.9), k_cat (2.7-5.6) and k_cat /K_M (3.4-5.9) support that large structural transitions govern GSH binding and deprotonation, which limits steady-state catalysis.

Developmental differences in memory reactivation relate to encoding and inference in the human brain

Despite the fact that children can draw on their memories to make novel inferences, it is unknown whether they do so through the same neural mechanisms as adults. We measured memory reinstatement as participants aged 7-30 years learned new, related information. While adults brought memories to mind throughout learning, adolescents did so only transiently, and children not at all. Analysis of trial-wise variability in reactivation showed that discrepant neural mechanisms-and in particular, what we interpret as suppression of interfering memories during learning in early adolescence-are nevertheless beneficial for later inference at each developmental stage. These results suggest that while adults build integrated memories well-suited to informing inference directly, children and adolescents instead must rely on separate memories to be individually referenced at the time of inference decisions.

Proteomic analysis suggests that monoterpenes in lemongrass disrupt Ca2+ homeostasis in Haemaphysalis longicornis leading to mitochondrial depolarization and cytotoxicity

Complex mixtures of bioactive ingredients in plant essential oils present complex chemistries which involve different modes of action. An increasing body of scientific reports has recently focused on the acaricidal activities of plant essential oils attributed to their monoterpene components, but information about their underlying molecular mechanism of action is scarce. Here, after the chemical analysis of lemongrass oil, a proteomic analysis of the ovary, salivary gland, and midgut of Haemaphysalis longicornis exposed to Cymbopogon citratus (lemongrass) essential oil was performed via data-independent acquisition mass spectrometry (DIA-MS) technology to further elucidate the molecular mechanisms involved. Pathway analysis reveals the activation of metabolic pathways mediated by oxidoreductases and transferases. Furthermore, the upregulation of various calcium-associated proteins and the upregulation of cytochrome c1, cytochrome c oxidase polypeptide IV, and programmed cell death protein 6-like isoform X1 suggest a cytotoxic mode of action via the formation of reactive oxygen species (ROS), mitochondrial Ca²⁺ overload, mitochondrial uncoupling, and depolarization, and ATP depletion leading to either apoptotic or necrotic death. Morphological alterations observed after the RNAi of a major detoxification enzyme (glutathione S-transferase) merit further investigation. Hence, the cytotoxic mode of action exhibited by C. citratus oil could be vital for the development of eco-friendly acaricide.

Transcriptome profile of Haemaphysalis longicornis (Acari: Ixodidae) exposed to Cymbopogon citratus essential oil and citronellal suggest a cytotoxic mode of action involving mitochondrial Ca2+ overload and depolarization

Haemaphysalis longicornis is an ixodid tick species of medical and veterinary importance. Investigation into the acaricidal activities of botanicals have increased recently but information about their molecular mechanism of action is scarce. Here, RNA-seq analysis of the ticks exposed to Cymbopogon citratus essential oil and citronellal was performed and the responsive genes were identified. More than 6.39 G clean reads with Q₂₀ ≥ 94.88% were obtained for each H. longicornis sample, with an average GC content of 50.94%. Using the Trinity method, 166,710 unigenes with a mean length of 869 bp and a maximum contig length of 29,156 bp were obtained. The upregulation of genes was concentration-dependent in most of the treated groups. Many genes responsive to C. citratus oil and citronellal were stress-related and they include genes associated with adrenergic signaling/calcium channels, cGMP-PKG signaling, apoptosis, focal adhesion, ECM-receptor interaction, ubiquitin-mediated proteolysis, mTOR signaling pathway, and longevity regulating pathway. The upregulation of genes (CACNAID, ADCY9, TPM1, and MYH6) associated with calcium channels suggests a neurotoxic mode of action, whereas, the upregulation of apoptosis-associated genes (CYC, DRONC, CASP7, CASP9, BCL2L1, bcl-xL, etc.) suggests a cytotoxic mode of action. The metabolism of C. citratus essential oil generates oxidative stress which increases the intra-mitochondrial free Ca²⁺ and triggers the formation of reactive oxygen species (ROS) that culminates to mitochondrial depolarization, ATP depletion, and either necrotic or apoptotic death. The neurotoxic and cytotoxic effects exhibited by the monoterpenes in H. longicornis is vital and could be exploited for the advancement of acaricide development and eco-friendly tick control.

Magnetic Resonance Imaging and Spectroscopy: New Noninvasive In Vivo Approaches in Toxicology Research

Magnetic Resonance Imaging (MRI) and Spectroscopy (MRS) give anatomical and biochemical information about a human patient or animal in a non-invasive manner. This unique quality permits the study of toxicological responses of an organ within an intact animal and in a manner in which many fewer animals are needed than by conventional methods of investigation. The use of MRI and MRS in the study of hepatotoxicants, particularly bromobenzene and ethanol, is reviewed. Bromobenzene causes localised hepatic oedema and bioenergetic deterioration; these changes were followed with time by 1H MRI and 31P MRS, respectively. Phosphocholine levels in the liver were found to increase dramatically during bromobenzene-induced damage, possibly related to an intracellular control mechanism in response to tissue damage. The ability of the bromobenzene-challenged liver to metabolise a fructose load was followed by dynamic 31P MRS. Chronic ethanol administration damages the liver. This toxicological process results in the accumulation of fat in the liver, which was followed by fat-selective 1H MRI. When ethanol is no longer administered to the subject, the fatty infiltration subsides, and this process was followed over 16 days in the same animal using fat-selective 1H MRI. Chronic ethanol renders the liver in situ more susceptible to hypoxic injury and less likely to recover afterwards, as shown by 31P MRS.

Transgenic mice and metabolomics for study of hepatic xenobiotic metabolism and toxicity

INTRODUCTION

The study of xenobiotic metabolism and toxicity has been greatly aided by the use of genetically modified mouse models and metabolomics.

AREAS COVERED

Gene knockout mice can be used to determine the enzymes responsible for the metabolism of xenobiotics in vivo and to examine the mechanisms of xenobiotic-induced toxicity. Humanized mouse models are especially important because there exist marked species differences in the xenobiotic-metabolizing enzymes and the nuclear receptors that regulate these enzymes. Humanized mice expressing CYPs and nuclear receptors including the pregnane X receptor, the major regulator of xenobiotic metabolism and transport were produced. With genetically modified mouse models, metabolomics can determine the metabolic map of many xenobiotics with a level of sensitivity that allows the discovery of even minor metabolites. This technology can be used for determining the mechanism of xenobiotic toxicity and to find early biomarkers for toxicity.

EXPERT OPINION

Metabolomics and genetically modified mouse models can be used for the study of xenobiotic metabolism and toxicity by: i) comparison of the metabolomics profiles between wild-type and genetically modified mice, and searching for genotype-dependent endogenous metabolites; ii) searching for and elucidating metabolites derived from xenobiotics; and iii) discovery of specific alterations of endogenous compounds induced by xenobiotics-induced toxicity.

