What Are the Potential Sites of Protein Arylation by N-Acetyl-p-benzoquinone Imine (NAPQI)?

A triple cysteine motif as major determinant of the modulation of neuronal KV7 channels by the paracetamol metabolite N-acetyl-p-benzo quinone imine

BACKGROUND AND PURPOSE

The analgesic action of paracetamol involves KV7 channels, and its metabolite N-acetyl-p-benzo quinone imine (NAPQI), a cysteine modifying reagent, was shown to increase currents through such channels in nociceptors. Modification of cysteine residues by N-ethylmaleimide, H2O2, or nitric oxide has been found to modulate currents through KV7 channels. The study aims to identify whether, and if so which, cysteine residues in neuronal KV7 channels might be responsible for the effects of NAPQI.

EXPERIMENTAL APPROACH

To address this question, we used a combination of perforated patch-clamp recordings, site-directed mutagenesis, and mass spectrometry applied to recombinant KV7.1 to KV7.5 channels.

KEY RESULTS

Currents through the cardiac subtype KV7.1 were reduced by NAPQI. Currents through all other subtypes were increased, either by an isolated shift of the channel voltage dependence to more negative values (KV7.3) or by such a shift combined with increased maximal current levels (KV7.2, KV7.4, KV7.5). A stretch of three cysteine residues in the S2-S3 linker region of KV7.2 was necessary and sufficient to mediate these effects.

CONCLUSION AND IMPLICATION

The paracetamol metabolite N-acetyl-p-benzo quinone imine (NAPQI) modifies cysteine residues of KV7 subunits and reinforces channel gating in homomeric and heteromeric KV7.2 to KV7.5, but not in KV7.1 channels. In KV7.2, a triple cysteine motif located within the S2-S3 linker region mediates this reinforcement that can be expected to reduce the excitability of nociceptors and to mediate antinociceptive actions of paracetamol.

Glutathione conjugation and protein modification resulting from metabolic activation of pesticide metalaxyl in vitro and in vivo

Metalaxyl (MTL), a germicidal agent, is widely used in agriculture. Due to the biological amplification effect, MTL entering the ecological environment would result in a threat to human health through the food chain. MTL is reportedly accumulated in liver. The objectives of the study included investigating the metabolic activation of MTL in liver and defining the mechanisms participating in the hepatotoxicity of MTL. The corresponding glutathione (GSH), N-acetylcysteine (NAC) conjugate, and cysteine conjugates were observed in liver microsomes, prepared from liver tissues of mice, containing MTL and GSH, NAC or cysteine. These conjugates were also detected in urine and bile of rats receiving MTL. Apparently, MTL was biotransformed to a quinone imine intermediate dose-dependently attacking the thiols and cysteine residues of protein. The bioactivation of MTL required cytochrome P450 enzymes, and CYP3A dominated the bio-activation of MTL.

Caesalpinia bonducella Counteracts Paracetamol-Instigated Hepatic Toxicity via Modulating TNF-α and IL-6/10 Expression and Bcl-2 and Caspase-8/3 Signalling

