Evidence for expression in human liver of halothane-induced neoantigens recognized by antibodies in sera from patients with halothane hepatitis

Chemical- and Drug-Induced Allergic, Inflammatory, and Autoimmune Diseases Via Haptenation

Haptens are small molecules that only elicit an immune response when bound to proteins. Haptens initially bind to self-proteins and activate innate immune responses by complex mechanisms via inflammatory cytokines and damage-associated molecular patterns and the subsequent upregulation of costimulatory signals such as cluster of differentiation 86 (CD86) on dendritic cells. Subsequent interactions between CD86 and CD28 on T cells are critically important for properly activating naive T cells and inducing interleukin 2 production, leading to the establishment of adaptive immunity via effector and memory T cells. Accumulating evidence revealed the involvement of haptens in the development of various autoimmune-like diseases such as allergic, inflammatory, and autoimmune diseases including allergic contact dermatitis, atopy, asthma, food allergy, inflammatory bowel diseases, hemolytic anemia, liver injury, leukoderma, and even antitumor immunity. Therefore, the development of in vitro testing alternatives to evaluate in advance whether a substance might lead to the development of these diseases is highly desirable. This review summarizes and discusses recent advances in chemical- and drug-induced allergic, inflammatory, and autoimmune diseases via haptenation and the possible molecular underlying mechanisms, as well as in vitro testing alternatives to evaluate in advance whether a substance might cause the development of these diseases.

Metabolic and Pharmaceutical Aspects of Fluorinated Compounds

The applications of fluorine in drug design continue to expand, facilitated by an improved understanding of its effects on physicochemical properties and the development of synthetic methodologies that are providing access to new fluorinated motifs. In turn, studies of fluorinated molecules are providing deeper insights into the effects of fluorine on metabolic pathways, distribution, and disposition. Despite the high strength of the C-F bond, the departure of fluoride from metabolic intermediates can be facile. This reactivity has been leveraged in the design of mechanism-based enzyme inhibitors and has influenced the metabolic fate of fluorinated compounds. In this Perspective, we summarize the literature associated with the metabolism of fluorinated molecules, focusing on examples where the presence of fluorine influences the metabolic profile. These studies have revealed potentially problematic outcomes with some fluorinated motifs and are enhancing our understanding of how fluorine should be deployed.

Acyl glucuronides: the good, the bad and the ugly

Acyl glucuronidation is the major metabolic conjugation reaction of most carboxylic acid drugs in mammals. The physiological consequences of this biotransformation have been investigated incompletely but include effects on drug metabolism, protein binding, distribution and clearance that impact upon pharmacological and toxicological outcomes. In marked contrast, the exceptional but widely disparate chemical reactivity of acyl glucuronides has attracted far greater attention. Specifically, the complex transacylation and glycation reactions with proteins have provoked much inconclusive debate over the safety of drugs metabolised to acyl glucuronides. It has been hypothesised that these covalent modifications could initiate idiosyncratic adverse drug reactions. However, despite a large body of in vitro data on the reactions of acyl glucuronides with protein, evidence for adduct formation from acyl glucuronides in vivo is limited and potentially ambiguous. The causal connection of protein adduction to adverse drug reactions remains uncertain. This review has assessed the intrinsic reactivity, metabolic stability and pharmacokinetic properties of acyl glucuronides in the context of physiological, pharmacological and toxicological perspectives. Although numerous experiments have characterised the reactions of acyl glucuronides with proteins, these might be attenuated substantially in vivo by rapid clearance of the conjugates. Consequently, to delineate a relationship between acyl glucuronide formation and toxicological phenomena, detailed pharmacokinetic analysis of systemic exposure to the acyl glucuronide should be undertaken adjacent to determining protein adduct concentrations in vivo. Further investigation is required to ascertain whether acyl glucuronide clearance is sufficient to prevent covalent modification of endogenous proteins and consequentially a potential immunological response.

Inhalational anesthesia: basic pharmacology, end organ effects, and applications in the treatment of status asthmaticus

The potent inhalational anesthetic agents are used on a daily basis to provide intraoperative anesthesia. Given their beneficial effects on airway tone and reactivity, they also have a role in the treatment of status asthmaticus that is refractory to standard therapy. Although generally not of clinical significance, these agents can affect various physiological functions. The potent inhalational anesthetic agents decrease mean arterial pressure and myocardial contractility. The decrease in mean arterial pressure reduces renal and hepatic blood flow. Secondary effects on end-organ function may result from the metabolism of these agents and the release of inorganic fluoride. The following article reviews the history of inhalational anesthesia, the physical structure of the inhalational anesthetic agents, their end-organ effects, reports of their use for the treatment of refractory status asthmaticus in the intensive care unit (ICU) patient, and special considerations for their administration in this setting including equipment for their delivery, scavenging, and monitoring.

Mechanisms, chemical structures and drug metabolism

From the studies that have been done by many laboratories over the last 2 decades, it is now clear that the toxicities produced by many drugs are due to their reactive metabolites. It is though that, in many cases, reactive metabolites cause toxicity by binding covalently to tissue proteins. However, until recently it was difficult to identify these protein targets. Due to the development of an immunochemical approach, this problem has been overcome, as is illustrated here by studies that have been conducted on the metabolic basis of the idiosyncratic hepatitis caused by the inhalation anaesthetic halothane. The major problem to solve in the future will be to determine how protein adduct formation leads to toxicity. It is possible that protein adduct formation may alter an important cellular function or may lead to immunopathology, as is thought to occur in the case of halothane hepatitis. If an allergic reaction is suspected, purified protein targets of reactive metabolites can serve as antigens for identifying sensitized individuals. This information can be used to prevent not only an allergic reaction to the drug, but possible cross-reactions to other drugs that are structurally related. Another important application of these studies is the design of safer alternative drugs that will not produce structurally similar toxic reactive metabolites.

The Endoplasmic Reticulum in Xenobiotic Toxicity

The endoplasmic reticulum (ER) is involved in an array of cellular functions that play important roles in xenobiotic toxicity. The ER contains the majority of cytochrome P450 enzymes involved in xenobiotic metabolism, as well as a number of conjugating enzymes. In addition to its role in drug bioactivation and detoxification, the ER can be a target for damage by reactive intermediates leading to cell death or immune-mediated toxicity. The ER contains a set of luminal proteins referred to as ER stress proteins (including GRP78, GRP94, protein disulfide isomerase, and calreticulin). These proteins help regulate protein processing and folding of membrane and secretory proteins in the ER, calcium homeostasis, and ER-associated apoptotic pathways. They are induced in response to ER stress. This review discusses the importance of the ER in molecular events leading to cell death following xenobiotic exposure. Data showing that the ER is important in both renal and hepatic toxicity will be discussed.

Therapeutic applications and uses of inhalational anesthesia in the pediatric intensive care unit

OBJECTIVE

To review the physical properties, end-organ effects, therapeutic applications, and delivery techniques of inhalational anesthetic agents in the pediatric intensive care unit.

DATA SOURCE

A computerized, bibliographic search regarding intensive care unit applications of inhalational anesthetic agents.

