In Vitro and in Vivo Studies of the Metabolic Activation of 8-Epidiosbulbin E Acetate

Identification of an Epoxide Metabolite of Amitriptyline In Vitro and In Vivo

Amitriptyline (ATL), a tricyclic antidepressant, has been reported to cause various adverse effects, particularly hepatotoxicity. The mechanisms of ATL-induced hepatotoxicity remain unknown. The study was performed to identify the olefin epoxidation metabolite of ATL and determine the possible toxicity mechanism. Two glutathione (GSH) conjugates (M1 and M2) and two N-acetylcysteine (NAC) conjugates (M3 and M4) were detected in rat liver microsomal incubations supplemented with GSH and NAC, respectively. Moreover, M1/M2 and M3/M4 were respectively found in ATL-treated rat primary hepatocytes and in bile and urine of rats given ATL. Recombinant P450 enzyme incubations demonstrated that CYP3A4 was the primary enzyme involved in the olefin epoxidation of ATL. Treatment of hepatocytes with ATL resulted in significant cell death. Inhibition of CYP3A attenuated the susceptibility to the observed cytotoxicity of ATL. The metabolic activation of ATL most likely participates in the cytotoxicity of ATL.

Mechanistic and predictive studies on the oxidation of furans by cytochrome P450: A DFT study

Furan-containing compounds distribute widely in food, herbal medicines, industrial synthetic products, and environmental media. These compounds can undergo oxidative metabolism catalyzed by cytochrome P450 enzymes (CYP450) within organisms, which may produce reactive products, possibly reacting with biomolecules to induce toxic effects. In this work, we performed DFT calculations to investigate the CYP450-mediated metabolic mechanism of furan-ring oxidation using 2-methylfuran as a model substrate, meanwhile, we studied the regioselective competition of another hydroxylation reaction involving methyl group of 2-methylfuran. As a result, we found the toxicological-relevant cis-enedione product can be produced from O-addition directly via a concerted manner without formation of an epoxide intermediate as traditionally believed. Moreover, our calculations demonstrate the kinetic and thermodynamic feasibility of both furan-ring oxidation and methyl hydroxylation pathways, although the former pathway is a bit more favorable. We then constructed a linear model to predict the rate-limiting activation energies (ΔE*) of O-addition with 11 diverse furan substates based on their adiabatic ionization potentials (AIPs) and condensation Fukui functions (CFFs). The results show a good predictive ability (R2=0.94, Q2CV=0.87). Therefore, AIP and CFF with clear physichem meanings relevant to the mechanism, emerge as pivotal molecular descriptors to enable the fast prediction of furan-ring oxidation reactivities for quick insight into the toxicological risk of furans, using just ground-state calculations.

Mechanism-Based Inactivation of Cytochrome P450 3A by Evodol

1. Evodol is one of the furanoids isolated from the fruits of Evodia rutaecarpa that has been widely prescribed for the treatment of gastrointestinal diseases in China. The aim of this study was to investigate the inhibitory effect of evodol on CYP3A.2. A 30-min preincubation of evodol with human liver microsomes raised an obvious left IC₅₀ shift, 3.9-fold for midazolam 1'-hydroxylation and 3.2-fold for testosterone 6β-hydroxylation. Evodol inactivated CYP3A in a time-, concentration- and NADPH-dependent manner, with K_I and k_inact of 5.1 μM and 0.028 min^-1 for midazolam 1'-hydroxylation and 3.0 μM and 0.022 min^-1 for testosterone 6β-hydroxylation.3. Co-incubation of ketoconazole attenuated the inactivation while inclusion of glutathione (GSH) and catalase/superoxide dismutase displayed no such protection.4. cis-Butene-1, 4-dial (BDA) intermediate derived from evodol were trapped by glutathione and N-acetyl-lysine in microsomes and characterized by HR-MS spectra. The BDA intermediate was believed to play a key role in CYP3A inactivation. CYP3A4 and 2C9 were the primary enzymes contributing to the bioactivation of evodol.5. To sum up, for the first time evodol was characterized as a mechanism-based inactivator of CYP3A.