Paracetamol is the most predominantly used antipyretic and analgesic drug. As paracetamol is metabolised mostly in the liver, both deliberate and unintentional overdoses of paracetamol are reported to provoke severe hepatotoxicity, including liver failure. Caesalpinia bonducella seed is well known for its medicinal and therapeutic properties. However, there is no report on its potential protective effects against paracetamol-instigated hepatotoxicity. Therefore, we studied the protective effects of aqueous seed extract of Caesalpinia bonducella (ASECB) on paracetamol-instigated hepatotoxicity in rats. Thirty female albino rats were divided into five groups: control, paracetamol-intoxicated, ASECB + paracetamol, silymarin + paracetamol, and ASECB alone. The rats were assessed for liver enzyme markers (alkaline phosphatase, alanine aminotransferase, aspartate aminotransferase, gamma-glutamyl transpeptidase), antioxidant activity (superoxide dismutase, catalase, reduced glutathione, glutathione peroxidase), lipid peroxidation (malondialdehyde), histopathological, cytokine levels (pro-inflammatory cytokines TNF-α and IL-6, and anti-inflammatory cytokine IL-10), and protein expression (pro-apoptotic markers caspase 3 and caspase 8 and anti-apoptotic marker Bcl-2) after the 8-day study period. Repercussions of paracetamol intoxication induced upregulation of liver enzyme markers, antioxidant depletion, malondialdehyde production, decreased expression of Bcl-2 and IL-10, and overexpression of apoptotic and pro-inflammatory mediators, which were attenuated by pre-treatment with ASECB. ASECB markedly mitigated paracetamol-instigated liver injury by suppressing caspase-8/3 signalling and inflammatory infiltration in liver tissue by significantly reducing TNF-α and IL-6. In conclusion, ASECB pre-treatment exerts potent liver protection against paracetamol-instigated hepatotoxicity evidenced by mitigation of oxidative stress, lipid peroxidation, inflammation, and apoptosis.

Advances in the understanding of acetaminophen toxicity mechanisms: a clinical toxicology perspective

INTRODUCTION

Acetaminophen (paracetamol) is a commonly used analgesic and antipyretic agent, which is safe in therapeutic doses. Acetaminophen poisoning due to self-harm or repeated supratherapeutic ingestion is a common cause of acute liver injury. Acetylcysteine has been a mainstay of treatment for acetaminophen poisoning for decades and is efficacious if administered early. However, treatment failures occur if administered late, in 'massive' overdoses or in high-risk patients.

AREAS COVERED

This review provides an overview of the mechanisms of toxicity of acetaminophen poisoning (metabolic and oxidative phase) and how this relates to the assessment and treatment of the acetaminophen poisoned patient. The review focuses on how these advances offer further insight into the utility of novel biomarkers and the role of proposed adjunct treatments.

EXPERT OPINION

Advances in our understanding of acetaminophen toxicity have allowed the development of novel biomarkers and a better understanding of how adjunct treatments may prevent acetaminophen toxicity. Newly proposed adjunct treatments like fomepizole are being increasingly used without robust clinical trials. Novel biomarkers (not yet clinically available) may provide better assessment of these newly proposed adjunct treatments, particularly in clinical trials. These advances in our understanding of acetaminophen toxicity and liver injury hold promise for improved diagnosis and treatment.

Desmin Reorganization by Stimuli Inducing Oxidative Stress and Electrophiles: Role of Its Single Cysteine Residue

The type III intermediate filament proteins vimentin and GFAP are modulated by oxidants and electrophiles, mainly through perturbation of their single cysteine residues. Desmin, the type III intermediate filament protein specific to muscle cells, is critical for muscle homeostasis, playing a key role in sarcomere organization and mitochondrial function. Here, we have studied the impact of oxidants and cysteine-reactive agents on desmin behavior. Our results show that several reactive species and drugs induce covalent modifications of desmin in vitro, of which its single cysteine residue, C333, is an important target. Moreover, stimuli eliciting oxidative stress or lipoxidation, including H2O2, 15-deoxy-prostaglandin J2, and CoCl2-elicited chemical hypoxia, provoke desmin disorganization in H9c2 rat cardiomyoblasts transfected with wild-type desmin, which is partially attenuated in cells expressing a C333S mutant. Notably, in cells lacking other cytoplasmic intermediate filaments, network formation by desmin C333S appears less efficient than that of desmin wt, especially when these proteins are expressed as fluorescent fusion constructs. Nevertheless, in these cells, the desmin C333S organization is also protected from disruption by oxidants. Taken together, our results indicate that desmin is a target for oxidative and electrophilic stress, which elicit desmin remodeling conditioned by the presence of its single cysteine residue.