MAIN RESULTS

Although the end-organ effects of inhalational anesthetic agents vary depending on the agent, general effects include a dose-related depression of ventilatory and cardiovascular function. With increasing anesthetic depth, there is a decrease in alveolar ventilation with a reduction in tidal volume and an increase in PaCO2 in spontaneously breathing patients. The potent inhalational anesthetic agents decrease mean arterial pressure and myocardial contractility. The decrease in mean arterial pressure reduces renal and hepatic blood flow. Secondary effects on end-organ function may result from the metabolism of these agents and the release of inorganic fluoride. Beneficial effects include sedation, amnesia, and anxiolysis, making these agents effective for sedation during mechanical ventilation. Bronchodilatory and anticonvulsant properties have led to their use as therapeutic agents in patients with refractory status asthmaticus and epilepticus. Issues regarding their delivery in the intensive care unit include equipment for their delivery, scavenging, and monitoring.

CONCLUSIONS

The literature contains reports of the therapeutic use of the potent inhalational anesthetic agents in the pediatric intensive care unit. Potential applications include sedation during mechanical ventilation as well as therapeutic use for the treatment of status asthmaticus and epilepticus.

Role of bioactivation in drug-induced hypersensitivity reactions

Drug-induced hypersensitivity reactions are a major problem in both clinical treatment and drug development. This review covers recent developments in our understanding of the pathogenic mechanisms involved, with special focus on the potential role of metabolism and bioactivation in generating a chemical signal for activation of the immune system. The possible role of haptenation and neoantigen formation is discussed, alongside recent findings that challenge this paradigm. Additionally, the essential role of costimulation is examined, as are the potential points whereby costimulation may be driven by reactive metabolites. The relevance of local generation of metabolites in determining the location and character of a reaction is also covered.

Monitoring drug-protein interaction

A variety of therapeutic drugs can undergo biotransformation via Phase I and Phase II enzymes to reactive metabolites that have intrinsic chemical reactivity toward proteins and cause potential organ toxicity. A drug-protein adduct is a protein complex that forms when electrophilic drugs or their reactive metabolite(s) covalently bind to a protein molecule. Formation of such drug-protein adducts eliciting cellular damages and immune responses has been a major hypothesis for the mechanism of toxicity caused by numerous drugs. The monitoring of protein-drug adducts is important in the kinetic and mechanistic studies of drug-protein adducts and establishment of dose-toxicity relationships. The determination of drug-protein adducts can also provide supportive evidence for diagnosis of drug-induced diseases associated with protein-drug adduct formation in patients. The plasma is the most commonly used matrix for monitoring drug-protein adducts due to its convenience and safety. Measurement of circulating antibodies against drug-protein adducts may be used as a useful surrogate marker in the monitoring of drug-protein adducts. The determination of plasma protein adducts and/or relevant antibodies following administration of several drugs including acetaminophen, dapsone, diclofenac and halothane has been conducted in clinical settings for characterizing drug toxicity associated with drug-protein adduct formation. The monitoring of drug-protein adducts often involves multi-step laboratory procedure including sample collection and preliminary preparation, separation to isolate or extract the target compound from a mixture, identification and determination. However, the monitoring of drug-protein adducts is often difficult because of short half-lives of the protein adducts, sampling problem and lack of sensitive analytical techniques for the protein adducts. Currently, chromatographic (e.g. high performance liquid chromatography) and immunological methods (e.g. enzyme-linked immunosorbent assay) are two major techniques used to determine protein adducts of drugs in patients. The present review highlights the importance for clinical monitoring of drug-protein adducts, with an emphasis on methodology and with a further discussion of the application of these techniques to individual drugs and their target proteins.

Drug bioactivation, covalent binding to target proteins and toxicity relevance

A number of therapeutic drugs with different structures and mechanisms of action have been reported to undergo metabolic activation by Phase I or Phase II drug-metabolizing enzymes. The bioactivation gives rise to reactive metabolites/intermediates, which readily confer covalent binding to various target proteins by nucleophilic substitution and/or Schiff's base mechanism. These drugs include analgesics (e.g., acetaminophen), antibacterial agents (e.g., sulfonamides and macrolide antibiotics), anticancer drugs (e.g., irinotecan), antiepileptic drugs (e.g., carbamazepine), anti-HIV agents (e.g., ritonavir), antipsychotics (e.g., clozapine), cardiovascular drugs (e.g., procainamide and hydralazine), immunosupressants (e.g., cyclosporine A), inhalational anesthetics (e.g., halothane), nonsteroidal anti-inflammatory drugs (NSAIDSs) (e.g., diclofenac), and steroids and their receptor modulators (e.g., estrogens and tamoxifen). Some herbal and dietary constituents are also bioactivated to reactive metabolites capable of binding covalently and inactivating cytochrome P450s (CYPs). A number of important target proteins of drugs have been identified by mass spectrometric techniques and proteomic approaches. The covalent binding and formation of drug-protein adducts are generally considered to be related to drug toxicity, and selective protein covalent binding by drug metabolites may lead to selective organ toxicity. However, the mechanisms involved in the protein adduct-induced toxicity are largely undefined, although it has been suggested that drug-protein adducts may cause toxicity either through impairing physiological functions of the modified proteins or through immune-mediated mechanisms. In addition, mechanism-based inhibition of CYPs may result in toxic drug-drug interactions. The clinical consequences of drug bioactivation and covalent binding to proteins are unpredictable, depending on many factors that are associated with the administered drugs and patients. Further studies using proteomic and genomic approaches with high throughput capacity are needed to identify the protein targets of reactive drug metabolites, and to elucidate the structure-activity relationships of drug's covalent binding to proteins and their clinical outcomes.

Retrospective study of post-anesthetic mild liver disorder associated with inhalation anesthetics, halothane and enflurane

The incidence of post-anesthetic mild liver disorder (PAMLD) was compared between 928 patients administered halothane and 1766 patients administered enflurane. They were selected from 19 504 surgical patients administered general anesthesia at Kyushu University Hospital over the past 6 years and 4 months. They had had normal liver function before operation and had no history of blood transfusion. Alanine aminotransferase (ALT) levels exceeding 70 IU. l(-1) within 180 days after operation were found in 226 patients in the halothane group (24.4%), and in 250 patients in the enflurane group (14.2%) ( P < 0.01). Both maximum ALT levels and duration of ALT elevation were higher and longer in the halothane group ( P < 0.01). These results suggest that, not only in the development of fulminant hepatitis but also in PAMLD, enflurane is less hepatotoxic than halothane.