Integrated biomarker responses in zebrafish exposed to sulfonamides

Dispersed pharmaceuticals such as sulfonamides pose a threat to aquatic ecosystems. We evaluated potential biomarkers of sulfonamide exposure using an extended zebrafish (Danio rerio) toxicity test. The tested sulfonamides induced obvious effects on spontaneous swimming activity and heartbeat rate in zebrafish. Glutathione S-transferase (GST) and malondialdehyde (MDA) were examined to reflect the biomarker response of zebrafish exposed to three sulfonamides (sulfamethoxazole, sulfadiazine (SDZ) and sulfadimidine). Both GST and MDA showed time-dependent responses to sulfonamide exposure. GST activity was significantly increased after exposure to sulfonamides for 3 days, while MDA concentration reached a maximum during the first day and then declined. These results suggest that MDA may be a more sensitive biomarker of sulfonamide toxicity than GST. These investigations demonstrated that SDZ was a typical inducer of metabolic enzymes, suggesting that it poses a potential ecotoxicological risk to aquatic ecosystems.

Cytochrome P450-mediated metabolism in the human gut wall

Objective

Although the human small intestine serves primarily as an absorptive organ for nutrients and water, it also has the ability to metabolise drugs. Interest in the small intestine as a drug-metabolising organ has been increasing since the realisation that it is probably the most important extrahepatic site of drug biotransformation.

Key findings

Among the metabolising enzymes present in the small intestinal mucosa, the cytochromes P450 (CYPs) are of particular importance, being responsible for the majority of phase I drug metabolism reactions. Many drug interactions involving induction or inhibition of CYP enzymes, in particular CYP3A, have been proposed to occur substantially at the level of the intestine rather than exclusively within the liver, as originally thought. CYP3A and CYP2C represent the major intestinal CYPs, accounting for approximately 80% and 18%, respectively, of total immunoquantified CYPs. CYP2J2 is also consistently expressed in the human gut wall. In the case of CYP1A1, large interindividual variation in the expression levels has been reported. Data for the intestinal expression of the polymorphic CYP2D6 are conflicting. Several other CYPs, including the common hepatic isoform CYP2E1, are expressed in the human small intestine to only a very low extent, if at all. The distribution of most CYP enzymes is not uniform along the human gastrointestinal tract, being generally higher in the proximal regions of the small intestine.

Summary

This article reviews the current state of knowledge of CYP enzyme expression in human small intestine, the role of the gut wall in CYP-mediated metabolism, and how this metabolism limits the bioavailability of orally administered drugs. Possible interactions between drugs and CYP activity in the small intestine are also discussed.

A small-molecule probe for hepatitis C virus replication that blocks protein folding

The hepatitis C virus (HCV) is a growing global health problem. Small molecules that interfere with host-viral interactions can serve as powerful tools for elucidating the molecular mechanisms of pathogenesis and defining new strategies for therapeutic development. Using a cell-based screen involving subgenomic HCV replicons, we identified the ability of 18 different abscisic acid (ABA) analogs, originally developed as plant growth regulators, to inhibit HCV replication. Three of these were further studied. One compound, here named origamicin, showed antiviral activity through the inhibition of host proteins involved in protein folding. Origamicin could therefore be an important tool for studying the maturation of both host and viral proteins. Herein we demonstrate an application for molecular scaffolds based on ABA for mammalian cell targets involved in protein folding.

Toxicity of hepatotoxins: new insights into mechanisms and therapy

Liver injury caused by hepatotoxins, such as carbon tetrachloride (CCl4), ethanol, and acetaminophen (APAP), is characterised by varying degrees of hepatocyte degeneration and cell death via either apoptosis or necrosis. The generation of reactive intermediate metabolites from the metabolism of hepatotoxins, and the occurrence of reactive oxygen species (ROS) during the inflammatory reaction account for a variety of pathophysiologic pathways leading to cell death, such as covalent binding, disordered cytosolic calcium homeostasis, glutathione (GSH) depletion, onset of mitochondrial permeability transition (MPT) and associated lipid peroxidation. The metabolism of hepatotoxins by cytochrome P-450 enzyme subtypes is a key step of the intoxication; therefore, enzyme inhibitors are shown to minimise the hepatotoxin-associated liver damage. Understanding the function of transcription factors, such as nuclear factor kappaB (NF-kappaB) in acute liver injury, may provide some answers as to the molecular mechanisms of toxic insults. Moreover, substantial evidence exists that MPT is involved in ROS-associated hepatocellular injury and new findings offer a novel therapeutic approach to attenuate cell damage by blocking the onset of MPT. Thus, oxidant stress and lipid peroxidation are crucial elements leading to hepatotoxin-associated liver injury. In addition to specific treatment for a given hepatotoxin, the general strategy for prevention and treatment of the damage includes reducing the production of reactive metabolites of the hepatotoxins, using anti-oxidative agents, and selectively targeting therapeutics to Kupffer cells or hepatocytes for on-going processes, which play a role in mediating a second phase of the injury.

Mechanism-based inactivation of cytochrome P450 3A4 by 4-ipomeanol

Earlier phase I and II clinical studies showed that 4-ipomeanol produced selective hepatotoxicity. To investigate the mechanism of bioactivation of 4-ipomeanol, we thoroughly studied the interaction of 4-ipomeanol with human cytochrome P450 3A4 (EC 1.14.14.1). 4-Ipomeanol produced a time- and concentration-dependent inactivation of P450 3A4. More than 80% of the P450 3A4 activity was lost after its incubation with 4-ipomeanol at the concentration of 75 microM in 12 min. The inactivation was characterized by a rate of inactivation (kinact) of 0.15 min(-1) and by an inactivation potency (KI) of 20 microM. In addition, the inhibition of P450 3A4 by 4-ipomeanol was NADPH-dependent and irreversible. Glutathione, catalase, and superoxide dismutase failed to protect P450 3A4 from inactivation by 4-ipomeanol. The presence of testosterone, a substrate of P450 3A4, protected the enzyme from inactivation. The estimated partition ratio of the inactivation was approximately 257. Covalent binding studies demonstrated that reactive metabolites of 4-ipomeanol modified P450 3A4 but not P450 reductase (EC 1.6.2.4). The stoichiometry of binding between reactive metabolites of radiolabeled 4-ipomeanol and P450 3A4 was approximately 1.5:1. In addition to P450 3A4, reactive metabolites of 4-ipomeanol were found to covalently bind to other proteins. 4-Ipomeanol failed to inactivate P450 1A2 in human liver microsomes. In conclusion, 4-ipomeanol irreversibly inhibited P450 3A4, and it was characterized as a mechanism-based inactivator of P450 3A4. This finding facilitates the understanding of the mechanism of bioactivation of 4-ipomeanol by human hepatic enzymes.