Integrated Imaging and Proteomic Sensors Resolve Proteome Aggregation in Liver Caused by Non-steroidal Anti-inflammatory Drug Overdose

Given the extreme heterogeneity and the loss of defined protein structures, misfolded and aggregated proteins are technically challenging to visualize and analyze. Herein, we assembled an integrated sensor system to resolve aggregated proteome in live cells and animal liver tissues that are overdosed by non-steroidal anti-inflammatory drugs (NSAIDs). A fluorogenic protein aggregation sensor (AggStain) first discovered the presence of aggregated proteome upon overdosing liver cells with NSAIDs. A solvatochromic protein aggregation sensor (AggRetina) further quantified the compactness (polarity) inside these cellular aggregates. Importantly, we exploited a proteomic sensor (AggLink) to selectively capture aggregated proteins upon NSAID overdose and profile their composition, revealing global collapse of cellular protein homeostasis. Finally, we detected subtle proteome aggregation in mouse liver tissue without obvious acute injury at a low NSAID dosage. Overall, we demonstrated an integrated sensor toolset for proteome aggregation studies and unveiled for the first time that NSAID overdose can cause proteome aggregation in liver cells and tissues.

Identifying chemicals based on receptor binding/bioactivation/mechanistic explanation associated with potential to elicit hepatotoxicity and to support structure activity relationship-based read-across

The liver is the most common target organ in toxicology studies. The development of chemical structural alerts for identifying hepatotoxicity will play an important role in in silico model prediction and help strengthen the identification of analogs used in structure activity relationship (SAR)- based read-across. The aim of the current study is development of an SAR-based expert-system decision tree for screening of hepatotoxicants across a wide range of chemistry space and proposed modes of action for clustering of chemicals using defined core chemical categories based on receptor-binding or bioactivation. The decision tree is based on ∼ 1180 different chemicals that were reviewed for hepatotoxicity information. Knowledge of chemical receptor binding, metabolism and mechanistic information were used to group these chemicals into 16 different categories and 102 subcategories: four categories describe binders to 9 different receptors, 11 categories are associated with possible reactive metabolites (RMs) and there is one miscellaneous category. Each chemical subcategory has been associated with possible modes of action (MOAs) or similar key structural features. This decision tree can help to screen potential liver toxicants associated with core structural alerts of receptor binding and/or RMs and be used as a component of weight of evidence decisions based on SAR read-across, and to fill data gaps.

The molecular mechanisms of acetaminophen-induced hepatotoxicity and its potential therapeutic targets

Acetaminophen (APAP), a widely used antipyretic and analgesic drug in clinics, is relatively safe at therapeutic doses; however, APAP overdose may lead to fatal acute liver injury. Currently, N-acetylcysteine (NAC) is clinically used as the main antidote for APAP poisoning, but its therapeutic effect remains limited owing to rapid disease progression and the general diagnosis of advanced poisoning. As is well known, APAP-induced hepatotoxicity (AIH) is mainly caused by the toxic metabolite N-acetyl-p-benzoquinone imine (NAPQI), and the toxic mechanisms of AIH are complicated. Several cellular processes are involved in the pathogenesis of AIH, including liver metabolism, mitochondrial oxidative stress and dysfunction, sterile inflammation, endoplasmic reticulum stress, autophagy, and microcirculation dysfunction. Mitochondrial oxidative stress and dysfunction are the major cellular events associated with APAP-induced liver injury. Many biomolecules involved in these biological processes are potential therapeutic targets for AIH. Therefore, there is an urgent need to comprehensively clarify the molecular mechanisms underlying AIH and to explore novel therapeutic strategies. This review summarizes the various cellular events involved in AIH and discusses their potential therapeutic targets, with the aim of providing new ideas for the treatment of AIH.

Paracetamol (acetaminophen) overdose and hepatotoxicity: mechanism, treatment, prevention measures, and estimates of burden of disease

INTRODUCTION

Paracetamol is one of the most used medicines worldwide and is the most common important poisoning in high-income countries. In overdose, paracetamol causes dose-dependent hepatotoxicity. Acetylcysteine is an effective antidote, however despite its use hepatotoxicity and many deaths still occur.