Use of molecular simulation for mapping conformational CYP2E1 epitopes

The identification of the epitopes recognized by autoantibodies against cytochrome P450s (CYPs) associated with drug-induced hepatotoxicity is difficult because of their conformational nature. In the present investigation, we used a novel approach based on the analysis of the whole molecule antigenic capacity following single amino acid substitutions to identify the conformational epitopes on CYP2E1. A molecular model of CYP2E1 was generated based on the CYP2C5 crystal structure, and potential motifs for amino acid exchanges were selected by computer simulation in the surface of alpha helices and beta sheets. Fourteen modified, apparently correctly folded CYP2E1 variants were produced in Escherichia coli and evaluated in immunoprecipitation experiments using sera with anti-CYP2E1 autoreactivity from 10 patients with halothane hepatitis and 12 patients with alcoholic liver disease. Ala substitution of Glu-248 and Lys-251 as well as of Lys-324, Lys-342, Lys-420, and Phe-421 severely decreased or abolished CYP2E1 recognition by the majority of both the halothane hepatitis and alcoholic liver disease sera, whereas the other substitutions had only minor effects. Based on the structural model, these substitutions identified two distinct epitopes on the CYP2E1 surface corresponding to the G-helix and an area formed by juxtaposition of the J' and K'' helices, respectively. The combined use of molecular modeling and single amino acid mutagenesis is thus a useful approach for the characterization of conformational epitopes recognized by autoantibodies.

Separation and detection methods for covalent drug–protein adducts

Covalent binding of reactive metabolites of drugs to proteins has been a predominant hypothesis for the mechanism of toxicity caused by numerous drugs. The development of efficient and sensitive analytical methods for the separation, identification, quantification of drug-protein adducts have important clinical and toxicological implications. In the last few decades, continuous progress in analytical methodology has been achieved with substantial increase in the number of new, more specific and more sensitive methods for drug-protein adducts. The methods used for drug-protein adduct studies include those for separation and for subsequent detection and identification. Various chromatographic (e.g., affinity chromatography, ion-exchange chromatography, and high-performance liquid chromatography) and electrophoretic techniques [e.g., sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), two-dimensional SDS-PAGE, and capillary electrophoresis], used alone or in combination, offer an opportunity to purify proteins adducted by reactive drug metabolites. Conventionally, mass spectrometric (MS), nuclear magnetic resonance, and immunological and radioisotope methods are used to detect and identify protein targets for reactive drug metabolites. However, these methods are labor-intensive, and have provided very limited sequence information on the target proteins adducted, and thus the identities of the protein targets are usually unknown. Moreover, the antibody-based methods are limited by the availability, quality, and specificity of antibodies to protein adducts, which greatly hindered the identification of specific protein targets of drugs and their clinical applications. Recently, the use of powerful MS technologies (e.g., matrix-assisted laser desorption/ionization time-of-flight) together with analytical proteomics have enabled one to separate, identify unknown protein adducts, and establish the sequence context of specific adducts by offering the opportunity to search for adducts in proteomes containing a large number of proteins with protein adducts and unmodified proteins. The present review highlights the separation and detection technologies for drug-protein adducts, with an emphasis on methodology, advantages and limitations to these techniques. Furthermore, a brief discussion of the application of these techniques to individual drugs and their target proteins will be outlined.

Underlying endotoxemia augments toxic responses to chlorpromazine: is there a relationship to drug idiosyncrasy?

Idiosyncratic reactions occur in a small fraction (typically <5%) of the population taking therapeutic drugs. Chlorpromazine (CPZ) is a phenothiazine, antipsychotic drug that has caused several idiosyncratic responses during its therapeutic use. Clinical evidence suggests that conditions associated with inflammation are risk factors for the appearance of these responses. Accordingly, we tested the hypothesis that an inflammatory stimulus, bacterial lipopolysaccharide (LPS), renders animals susceptible to CPZ-induced idiosyncratic reactions seen in humans. Male Sprague-Dawley rats (200-250 g) were fasted for 24 h. A small dose of LPS (7.4 x 10(6) EU/kg from Escherichia coli) or its vehicle (saline) was administered by tail vein 2 h before an intraperitoneal injection of CPZ (70 mg/kg) or its vehicle (saline). Cholestasis and hepatocellular necrosis were evaluated as increased concentrations of serum bile acids and bilirubin and increased activities of alkaline phosphatase, gamma-glutamyltransferase, alanine aminotransferase, and aspartate aminotransferase. With the exception of bile acids, these serum markers were elevated in animals treated with LPS/CPZ. Histopathological lesions in liver sections were consistent with these findings. Elevated serum creatine kinase activity, which is associated with human idiosyncratic responses to phenothiazines, was also found in animals treated with LPS/CPZ, but not with either LPS or CPZ alone. These results raise the possibility that concurrent, modest inflammation may underlie susceptibility of individuals to certain idiosyncratic reactions and may form the basis for an animal model with which to understand and predict drug idiosyncrasy.

The role of reactive drug metabolites in immune-mediated adverse drug reactions

OBJECTIVE

To highlight recent advances in the understanding of adverse drug reactions (ADRs), with a focus on models outlining interactions between drug metabolism, disease processes, and immunity. Specific mechanisms that identify the metabolic pathways responsible for drug bioactivation to reactive drug metabolites (RDMs) involved in the initiation and propagation of specific immune-mediated hypersensitivity reactions are discussed. Drug classes well known to be associated with immune-mediated ADRs are reviewed and the clinical implications of current research are discussed.

DATA SOURCES

Original experimental research and immunologic review articles relevant to ADR diagnosis and etiology.

DATA EXTRACTION

Results of relevant in vitro experiments and clinical reactions to drug therapy were compiled and reviewed. Critical discoveries concerning the identification of RDMs involved in ADRs were highlighted, with respect to RDM involvement in the production of an immune response to drug haptens.

DATA SYNTHESIS

Drug adverse effects are classified according to clinical characteristics, immune interactions, and mechanistic similarities. Cytochrome P450 bioactivation of drug molecules to RDMs is a prerequisite to many ADRs. An electrophilic metabolite may react with cellular macromolecules (i.e., lipids, proteins, nucleic acids), resulting in direct cellular damage and organ toxicity. Covalent binding of an RDM to cellular macromolecules may also result in the formation of a hapten that is capable of eliciting a cellular or humoral immune response against drug or protein epitopes, culminating in the characteristic symptoms of hypersensitivity reactions. Mechanistic details concerning the identification of stable protein-metabolite conjugates and their interaction with the immune system remain unclear. Genetic imbalance between bioactivation and detoxification pathways, as well as reduced cellular defense against RDMs due to disease or concomitant drug therapy, act as risk factors to the onset and severity of ADRs.

CONCLUSIONS

Adverse reactions to drug therapy cause significant morbidity and mortality. Identification of the pathways involved in drug bioactivation and detoxification may elucidate the potential of chemical agents to induce immune-mediated ADRs. Understanding the mechanisms of ADRs to current xenobiotics is helpful in the prevention and management of ADRs, and may prove useful in the design of novel therapeutic agents with reduced incidence of severe adverse events.