Hepatotoxicity and mechanism of action of haloalkanes: carbon tetrachloride as a toxicological model

The use of many halogenated alkanes such as carbon tetrachloride (CCl4), chloroform (CHCl3) or iodoform (CHI3), has been banned or severely restricted because of their distinct toxicity. Yet CCl4 continues to provide an important service today as a model substance to elucidate the mechanisms of action of hepatotoxic effects such as fatty degeneration, fibrosis, hepatocellular death, and carcinogenicity. In a matter of dose,exposure time, presence of potentiating agents, or age of the affected organism, regeneration can take place and lead to full recovery from liver damage. CCl4 is activated by cytochrome (CYP)2E1, CYP2B1 or CYP2B2, and possibly CYP3A, to form the trichloromethyl radical, CCl3*. This radical can bind to cellular molecules (nucleic acid, protein, lipid), impairing crucial cellular processes such as lipid metabolism, with the potential outcome of fatty degeneration (steatosis). Adduct formation between CCl3* and DNA is thought to function as initiator of hepatic cancer. This radical can also react with oxygen to form the trichloromethylperoxy radical CCl3OO*, a highly reactive species. CCl3OO* initiates the chain reaction of lipid peroxidation, which attacks and destroys polyunsaturated fatty acids, in particular those associated with phospholipids. This affects the permeabilities of mitochondrial, endoplasmic reticulum, and plasma membranes, resulting in the loss of cellular calcium sequestration and homeostasis, which can contribute heavily to subsequent cell damage. Among the degradation products of fatty acids are reactive aldehydes, especially 4-hydroxynonenal, which bind easily to functional groups of proteins and inhibit important enzyme activities. CCl4 intoxication also leads to hypomethylation of cellular components; in the case of RNA the outcome is thought to be inhibition of protein synthesis, in the case of phospholipids it plays a role in the inhibition of lipoprotein secretion. None of these processes per se is considered the ultimate cause of CCl4-induced cell death; it is by cooperation that they achieve a fatal outcome, provided the toxicant acts in a high single dose, or over longer periods of time at low doses. At the molecular level CCl4 activates tumor necrosis factor (TNF)alpha, nitric oxide (NO), and transforming growth factors (TGF)-alpha and -beta in the cell, processes that appear to direct the cell primarily toward (self-)destruction or fibrosis. TNFalpha pushes toward apoptosis, whereas the TGFs appear to direct toward fibrosis. Interleukin (IL)-6, although induced by TNFalpha, has a clearly antiapoptotic effect, and IL-10 also counteracts TNFalpha action. Thus, both interleukins have the potential to initiate recovery of the CCl4-damaged hepatocyte. Several of the above-mentioned toxication processes can be specifically interrupted with the use of antioxidants and mitogens, respectively, by restoring cellular methylation, or by preserving calcium sequestration. Chemicals that induce cytochromes that metabolize CCl4, or delay tissue regeneration when co-administered with CCl4 will potentiate its toxicity thoroughly, while appropriate CYP450 inhibitors will alleviate much of the toxicity. Oxygen partial pressure can also direct the course of CCl4 hepatotoxicity. Pressures between 5 and 35 mmHg favor lipid peroxidation, whereas absence of oxygen, as well as a partial pressure above 100 mmHg, both prevent lipid peroxidation entirely. Consequently, the location of CCl4-induced damage mirrors the oxygen gradient across the liver lobule. Mixed halogenated methanes and ethanes, found as so-called disinfection byproducts at low concentration in drinking water, elicit symptoms of toxicity very similar to carbon tetrachloride, including carcinogenicity.

Potential roles of hepatic heat shock protein 25 and 70i in protection of mice against acetaminophen-induced liver injury

The aim of the present study was to assess the contribution of the level of expression of heat shock protein 25 (HSP25), 60 (HSP60), 70 (HSC70) and 70i (HSP70i) in mouse livers after a lethal dose of acetaminophen (APAP) to their survival. We examined changes in survival ratio, plasma APAP level and alanine aminotransferase (ALT) activity, and hepatic reduced glutathione (GSH), HSP25, HSP60, HSC70 and HSP70i levels following treatment of mice with APAP (500 mg/kg, p.o.). The plasma APAP level increased rapidly, and reached a maximum 0.5 h after APAP treatment. Hepatic GSH decreased rapidly, and was almost completely depleted 1 h after APAP treatment. Plasma ALT activity, an index of liver injury, significantly increased from 3 h onwards after APAP treatment. The survival ratios 9 h, 24 h and 48 h after APAP treatment were 96%, 38% and 36%, respectively. We found a remarkable difference in the patterns of hepatic HSP25 and HSP70i induction in mice that survived after APAP treatment. HSP70i levels increased from 1 h onwards after APAP treatment in a time-dependent manner, and reached a maximum at 9 h. In contrast, HSP25 could be detected just 24 h after APAP treatment, and maximal accumulation was observed at 48 h. Other HSPs examined were unchanged. Notably, the survival ratio dropped by only 2% after HSP25 expression. Recently, a novel role for HSP25 as an anti-inflammatory factor was suggested. We have already shown that 48-h treatment with APAP induces severe centrilobular necrosis with inflammatory cell infiltration in mouse livers. Taken together, the level of expression of hepatic HSP25 may be a crucial determinant of the fate of mice exposed to APAP insult.

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Production of intracellular 35S-glutathione by rat and human hepatocytes for the quantification of xenobiotic reactive intermediates

The quantification and identification of xenobiotic reactive intermediates is difficult in the absence of highly radiolabeled drug. We have developed a method for identifying these intermediates by measuring the formation of adducts to intracellularly generated radiolabeled glutathione (GSH). Freshly isolated adherent rat and human hepatocytes were incubated overnight in methionine and cystine-free ('thio-free') medium. They were then exposed to 100 microM methionine and 10 microCi 35S-labeled methionine in otherwise thio-free medium to replete cellular GSH pools with intracellularly generated 35S-labeled GSH. After 3 h, acetaminophen was added as a test compound and the cells were incubated for an additional 24 h. Intracellular GSH and its specific activity were quantified after reaction with monobromobimane followed by HPLC analysis with fluorescence and radiochemical detection. Radiolabeled GSH was detectable at 3 h and maintained high specific activity and physiological concentrations for up to 24 h. Incubation medium from acetaminophen treated and nontreated hepatocytes were analyzed for radiolabeled peaks by HPLC using radiochemical detection. Radiolabeled peaks not present in nontreated hepatocytes were identified as acetaminophen GSH adducts by LC-MS. Formation of acetaminophen 35S-GSH adducts by rat hepatocytes containing endogenously synthesized 35S-GSH was increased with acetaminophen concentrations ranging from 500 to 2 mM.

Identification of seven proteins in the endoplasmic reticulum as targets for reactive metabolites of bromobenzene

The hepatotoxicity of bromobenzene is strongly correlated with the covalent binding of chemically reactive metabolites to cellular proteins, but up to now relatively few hepatic protein targets of these reactive metabolites have been identified. To identify additional hepatic protein targets we injected an hepatotoxic dose of [14C]bromobenzene to phenobarbital-pretreated male Sprague-Dawley rats ip. After 4 h, their livers were removed and homogenized, and the homogenates fractionated by differential ultracentrifugation. The highest specific radiolabeling (6.1 nmol equiv 14C/mg of protein) was observed in a particulate fraction (P25) sedimented at 25000g from a 6000g supernatant fraction. Proteins in this fraction were separated by two-dimensional electrophoresis and, after transblotting, analyzed for radioactivity by phosphorimaging. More than 20 radiolabeled protein spots were observed in the blots. For 17 of these spots, peptide mass maps were obtained using in-gel digestion with trypsin, followed by MALDI-TOF mass spectrometric analysis of the resulting peptide mixtures. By searching genomic databases, the 17 sets of MS-derived peptide masses were found to match predicted tryptic fragments of just 7 proteins. Spots 1-4 matched with 78 kDa glucose regulated protein (GRP78), protein disulfide isomerase isozyme A1 (PDIA1), endoplasmic reticulum protein ERp29, and PDIA6, respectively. Spots 5 and 6, 7-11, and 12-17 presented as apparent "charge trains" of spots, each of which gave peptide mixtures closely similar to those of other spots within the train. The proteins present in these sets of spots were identified as transthyretin, serum albumin precursor and PDIA3, respectively. The possible relationship of the adduction of these proteins to the toxicological outcome is discussed.

