Covalent Protein Modification by Reactive Drug Metabolites Using Online Electrochemistry/Liquid Chromatography/Mass Spectrometry

Versatile Tools for Understanding Electrosynthetic Mechanisms

Electrosynthesis is a popular, green alternative to traditional organic methods. Understanding the mechanisms is not trivial yet is necessary to optimize reaction processes. To this end, a multitude of analytical tools is available to identify and quantitate reaction products and intermediates. The first portion of this review serves as a guide that underscores electrosynthesis fundamentals, including instrumentation, electrode selection, impacts of electrolyte and solvent, cell configuration, and methods of electrosynthesis. Next, the broad base of analytical techniques that aid in mechanism elucidation are covered in detail. These methods are divided into electrochemical, spectroscopic, chromatographic, microscopic, and computational. Technique selection is dependent on predicted reaction pathways and electrogenerated intermediates. Often, a combination of techniques must be utilized to ensure accuracy of the proposed model. To conclude, future prospects that aim to enhance the field are discussed.

Investigation of Clozapine and Olanzapine Reactive Metabolite Formation and Protein Binding by Liquid Chromatography-Tandem Mass Spectrometry

Drug-induced toxicity has, in many cases, been linked to oxidative metabolism resulting in the formation of reactive metabolites and subsequent covalent binding to biomolecules. Two structurally related antipsychotic drugs, clozapine (CLZ) and olanzapine (OLZ), are known to form similar nitrenium ion reactive metabolites. CLZ-derived reactive metabolites have been linked to agranulocytosis and hepatotoxicity. We have studied the oxidative metabolism of CLZ and OLZ as well as two known metabolites of CLZ, desmethyl-CLZ (DCLZ), and CLZ-N-oxide (CLZ-NO), using in vitro rat liver microsomal (RLM) incubations with glutathione (GSH) trapping of reactive metabolites and liquid chromatography-high resolution tandem mass spectrometry (LC-HRMS/MS). Reactive metabolite binding to selected standard peptides and recombinant purified human proteins was also evaluated. Bottom-up proteomics was performed using two complementary proteases, prefractionation of peptides followed by LC-HRMS/MS for elucidating modifications of target proteins. Induced RLM was selected to form reactive metabolites enzymatically to assess the complex profile of reactive metabolite structures and their binding potential to standard human proteins. Multiple oxidative metabolites and several different GSH adducts were found for CLZ and OLZ. Modification sites were characterized on human glutathione S-transferase (hGST) alpha 1 (OLZ-modified at Cys112), hGST mu 2 (OLZ at Cys115), and hGST pi (CLZ, DCLZ, CLZ-NO and OLZ at Cys170), human microsomal GST 1 (hMGST1, CLZ and OLZ at Cys50), and human serum albumin (hSA, CLZ at Cys34). Furthermore, two modified rat proteins, microsomal GST 1 (CLZ and OLZ at Cys50) and one CYP (OLZ-modified, multiple possible isoforms), from RLM background were also characterized. In addition, direct effects of the reactive metabolite modifications on proteins were observed, including differences in protease cleavage specificity, chromatographic behavior, and charge-state distributions.

Oxidative metabolism of typical phenolic compounds of Danshen by electrochemistry coupled to quadrupole time-of-flight tandem mass spectrometry

An electrochemistry coupled to online quadrupole time-of-flight tandem mass spectrometry (EC/Q-TOF/MS) was applied to investigate the oxidative transformation and metabolic pathway of five phenolic acids in Danshen sample. Simulation of the phase I oxidative metabolism was carried out in an electrochemical reactor equipped with a glassy carbon working electrode. The phase II reactivity of the generated oxidative products towards biomolecules (such as glutathione) was investigated by ways of covalent adduct formation experiments. The results obtained by EC/MS were compared with well-known in vitro studies by conducting rat liver microsome incubations. Structures of the electrochemically produced metabolites were identified by accurate mass measurement and previously results in vivo metabolites. It was indicated that the electrochemical oxidation was in good accordance with similar products found in vivo experiments. In conclusion, this work confirmed that EC/Q-TOF/MS was a promising analytical tool in the prediction of metabolic transformations of functional foods.