Kupffer cells from halothane-exposed guinea pigs carry trifluoroacetylated protein adducts

The anesthetic, halothane, is bioactivated by the liver cytochrome P450 system to trifluoroacetyl-chloride, which can readily acylate liver protein. Covalent binding of the trifluoroacetyl moiety may result in hapten formation leading to the induction of an immune response and ultimately halothane hepatitis. In this study the presence of trifluoroacetylated-protein adducts in Kupffer cells was investigated to learn how the immune system might come in contact with the proteins. Guinea pigs were exposed to 1.0% halothane, 40% oxygen for 4 h. Kupffer cells were isolated on days 1 through 9 post-exposure, by liver perfusion and purification by elutriation. Using gel electrophoresis and Western blotting techniques, it has been demonstrated that Kupffer cells obtained from halothane-treated guinea pigs, do carry trifluoroacetyl-protein adducts as recognized by an anti-trifluoroacetyl-rabbit serum albumin antibody. Apparent molecular weights of polypeptides bound by trifluoroacetyl were of a wide range, 25-152 kDa. Bands were most prominent in the larger Kupffer cells with more appearing at lower molecular weights. Trifluoroacetyl-protein adducts were not detected in lung, spleen, lymph node or peripheral blood macrophages. This work suggests a role for Kupffer cells in the presentation of altered proteins in the liver to cells of the immune system.

Covalent binding of xenobiotics to specific proteins in the liver

Chemicals that cause toxicity though a direct mechanism, such as acetaminophen, covalently bind to a select group of proteins prior to the development of toxicity, and these proteins may be important in the initiation of the events that lead to the hepatotoxicity. Disruption of the cell is measured by release of intracellular proteins such as alanine aminotransferase and occurs late in the time course following a hepatotoxic dose of a direct toxin. Prior to this disruption, there appears to be a large number of proteins covalently modified by a reactive metabolite. There are at least two possible mechanisms that may cause the toxicity. First, some critical protein is a target of the reactive metabolite. Disruption of the enzymatic function (or a critical pathway for a regulatory protein) may lead directly to cell death. With the direct hepatotoxin acetaminophen, there is a decrease in the activity of several of the early target proteins, but how this disruption of critical proteins leads to the toxicity is still unclear. The early targets appear to be proteins with accessible nucleophilic sulfhydryl groups, and usually the target has a high concentration of the protein within the cell. It is possible that the binding to some of these proteins represents a detoxification protecting more critical targets within the cell. A second mechanism for the direct toxicity is that more and more proteins become targets in the time course following administration of a direct toxin, and eventually the cells machinery is overwhelmed. The cell can then no longer function, or there is a disruption the redox balance within the cell due to the decreased function of numerous proteins. In contrast to the direct-acting toxins, the chemical-protein conjugates that initiate toxicity through an activation of the immune system appear to have a limited number of target proteins and are localized within one subcellular fraction. Halothane produces adducts almost exclusively in the microsomal fraction, and these adducts appear to be limited to selective proteins with high concentrations in this fraction. The substitution level is an important factor in the development of an immune response. Halothane hepatitis patients' antibodies primarily recognize proteins with a high substitution level. For halothane and diclofenac, the proteins are accessible to the immune system through exposure on the plasma membrane. Trichloroethylene binds primarily to a 50-kDa microsomal protein, and preliminary evidence has been presented which indicates that a trichloroethylene-protein conjugate is released into the blood following exposure, where contact with the immune system can occur. In order to elicit an immune response the immune system requires multiple exposure to the chemical-protein conjugates. With halothane hepatitis and with diclofenac hepatitis, as well as occupational and environmental exposure to trichloroethylene, there are multiple exposures leading to repeat presentation of the protein adducts to the immune system; this situation is not generally found with acetaminophen overdose patients. In summary, direct toxicants such as acetaminophen covalently bind to selected targets which may be critical to the development of hepatotoxicity, and they later form adducts with numerous proteins which may overwhelm the cell's capacity to maintain homeostasis, leading to loss of vital function and cell death (Fig.3). In contrast, indirect toxicants that elicit an immune-mediated toxicity such as halothane, and possibly diclofenac and trichloroethylene, appear to have a limited number of protein targets with a high substitution level, and the immune system is exposed repeatedly to the modified proteins.

Protein targets of xenobiotic reactive intermediates

Many xenobiotics are metabolically activated to electrophilic intermediates that form covalent adducts with proteins; the mechanism of toxicity is either intrinsic or idiosyncratic in nature. Many intrinsic toxins covalently modify cellular proteins and somehow initiate a sequence of events that leads to toxicity. Major protein adducts of several intrinsic toxins have been identified and demonstrate significant decreases in enzymatic activity. The reactivity of intermediates and subcellular localization of major targets may be important in the toxicity. Idiosyncratic toxicities are mediated through either a metabolic or immune-mediated mechanism. Xenobiotics that cause hypersensitivity/autoimmunity appear to have a limited number of protein targets, which are localized within the subcellular fraction where the electrophile is produced, are highly substituted, and are accessible to the immune system. Metabolic idiosyncratic toxins appear to have limited targets and are localized within a specific subcellular fraction. Identification of protein targets has given us insights into mechanisms of xenobiotic toxicity.

Detection of autoantibodies directed against human hepatic endoplasmic reticulum in sera from patients with halothane-associated hepatitis

1. Previous studies have demonstrated the presence of antibodies to trifluoroacetylated hepatic proteins (TFA-proteins) in sera from patients with the severe form of halothane-associated hepatitis (halothane hepatitis). The TFA-proteins are produced via cytochrome P450-mediated metabolism of halothane to the reactive species TFA-chloride. 2. To investigate the presence of autoantibodies (which recognize various non-TFA-modified human hepatic polypeptides) in patients with halothane hepatitis immunoblotting experiments were performed using microsomal fractions prepared freshly from livers of five different (halothane-free) tissue donors. Blots were developed using 15 well-characterised sera from patients with halothane hepatitis. 3. Autoantibodies to human hepatic polypeptides were detected in most, but not all, of the patients' sera. The pattern of antibody reactivity varied markedly between sera. Although no common pattern of antibody recognition was observed, polypeptides of molecular mass between 60 and 80 kDa were the predominant targets. A similar protein recognition pattern was seen when each positive serum was tested against the five individual human liver samples. 4. Such autoantibodies were not detected in sera from 16 normal human blood donors, but were detected in three of six sera from patients exposed to halothane without developing hepatitis. 5. The autoantibodies are thought to arise in patients exposed to halothane as a consequence of a halothane-induced immune response to chemically-modified proteins. Such antibodies could contribute to the complex pathological processes involved in halothane hepatitis.