Feature MS fragments-based method for identification of toxic furanoids in biological samples

Numerous furan-containing compounds have been reported to be toxic. The toxicity may be attributed to the metabolic activation of the furan ring to cis-enediones. Identification of unknown furans that undergo bioactivation is challenging. Here, we present a novel approach that enables non-targeted profiling of bioactivation of unknown furanoids both in vitro and in vivo. Cyclic pyrrole-glutathione conjugate was the predominant product of cis-enediones with glutathione. The shared glutathione substructure of conjugates was capable of generating four constant and signature fragments under collision-induced dissociation (CID) in the mass spectrometer, including neutral loss fragments 103.0269 Da and 146.0691 Da and product ions at m/z 130.0499 and 177.0328. The unique structure and high abundance of conjugates in combination with the consistency and specificity of CID fragmentation brought extraordinarily high selectivity and reliability for the four fragments as a fingerprint of bioactivated furanoids. The bioactivated furanoids can be identified by screening the four fragments in high-resolution MS/MS datasets using the neutral loss filtering and diagnostic fragmentation filtering of data post-acquisition software MZmine. The simultaneous formation of four individual signal points in the filtering channel with the same precursor ion and retention time was assigned to be furanoids. The method has been rigorously validated. In the pooled urine samples from nine model furanoids-treated mice, nine cis-enediones from the parent furanoids and two from furanoid metabolites were accurately detected and identified. The method showed great performance in non-targeted profiling bioactivated furanoids and their metabolites in urine samples of herbal extract-treated mice.

Diosbulbin B: An important component responsible for hepatotoxicity and protein covalent binding induced by Dioscorea bulbifera L

BACKGROUND

Dioscorea bulbifera L. (DBL) is an herbal medicine used for the treatment of thyroid diseases and tumors in China. However, the hepatotoxicity of DBL limits its wide safe use. Diosbulbin B (DSB) is the most abundant diterpene lactone occurring in DBL. Numbers of studies showed that this furanoterpenoid plays an important role in DBL-induced liver injury and that DSB is metabolized to a cis-enedial intermediate which reacts with protein to form protein covalent binding and induces hepatotoxicity.

PURPOSE

The present study aimed to define the association of DSB content in DBL with the severity of DBL hepatotoxicity to ensure the safe use of the herbal medicine in clinical practice and to determine the role of DSB in DBL-induced liver injury.

METHODS

Chemical chromatographic fingerprints of DBL were established by UPLC-MS/MS. Their hepatotoxicity potencies were evaluated in vitro and in vivo. Metabolic activation of DSB was evaluated by liver microsomal incubation. Protein modification was assessed by mass spectrometry and immunostaining.

RESULTS

The contents of DSB in DBL herbs collected from 11 locations in China varied dramatically with as much as 47-fold difference. The hepatotoxicity potencies of DBL herbs were found to be proportional to the contents of DSB. Intensified protein adduction derived from the reactive metabolite of DSB was observed in mice administered DBL with high contents of DSB.

CONCLUSION

The findings not only demonstrated that contents of DSB can be quite different depending on harvest location and special attention needs to pay for quality control of DBL but also suggest DSB is a key contributor for DBL-induced hepatotoxicity.

Asparagine and Glutamine Residues Participate in Protein Covalent Binding by Epoxide Metabolite of 8-Epidiosbulbin E Acetate In Vitro and In Vivo

Dioscorea Bulbifera L. (DBL), an effective traditional Chinese medicine, has been restricted because of multiple reports that it can cause severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the main components of DBL, can induce severe liver injury. It has been reported that EEA can be metabolized by CYP3A to the corresponding cis-enedial intermediate which alkylates the lysine residues of proteins to form pyrroline derivatives. The present study unexpectedly found that the reactive intermediate reacted with the amide groups of asparagine (Asn) and glutamine (Gln) residues of hepatic proteins of mice treated with EEA. The amide-derived protein modification increased with the increase in the dose administered. Like the adduction of the primary amine of lysine residues, the electrophilic metabolite reacted with the amide groups of Asn and Gln residues to offer the corresponding pyrrolines. The structures of the pyrrolines were confirmed by mass spectrometry and nuclear magnetic resonance spectroscopy.