AREAS COVERED

This review summarizes paracetamol overdose and toxicity (including mechanisms, risk factors, risk assessment, and treatment). In addition, we summarize the epidemiology of paracetamol overdose worldwide. A literature search on PubMed for poisoning epidemiology and mortality from 1 January 2017 to 26 October 2022 was performed to estimate rates of paracetamol overdose, liver injury, and deaths worldwide.

EXPERT OPINION

Paracetamol is widely available and yet is substantially more toxic than other analgesics available without prescription. Where data were available, we estimate that paracetamol is involved in 6% of poisonings, 56% of severe acute liver injury and acute liver failure, and 7% of drug-induced liver injury. These estimates are limited by lack of available data from many countries, particularly in Asia, South America, and Africa. Harm reduction from paracetamol is possible through better identification of high-risk overdoses, and better treatment regimens. Large overdoses and those involving modified-release paracetamol are high-risk and can be targeted through legislative change.

The protein adduction derived from reactive metabolites of multiple furanoids in cortex Dictamni-treated mice

The association of herb medicine Cortex Dictamni (CD) with severe even fatal hepatotoxicity has been widely reported. Recently, we demonstrated that the metabolic activation of at least ten furanoids in CD was responsible for the liver injury caused by the ethanol extract of CD (ECD) in mice. Protein adduction by reactive metabolites is considered to initiate the process of liver injury. Unlike single chemicals, the mode of and the details of protein modification by multiple components in an herb is unclear. This study aimed to characterize protein adductions derived from the reactive metabolite of furanoids in ECD-treated mice and define the association of protein adduction with liver injury. The hepatic cysteine- and lysine-based protein adducts derived from epoxide or cis-enedione of at least six furanoids were identified in mice. The furanoids with an earlier serum content T_max were mainly to bind with hepatic glutathione and no protein adducts were formed except for dictamnine. The hepatic proteins were modified by the later absorbed furanoids. The levels of hepatic protein adduct were correlated with the degree of liver injury. In addition, the reactive metabolites of different furanoids can simultaneously bind to the model peptide by the identical reactive moiety, indicating the additive effects of the individual furanoids in the modification of hepatic proteins. In conclusion, hepatic protein adduction by multiple furanoids may play a role in ECD-induced liver injury. The earlier absorbed furanoids were mainly to bind with glutathione whereas the hepatic proteins were modified by the later furanoids.

Estimation of ZYKR1 in human urine and plasma utilizing LC-MS/MS positive electrospray ionization; a kappa opioid receptor (KOR) agonist

ZYKR1, a short chain novel peptide with selective kappa opioid receptor agonist activity used as analgesics for the treatment of pain management. A sensitive and selective LC-MS/MS assay was developed and validated for estimation of ZYKR1 in human urine and plasma. ZY17258, an analogue compound was used as an internal standard. ZYKR1 was quantified using a selective reaction monitoring in electrospray ionization positive mode. The chromatographic separation was performed using mobile phase consisted of 0.05% v/v formic acid in water and methanol in gradient elution by analytical column Kinetex C₈, 100 A°, 5 µm, 100 × 4.6 mm with 8.0 min analytical run time. Solid Phase extraction technique was used for purification of ZYKR1 and IS from human urine and plasma. The calibration curves were linear over range of 0.300 ng/mL to 300 ng/mL and 0.500 ng/mL to 500 ng/mL for human urine and plasma, respectively. No matrix effect and no significant carryover were observed. The extraction recovery was consistent and ranged from about 85% to 93% in human urine and in plasma respectively. Inter-day and intra-day accuracy (bias, %) and precision (CV, %) was -11.11 to 5.91 % and -2.25 to 6.65 % in human urine and -2.74 to 7.17 % and 2.24 to 15.18 % in plasma respectively were well within the acceptance criteria. Both the assays were devoid of endogenous matrix interference and commonly used concomitant drug interference. The validated assays were used for estimation of ZYKR1 from clinical pharmacokinetic study sample bioanalysis in healthy human subjects.