Molecular mimicry in halothane hepatitis: biochemical and structural characterization of lipoylated autoantigens

Exposure of human individuals to halothane causes, in about 20% of all cases, a mild transient form of hepatotoxicity. A small subset of exposed individuals, however, develops a potentially severe and life-threatening form of hepatic damage, coined halothane hepatitis. Halothane hepatitis is thought to have an immunological basis. Sera of afflicted individuals contain a wide variety of autoantibodies against hepatic proteins, in both trifluoroacetylated form (CF3CO-proteins) and, at least in part, in native form. CF3CO-proteins are elicited in the course of oxidative biotransformation of halothane, and include the trifluoroacetylated forms of protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78, and ERp99. Current evidence suggests that CF3CO-proteins arise in all halothane-exposed individuals; however, the vast majority of individuals appear to immunochemically tolerate CF3CO-proteins. The lack of immunological responsiveness of these individuals towards CF3CO-proteins might be due to tolerance, induced through the occurrence of structures in the repertoire of self-determinants, which immunochemically and structurally mimic CF3CO-proteins very closely. In fact, lipoic acid, the prosthetic group of the constitutively expressed E2 subunits of the family of mammalian 2-oxoacid dehydrogenase complexes and of protein X, was shown by immunochemical and molecular modelling analysis to be a perfect structural mimic of N6-trifluoroacetyl-L-lysine (CF3 CO-Lys), the major haptenic group of CF3CO-proteins. As a consequence of molecular mimicry, autoantibodies in patients' sera not only recognize CF3CO-proteins, but also the E2 subunit proteins of the 2-oxoacid dehydrogenase complexes and protein X, as autoantigens associated with halothane hepatitis. Furthermore, a fraction of patients with halothane hepatitis exhibit irregularities in the hepatic expression levels of these native, not trifluoroacetylated autoantigens. Collectively, these data suggest that molecular mimicry of CF3CO-Lys by lipoic acid, or the impairment thereof, might play a role in the susceptibility of individuals for the development of halothane hepatitis.

Human hepatocytes express trifluoroacetylated neoantigens after in vitro exposure to halothane

Biotransformation of anaesthetic halothane by cytochrome P450-dependent monooxygenases resulted in the production of reactive intermediate trifluoroacetyl (TFA) halide, capable of covalently binding to hepatocyte proteins. TFA-modified liver proteins can act as antigens and are implicated in the pathogenesis of halothane hepatitis in humans. The aim of this study was to investigate the formation of TFA-neoantigens in halothane-treated primary cultures of adult human hepatocytes and to evaluate the usefulness of this in vitro model for studying immune-mediated halothane hepatotoxicity. Cultured human hepatocytes were incubated with halothane under constant temperature, atmosphere and anaesthetic concentration conditions. The results obtained show that halothane-treated hepatocytes isolated from seven different donors produced TFA-antigens as detected by immunocytochemical and western immunoblot analysis using rabbit anti-TFA antiserum. TFA-adducts were localized mainly in the endoplasmic reticulum and in small amounts on the plasma membrane of parenchymal cells. By immunoblotting, several neoantigens, with molecular masses from 42 to 100 kDa, were detected in halothane-exposed hepatocytes. These observations are consistent with the formation of TFA-adducts through metabolism of the anaesthetic and suggest that primary cultures of human hepatocytes represent a suitable in vitro model to study the pathogenesis of immune-mediated halothane hepatotoxicity.

Formation of trifluoroacetylated protein antigens in cultured rat hepatocytes exposed to halothane in vitro

Immune responses to novel, halothane metabolite-modified protein antigens (tri-fluoroacetylated proteins; TFA-proteins) have been implicated in the pathogenesis of halothane hepatitis. The aim of the present study was to investigate and characterize expression of TFA-proteins in cultures of rat hepatocytes which were exposed to halothane in vitro. Following exposure to halothane, the hepatocytes were harvested, then subcellular fractions were prepared and were analysed by immunoblotting for expression of antigens recognized by a rabbit anti-TFA antiserum, and by antibodies in sera from two patients with halothane hepatitis. Hepatocytes exposed to halothane in vitro were shown to express novel microsomal protein antigens, which exhibited molecular masses that were identical to the molecular masses of the major TFA-protein antigens expressed in vivo, in livers of halothane-treated rats (100, 80 and 60 kDa). Experiments in which hepatocytes were exposed to halothane in the presence of SKF-525A, or were exposed to deuterated halothane in place of halothane, confirmed that these novel antigens were TFA-modified proteins whose generation required cytochrome P450-mediated metabolism of halothane. The maximal levels of TFA-antigens expressed in vitro were about 30% of the levels expressed in halothane-treated rats in vivo. Maximal expression of the TFA-antigens in vitro occurred when hepatocytes were exposed to halothane at doses which yielded concentrations of the drug in culture medium of about 13 microM. Expression of the antigens in vitro occurred slowly, with an apparent half-time of about 8 hr. Overall, these results demonstrate that the properties of the TFA-antigens expressed in cultured hepatocytes in vitro closely resemble the properties exhibited by the antigens expressed in vivo, in livers of halothane-treated rats.

Identification of the dihydrolipoamide acetyltransferase subunit of the human pyruvate dehydrogenase complex as an autoantigen in halothane hepatitis. Molecular mimicry of trifluoroacetyl-lysine by lipoic acid

Trifluoroacetylated (CF3CO-) proteins, elicited upon exposure of animals or humans to halothane, were recognized by anti-CF3CO antibody, monospecific for the hapten derivative N6-trifluoroacetyl-L-lysine. Anti-CF3CO antibodies cross-reacted with the dihydrolipoamide acetyltransferase (E2 subunit) of pyruvate dehydrogenase, indicating that epitopes on the E2 subunit of pyruvate dehydrogenase molecularly mimic those on CF3CO-proteins. Lipoic acid, the prosthetic group of the E2 subunit of pyruvate dehydrogenase was essential in this process, in that only the lipoylated form of the recombinantly expressed inner lipoyl domain of the human E2 subunit of pyruvate dehydrogenase, but not the unlipolyated form, was recognized by anti-CF3CO antibody. Furthermore, based on a high degree of structural relatedness, both CF3CO-Lys and (6RS)-lipoic acid, as well as the lipoylated peptide ETDK(lipoyl)ATIG specifically inhibited the recognition by anti-CF3CO antibody of the E2 subunit of pyruvate dehydrogenase, of trifluoroacetylated rabbit serum albumin and of human liver CF3CO-proteins. In sera of patients with halothane hepatitis, autoantibodies with properties identical to those of anti-CF3CO antibody were identified which could not discriminate between CF3CO-proteins and the E2 subunit of pyruvate dehydrogenase. These data suggest that the E2 subunit pyruvate of dehydrogenase is an autoantigen in halothane hepatitis and that molecular mimicry of CF3CO-proteins by the E2 subunit of pyruvate dehydrogenase is due to the similar structures of CF3CO-Lys and lipoic acid.

Glutathione-S-transferase is a target for covalent modification by a halothane reactive intermediate in the guinea pig liver

The anesthetic halothane is bioactivated by the liver cytochrome P450 system to the reactive intermediate, trifluoroacetyl chloride, which can acylate liver protein. Cytosolic glutathione-S-transferase (GST) was identified as a major target for protein adduct formation in guinea pig liver slices exposed to halothane. To determine if GST is also a target in vivo, male Hartley guinea pigs were exposed to 1% halothane in 40% O2 for 4 h. At 10 h post exposure, livers were removed and microsomal and cytosolic fractions prepared. Past studies have shown these conditions resulted in maximal covalent binding of halothane intermediates to hepatic protein. Protein was isolated by ethanol precipitation and washed with trichloroacetic acid to remove unbound metabolites. Cytosolic GST was isolated by gel filtration and S-hexyl-glutathione affinity chromatography to electrophoretic purity. Protein adducts were quantified using a covalently bound fluorine assay. Covalent binding of a halothane intermediate to cytosolic and microsomal protein was determined as 2.0 +/- 0.4 and 13.2 +/- 2.3 nmol F/mg protein, respectively. Liver glutathione depletion by buthionine sulfoximine pretreatment produced an increase in covalent binding only to cytosolic proteins (3.3 +/- 0.4 nmol F/mg protein). Adduct formation to cytosolic GST was determined to be 4.7 +/- 1.6 nmol F/mg protein. Glutathione-S-transferase is a target for covalent modification in the liver following an inhalation exposure to halothane.