Evidence for adduction of biologic amines with reactive metabolite of 8-epidiosbulbin E acetate in vitro and in vivo

Dioscorea bulbifera L. (DBL) is one of traditional Chinese medicines and has been used for the treatment of goiter, tumor and carbuncles. However, clinic application of the herbal medicine has been limited, due to reported severe hepatotoxicity. 8-Epidiosbulbin E acetate (EEA), one of the major components of DBL, can cause severe liver damage. The furan ring of EEA is metabolized by CYP3A4 to a cis-enedial reactive intermediate prone to react amino and/or thiol groups of amino acid residues. In this study, we investigated the interaction of the reactive intermediate with biologic amines. EEA-derived biologic amine adducts, including spermidine, spermine, putrescine, ornithine, lysine and glutamine were detected in cultured mouse primary hepatocytes treated with EEA. Only spermidine adduct was observed in bile of mice given EEA. The detection of the adducts was established by labeling with bromobenzyl mercaptan and LC-MS/MS analysis. Exposure of EEA resulted in concentration dependent cytotoxicity in hepatocytes. Pretreatment with spermidine attenuated the susceptibility of cells to the cytotoxicity of EEA, because of the compensation of the depleted spermidine.

A Metabolic Activation-Based Chemoproteomic Platform to Profile Adducted Proteins Derived from Furan-Containing Compounds

Human exposure to widespread furan-containing compounds (FCCs) has drawn much attention due to the high risk of their toxicities. Identifying adducted proteins resulting from the metabolic activation of FCCs is the core to learning the mechanism of FCCs' toxic action. We succeeded in establishing a metabolic activation-based chemoproteomic platform to map FCC-derived protein adducts in cultured primary hepatocytes treated with FCCs and to pinpoint the modification sites, using click chemistry but without alkynylation or azidation of FCCs to be investigated. The proposed platform was systematically verified by biomimetic synthesis, liver microsomal incubation, and primary hepatocyte culture. A mixture of furan, 2-methylfuran, and 2,5-dimethylfuran as model was tested by use of the established platform. A total of hepatic 171 lysine-based adducted proteins and 145 cysteine-based adducted proteins by the reactive metabolites of the three FCCs were enriched and identified (Byonic score ≥ 100). The target proteins were found to mainly participate in ATP synthesis. The technique was also successfully applied to furan-containing natural products. The established platform made it possible to profile covalently adducted proteins, because of potential exposure to a vast inventory of over two million of FCCs documented.

Metabolic Activation of Perampanel Mediated by CYP1A2

Perampanel (PRP), a noncompetitive α-amino-3-hydroxy-5-methyl-4-isoxazolepropanoic acid (AMPA) receptor antagonist with high selectivity, has been used as a new adjuvant for the treatment of fractional seizures with or without primary generalized tonic-clonic seizures and secondary generalized seizures in epilepsy patients over the age of 12. Adverse events such as liver injury have been reported during the clinical application of PRP. The purpose of the study is to explore the in vitro and in vivo metabolic activation of PRP. Two GSH conjugates were detected in rat liver microsomal incubations containing PRP, GSH, and NADPH. The two GSH conjugates were both obtained from the bile of rats and rat primary hepatocytes after exposure to PRP. Similar microsomal incubations complemented with N-acetylcysteine (NAC) in place of GSH offered two NAC conjugates. As expected, the NAC conjugates were detected in the urine of PRP-treated rats. One of the two NAC conjugates was identified as NAC conjugate 12 verified by chemical synthesis. The individual human recombinant P450 enzyme incubation assay demonstrated that CYP1A2 dominated the catalysis for the metabolic activation of PRP. Pretreatment with α-naphthoflavone (NTF) decreased the formation of PRP-derived GSH conjugates in both livers of rats and cultured primary hepatocytes after being treated with PRP. Additionally, NTF was found to decrease the susceptibility of primary hepatocytes to the cytotoxicity of PRP. The findings indicate that PRP was metabolized to the corresponding epoxide, which could participate in PRP-induced cytotoxicity.