Instrumentation and applications of electrochemistry coupled to mass spectrometry for studying xenobiotic metabolism: A review

The knowledge of metabolic pathways and biotransformation of xenobiotics, artificial substances foreign to the entire biological system, is crucial for elucidation of degradation routes of potentially toxic substances. Nowadays, there are many methods to simulate xenobiotic metabolism in the human body in vitro. In this review, the metabolism of various substances in the human body is described, followed by a summary of methods used for prediction of metabolic pathways and biotransformation. Above all, focus is placed on the coupling of electrochemistry to mass spectrometry, which is still a relatively new technique. This promising tool can mimic both oxidative phase I and conjugative phase II metabolism. Different experimental arrangements, with or without a separation step, and various applications of this technique are illustrated and critically reviewed.

A Disposable Microfluidic Device with a Screen Printed Electrode for Mimicking Phase II Metabolism

Human metabolism is investigated using several in vitro methods. However, the current methodologies are often expensive, tedious and complicated. Over the last decade, the combination of electrochemistry (EC) with mass spectrometry (MS) has a simpler and a cheaper alternative to mimic the human metabolism. This paper describes the development of a disposable microfluidic device with a screen-printed electrode (SPE) for monitoring phase II GSH reactions. The proposed chip has the potential to be used as a primary screening tool, thus complementing the current in vitro methods.

Paraquat-Melanin Redox-Cycling: Evidence from Electrochemical Reverse Engineering

Parkinson's disease is a neurodegenerative disorder associated with oxidative stress and the death of melanin-containing neurons of the substantia nigra. Epidemiological evidence links exposure to the pesticide paraquat (PQ) to Parkinson's disease, and this link has been explained by a redox cycling mechanism that induces oxidative stress. Here, we used a novel electrochemistry-based reverse engineering methodology to test the hypothesis that PQ can undergo reductive redox cycling with melanin. In this method, (i) an insoluble natural melanin (from Sepia melanin) and a synthetic model melanin (having a cysteinyldopamine-melanin core and dopamine-melanin shell) were entrapped in a nonconducting hydrogel film adjacent to an electrode, (ii) the film-coated electrode was immersed in solutions containing PQ (putative redox cycling reductant) and a redox cycling oxidant (ferrocene dimethanol), (iii) sequences of input potentials (i.e., voltages) were imposed to the underlying electrode to systematically engage reductive and oxidative redox cycling, and (iv) output response currents were analyzed for signatures of redox cycling. The response characteristics of the PQ-melanin systems to various input potential sequences support the hypothesis that PQ can directly donate electrons to melanin. This observation of PQ-melanin redox interactions demonstrates an association between two components that have been individually linked to oxidative stress and Parkinson's disease. Potentially, melanin's redox activity could be an important component in understanding the etiology of neurological disorders such as Parkinson's disease.

Observation of electrochemically generated nitrenium ions by desorption electrospray ionization mass spectrometry

We report the observation of the electrochemically generated nitrenium ions of 4,4'-dimethyoxydiphenylamine and di-p-tolylamine in solution by mass spectrometry. This setup takes inspiration from desorption electrospray ionization mass spectrometry to sample directly from the surface of a rotating waterwheel working electrode for mass spectrometric analysis. Detection of the 4,4'-dimethyoxydiphenylamine nitrenium ion was expected based upon para-methoxy resonance stabilization, whereas observation of the di-p-tolylamine nitrenium ion might be unexpected because resonance stabilization from the para-substituted position is unavailable. However, the short timescale analysis of the setup allows for the isolation of the di-p-tolylamine nitrenium ion, which is electrogenerated in solution and detected mass spectrometrically.