Identification of three protein targets for reactive metabolites of bromobenzene in rat liver cytosol

The hepatotoxicity of bromobenzene and many other simple organic chemicals is believed to be associated with covalent binding of chemically reactive metabolites to cellular proteins. Recently, a rat liver microsomal esterase was shown to be targeted by bromobenzene metabolites formed in vitro [Rombach, E. M., and Hanzlik, R. P. (1998) Chem. Res. Toxicol. 11, 178-184]. To identify protein targets for bromobenzene metabolites in cytosol, we incubated liver microsomes and glutathione-depleted liver cytosol from phenobarbital-treated rats with [(14)C]bromobenzene in vitro. In a separate experiment, we intraperitoneally injected a hepatotoxic dose of [(14)C]bromobenzene to phenobarbital-treated rats. The cytosol fractions from both experiments were recovered and analyzed for protein-bound radioactivity. Under the conditions that were used, 2.6 and 3.9 nmolar equiv of bromobenzene/mg of cytosolic protein was bound in vitro and in vivo, respectively. Denaturing polyacrylamide gel electrophoresis of these cytosolic proteins followed by phosphor imaging analysis revealed several radiolabeled protein bands over a broad molecular mass range, the patterns observed in vitro and in vivo being generally similar to each other. Cytosolic proteins labeled in vitro were separated by ion exchange chromatography and electrophoresis, and three major radioactive bands with estimated molecular masses of ca. 14, 25, and 30 kDa were in-gel digested with trypsin, followed by on-line HPLC electrospray ionization mass spectrometry of the resulting peptide mixtures. For the three protein bands, the observed peptide masses were found to match the predicted tryptic fragments of liver fatty acid binding protein, glutathione transferase subunit A1, and carbonic anhydrase isoform III, respectively, with 83, 45, and 59% coverage of the corresponding complete sequences. The possible relationship of the adduction of these proteins to the toxicological outcome is discussed.

Biotransformation of styrene in mice. Stereochemical aspects

Biotransformation of styrene and its toxic metabolite, phenyloxirane (1), in mice in vivo was studied. Mice were treated with single intraperitoneal doses of styrene (400 mg/kg of body weight), and with (R)-, (S)-, or racemic styrene oxide (150 mg/kg of body weight). Profiles of neutral and acidic metabolites were determined by GC/MS. Mandelic acid (3) and two mercapturic acids, N-acetyl-S-(2-hydroxy-2-phenylethyl)cysteine (5) and N-acetyl-S-(2-hydroxy-1-phenylethyl)cysteine (6), were found to be major urinary metabolites of both styrene and phenyloxirane. 1-Phenylethane-1,2-diol (2) was the main neutral metabolite. The rate of excretion of this metabolite, as determined by GC, was 5-10 times lower than that of mandelic acid. Several minor acidic metabolites were also identified. Among them, novel phenolic metabolites, namely, 2-(4-hydroxyphenyl)ethanol (7), (4-hydroxyphenyl)acetic acid (11), and two isomeric hydroxymandelic acids (12), are of toxicological significance. Main stereogenic metabolites were isolated as methyl esters from extracts of pooled acidified urine treated with diazomethane. The mandelic acid that was obtained was converted to diastereomeric Mosher's derivatives prior to analysis by NMR. Mercapturic acids were analyzed directly by (13)C NMR. Pure enantiomers of 1 were metabolized predominantly but not exclusively to corresponding enantiomers of 3. Styrene yielded predominantly (S)-mandelic acid. Fractions of mercapturic acids 5 and 6 isolated from urine amounted to 12-15% of the dose for all compounds that were administered. Conversion to mercapturic acids was highly regio- and stereoselective, yielding predominantly regioisomer 5. Styrene, as compared to racemic phenyloxirane, yielded slightly more diastereomers arising from (S)-1 than from (R)-1. These data can be explained by formation of a moderate excess of the less mutagenic (S)-1 in the metabolic activation of styrene in mice in vivo.

The disposition and metabolism of 1,3-dibromobenzene in the rat

The distribution, excretion and metabolism of 1,3-dibromobenzene following a single i.p. administration to rats 100 or 300 mg/kg was investigated using radiotracer [3H] and GC-MS technique. After 72 hours about 74 to 90% were excreted in urine. The highest radioactivity was observed in the liver, kidneys and fat tissue. Later on a steady decline of radioactivity was apparent in all investigated tissues except for blood cells and the sciatic nerve, where constant levels were noted. In urine the following substances were identified and quantified by GC peak areas: unchanged 1,3-DBB (18%), dibromophenols (34%), dibromothiophenols (28%), dibromothioanisole (1.8%), bromophenol (5.5%), bromohydroxythiophenols (5%), and bromohydroxythioanisole (7.5%).

The nature of halogen substitution determines the mode of cytotoxicity of halopropanols

The cytochrome P450-dependent generation of reactive metabolites from 1,3-dichloropropanol and 1,3-dibromopropanol was assessed in a microsomal thiol depletion assay, while the toxicity of these compounds was assessed in rat hepatocyte cultures and in the 3T3 cell line. Thiol-depleting metabolites of both compounds were generated in the microsomal assay; however, only dibromopropanol extensively depleted glutathione when glutathione S-transferase was used as the enzyme source. The cytotoxicity of dichloropropanol was both cytochrome P450- and glutathione-dependent, whereas that of dibromopropanol was glutathione-dependent but largely independent of cytochrome P450. These results indicate that the mechanisms underlying the cytotoxicity of halopropanols are dependent on the nature of the halogen substitution and that microsomal and cellular assays for reactive metabolite generation may yield conflicting results.

Identification of a rat liver microsomal esterase as a target protein for bromobenzene metabolites

The hepatotoxicity of bromobenzene and many other simple organic molecules has been associated with their biotransformation to chemically reactive metabolites and the subsequent covalent binding of those metabolites to cellular macromolecules. To identify proteins targeted by bromobenzene metabolites, we incubated [14C]bromobenzene in vitro with liver microsomes from phenobarbital-induced rats under conditions which typically led to covalent binding of 2-4 nmol equiv of bromobenzene/mg of protein. Microsomal proteins were solubilized with detergent, separated by chromatography and electrophoresis, and analyzed for 14C by phosphorimaging of stained blots. Much of the radioactivity was associated with several bands of proteins of ca. 50-60 kDa, plus another prominent band around 70 kDa, but labeling density appeared to vary considerably overall. A major radiolabeled protein was purified by preparative electrophoresis and submitted to automated Edman microsequencing. Its N-terminal sequence was found to correspond to that of a known rat liver microsomal carboxylesterase (E.C. 3.1.1.1) previously identified as a target for reactive metabolites of halothane. The extent to which covalent modification of this protein by reactive metabolites contributes to the production of hepatotoxic effects remains to be determined.

Metabolism and toxicity of 2-methylpropene (isobutene)--a review

2-Methylpropene (MP) or isobutene is a gaseous chemical used on a large scale in the synthetic rubber industry. The present review covers the rather scarce literature on MP with respect to its metabolic fate and toxicity in laboratory animals and humans. It has been shown both in vivo and in vitro that MP is metabolized to the primary metabolite 2-methyl-1,2-epoxypropane (MEP) by rodent and human liver tissue. The formation of this reactive epoxide intermediate is catalyzed by CYP2E1, while epoxide hydrolase and glutathione S-transferase appear to be involved in its inactivation. In rats, the capacity to absorb and metabolize MP is saturable. MP is oxidized to compounds that are mainly excreted in urine. Data indicate that rodents can tolerate low levels of MP without apparent toxicity. The primary metabolite MEP, however, is able to produce genetic damage in both prokaryotic and eukaryotic cells in vitro. MP is thus not toxic per se but elicits metabolic activation to become potentially harmful. Consequently, the balance between formation and detoxification of MEP plays a key role in determining the potential toxicity of the parent compound. Obviously, further research, including repeated exposure toxicity studies, is required before an estimation of the risk for man can be made.