Metabolic Activation of the Toxic Natural Products From Herbal and Dietary Supplements Leading to Toxicities

Currently, herbal and dietary supplements have been widely applied to prevent and treat various diseases. However, the potential toxicities and adverse reactions of herbal and dietary supplements have been increasingly reported, and have gradually attracted widespread attention from clinical pharmacists and physicians. Metabolic activation of specific natural products from herbal and dietary supplements is mediated by hepatic cytochrome P450 or intestinal bacteria, and generates chemical reactive/toxic metabolites that bind to cellular reduced glutathione or macromolecules, and form reactive metabolites-glutathione/protein/DNA adducts, and these protein/DNA adducts can result in toxicities. The present review focuses on the relation between metabolic activation and toxicities of natural products, and provides updated, comprehensive and critical comment on the toxic mechanisms of reactive metabolites. The key inductive role of metabolic activation in toxicity is highlighted, and frequently toxic functional groups of toxic natural products were summarized. The biotransformation of drug cytochrome P450 or intestinal bacteria involved in metabolic activation were clarified, the reactive metabolites-protein adducts were selected as biomarkers for predicting toxicity. And finally, further perspectives between metabolic activation and toxicities of natural products from herbal and dietary supplements are discussed, to provide a reference for the reasonable and safe usage of herbal and dietary supplements.

In Vitro and In Vivo Studies of Metabolic Activation of Marrubiin, a Bioactive Constituent from Marrubium Vulgare

Marrubiin, a furanoid compound, is a well-known diterpenoid lactone isolated from Marrubium vulgare, which displays a wide spectrum of pharmacological effects and potential hepatotoxicity. Considering that marrubiin contains a structural alert, furan ring, metabolic activation may be one of the major metabolic pathways, and the reactive metabolite may be involved in the hepatotoxicity. The present study was carried out to investigate the bioactivation mechanism of marrubiin in rats and humans. Marrubiin was initially metabolized into cis-butene-1,4-dial intermediate, which was readily trapped by glutathione (GSH) and N-acetyl-lysine (NAL) in the microsomal incubations supplemented with NADPH. A total of nine conjugates were detected and identified by high-resolution mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy. M1-M3 and M6 and M7 were characterized as mono-GSH conjugates, and M4 and M5 were identified as bis-GSH conjugates. M8 and M9 were identified as NAL conjugates. In rat bile, five GSH conjugates (M1-M3; M6 and M7) were detected. M1, M8, and M9 were chemically synthesized, and their structures were characterized by 13C NMR. Sulfaphenazole, ticlopidine, and ketoconazole displayed significant inhibitory effect on the bioactivation of marrubiin. Further phenotyping revealed that CYP2C9, CYP2C19, and CYP3A4 were the primary enzymes catalyzing the bioactivation of marrubiin. The current study provides evidence for the CYP-dominated bioactivation of marrubiin to the corresponding cis-butene-1,4-dial intermediate, which enables us to better understand the potential side effects caused by marrubiin.