Evaluation of KP-1199: a novel acetaminophen analog for hemostatic function and antinociceptive effects

BACKGROUND

Acetaminophen (APAP) is a widely self-prescribed analgesic for mild to moderate pain, but overdose or repeat doses can lead to liver injury and death. Kalyra Pharmaceuticals has developed a novel APAP analog, KP-1199, currently in Phase 1 clinical studies, which lacks hepatotoxicity. In this study, the authors evaluated the antinociceptive effect of KP-1199 on thermal injury-induced nociceptive behaviors as well as hemostatic parameters using human blood samples.

METHODS

Full-thickness thermal injury was induced in anesthetized adult male Sprague-Dawley rats. On day 7 post-injury, KP-1199 (30 and 60 mg/kg) or APAP (60 mg/kg) was administered orally. Antinociception of KP-1199 and APAP were assessed at multiple time points using Hargreaves' test. In separate experiments, human whole blood was collected and treated with either KP-1199, APAP, or Vehicle (citrate buffer) at 1× (214 μg/ml) and 10× (2140 μg/ml) concentrations. The treated blood samples were assessed for: clotting function, thrombin generation, and platelet activation.

RESULTS

APAP did not produce antinociceptive activity. KP-1199 treatment significantly increased the nociceptive threshold, and the antinociceptive activity persisted up to 3 h post-treatment. In human samples, 10× APAP caused significantly prolonged clotting times and increased platelet activation, whereas KP-1199 had caused no negative effects on either parameter tested.

CONCLUSION

These results suggest that KP-1199 possesses antinociceptive activity in a rat model of thermal injury. Since KP-1199 does not induce platelet activation or inhibit coagulation, it presents an attractive alternative to APAP for analgesia, especially for battlefield or surgical scenarios where blood loss and blood clotting are of concern.

Chemical probes for drug-induced liver injury imaging

Drug-induced liver injury (DILI) has been a long-standing concern of modern medicine, and the single most frequent reason for drug nonapprovals and postapproval restrictions or withdrawals. Chemical probes for early diagnosis of DILI has triggered a tremendous interest in the field of molecular imaging. In this review, we make a brief summary of the recently developed chemical probes and their applications in DILI imaging with special attention to the design of chemical probes, mechanism of their actions and their performances in DILI imaging.

What are the Potential Sites of DNA Attack by N-Acetyl-p-benzoquinone Imine (NAPQI)?

Metabolic bioactivation of small molecules can produce electrophilic metabolites that can covalently modify proteins and DNA. Paracetamol (APAP) is a commonly used over-the-counter analgesic, and its hepatotoxic side effects have been postulated to be due to the formation of the electrophilic metabolite N-acetylbenzoquinone imine (NAPQI). It has been established that NAPQI reacts to form covalent bonds to the side-chain functional groups of cysteine, methionine, tyrosine, and tryptophan residues. While there have been scattered reports that APAP can form adducts with DNA the nature of these adducts have not yet been fully characterised. Here the four deoxynucleosides, deoxyguanosine (dG), deoxyadenosine (dA), deoxycytidine (dC), and deoxythymidine (dT) were reacted with NAPQI and the formation of adducts was profiled using liquid chromatography–mass spectrometry with positive-ion mode electrospray ionisation and collision-induced dissociation. Covalent adducts were detected for dG, dA, and dC and tandem mass spectrometry (MS/MS) spectra revealed common neutral losses of deoxyribose (116 amu) arising from cleavage of the glyosidic bond with formation of the modified nucleobase. Of the four deoxynucleosides, dC proved to be the most reactive, followed by dG and dA. A pH dependence was found, with greater reactivity being observed at pH 5.5. The results of density functional theory calculations aimed at understanding the relative reactivities of the four deoxynucleosides towards NAPQI are described.