Hepatotoxicity in drug development: detection, significance and solutions

Despite considerable progress in the understanding of the mechanism of liver toxicity we are not yet able to design non-hepatotoxic molecules rationally. Also, there is no "universal" in vitro primary screening approach for early identification of hepatotoxic molecules. In most cases hepatotoxicity is detected at later stages of drug development in animal toxicity studies or clinical trials. Although the liver is the most common target organ for drug candidates in animal toxicity studies, hepatotoxicity rarely leads to cessation of drug development during the preclinical phase. Indeed, contrary to other target organs, liver toxicity is usually reversible and can be monitored in man by sensitive serum enzyme tests. Therefore in many cases a compound found hepatotoxic in an animal species will be tested in man for a definitive assessment of its hepatotoxic potential. Liver toxicity in man may be acceptable when a drug has major therapeutic potential. In this situation mechanistic studies are essential to assess the risk in man and in some cases to identify protective agents. When liver toxicity leads to project termination a secondary screening approach may be envisaged if biologically active analogs are available.

Protective effects of trolox C, vitamin C, and catalase on bromobenzene-induced damage to rat hepatocytes

BACKGROUND/METHODS

The protective effects of trolox C (water-soluble vitamin E), vitamin C, and catalase on bromobenzene (BB)-induced toxicity to isolated rat hepatocytes were evaluated. The glutathione (GSH) content of the hepatocytes exposed to BB was measured.

RESULTS

BB caused acute damage to the cells during 2 h of incubation (short) when BB was added directly to the culture wells, whereas a late-occurring and time-dependent increase in lactate dehydrogenase (LDH) leakage rate was observed during 24 h of incubation (long) when BB was dissolved in a different way. Incubation of the cells with trolox C (0.5-2.0 mM) prevented the hepatocellular damage induced by BB at 2.4 mM during the long-term incubation. Vitamin C (0.1-1.0 mM) had a protective effect on BB-induced toxicity during both the short- (BB, 1.6 mM) and the long- (BB, 2.4 mM) term incubations. Catalase (3200 U/ml) also showed a beneficial effect on the cells during the short-term BB exposure. Trolox C (2.0 mM) and vitamin C (0.5 mM) restored BB-induced GSH depletion in the cells.

CONCLUSIONS

BB induced two patterns of LDH leakage from isolated hepatocytes on the basis of different ways of BB exposure and incubation periods. Trolox C, vitamin C, and catalase exerted protective effects on BB-induced toxicity during short- or/and long-term incubations. The effects were concentration-dependent. Restoration of GSH content in BB-exposed hepatocytes suggests that trolox C and vitamin C could reduce GSH consumption during BB metabolism and exert an antioxidant effect.

Comparative metabolism and disposition of ethoxyquin in rat and mouse. II. Metabolism

1. The major pathways of ethoxyquin (EQ) metabolism in both the rat and mouse are O-deethylation and conjugation to endogenous substrates. 2. The two major EQ-derived metabolites excreted in rat urine were in the form of sulphate conjugates, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline sulphate, and 1,2,3,4-tetrahydro-3,6-dihydroxy-4-methylene-2,2-dimethylquinoline sulphate. The latter apparently arises from an intramolecular rearrangement of the 3,4-epoxide of ethoxyquin. 3. Mouse urine contained one major glucuronide, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline glucuronide as well as one major sulphate conjugate, 1,2-dihydro-6-hydroxy-2,2,4-trimethylquinoline sulphate. 4. EQ-derived radioactivity was excreted in rat bile, mainly as GSH conjugates, with little unchanged EQ present. Two of the biliary metabolites are glutathione conjugates of ethoxyquin 3,4-epoxide; the third appears to be a conjugate of either ethoxyquin 7,8-epoxide or 2,2,4-trimethylquinol-6-one.

Toxicity of dichloropropanols in rat hepatocyte cultues

Exposure of humans to dichloropropanols has been shown to result in fulminant hepatic necrosis. These compounds have also been shown to be hepatotoxic in rats. In this study, 1,3-dichloropropanol, but not 2,3-dichloropropanol, was shown to be toxic to 24 h cultures of rat hepatocytes. The toxicity was inhibited by pre-treatment of cultures with a cytochrome P450 inhibitor and enhanced by prior depletion of cellular glutathione. In addition, at equimolar concentrations both isomers were shown to deplete glutathione, although the extent of depletion was greater with the 1,3-isomer. 1,3-Dichloropropanol also depleted ATP and reduced the mitochondrial membrane potential. The effects on ATP, glutathione and membrane potential could be inhibited by the cytochrome P450 inhibitor. It is concluded that the toxicity of 1,3-dichloropropanol is mediated by cytochrome P450 and involves depletion of glutathione and loss of mitochondrial function.

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.

Hepatobiliary function and toxicity in vitro using isolated hepatocyte couplets

1. Hepatocyte couplets can be routinely prepared from rat liver to produce a suitable in vitro model for polarized primary cells. 2. Centrifugal elutriation provides a means of producing enriched subpopulations of periportal and perivenous couplets from the same liver, thus providing a means of studying the influence of zonal heterogeneity on hepatobiliary function. 3. The maintenance of structural and secretory polarity demonstrated by hepatocyte couplets provides a convenient in vitro system for mechanistic studies of factors both regulatory and adversely affecting hepatobiliary functions. 4. Couplets are also uniquely appropriate for specific studies of regulation at the biliary pole, on the performance of junctions and on the maintenance and rate of transcytotic movement. 5. The possibility also exists that effects of an in vivo pre-exposure to agents causing hepatobiliary dysfunction can be assessed in couplets ex vivo.

Protective effects of calcium channel blockers on acute bromobenzene toxicity to isolated rat hepatocytes. Inhibition of phenylephrine-induced calcium oscillations

BACKGROUND AND METHODS

Protective effects of verapamil, nifedipine, diltiazem, and ethylene glycol tetraacetic acid (EGTA) on acute bromobenzene (BB) toxicity to rat hepatocytes were evaluated, and cytosolic [Ca2+]i was monitored in single BB-exposed rat hepatocytes. Additionally, the effect of nifedipine on phenylephrine-stimulated calcium oscillations was investigated.

RESULTS

BB at 0.8-2.4 mM increased the lactate dehydrogenase (LDH) leakage rate dose-dependently. Pretreatment with verapamil (25-35 microM), nifedipine (35-45 microM), diltiazem (25 microM), or EGTA (1.5-5 mM) markedly attenuated the BB-induced (1.6 mM) LDH leakage rate during 2 h of incubations. BB did not cause any detectable acute change in [Ca2+]i. BB interfered with phenylephrine-stimulated calcium oscillations, by blocking the oscillations in 58% of the cells and reducing the oscillation frequency in the rest. Nifedipine (100 and 200 microM) blocked the phenylephrine-induced calcium oscillations completely in 55% and 88% of the cells, respectively.