Immunochemical Detection of Protein Modification Derived from Metabolic Activation of 8-Epidiosbulbin E Acetate

Furanoid 8-epidiosbulbin E acetate (EEA) is one of the most abundant diterpenoid lactones in herbal medicine Dioscorea bulbifera L. (DB). Our early work proved that EEA could be metabolized to EEA-derived cis-enedial (EDE), a reactive intermediate, which is required for the hepatotoxicity observed in experimental animals exposed to EEA. Also, we found that EDE could modify hepatic protein by reaction with thiol groups and/or primary amines of protein. The present study was inclined to develop polyclonal antibodies to detect protein modified by EDE. An immunogen was prepared by reaction of EDE with keyhole limpet hemocyanin (KLH), and polyclonal antibodies were raised in rabbits immunized with the immunogen. Antisera collected from the immunized rabbits demonstrated high titers evaluated by enzyme-linked immunosorbent assays (ELISAs). Immunoblot analysis showed that the polyclonal antibodies recognized EDE-modified bovine serum albumin (BSA) in a hapten load-dependent manner but did not cross-react with native BSA. Competitive inhibition experiments elicited high selectivity of the antibodies toward EDE-modified BSA. The antibodies allowed us to detect and enrich EDE-modified protein in liver homogenates obtained from EEA-treated mice. The developed immunoprecipitation technique, along with mass spectrometry, enabled us to succeed in identifying multiple hepatic proteins of animals given EEA. We have successfully developed polyclonal antibodies with the ability to recognize EDE-derived protein adducts, which is a unique tool for us to define the mechanisms of toxic action of EEA.

Metabolomic-transcriptomic landscape of 8-epidiosbulbin E acetate -a major diterpenoid lactone from Dioscorea bulbifera tuber induces hepatotoxicity

Studies have shown that 8-epidiosbulbin E acetate (EEA), a major diterpenoid lactone in the tuber of Dioscorea bulbifera, can induce hepatotoxicity in vivo. However, the underlying mechanisms remain unknown. Using the integrated transcriptomic and metabolomics method, in this study we investigated the global effect of EEA exposure on the transcriptomic and metabolomic profiles in mice. The abundance of 7131 genes and 42 metabolites in the liver, as well as 43 metabolites in the serum were altered. It should be noted that EEA mainly damaged hepatic cells through the aberrant regulation of multiple systems primarily including bile acid metabolism, and taurine and hypotaurine metabolism. In addition, an imbalance of bile acid metabolism was found to play a key pat in response to EEA-triggered hepatotoxicity. In summary, these findings contributed to understanding the underlying mechanisms of EEA hepatotoxicity.

Liver-specific metabolomics characterizes the hepatotoxicity of Dioscorea bulbifera rhizome in rats by integration of GC-MS and 1H-NMR

ETHNOPHARMACOLOGICAL RELEVANCE

Dioscorea bulbifera rhizome (DBR), one type of herbal medicine, is extensively used in both Indian and Chinese system of traditional medicine. It has been effective in treating various diseases, such as sore throat, struma, and tumors. However, more and more clinical investigations have suggested that DBR can cause liver injury.

AIM OF THE STUDY

In the present study, we aimed to characterize the corresponding molecular changes of liver dysfunction and reveal overall metabolic and physiological mechanisms of the subchronic toxic effect of DBR.

MATERIALS AND METHODS

A liver-specific metabolomics approach integrating GC-MS and ¹H-NMR was developed to assess the hepatotoxicity in rats after DBR exposure for 12 weeks. Multivariate statistical analysis and pattern recognition were employed to examine different metabolic profiles of liver in DBR-challenged rats.

RESULTS

A total of 61 metabolites were screened as significantly altered metabolites, which were distributed in 43 metabolic pathways. The correlation network analysis indicated that the hub metabolites of hepatotoxicity could be mainly linked to amino acid, lipid, purine, pyrimidine, bile acid, gut microflora, and energy metabolisms. Notably, purine, pyrimidine, and gut microflora metabolisms might be novel pathways participating in metabolic abnormalities in rats with DBR-triggered hepatic damage.

CONCLUSIONS

Our results primarily showed that the liver-specific metabolic information provided by the different analytical platforms was essential for identifying more biomarkers and metabolic pathways, and our findings provided novel insights into understand the mechanistic complexity of herb-induced liver injury.