Pharmacokinetic changes of tramadol in rats with hepatotoxicity induced by ethanol and acetaminophen in perfused rat liver model

Tramadol is an opioid agonist with activation monoaminergic properties. It can be administered orally, rectally, intravenously, or intramuscularly as a centrally acting analgesic. Liver injury can lead to changes in the metabolism of tramadol. In this study, the rate of tramadol metabolism in rats with damaged liver induced by ethanol and acetaminophen was assessed in a recirculation perfusion system. Acetaminophen is a mild analgesic and antipyretic agent, which can cause centrilobular hepatic necrosis in toxic doses, whereas alcohol causes death due to liver diseases. Alcoholic liver disease (ALD), such as alcoholic fatty liver, alcoholic hepatitis, and alcoholic fibrosis, is the most common liver disease. The aim of this study was to investigate the alteration in tramadol metabolism in different hepatotoxicity conditions in animal models. Male rats were randomly assigned to three groups. The control group received normal saline, group 2 received acetaminophen at the dose of 250 mg/kg/day, and group 3 received ethanol at the beginning dose of 3 g/kg/day, which was slowly increased to 6 g/kg/day. Tramadol was added to the perfusion solution at the concentration of 500 ng/mL. Samples were collected during 180 min, and analyte concentrations were determined by the High-Performance Liquid Chromatography (HPLC) method. The concentration of tramadol and its three main metabolites, O-desmethyltramadol (M1), N-desmethyltramadol (M2), and N,O-didesmethyltramadol (M5), were determined in perfusate samples. Ethanol and acetaminophen significantly affected the pattern of weight gain and liver weights before perfusion and caused a significant increase in enzyme activities. Moreover, histopathologic examination revealed that ethanol and acetaminophen caused liver damage. An increase in the elimination half-life and reduced clearance rate of tramadol were seen in the acetaminophen and ethanol groups, in comparison to the control group. Additionally, significant reductions in the Area Under the Curve (AUC) of metabolites of tramadol (M1, M2, and M5) were observed in the acetaminophen and ethanol groups in the perfused rat liver model. Liver damage caused by ethanol and acetaminophen during 45 days in animals leads to a significant reduction in the level of tramadol metabolites. Therefore, in patients with liver damage caused by ethanol and acetaminophen, caution needs to be considered when prescribing tramadol.

Mechanisms of soft and hard electrophile toxicities

Electron-deficient chemicals (electrophiles) react with compounds that have one or more unshared valence electron pairs (nucleophiles). The resulting covalent reactions between electrophiles and nucleophiles (e.g., Michael addition, S_N2 reactions) are important, not only to Organic Chemistry, but also to the fields of Molecular Biology and Toxicology. Specifically, covalent bond formation is the operational basis of many critically important cellular processes; e.g., enzyme function, neurotransmitter release, and membrane-vesicle fusion. Given this context it is understandable that these reactions are also relevant to Toxicology, since a significant number of xenobiotic chemicals are toxic electrophiles that can react with endogenous nucleophilic residues. Therefore, the purpose of this Review is to discuss electrophile-nucleophile chemistry as it pertains to cell injury and resulting organ toxicity. Our discussion will involve an introduction to the Hard and Soft, Acids and Bases (HSAB) theory of Pearson. The HSAB concept provides a framework for calculation of quantum chemical parameters that classify the electrophile and nucleophile covalent components according to their respective electronic nature (softness/hardness) and reactivity (electrophilicity/nucleophilicity). The calculated quantum indices in conjunction with corroborative in vivo, in chemico (cell free) and in vitro research can offer an illuminating approach to mechanistic discovery. Accordingly, we will provide examples that demonstrate how this approach has been used to discern mechanisms and sites of electrophile action.