CONCLUSIONS

The findings demonstrate that verapamil, nifedipine, diltiazem, and EGTA significantly protect rat hepatocytes against BB toxicity. BB interferes with phenylephrine-stimulated calcium oscillations. Nifedipine inhibits the oscillations at doses higher than those exerting a protective effect.

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.

Interindividual variation in biotransformation and cytotoxicity of bromobenzene as determined in primary hepatocyte cultures derived from monkey and human liver

1. Bromobenzene-evoked hepatotoxicity resulting from cytochrome P450-mediated epoxidation has been studied extensively in rodents in vivo and in rodent hepatocytes. In this paper we present data concerning the formation of bromphenols, glutathione (GSH) depletion and cytotoxicity observed in primate hepatocytes in primary culture after exposure to bromobenzene (BrB). 2. After pre-incubation for 2 or 24 h, hepatocytes were exposed to BrB in concentrations up to 2 mM for 4 or 24 h. 3. In both human and cynomolgus monkey hepatocytes BrB cytotoxicity and GSH depletion were found after exposure to 2 mM BrB. The degree of the observed effects was not influenced by the duration of pre-incubation and/or exposure periods. 4. Major inter-individual differences were observed, which could not be attributed to differences in cytochrome P450-mediated bioactivation rates. This suggests that the variation in individual susceptibility to BrB may be related to inter-individual differences in the activity of de-activating (metabolic) pathways. 5. The study of the background of these inter-individual differences may contribute to a more complete understanding of the factors ruling sensitivity to BrB or related chemicals.

In vivo and in vitro 31P magnetic resonance spectroscopic studies of the hepatic response of healthy rats and rats with acute hepatic damage to fructose loading

The hepatic response to a fructose challenge for control rats, and rats subjected to an acute sublethal dose of carbon tetrachloride (CCl4) or bromobenzene (BB), was compared using dynamic in vivo 31P MRS. Fructose loading conditions were used in which control rats showed only a modest increase in hepatic phosphomonoester (PME), and a small decrease in ATP, Pi, and intracellular pH after fructose administration. Both CCl4 and BB-treated rats showed a much greater fructose-induced accumulation of PME than did controls. Trolox C, a free radical scavenger, prevented most of this PME increase. BB-treated rats, given sufficient time to recover from the hepatotoxic insult, responded to the fructose load similarly to controls. Liver aldolase activities of control, toxicant-treated rats, and toxicant plus Trolox C-treated rats correlated inversely with PME accumulation after fructose loading (correlation coefficient: -0.834, P < 0.05). Perchloric acid extracts of rat livers studied by in vitro 31P MRS confirmed that the PME accumulation after fructose loading is mainly due to an increase in fructose 1-phosphate. These studies are consistent with the aldolase-catalyzed cleavage of fructose 1-phosphate being rate-limiting in hepatic fructose metabolism, and that the CCl4 and BB treatment modify and inactivate the aldolase enzyme.

Dissociation of covalent protein adduct formation from oxidative injury in cultured hepatocytes exposed to cocaine

1. The relationship between the oxidative and the alkylating properties of cocaine was investigated in primary cultures of hepatocytes derived from phenobarbital-pretreated rats. 2. The cytotoxic effects (LDH release) of 300 microM cocaine were preceded by depletion of intracellular glutathione (GSH) and concomitant increases of oxidized glutathione (GSSG). Furthermore, exposure to [3H]-cocaine was associated with the formation of covalent protein adducts which plateaued between 2 and 7 h and which remained stable for at least 24 h. 3. The addition of the thiol-reducing agent dithiothreitol (DTT, 0.5 mM) protected against cocaine-induced LDH release without altering the time course and extent of cocaine covalent protein adduct formation. Similarly, when DTT was added after short-term exposure to cocaine in Krebs-Henseleit buffer, the loss of viability could be prevented, indicating that alterations in the thiol redox equilibrium, and not covalent protein adduct formation per se, may be crucial for the development of hepatocyte injury. In contrast, high concentrations (2.5-5.0 mM) of DTT inhibited both cocaine bioactivation and covalent binding and thus protected through prealkylative mechanisms. 4. Data demonstrate that cocaine-induced acute lethal hepatocyte injury was mediated by non-alkylative mechanisms, and that covalent adduct formation could be clearly dissociated from the consequences of oxidative stress that lead to cell killing.

Studies on the mechanism of 4-aminophenol-induced toxicity to renal proximal tubules

4-Aminophenol (PAP) is known to cause nephrotoxicity in the rat where it produces selective necrosis to renal proximal tubules. The aim of this work was to investigate the toxicity of PAP and its known nephrotoxic metabolite 4-amino-3-S-glutathionylphenol using a well defined suspension of rabbit renal proximal tubules. PAP at a concentration of 0.5 mM and 1 mM caused proximal tubule cell death (measured by lactate dehydrogenase release) in a time-dependent manner over a 4-h exposure. In contrast, 4-amino-3-S-glutathionylphenol at 1 mM produced no proximal tubule cell death over a similar 4-h exposure. At 2 h, 1 mM PAP inhibited proximal tubule respiration by 30% and decreased cellular adenosine triphosphate (ATP) levels by 60%. These events preceded cell death. The addition of PAP to proximal tubules led to a rapid depletion of cellular glutathione, exposure to 0.5 mM causing a 50% depletion within 1 h. The cytochrome P-450 inhibitors SKF525A (1 mM) and metyrapone (1 mM), the iron chelator deferoxamine (1 mM) and the antioxidant N,N'-phenyl-1,4-phenylenediamine (2 microM) had no effect on PAP-induced cell death. However ascorbic acid (0.1 mM), afforded a marked protection against the depletion of cellular glutathione and completely protected against the cell death produced by 1 mM-PAP. These results indicate that oxidation of PAP to generate a metabolite that can react with glutathione is an important step in the toxicity, while mitochondria appear to be a critical target for the reactive intermediate formed.

A comparative study of the toxicity of chemically reactive xenobiotics towards adherent cell cultures: selective attenuation of menadione toxicity by buthionine sulphoximine pretreatment

Metabolic activation to reactive intermediates is a prerequisite for many forms of chemically-induced toxicity. Hepa 1c1c-9 cells were exposed to varying concentrations of several reactive metabolites implicated in adverse drug reactions and the toxicity of the compounds assessed using applied fluorescence technology. Cytotoxicity was assessed using the fluorescence of 2', 7'-bis-(2-carboxyethyl)-5-(6)-carboxy-fluorescein as an index of cell viability. The role of glutathione in cellular defence against these chemicals was investigated by pretreating the target cells overnight with buthionine sulphoximine, a specific inhibitor of glutathione synthesis. Depletion of intracellular glutathione augmented the toxicity of N-acetyl-p-benzoquinone imine (1.5-3-fold at 100 and 10 microM). Toxicity produced by the hydroxylamine of sulphamethoxazole (500 microM) was dependent entirely on pretreatment of the cells with buthionine sulphoximine (% cell death = 33 +/- 16 compared with 0 +/- 4 in untreated cells, P < 0.05). By contrast, the lethal effects of the model quinone, menadione, were attenuated markedly following glutathione depletion. The data obtained suggest that this assay, previously used with suspension cultures, may be useful in the rapid in-vitro screening of putative reactive intermediates. Moreover, the application of such methodology should prove beneficial for the elucidation of cellular mechanisms of defence and detoxification.