Antimalarial benzoheterocyclic 4-aminoquinolines: Structure-activity relationship, in vivo evaluation, mechanistic and bioactivation studies

A novel class of benzoheterocyclic analogues of amodiaquine designed to avoid toxic reactive metabolite formation was synthesized and evaluated for antiplasmodial activity against K1 (multidrug resistant) and NF54 (sensitive) strains of the malaria parasite Plasmodium falciparum. Structure-activity relationship studies led to the identification of highly promising analogues, the most potent of which had IC50s in the nanomolar range against both strains. The compounds further demonstrated good in vitro microsomal metabolic stability while those subjected to in vivo pharmacokinetic studies had desirable pharmacokinetic profiles. In vivo antimalarial efficacy in Plasmodium berghei infected mice was evaluated for four compounds, all of which showed good activity following oral administration. In particular, compound 19 completely cured treated mice at a low multiple dose of 4×10mg/kg. Mechanistic and bioactivation studies suggest hemozoin formation inhibition and a low likelihood of forming quinone-imine reactive metabolites, respectively.

Comparison of trapping profiles between d-peptides and glutathione in the identification of reactive metabolites

Qualitative trapping profile of reactive metabolites arising from six structurally different compounds was tested with three different d-peptide isomers (Peptide 1, gly–tyr–pro–cys–pro–his-pro; Peptide 2, gly–tyr–pro–ala–pro–his–pro; Peptide 3, gly–tyr–arg–pro–cys–pro–his–lys–pro) and glutathione (GSH) using mouse and human liver microsomes as the biocatalyst. The test compounds were classified either as clinically “safe” (amlodipine, caffeine, ibuprofen), or clinically as “risky” (clozapine, nimesulide, ticlopidine; i.e., associated with severe clinical toxicity outcomes). Our working hypothesis was as follows: could the use of short different amino acid sequence containing d-peptides in adduct detection confer any add-on value to that obtained with GSH? All “risky” agents’ resulted in the formation of several GSH adducts in the incubation mixture and with at least one peptide adduct with both microsomal preparations. Amlodipine did not form any adducts with any of the trapping agents. No GSH and peptide 2 and 3 adducts were found with caffeine, but with peptide 1 one adduct with human liver microsomes was detected. Ibuprofen produced one Peptide 1-adduct with human and mouse liver microsomes but not with GSH. In conclusion, GSH still remains the gold trapping standard for reactive metabolites. However, targeted d-peptides could provide additional information about protein binding potential of electrophilic agents, but their clinical significance needs to be clarified using a wider spectrum of chemicals together with other safety estimates.

Mass spectrometric methods for monitoring redox processes in electrochemical cells

Electrochemistry (EC) is a mature scientific discipline aimed to study the movement of electrons in an oxidation-reduction reaction. EC covers techniques that use a measurement of potential, charge, or current to determine the concentration or the chemical reactivity of analytes. The electrical signal is directly converted into chemical information. For in-depth characterization of complex electrochemical reactions involving the formation of diverse intermediates, products and byproducts, EC is usually combined with other analytical techniques, and particularly the hyphenation of EC with mass spectrometry (MS) has found broad applicability. The analysis of gases and volatile intermediates and products formed at electrode surfaces is enabled by differential electrochemical mass spectrometry (DEMS). In DEMS an electrochemical cell is sampled with a membrane interface for electron ionization (EI)-MS. The chemical space amenable to EC/MS (i.e., bioorganic molecules including proteins, peptides, nucleic acids, and drugs) was significantly increased by employing electrospray ionization (ESI)-MS. In the simplest setup, the EC of the ESI process is used to analytical advantage. A limitation of this approach is, however, its inability to precisely control the electrochemical potential at the emitter electrode. Thus, particularly for studying mechanistic aspects of electrochemical processes, the hyphenation of discrete electrochemical cells with ESI-MS was found to be more appropriate. The analytical power of EC/ESI-MS can further be increased by integrating liquid chromatography (LC) as an additional dimension of separation. Chromatographic separation was found to be particularly useful to reduce the complexity of the sample submitted either to the EC cell or to ESI-MS. Thus, both EC/LC/ESI-MS and LC/EC/ESI-MS are common.