Cytochrome P450 mediated metabolic activation of chrysophanol

Chrysophanol, a major anthraquinone component occurring in many traditional Chinese herbs, is accepted as important active component with various pharmacological actions such as antibacterial and anticancer activity. Previous studies demonstrated that exposure to chrysophanol induced cytotoxicity, but the mechanisms of the toxic effects remain unknown. In the present metabolism study, three oxidative metabolites (M1-M3, aloe-emodine, 7-hydroxychrysophanol, and 2-hydroxychrysophanol) and five GSH conjugates (M4-M8) were detected in rat and human liver microsomal incubations of chrysophanol supplemented with GSH, and the formation of the metabolites was NADPH dependent except M4 and M5. M4 and M5 were directly derived from parent compound chrysophanol, M6 arose from M2, and M7 and M8 resulted from the oxidation of M4 and M5. Metabolites M5 and M6 were also observed in bile of rats after exposure to chrysophanol, M1-M3 and one NAC conjugate (M9) were detected in urine of rats administrated chrysophanol, and urinary metabolite M9 originated from the degradation of biliary GSH conjugation M6. Recombinant P450 enzyme incubation and microsome inhibition studies demonstrated that P450 1A2 was the primary enzyme responsible for the metabolic activation of chrysophanol and that P450 2B6 and P450 3A4 also participated in the generation of the oxidative metabolites. These findings helped us to understand the mechanisms of chrysophanol-induced cytotoxicity.

Detection and Structural Characterization of Nucleophiles Trapped Reactive Metabolites of Limonin Using Liquid Chromatography-Mass Spectrometry

Limonin (LIM), a furan-containing limonoid, is one of the most abundant components of Dictamnus dasycarpus Turcz. Recent studies demonstrated that LIM has great potential for inhibiting the activity of drug-metabolizing enzymes. However, the mechanisms of LIM-induced enzyme inactivation processes remain unexplored. The main objective of this study was to identify the reactive metabolites of LIM using liquid chromatography-mass spectrometry. Three nucleophiles, glutathione (GSH), N-acetyl cysteine (NAC), and N-acetyl lysine (NAL), were used to trap the reactive metabolites of LIM in in vitro and in vivo models. Two different types of mass spectrometry, a hybrid quadrupole time-of-flight (Q-TOF) mass spectrometry and a LTQ velos Pro ion trap mass spectrometry, were employed to acquire structural information of nucleophile adducts of LIM. In total, six nucleophile adducts of LIM (M1-M6) with their isomers were identified; among them, M1 was a GSH and NAL conjugate of LIM, M2-M4 were glutathione adducts of LIM, M5 was a NAC and NAL conjugate of LIM, and M6 was a NAC adduct of LIM. Additionally, CYP3A4 was found to be the key enzyme responsible for the bioactivation of limonin. This metabolism study largely facilitates the understanding of mechanisms of limonin-induced enzyme inactivation processes.

Metabolism of five diterpenoid lactones from Dioscorea bulbifera tubers in zebrafish

Diterpenoid lactones (DLs) have been reported to be the main hepatotoxic constituents in Dioscorea bulbifera tubers (DBT), a traditional Chinese medicinal herb. The acquisition of early information regarding its metabolism is critical for evaluating the potential hepatotoxicity of DLs. We investigated, for the first time, the main metabolites of diosbulbin A (DIOA), diosbulbin C (DIOC), diosbulbin (DIOG), diosbulbin (DIOM) and diosbulbin (DIOF) in adult zebrafish. By using ultra-high performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UHPLC-QTOF MS), 6, 2, 7, 5 and 4 metabolites of DIOA, DIOC, DIOF, DIOM and DIOG were identified in the zebrafish body and the aqueous solution, respectively. Both phase-I and phase-II metabolites were observed in the metabolic profiles and the metabolic pathways involved in hydroxyl reduction, glucuronidation, glutathione conjugation and sulfation. The above results indicated that hepatocytic metabolism might be the major route of clearance for DLs. This study provided important information for the understanding of the metabolism of DLs in DBT.