Effects of 3-methylcholanthrene or diethyl maleate on the hepatotoxicity of acetaminophen

This report presents a set of investigations on the hepatotoxic action of acetaminophen (AA). Male mice of Balb C strain were given [3H]acetaminophen in doses of 100, 300 and 600 mg kg-1 with or without pretreatment with 3-methylcholanthrene (3MCh) or diethyl maleate (DEM). The results of this study show that AA administered in moderate doses brings about necrotic changes due to adduct formation with macromolecules. Adduct formation was inversely correlated with the level of glutathione. Both modifiers enhanced hepatic necrosis and lethality. Diethyl maleate exerted these effects without enhancing covalent binding to macromolecules, while 3MCh increased both adduct formation and lipid peroxidation.

Synthesis of [2-3H-ethyl]S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine and its use in covalent-binding studies

Metabolism of S-(2-chloro-1,1,2-trifluoroethyl)-L-cysteine (CTFC) yields chlorofluorothioacetyl fluoride, which reacts with cellular proteins to form stable lysine adducts. Little is known about the subcellular localization of these protein adducts or about their role in CTFC-induced nephrotoxicity. A method for the synthesis of CTFC and other cysteine S-conjugates labeled with 3H at the S-alkyl or S-alkenyl position would be useful in studies of S-conjugate metabolism and toxicity. Reaction of L-cysteine, chlorotrifluoroethene, 1,8-diazabicyclo[5.4.0]undec-7-ene, and 3H-labeled water followed by repeated crystallization yielded radiochemically pure [3H]CTFC (235 mg, 20% yield; sp act 1.07 x 10(9) Bq/mmol), which was identical to CTFC by TLC, 1H NMR, and 19F NMR. 3H NMR revealed a doublet of triplets at 6.5 ppm with geminal and vicinal T-F couplings of 51.5 and 6.0 Hz, respectively, consistent with the proposed structure. When 2H-labeled water was used, [2H]CTFC was formed, and its structure was confirmed by 1H and 19F NMR, FAB-MS, and TLC. Analysis of renal and hepatic subcellular fractions of rats given 1, 10, or 100 mumol/kg [3H]CTFC showed a dose-dependent binding of 3H-containing metabolites to liver and kidney proteins.

Bromobenzene-glutathione excretion into bile reflects toxic activation of bromobenzene in rats

This investigation was designed to determine whether biliary excretion of bromobenzene(BB)-glutathione(GSH) conjugate can be used as an index of in vivo activation of BB. In order to test this hypothesis, the effect of chemicals known to alter the toxicity and biotransformation of BB (i.e., cytochrome P-450 inducers and inhibitors) on the biliary excretion of BB-GSH was studied in rats. BB-GSH was the major BB metabolite in bile. A linear relationship was observed between the dosage of BB administered and BB-GSH excreted into bile, up to a dosage of 250 mumol/kg of BB. Of the inducers tested, phenobarbital, which is known to increase the toxicity of BB, dramatically increased (700%) the rate of biliary excretion of BB-GSH over that in control animals. In contrast, 3-methylcholanthrene, which is known to decrease the hepatotoxicity of BB, decreased the biliary excretion of BB-GSH (56%). Inhibitors of P-450, such as SKF 525-A and piperonyl butoxide which are known to decrease the activation and hepatotoxicity of BB, also decreased the biliary excretion of BB-GSH. These findings are in agreement with the hypothesis that the biliary excretion of BB-GSH reflects the formation of the reactive BB metabolite in liver and the rate of biliary excretion can be used to determine factors that are important in determining the toxicity of BB.

Pharmacokinetic data in the evaluation of the safety of food and color additives

Safety evaluation of food and color additives intended for human use is usually based on toxicity data obtained from animal studies; human data are rarely available. The extrapolation of animal data to humans is often controversial. The important role that pharmacokinetic data could play in the safety evaluation of food and color additives is now widely recognized. This paper reviews the current scientific knowledge concerning the application of properly designed pharmacokinetic studies to the evaluation of the safety of food and color additives. In principle, pharmacokinetic data can be useful not only in designing, interpreting, and extrapolating animal toxicity studies to humans, but also in providing insight into the mechanisms of toxicity. Examples of such applications are provided.

Proton magnetic resonance imaging and phosphorus-31 magnetic resonance spectroscopy studies of bromobenzene-induced liver damage in the rat

Respiratory-gated proton magnetic resonance imaging was used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity in the perihilar region of the liver was seen 24-48 hr after an intraperitoneal injection of bromobenzene. Localized proton magnetic resonance spectra from within this region indicated that the increased proton signal intensity was not due to accumulation of fat in the liver, but primarily due to a longer T2 for the proton resonance of water. This is consistent with acute edema in this localized region. In vivo 31P magnetic resonance spectroscopy studies of the same rat livers in situ were performed. Spectroscopic conditions were determined whereby localized, quantitative 31P spectra could be obtained. Using these methods, 10 mmol/kg bromobenzene was found after 24 hr to cause a number of statistically significant (p less than 0.05) effects: a decrease in adenosine 5'-triphosphate levels from 4.1 +/- 0.5 to 3.0 +/- 0.5 mM, a decrease in phosphodiester levels from 11.3 +/- 0.9 to 9.3 +/- 0.7 mM and an increase in the phosphomonoesters from 3.0 +/- 0.4 to 5.5 +/- 1.2 mM (mean +/- standard deviation). High resolution in vitro 31P spectra of perchloric acid extracts of these rat livers showed that the increased phosphomonoester resonance was due to a selective 4.3-fold increase in phosphocholine. Thus, our in vivo and in vitro 31P magnetic resonance spectra are consistent with the hypothesis that a phosphatidylcholine-specific phospholipase C (generating phosphocholine and diacylglycerol) is activated during tissue damage. Both the imaging and spectroscopy results obtained with bromobenzene closely resemble CCl4-induced liver changes previously reported, and may reflect a generalized response of the liver to any acutely acting toxic chemical.

The response of the rat liver in situ to bromobenzene--in vivo proton magnetic resonance imaging and 31P magnetic resonance spectroscopy studies

Proton magnetic resonance imaging (MRI) and 31P magnetic resonance spectroscopy (MRS) have been used to study the response of the rat liver in situ to bromobenzene, a classic hepatotoxicant. A localized region of high proton signal intensity was seen in the perihilar region of the liver 24 hr after injection of a sublethal dose of bromobenzene. The signal intensity of the entire liver was increased at 48 hr with a gradual return approaching control values by 120 hr. These results are consistent with acute hepatic edema followed by repair of the damaged tissue. In vivo 31P MRS studies of the same rat livers were performed under conditions whereby localized, quantitative spectra could be obtained without surgical intervention. Initial concentrations of the major endogenous phosphorus-containing metabolites within the livers of control rats were 2.97 +/- 0.43 mM for the phosphomonoesters (PME), 2.92 +/- 0.56 mM for inorganic phosphate, 11.3 +/- 1.0 mM for phosphodiesters (PDE), 4.09 +/- 0.54 mM for ATP, and 0.56 +/- 0.50 mM for ADP and the intracellular pH was 7.39 +/- 0.14 (mean +/- SD, n = 10). Bromobenzene was found to cause statistically significant (p less than 0.05) changes in several of these metabolites: a decrease in hepatic ATP levels (20% at 24 hr; 27% at 48 hr), a decrease in PDE levels (15% at 24 hr; 18% at 48 hr), and an increase in the PME (63% at 24 hr; 84% at 48 hr). Both the proton MRI and the 31P MRS changes have an onset of 15-20 hr and maximum effect at 25-60 hr, but the MRS changes returned to normal well before the MRI changes. The decreased ATP levels indicate deleterious effects of bromobenzene on the bioenergetic status of the liver in situ, while the increase in PME, due to a selective increase in phosphocholine, suggests the activation of a phosphatidylcholine-specific phospholipase C in response to tissue damage. Trolox C, a potent inhibitor of lipid peroxidation, prevented the bromobenzene-induced hepatic edema (i.e., the increase in proton MRI signal intensity) and the bioenergetic deterioration (i.e., the decrease in ATP levels). However, the bromobenzene-induced increase in PME levels was not prevented by Trolox C. These results indicate that the process of lipid peroxidation plays a significant role in the hepatotoxicity of bromobenzene within the intact animal.