An enzyme-linked immuno-mass spectrometric assay with the substrate adenosine monophosphate

An enzyme-linked immuno-mass spectrometric assay (ELIMSA) with the specific detection probe streptavidin conjugated to alkaline phosphatase catalyzed the production of adenosine from the substrate adenosine monophosphate (AMP) for sensitive quantification of prostate-specific antigen (PSA) by mass spectrometry. Adenosine ionized efficiently and was measured to the femtomole range by dilution and direct analysis with micro-liquid chromatography, electrospray ionization, and mass spectrometry (LC-ESI-MS). The LC-ESI-MS assay for adenosine production was shown to be linear and accurate using internal (13)C(15)N adenosine isotope dilution, internal (13)C(15)N adenosine one-point calibration, and external adenosine standard curves with close agreement. The detection limits of LC-ESI-MS for alkaline phosphatase-streptavidin (AP-SA, ∼190,000 Da) was tested by injecting 0.1 μl of a 1 pg/ml solution, i.e., 100 attograms or 526 yoctomole (5.26E-22) of the alkaline-phosphatase labeled probe on column (about 315 AP-SA molecules). The ELIMSA for PSA was linear and showed strong signals across the picogram per milliliter range and could robustly detect PSA from all of the prostatectomy patients and all of the female plasma samples that ranged as low as 70 pg/ml with strong signals well separated from the background and well within the limit of quantification of the AP-SA probe. The results of the ELIMSA assay for PSA are normal and homogenous when independently replicated with a fresh standard over multiple days, and intra and inter diem assay variation was less than 10 % of the mean. In a blind comparison, ELIMSA showed excellent agreement with, but was more sensitive than, the present gold standard commercial fluorescent ELISA, or ECL-based detection, of PSA from normal and prostatectomy samples, respectively.

Pyridoxamine-5-phosphate enzyme-linked immune mass spectrometric assay substrate for linear absolute quantification of alkaline phosphatase to the yoctomole range applied to prostate specific antigen

There is a need to measure proteins that are present in concentrations below the detection limits of existing colorimetric approaches with enzyme-linked immunoabsorbent assays (ELISA). The powerful enzyme alkaline phosphatase conjugated to the highly specific bacterial protein streptavidin binds to biotinylated macromolecules like proteins, antibodies, or other ligands and receptors with a high affinity. The binding of the biotinylated detection antibody, with resulting amplification of the signal by the catalytic production of reporter molecules, is key to the sensitivity of ELISA. The specificity and amplification of the signal by the enzyme alkaline phosphatase in ELISA together with the sensitivity of liquid chromatography electrospray ionization and mass spectrometry (LC-ESI-MS) to detect femtomole to picomole amounts of reporter molecules results in an ultrasensitive enzyme-linked immune mass spectrometric assay (ELIMSA). The novel ELIMSA substrate pyridoxamine-5-phosphate (PA5P) is cleaved by the enzyme alkaline phosphatase to yield the basic and hydrophilic product pyridoxamine (PA) that elutes rapidly with symmetrical peaks and a flat baseline. Pyridoxamine (PA) and (13)C PA were both observed to show a linear relationship between log ion intensity and quantity from picomole to femtomole amounts by liquid chromatography-electrospray ionization and mass spectrometry. Four independent methods, (i) internal (13)C isotope PA dilution curves, (ii) internal (13)C isotope one-point calibration, (iii) external PA standard curve, and (iv) external (13)C PA standard curve, all agreed within 1 digit in the same order of magnitude on the linear quantification of PA. Hence, a mass spectrometer can be used to robustly detect 526 ymol of the alkaline phosphatase streptavidin probe and accurately quantify zeptomole amounts of PSA against log linear absolute standard by micro electrospray on a simple ion trap.