Identification of protein disulfide isomerase and calreticulin as autoimmune antigens in LEC strain of rats

Long Evans Cinnamon (LEC) rats, showing spontaneous hereditary hepatitis and hepatic carcinoma, were found to possess autoimmune antibodies to liver microsomal proteins, particularly to proteins with the molecular weight of 56kD and 55kD. The antibodies occurred in association with acute lethal hepatitis in the LEC rats in our previous study. Two-dimensional immunoblot analysis of the antigenic proteins revealed that the 56kDa and 55kDa proteins showed 4.2 and 4.0 pI values and were estimated to be protein disulfide isomerase (PDI) and calreticulin, respectively, from NH2-terminal amino acid sequence analysis. These proteins were further identified by immunoblot analyses using purified proteins and specific antibodies. PDI was a major autoimmune antigenic protein.

Autoantibodies to hepatic microsomal carboxylesterase in halothane hepatitis

Halothane hepatitis can be life-threatening, and this severe adverse reaction may arise via an immune process. We have detected autoantibodies to purified human liver microsomal carboxylesterase in sera of 17 out of 20 patients with halothane hepatitis (85%) but not in 9 halothane-exposed controls and in only 2 (at low levels) of 33 patients with liver disease due to other causes. Immunohistochemical studies localised the carboxylesterase predominantly to the centrilobular region of liver sections, which is consistent with the area affected by halothane hepatitis. Human hepatic microsomal carboxylesterase is a target antigen in halothane hepatitis, and an immune response to this protein may be involved in the liver damage observed.

Association of anti-58 kDa endoplasmic reticulum antibodies with halothane hepatitis

We recently showed that when rats were administered the inhalation anesthetic halothane, a 58 kDa liver endoplasmic reticulum protein became covalently trifluoroacetylated by the trifluoroacetyl chloride metabolite of halothane. Although the 58 kDa protein showed 99% identity to that of the deduced amino acid sequence of a cDNA reported to correspond to phosphatidylinositol-specific phospholipase C-alpha, it did not have phosphatidylinositol-specific phospholipase C activity. It was concluded that the reported cDNA of phosphatidylinositol-specific phospholipase C-alpha actually encoded for the 58 kDa endoplasmic reticulum protein of unknown function. Other researchers have come to the same conclusion and have shown that the 58 kDa protein has protein disulfide-isomerase and protease activities. We now report that patients with halothane hepatitis have serum antibodies that react with both purified trifluoroacetylated and native rat liver 58 kDa proteins. These results suggest that when patients are exposed to halothane a human liver orthologue of the rat liver trifluoroacetylated-58 kDa protein is formed. In certain patients, this protein may become immunogenic and lead to the formation of specific antibodies and or specific T-cells, which may react with both trifluoroacetylated and native 58 kDa proteins, and ultimately be responsible, at least in part, for the hepatitis caused by halothane.

Leakage of mitochondrial and cytosolic GOT in isolated rat hepatocytes exposed to halothane: an immunohistochemical study

The leakage of mitochondrial (m-) and cytosolic (s-) aspartate aminotransferases (glutamic-oxaloacetic transaminase: GOT) from isolated hepatocytes exposed to halothane was investigated immunohistochemically. In control isolated hepatocytes, a large number of cells were immunopositive (m-GOT, 85%; s-GOT, 98.5%) at various intensities. Reaction products of m-GOT-immunohistochemistry (m-GOT-I) were seen exclusively in mitochondria, while those of s-GOT (s-GOT-I) were seen in the cytoplasm. When treated with low concentrations of halothane (up to 0.75%), the number of strongly m-GOT-I-immunopositive cells was reduced to half, while that of s-GOT-I showed no noticeable change. The number of m-GOT-I-immunonegative cells showed a negligible increase, while that of s-GOT-I increased gradually. At higher concentrations of halothane (1% or more), strongly immunopositive cells in both m- and s-GOT-I almost disappeared, while immunonegative cell numbers predominantly increased. This study showed that the isolated hepatocyte system was not homogeneous with regard to the quantity of GOT isozymes, and that halothane could induce the leakage of these isozymes from hepatocytes, along with inducing ultrastructural abnormalities, even at the lowest concentration used (0.5%). Furthermore, the data appear to indicate that the sensitivity of isolated hepatocytes to halothane is dependent on the nature of the hepatocyte itself and on the nature of both types of GOT isozymes.

Production of antibodies that recognize the heterogeneity of immunoreactive sites in human hemoglobin chemically modified by acetaldehyde

Human hemoglobin (Hgb) was incubated with acetaldehyde under two different conditions: (a) in the presence of 250 mM acetaldehyde for 1 hr then reduced with 100 mM NaCNBH3 for an additional 4 hr at room temperature; and (b) in the presence of 500 mM acetaldehyde for 10 days at room temperature and then reduced with 1 mM NaBH4 for 1 hr. It was found that 44% and 27% of free amino groups in Hgb-acetaldehyde adduct (AA) remained unmodified when Hgb was treated under conditions (a) and (b), respectively. SDS-PAGE analysis revealed that the molecular weight of Hgb-AA(a) [Hgb modified under condition (a)] was slightly greater than that of unmodified Hgb and extensive protein cross-linking had occurred in Hgb-AA(b) [Hgb modified under condition (b)]. Electrophoresis on agarose gel showed the order of negative charge was Hgb-AA(b) > Hgb-AA(a) > unmodified Hgb. Polyclonal antibody raised in rabbits using keyhole limpet hemocyanin as the carrier protein modified by acetaldehyde under condition (a) [i.e., KLH-AA(a)] preferentially recognized Hgb-AA(a), whereas antibody raised using KLH-AA(b) as the immunogen recognized only Hgb-AA(b). In conclusion, antibodies raised with protein-AA antigens produced under different conditions recognize different epitopes.

Drug metabolism and genetic polymorphism in subjects with previous halothane hepatitis

To test the hypothesis that halothane hepatitis is caused by a combination of altered drug metabolism and an immunoallergic disposition, the metabolism of antipyrine, metronidazole, sparteine, phenytoin, and racemic R- and S-mephenytoin was investigated in seven subjects with previous halothane hepatitis. The HLA tissue types and the complement C3 phenotypes were also determined. The metabolism of antipyrine and metronidazole was within normal range in all subjects, and they were all fast or extensive metabolizers of sparteine, mephenytoin, and phenytoin. HLA tissue types were unremarkable. Five of the seven subjects had complement C3 phenotypes F or FS. In the general population phenotype S is the most common, but the difference in complement C3 phenotypes is not statistically significant (p = 0.07). We conclude, although in a limited number of patients, that subjects with previous halothane hepatitis do not appear to be different from controls with regard to drug metabolism and HLA tissue type. The possibility of a higher frequency of complement C3 phenotype F and FS needs further investigation.