Biomechanisms of cocaine-induced hepatocyte injury mediated by the formation of reactive metabolites

Cocaine is an intrinsic hepatotoxin in laboratory animals, and there is growing evidence that high doses of cocaine can precipitate hepatic necrosis in humans. The rodent model of cocaine hepatotoxicity is commensurate with the concept that a multistep mainly cytochrome P-450 dependent N-oxidative pathway is responsible for the expression of hepatocellular injury. Among the possible biomechanisms by which cocaine exerts its cytotoxic effects, direct oxidative damage by reactive oxygen species generated by redox cycling during the metabolic cascade seems most important. The role of the ensuing lipid peroxidation and protein thiol oxidation is less clear. Similarly, the functional role of irreversible (covalent) binding of a not yet defined electrophilic cocaine intermediate to hepatocellular proteins remains enigmatic so long as the critical molecular targets have not been identified. Finally, glutathione plays a pivotal protective role against cocaine-induced hepatic injury. Interactions with ethanol or inducers of the expression of the cytochrome P-450IIB subfamily can potentiate cocaine hepatotoxicity. Thus, the net amount of the ultimate reactive species seems to determine the severity of the hepatic lesions and to be responsible for the marked interspecies, interstrain, and sex differences. Recent advances in culture techniques of hepatocytes and precision-cut liver slices from various species including man have made it possible to correlate cocaine biotransformation with cytotoxicity and to selectively study the putative cellular mechanisms. Clearly, more studies are necessary to further illuminate our understanding of the role of the biochemical and molecular events precipitating hepatic necrosis during cocaine-mediated hepatotoxicity.

Changes in glutathione and cellular energy as potential mechanisms of papaverine-induced hepatotoxicity in vitro

The purpose of this study was to elucidate the mechanism of hepatotoxicity of papaverine hydrochloride (papaver) in vitro. To evaluate the role of metabolism in the toxicity of papaver, cells were pretreated with SKF-525A or benzyl imidazole (cytochrome P450 system inhibitors) for 24 hr at 1 x 10(-5) or 1 x 10(-4) M, respectively, or with phenobarbital sodium (cytochrome P450 system inducer) for 3 days at 2 x 10(-3) M. Cells then were exposed to concentrations of papaver ranging from 1 x 10(-5) to 1 x 10(-3) M for 4 to 24 hr. Cytotoxicity was evaluated by enzyme leakage (lactate dehydrogenase) and by energy status of the cells (ATP/ADP). The role of biological reactive intermediates in the toxicity of papaver was investigated by measuring changes in cellular reduced glutathione levels (GSH), by inhibiting GSH synthesis, and by determining the production of lipid peroxidation (LPX). Papaverine produced concentration- and time-dependent increases in enzyme leakage, with significant effects occurring by the 8-hr exposure period. Pretreatment with SKF-525A or benzyl imidazole increased enzyme leakage induced by papaver especially at a later time frame (24 hr), but pretreatment with phenobarbital delayed the onset of cytotoxicity from 8 to 12 hr. Decreases in GSH levels paralleled the time course of enzyme leakage. However, the administration of buthionine sulfoximine to cell cultures dramatically decreased the time by which papaver induced cellular injury (2 hr vs 8 hr). Changes in cellular energy status (ATP/ADP) were also detected earlier than enzyme leakage (4 hr vs 8 hr). In contrast, no significant production of lipid peroxidation was noted in papaver-treated cultures. We suggest that the mechanism of papaver-induced hepatotoxicity may be related to alterations in glutathione balance of the cells and to disruption of energy homeostasis.

Tissue acylation by the chlorofluorocarbon substitute 2,2-dichloro-1,1,1-trifluoroethane

Hydrochlorofluorocarbons (HCFCs) are being developed as substitutes for ozone-depleting chlorofluorocarbons (CFCs); because widespread human exposure to HCFCs may be expected, it is important to evaluate their toxicities thoroughly. Here we report studies on the bioactivation of the CFC substitute 2,2-dichloro-1,1,1-trifluoroethane (HCFC-123) to an electrophilic intermediate that reacts covalently with liver proteins. HCFC-123 and its analog halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) were studied in rats by 19F NMR spectroscopy, and we found that a trifluoroacetylated lysine adduct was formed with liver proteins. Also, the pattern of proteins immunoreactive with hapten-specific anti-trifluoroacetylprotein antibodies was identical in livers of HCFC-123- and halothane-exposed rats. Because halothane causes an idiosyncratic, and sometimes fatal, hepatitis that is associated with an immune response against several trifluoroacetylated liver proteins, the present findings raise the possibility that humans exposed to HCFC-123 or structurally related HCFCs may be at risk of developing an immunologically mediated hepatitis.

Cellular toxicity of sulfamethoxazole reactive metabolites--I. Inhibition of intracellular esterase activity prior to cell death

Reactive metabolites produced by oxidative metabolism of the parent compound are considered responsible for the toxicity of a number of drugs, including idiosyncratic reactions to sulfonamide antibiotics. Using sulfamethoxazole hydroxylamine (SMX-HA) as a model compound, we report the use of a pH-sensitive fluorescent probe, 2',7'-biscarboxyethyl-5(6)-carboxyfluorescein (BCECF), to identify early subcellular targets of chemically synthesized, toxic drug metabolites in peripheral blood mononuclear cells. When toxicity was assessed with this probe immediately after a 2-hr drug challenge, SMX-HA produced a concentration-dependent decrease in cellular fluorescence which was not accompanied by the development of compromised cell membrane integrity until 18 hr later. Dissipation of pH gradients across the cell membrane with nigericin and monensin demonstrated that decreased intracellular pH was only a small component of SMX-HA-induced toxicity. Loading cells with BCECF 30 min prior to SMX-HA challenge produced only a 3% decrease in cellular fluorescence at an SMX-HA concentration of 1 mM, whereas addition of BCECF after drug challenge resulted in a 71% decrease in fluorescence, consistent with a direct drug effect on cellular esterase activity. This was confirmed by monitoring BCECF cleavage in cell lysates in the presence and absence of SMX-HA. These studies demonstrate that inhibition of cellular esterase activity accounted for the observed loss of cellular fluorescence after drug exposure. Since changes in cellular fluorescence at 2 hr correlated well with cell death at 18 hr, we conclude that SMX-HA inhibition of intracellular esterase activity is an early event in the process that terminates in metabolite-induced cell death.