Genetic Association of Single Nucleotide Polymorphisms with Acetaminophen-Induced Hepatotoxicity

Acetaminophen is commonly used to reduce pain and fever. Unfortunately, overdose of acetaminophen is a leading cause of acute liver injury and failure in many developed countries. The majority of acetaminophen is safely metabolized in the liver and excreted in the urine; however, a small percentage is converted to the highly reactive N-acetyl-p-benzoquinone imine (NAPQI). At therapeutic doses, NAPQI is inactivated by glutathione S-transferases, but at toxic levels, excess NAPQI forms reactive protein adducts that lead to hepatotoxicity. Individual variability in the response to both therapeutic and toxic levels of acetaminophen suggests a genetic component is involved in acetaminophen metabolism. In this review, we evaluate the genetic association studies that have identified 147 single nucleotide polymorphisms linked to acetaminophen-induced hepatotoxicity. The identification of novel genetic markers for acetaminophen-induced hepatotoxicity provides a rich resource for further evaluation and may lead to improved prognosis, prevention, and treatment.

The effects of colostrum on some biochemical parameters in the experimental intoxication of rats with paracetamol

In the current study, the possible prophylactic and therapeutic effects of colostrum (COL) on acute organ injury caused by paracetamol (PAR) in rats were evaluated. Within the scope of this study, a 2-month-old male (150-200 g) 70 Wistar Albino rat was used and a total of seven groups were designed. The first group (CNT) was maintained for control purposes. The second group (COL-1) was given COL for 1 day, at a dose of 500 mg/kg at 6-h intervals, and blood and tissue sampling was performed at 24 h. The third group (COL-7) received COL for 7 days, at a dose of 500 mg/kg at 6-h intervals on day 1 and at a daily dose of 500 mg/kg on the following days, and blood and tissue samples were taken at the end of seventh day. The fourth group (PAR-1) was administered with PAR at a dose of 1.0 g/kg bw and was blood and tissue sampled at 24 h. The fifth group (PAR-7) received PAR at a dose of 1.0 g/kg bw on day 1 and was blood and tissue was removed at the end of day 7. The sixth group (PAR+COL-1) was administered with a combination of PAR (1 g/kg bw) and COL (500 mg/kg at 6-h intervals), and blood and tissue samples were collected at 24 h. The seventh group (PAR+COL-7) received 1.0 g/kg bw of PAR on day 1 and was given COL throughout the 7-day study period (at a dose of 500 mg/kg at 6-h intervals on day 1 and at a daily dose of 500 mg/kg on the following days). In the seventh group, blood and tissue samples were taken at the end of seventh day. Alkaline phosphatase (ALP), alanine aminotransferase (ALT), aspartate aminotransferase (AST), blood urea nitrogen (BUN), glucose, creatinine, triglyceride, total bilirubin, total protein and albumin levels/activities were analysed in the serum samples. The malondialdehyde (MDA), nitric oxide (NO), superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GSH-Px) levels/activities, known as oxidative stress parameters, were assayed for tissue homogenates and blood (erythrocytes/plasma); in addition, enzyme activities of GSH S-transferase (GST), cytochrome P4502E1 (CYP2E1), NADH-cytochrome b5 reductase (CYTB5), glucose-6-phosphate dehydrogenase (G6PD), NADPH-cytochrome P450 C reductase (CYTC) and glutathione (GSH) levels/activities defined as drug metabolising parameters were measured in liver homogenates. In result, it was determined that PAR caused significant alterations in some biochemical and lipid peroxidation parameters and the activities/levels of drug metabolising parameters in the liver and that COL normalised some of these parameters and reduced PAR-induced tissue damage.

Reactive metabolites of acetaminophen activate and sensitize the capsaicin receptor TRPV1