Metabolic activation of halothane to neoantigens in C57Bl/10 mice: immunochemical studies

C57Bl/10 mice were given halothane (10 mmol/kg, intraperitoneally) and microsomal proteins were analysed for the presence of trifluoroacetylated (TFA) neoantigens by SDS-gel electrophoresis followed by immunoblotting using a polyclonal anti-TFA antibody. In microsomal preparations from liver, lung and olfactory tissues, a 54 kDa TFA adduct was detectable 1 h after dosing. After 3-48 h, multiple bands were detected in liver (45-100 kDa) and in the lung (26-57 kDa) and in one experiment in which [14C]halothane was given, several immunoreactive bands from liver microsomes were shown to contain a covalently bound metabolite of the drug. In olfactory tissue, initially (1 h), a major band of 54 kDa and a less prominent component of about 50 kDa were seen. The number of bands increased at later times but the additional bands were far fewer than in liver. The rate of decay of the 54 kDa adduct was also measured in both liver and olfactory microsomes and found to be compatible with the reported turnover of total liver cytochrome P-450. 24 h after treating mice with halothane (10 mmol/kg), no TFA neoantigens could be detected on the outer cell surface of isolated viable hepatocytes when analysed by fluorescence activated flow cytometry. In contrast, non-viable cells, or those fixed in acetone were all positive. Using immunohistochemistry, TFA neoantigens were demonstrated in the centrilobular area of the liver, the non-ciliated bronchiolar epithelial (Clara) cells of the lung, proximal tubular cells of the kidney and the respiratory and olfactory epithelium of nasal tissues.

Mechanisms of halothane toxicity: novel insights

Exposure of individuals to halothane causes, in 20% of patients, a mild form of hepatotoxicity. In contrast, a very small subset of individuals only develops halothane hepatitis, which is thought to have an immunological basis. Sera of halothane hepatitis patients contain antibodies directed against some discrete liver trifluoroacetyl (TFA)-protein adducts, which arise upon oxidative biotransformation of halothane and include protein disulfide isomerase, microsomal carboxylesterase, calreticulin, ERp72, GRP 78 and ERp99. No immune response occurs in the majority of human individuals, although evidence suggests that TFA-protein adducts arise in all halothane-exposed individuals. The lack of immunological responsiveness of individuals might be due to tolerance, induced by a presumed repertoire of self-peptides that molecularly mimic TFA-protein adducts. Thus, constitutively expressed proteins of 52 and 64 kDa have been identified that confer molecular mimicry of TFA-protein adducts. The 64 kDa protein corresponds to the E2 subunit of the mitochondrial pyruvate dehydrogenase complex. Lipoic acid, the prosthetic group of the E2 subunit, is involved in the molecular mimicry process. A fraction of halothane hepatitis patients exhibit irregularities in the expression levels of the 52 kDa protein and the E2 subunit protein. Molecular mimicry of TFA-protein adducts by the 52 kDa protein and the E2 subunit protein might play a role in the susceptibility of individuals to development of halothane hepatitis.

Molecular mimicry of trifluoroacetylated human liver protein adducts by constitutive proteins and immunochemical evidence for its impairment in halothane hepatitis

A monospecific antibody (anti-CF3CO antibody) was obtained by affinity chromatography on a N epsilon-trifluoroacetyl-L-lysine (CF3CO-Lys) matrix of a rabbit polyclonal antiserum, directed against trifluoroacetylated protein adducts (CF3CO-proteins). The anti-CF3CO antibody recognized distinct CF3CO-proteins on immunoblots of a liver biopsy obtained from a human individual 10 h after halothane anaesthesia. Cross-reactive proteins of 52 kDa and 64 kDa were recognized on immunoblots of livers obtained from human individuals not exposed to halothane. Recognition of both CF3CO-proteins and the 52-kDa and 64-kDa cross-reactive proteins was abolished in the presence of 1 mM CF3CO-Lys. Anti-CF3CO antibody, affinity-adsorbed to the 52-kDa or the 64-kDa cross-reactive proteins of human liver, recognized the majority of target CF3CO-proteins on immunoblots of the human liver biopsy of an individual exposed to halothane. Liver biopsies of 5 out of 7 (71%) patients with halothane hepatitis exhibited an absence or low amounts of immunorecognizable 52-kDa and/or 64-kDa cross-reactive proteins. In contrast, of 22 control human individuals tested, all liver tissue samples were positive for the 52-kDa and/or the 64-kDa cross-reactive proteins. These data indicate that epitopes on the cross-reactive proteins of 52 kDa and 64 kDa of human liver bear strong immunochemical resemblance to epitopes on human liver CF3CO-proteins. Low-level expression of the cross-reactive proteins of 52 kDa and 64 kDa is discussed as one possible factor in human susceptibility to halothane hepatitis.

Idiosyncratic drug reactions: a mechanistic evaluation of risk factors

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The topography of trifluoroacetylated protein antigens in liver microsomal fractions from halothane treated rats

Sera from patients with halothane hepatitis contain immunoglobulin G (IgG) antibodies to trifluoroacetylated liver microsomal proteins of 100, 76, 59, 57 and 54 kDa, which are produced as a consequence of metabolism of halothane to trifluoroacetyl halide by cytochrome(s) P450. In the present study, the membrane topographies of the various antigens in rat liver microsomal fractions were investigated. Liver microsomal fractions from rats treated with halothane in vivo, and rat liver microsomal fractions which had been incubated with halothane in vitro, were used as the source of trifluoroacetyl antigens. The antigens were detected by immunoblotting. Whereas the 100, 76, 59 and 57 kDa antigens were solubilized from the microsomal membrane by either 0.1 M sodium carbonate or 0.1% (w/v) sodium deoxycholate, the 54 kDa antigen was not solubilized by 0.1% (w/v) sodium deoxycholate. In intact microsomal fractions, the 100, 76, 59 and 57 kDa antigens were not degraded appreciably by trypsin unless detergent was added to permeabilize the microsomal membrane. These results indicate that the 54 kDa antigen is an integral membrane protein, whereas the 100, 76, 59 and 57 kDa antigens are peripheral membrane proteins situated within the lumen of microsomal vesicles, and hence presumably located within the lumen of the endoplasmic reticulum in vivo.