Electrochemistry/liquid chromatography/mass spectrometry to demonstrate irreversible binding of the skin allergen p-phenylenediamine to proteins

RATIONALE

para-Phenylenediamine (PPD) is a potent and well-known allergen, which is commonly used in hair or fur dyes and can cause severe allergic contact dermatitis. In this work, the skin-sensitizing potential of PPD with respect to the conjugation of proteins was evaluated using an approach without animal testing.

METHODS

Electrochemistry (EC) coupled offline to liquid chromatography (LC) and electrospray ionization mass spectrometry (ESI-MS) was employed to convert the pre-hapten PPD into its reactive hapten analogs. A previous study had already shown that this purely instrumental method is suitable for accelerating and simulating the various oxidation processes, which PPD may undergo, and that the emerging products are prone to react with soft thiol groups of small nucleophiles like glutathione and cysteine.

RESULTS

This investigation was extended by successfully demonstrating adduct formation between EC-generated PPD oxidation products and the three proteins β-lactoglobulin A (β-LGA), human serum albumin and human hemoglobin. A tryptic digest of modified β-LGA provided evidence for irreversible protein binding of monomeric PPD, a PPD dimer and the PPD trimer known as Bandrowski's Base. It was shown that the main oxidation product p-phenylene quinone diimine, and the reactive oligomerized species, primarily attack the free thiol function of proteins rather than other nucleophilic amino acid residues.

CONCLUSIONS

The pre-hapten PPD was efficiently activated upon EC oxidation and the resulting species were further reacted with different proteins leading to diverse hapten-protein complexes. Thereby, problems related to the complex matrix present in conventional in vitro or in vivo methods could effectively be avoided.

Formation and characterization of covalent guanosine adducts with electrochemistry-liquid chromatography-mass spectrometry

Chemicals can interact with the genetic material giving rise to the formation of covalent adducts. These alterations can lead to adverse consequences, including cancer, reproductive impairment, development anomalies, or genetic diseases. In search for an assay allowing identification of hazardous compounds that might form covalent adducts with nucleic acids, electrochemistry (EC)/liquid chromatography (LC)/mass spectrometry (MS) is presented. EC/LC/MS is a purely instrumental approach. EC is used for oxidative activation, LC for the fractionation of the reaction mixture, and MS for the detection and characterization of the reaction products. To test the system capabilities, we investigated the formation of covalent adducts produced by guanosine and acetaminophen (APAP). Electrochemical activation of mixtures of guanosine and APAP gave rise to the formation of four isomers of (guanosine + APAP-2H). Mass voltammograms as well as dose–response-curves were used to obtain insights in the mechanism of adduct formation. These experiments revealed that a mechanism involving radical intermediates is favored. The initial step of adduct formation is the conversion of both APAP and guanosine into radicals via one-electron–one-proton reactions. Among different competing reaction pathways, the generated radical intermediates undergo intermolecular reactions to form covalent adducts between guanosine and APAP.

Mass spectrometry of peptides and proteins from human blood

It is difficult to convey the accelerating rate and growing importance of mass spectrometry applications to human blood proteins and peptides. Mass spectrometry can rapidly detect and identify the ionizable peptides from the proteins in a simple mixture and reveal many of their post-translational modifications. However, blood is a complex mixture that may contain many proteins first expressed in cells and tissues. The complete analysis of blood proteins is a daunting task that will rely on a wide range of disciplines from physics, chemistry, biochemistry, genetics, electromagnetic instrumentation, mathematics and computation. Therefore the comprehensive discovery and analysis of blood proteins will rank among the great technical challenges and require the cumulative sum of many of mankind's scientific achievements together. A variety of methods have been used to fractionate, analyze and identify proteins from blood, each yielding a small piece of the whole and throwing the great size of the task into sharp relief. The approaches attempted to date clearly indicate that enumerating the proteins and peptides of blood can be accomplished. There is no doubt that the mass spectrometry of blood will be crucial to the discovery and analysis of proteins, enzyme activities, and post-translational processes that underlay the mechanisms of disease. At present both discovery and quantification of proteins from blood are commonly reaching sensitivities of ∼1 ng/mL.