The irritant receptor TRPA1 was suggested to mediate analgesic, antipyretic but also pro-inflammatory effects of the non-opioid analgesic acetaminophen, presumably due to channel activation by the reactive metabolites parabenzoquinone (pBQ) and N-acetyl-parabenzoquinonimine (NAPQI). Here we explored the effects of these metabolites on the capsaicin receptor TRPV1, another redox-sensitive ion channel expressed in sensory neurons. Both pBQ and NAPQI, but not acetaminophen irreversibly activated and sensitized recombinant human and rodent TRPV1 channels expressed in HEK 293 cells. The reducing agents dithiothreitol and N-acetylcysteine abolished these effects when co-applied with the metabolites, and both pBQ and NAPQI failed to gate TRPV1 following substitution of the intracellular cysteines 158, 391 and 767. NAPQI evoked a TRPV1-dependent increase in intracellular calcium and a potentiation of heat-evoked currents in mouse spinal sensory neurons. Although TRPV1 is expressed in mouse hepatocytes, inhibition of TRPV1 did not alleviate acetaminophen-induced hepatotoxicity. Finally, intracutaneously applied NAPQI evoked burning pain and neurogenic inflammation in human volunteers. Our data demonstrate that pBQ and NAQPI activate and sensitize TRPV1 by interacting with intracellular cysteines. While TRPV1 does not seem to mediate acetaminophen-induced hepatotoxicity, our data identify TRPV1 as a target of acetaminophen with a potential relevance for acetaminophen-induced analgesia, antipyresia and inflammation.

Nontargeted Identification of Reactive Metabolite Protein Adducts

Metabolic bioactivation of many different chemicals results in the formation of highly reactive compounds (chemically reactive metabolites, CRMs) that can lead to toxicity via binding to macromolecular targets (e.g., proteins or DNA). There is a need to develop robust, rapid, and nontargeted analytical techniques to determine the identity of the protein targets of CRMs and their sites of modification. Here, we introduce a nontargeted methodology capable of determining both the identity of a CRM formed from an administered compound as well as the protein targets modified by the reactive metabolite in a single experiment without prior information. Acetaminophen (N-acetyl-p-aminophenol, APAP) and ¹³C₆-APAP were incubated with rat liver microsomes, which are known to bioactivate APAP to the reactive metabolite N-acetyl-p-benzoquinone imine (NAPQI). Global tryptic digestion followed by liquid chromatographic/mass spectrometric (LC/MS) analysis was used to locate "twin" ion peaks of peptides adducted by NAPQI and for shotgun proteomics via tandem mass spectrometry (MS/MS). By the development of blended data analytics software called Xenophile, the identity of the amino acid residue that was adducted can be established, which eliminates the need for specific parametrization of protein database search algorithms. This combination of experimental design and data analysis software allows the identity of a CRM, the protein target, and the amino acid residues that are modified to be rapidly established directly from experimental data. Xenophile is freely available from https://github.com/mgleeming/Xenophile .

Enolate-Forming Compounds as a Novel Approach to Cytoprotection

Evidence from laboratory studies and clinical trials suggests that plant-derived polyphenolic compounds such as curcumin, resveratrol, or phloretin might be useful in the treatment of certain diseases (e.g., Alzheimer's disease) and acute tissue injury states (e.g., spinal cord trauma). However, despite this potential, the corresponding chemical instability, toxic potential, and low bioavailability of these compounds could limit their ultimate clinical relevance. We have shown that pharmacophores of curcumin (e.g., 2-acetylcyclopentanone) and phloretin (e.g., 2',4',6'-trihydroxyacetophenone; THA) can provide cytoprotection in cell culture and animal models of oxidative stress injury. These pharmacophores are 1,3-dicarbonyl and polyphenol derivatives, the enol groups of which can ionize in biological solutions to form an enolate. This carbanionic moiety can chelate metal ions and, as a nucleophile, can scavenge toxic electrophiles (e.g., acrolein, 4-hydroxy-2-nonenal, and N-acetyl-p-benzoquinone imine) involved in many pathogenic conditions. Aromatic derivatives such as THA can also trap free oxygen and nitrogen radicals and thereby provide another layer of cytoprotection. The multifunctional character of these enolate-forming compounds suggests an ability to block pathogenic processes (e.g., oxidative stress) at several steps. The purpose of this review is to discuss research supporting our theory that enolate formation is a significant cytoprotective property that represents a platform for development of pharmacotherapeutic approaches to a variety of toxic and pathogenic conditions. Our discussion will focus on mechanism and structure-activity studies that define enolate chemistry and their corresponding relationships to cytoprotection.