The kidney as a novel target tissue for protein adduct formation associated with metabolism of halothane and the candidate chlorofluorocarbon replacement 2,2-dichloro-1,1,1-trifluoroethane

Hydrochlorofluorocarbons (HCFCs) have been identified as chemical replacements of the widely used chlorofluorocarbons (CFCs) that are implicated in stratospheric ozone depletion. Many HCFCs are structural analogues of the anesthetic agent halothane and may follow a common pathway of biotransformation and formation of adducts to protein-centered and other cellular nucleophiles. Exposure of rats to a single dose of halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) or of the candidate CFC substitute HCFC 123 (2,2-dichloro-1,1,1-trifluoroethane) led to the formation of trifluoroacetylated protein adducts (CF3CO-proteins) not only in the liver, but also in the kidney as a novel target tissue for protein trifluoroacetylation. CF3CO-proteins in the kidney amounted to about 5% of those formed in the liver of the same animal. The amount of CF3CO-proteins formed within the kidney was roughly reflected by the capacity of metabolism of halothane or HCFC 123 by rat kidney microsomes in vitro which amounted to about 10% of that observed with liver microsomes. By immunohistochemistry, CF3CO-proteins in the kidney were mainly localized in the tubular segments of the cortex. In the liver, the density of CF3CO-proteins decreased from the central vein towards the portal triad. In vitro incubation of rat liver microsomes with halothane or HCFC 123 resulted in extensive formation of CF3CO-proteins and reproduced faithfully the pattern of liver CF3CO-proteins obtained in vivo. CF3CO-proteins generated in vitro were immunochemically not discernible from those generated in vivo. Glutathione (5 mM) and cysteine (5 mM) virtually abolished CF3CO-protein formation; the release of Br- from halothane and Cl- from HCFC 123 was reduced to much lesser a degree. S-Methyl-glutathione, N-acetyl-cysteine, methionine, and N-acetyl-methionine only slightly affected the formation of CF3CO-proteins or metabolism of either substrate. The data suggest that metabolism and concomitant CF3CO-protein formation of halothane or of candidate CFC replacements like HCFC 123 is not restricted to the liver but also takes place in the kidney. Furthermore, an in vitro system for CF3CO-protein formation has been developed and used to show that protein-centered and glutathione-centered nucleophilic sites compete for intermediates of metabolism of halothane or of HCFC 123.

Halothane metabolism. Impairment of hepatic omega-oxidation of leukotrienes in vivo and in vitro

Omega-oxidation of leukotrienes is the initial step of hepatic degradation and thus inactivation of these proinflammatory mediators. Omega-oxidation is followed by beta-oxidation of leukotrienes from the omega-end. After exposure of rats to a single dose of the anesthetic agent halothane, a transient decrease in leukotriene omega-oxidation was induced both in vivo and in vitro. In untreated rats, 44.1 +/- 6.0% of N-[3H]acetylleukotriene E4 injected intravenously was recovered unchanged in bile collected for 60 min in vivo; 46.5 +/- 3.0% was recovered as omega-/beta-oxidation products, of which 24.7 +/- 4.5% were associated with beta-oxidation products only (mean +/- SEM; n = 5). In rats receiving a single dose of halothane 18 h before the experiment, recovery of unchanged N-[3H]acetylleukotriene E4 was significantly increased to 79.8 +/- 4.8%, while the fraction of omega-/beta-oxidation products decreased to 9.0 +/- 1.7% (n = 5); 90 h after exposure to halothane, N-[3H]acetylleukotriene E4 recovery decreased to 30.0 +/- 3.0% and omega-/beta-oxidation products amounted to 49.1 +/- 3.8%; the fraction of beta-oxidation products was significantly increased to 43.1 +/- 3.4% (n = 5). Ten days after exposure of rats to halothane, the recoveries of N-[3H]acetylleukotriene E4, of omega-/beta-oxidation products, and of beta-oxidation products alone, returned to almost normal values. Microsomal fractions obtained from rat hepatocytes catalyzed the NADPH- and O2-dependent leukotriene omega-oxidation in vitro. The formation of omega-hydroxy-metabolites of leukotriene B4, leukotriene E4, and N-acetylleukotriene E4 was decreased by 50% in microsomal fractions obtained from rats 18 h and 90 h after halothane treatment, and returned back to control levels in microsomal fractions obtained 10 days after halothane treatment. The Km value of leukotriene B4 omega-oxidation revealed no significant change in enzyme affinity towards leukotriene B4; in contrast, as reflected by the reduction of the Vmax value by 65%, a decrease in the amount of the active enzyme in microsomes obtained from rats 18 h after halothane treatment was observed. Halothane-metabolism-dependent trifluoroacetylation of hepatic proteins may mediate this process. Thus, the time course of the density on immunoblots of trifluoroacetylated protein adducts paralleled that of the transient decrease in leukotriene omega-oxidation. In contrast to its omega-oxidation, leukotriene B4 synthesis from 5-hydroperoxyeicosatetraenoate was not inhibited in hepatocyte homogenates obtained from rats pretreated with halothane. The data suggest that metabolism of halothane causes a transient derangement of hepatic leukotriene homeostasis in vivo.

Exposure to the chlorofluorocarbon substitute 2,2-dichloro-1,1,1- trifluoroethane and the anesthetic agent halothane is associated with transient protein adduct formation in the heart

Hydrochlorofluorocarbons (HCFCs) that are structural analogues of the anesthetic agent halothane may follow a common pathway of bioactivation and formation of adducts to cellular targets of distinct tissues. Exposure of rats to a single dose of HCFC 123 (2,2-dichloro- 1,1,1-trifluoroethane) or its structural analogue halothane (2-bromo-2-chloro-1,1,1-trifluoroethane) in vivo resulted in the formation of one prominent trifluoroacetylated protein adduct (TFA-protein adduct) in the heart. In contrast, a variety of distinct TFA-protein adducts were formed in the liver and the kidney of the same animals. The TFA-protein adduct in the heart was processed rapidly; t1/2 of the intact TFA-protein adduct was less than 12 h.

Formation and identification of protein adducts to cytosolic proteins in guinea pig liver slices exposed to halothane

The anesthetic halothane can be bioactivated to the reactive intermediate, trifluoroacetyl chloride, which can covalently bind to liver protein. The product of this reaction is trifluoroacetyl-N-epsilon-lysine which can act as a foreign epitope in altering both protein immunogenicity and antigenicity. An in vitro liver slice system was used to study the formation of protein adducts following exposure to halothane. Liver slices (30-35 mg wet weight, 250-300 microns thick) from adult male Hartley guinea pigs (600-800 g) were exposed to [14C]halothane (0.6-0.9 microCi, 1.0-1.7 mM) in 95% O2/5% CO2 for 1, 6 and 12 h. The slices were homogenized and subcellular fractions prepared. Proteins were resolved by electrophoresis and bound radioactivity was detected by scintillation counting and autoradiography. Greater than 80% of detectable radioactivity to whole liver cell protein was localized in the 20-30-kDa range and increased in a linear fashion over the 12-h incubation period. Covalent binding was localized to two proteins of 27 kDa and 26 kDa present in the cytosolic compartment. Purification followed by N-terminal amino acid sequence analysis of the 27-kDa protein has identified it to be homologous with glutathione-S-transferase b. This cytosolic protein appears to be the major target for trifluoroacetylation in liver slices exposed to halothane.