Electrochemistry-mass spectrometry unveils the formation of reactive triclocarban metabolites

Triclocarban (3,4,4'-trichlorocarbanilide, TCC) is a widely used antibacterial agent in personal care products and is frequently detected as an environmental pollutant in waste waters and surface waters. In this study, we report novel reactive metabolites potentially formed during biotransformation of TCC. The oxidative metabolism of TCC has been predicted using an electrochemical cell coupled online to liquid chromatography and electrospray ionization mass spectrometry. The electrochemical oxidation unveils the fact that hydroxylated metabolites of TCC may form reactive quinone imines. Moreover, a so-far unknown dechlorinated and hydroxylated TCC metabolite has been identified. The results were confirmed by in vitro studies with human and rat liver microsomes. The reactivity of the newly discovered quinone imines was demonstrated by their covalent binding to glutathione and macromolecules, using β-lactoglobulin A as a model protein. The results regarding the capability of the electrochemical cell to mimic the oxidative metabolism of TCC are discussed. Moreover, the occurrence of reactive metabolites is compared with findings from earlier in vivo studies and their relevance in vivo is argued.

Electrochemical simulation of oxidation processes involving nucleic acids monitored with electrospray ionization-mass spectrometry

Oxidation is commonly involved in the alteration of nucleic acids giving rise to diverse effects including mutation, cell death, malignancy, and aging. We demonstrate that electrochemistry represents an efficient and fast method to mimic oxidative modification of nucleic acids occurring in biological systems. Oxidation reactions were performed in a thin-layer cell employing a conductive diamond electrode as the working electrode and were monitored with electrospray ionization-mass spectrometry. Mass voltammograms were acquired for guanosine, adenosine, cytidine, and uridine. The observed oxidation potentials increased in the order guanosine << adenosine < cytidine < uridine. Oxidation products of guanosine were characterized using high-resolution (tandem) mass spectrometry performed with a quadrupole-quadrupole time-of-flight instrument. On the basis of these experiments, it was concluded that the initial electrode reaction involves a one-electron, one-proton step to give a free radical. The primary oxidation product represents the starting point for a number of follow-up reactions, including guanosine dimerization as well as further oxidation to 8-hydroxyguanosine. Similar results were obtained for guanosine monophosphate and the corresponding dinucleotide. Furthermore, the guanosine radical was identified as an important intermediate for the formation of a covalent adduct with acetaminophen. This observation sheds new light on the mechanism of adduct formation as it demonstrates that oxidative activation of both the nucleobase and the adduct-forming agent is necessary to observe a detectable amount of adduct species.

Assessing covalent binding of reactive drug metabolites by complete protein digestion and LC–MS analysis

Background: Covalent binding by reactive drug metabolites represents a poorly understood cause of drug toxicity. Currently, assessing protein covalent binding usually entails the use of radioactive drug and therefore has limited applicability in drug discovery. Several marketed drugs are known to form reactive metabolites and have been shown to covalently bind to proteins. Results: In this article, we describe a new method for the analysis of reactive metabolite–protein binding by MS using a strategy of complete digestion of microsomal proteins into free amino acids. Immobilized pronase was found to be the best method for complete digestion in terms of stability of amino acid modifications as well as minimized spectral background. Conclusion: Modified cysteine residues were identified for four tested drug compounds known to form reactive metabolites following in vitro microsomal incubations and accurate mass measurements by LC–MS analysis.