Identification and Quantification of Protein Adducts Formed by Metabolites of 1-Methoxy-3-indolylmethyl Glucosinolate in Vitro and in Mouse Models

Formation of DNA Adducts by 1-Methoxy-3-indolylmethylalcohol, a Breakdown Product of a Glucosinolate, in the Mouse: Impact of the SULT1A1 Status-Wild-Type, Knockout or Humanised

We previously found that feeding rats with broccoli or cauliflower leads to the formation of characteristic DNA adducts in the liver, intestine and various other tissues. We identified the critical substances in the plants as 1-methoxy-3-indolylmethyl (1-MIM) glucosinolate and its degradation product 1-MIM-OH. DNA adduct formation and the mutagenicity of 1-MIM-OH in cell models were drastically enhanced when human sulfotransferase (SULT) 1A1 was expressed. The aim of this study was to clarify the role of SULT1A1 in DNA adduct formation by 1-MIM-OH in mouse tissues in vivo. Furthermore, we compared the endogenous mouse Sult1a1 and transgenic human SULT1A1 in the activation of 1-MIM-OH using genetically modified mouse strains. We orally treated male wild-type (wt) and Sult1a1-knockout (ko) mice, as well as corresponding lines carrying the human SULT1A1-SULT1A2 gene cluster (tg and ko-tg), with 1-MIM-OH. N2-(1-MIM)-dG and N6-(1-MIM)-dA adducts in DNA were analysed using isotope-dilution UPLC-MS/MS. In the liver, caecum and colon adducts were abundant in mice expressing mouse and/or human SULT1A1, but were drastically reduced in ko mice (1.2-10.6% of wt). In the kidney and small intestine, adduct levels were high in mice carrying human SULT1A1-SULT1A2 genes, but low in wt and ko mice (1.8-6.3% of tg-ko). In bone marrow, adduct levels were very low, independently of the SULT1A1 status. In the stomach, they were high in all four lines. Thus, adduct formation was primarily controlled by SULT1A1 in five out of seven tissues studied, with a strong impact of differences in the tissue distribution of mouse and human SULT1A1. The behaviour of 1-MIM-OH in these models (levels and tissue distribution of DNA adducts; impact of SULTs) was similar to that of methyleugenol, classified as "probably carcinogenic to humans". Thus, there is a need to test 1-MIM-OH for carcinogenicity in animal models and to study its adduct formation in humans consuming brassicaceous foodstuff.

Feeding Brassica vegetables to rats leads to the formation of characteristic DNA adducts (from 1-methoxy-3-indolylmethyl glucosinolate) in many tissues

Juices of Brassica vegetables are mutagenic and form characteristic DNA adducts in bacteria and mammalian cells. In this study, we examined whether such adducts are also formed in vivo in animal models. Rats fed raw broccoli ad libitum in addition to normal laboratory chow for 5 weeks showed one major adduct spot and sometimes an additional minor adduct spot in liver, kidney, lung, blood and the gastrointestinal tract, as determined by ³²P-postlabelling/thin-layer chromatography. Adducts with the same chromatographic properties were formed when herring sperm DNA (or dG-3’-phosphate) was incubated with 1-methoxy-3-indolylmethyl glucosinolate (phytochemical present in Brassica plants), in the presence of myrosinase (plant enzyme that hydrolyses glucosinolates to bioactive breakdown products). UPLC–MS/MS analysis corroborated this finding: 1-Methoxy-3-indolylmethyl-substituted purine nucleosides were detected in the hepatic DNA of broccoli-fed animals, but not in control animals. Feeding raw cauliflower led to the formation of the same adducts. When steamed rather than raw broccoli was used, the adduct levels were essentially unchanged in liver and jejunum, but elevated in large intestine. Due to inactivation of myrosinase by the steaming, higher levels of the glucosinolates may have reached the large bowl to be activated by glucosidases from intestinal bacteria. In conclusion, the consumption of common Brassica vegetables can lead to the formation of substantial levels of DNA adducts in animal models. The adducts can be attributed to a specific phytochemical, neoglucobrassicin (1-methoxy-3-indolylmethyl glucosinolate).

Quo vadis blood protein adductomics?

Chemicals are measured regularly in air, food, the environment, and the workplace. Biomonitoring of chemicals in biological fluids is a tool to determine the individual exposure. Blood protein adducts of xenobiotics are a marker of both exposure and the biologically effective dose. Urinary metabolites and blood metabolites are short term exposure markers. Stable hemoglobin adducts are exposure markers of up to 120 days. Blood protein adducts are formed with many xenobiotics at different sites of the blood proteins. Newer methods apply the techniques developed in the field of proteomics. Larger adducted peptides with 20 amino acids are used for quantitation. Unfortunately, at present the methods do not reach the limits of detection obtained with the methods looking at single amino acid adducts or at chemically cleaved adducts. Therefore, to progress in the field new approaches are needed.

Mode of action-based risk assessment of genotoxic carcinogens

The risk assessment of chemical carcinogens is one major task in toxicology. Even though exposure has been mitigated effectively during the last decades, low levels of carcinogenic substances in food and at the workplace are still present and often not completely avoidable. The distinction between genotoxic and non-genotoxic carcinogens has traditionally been regarded as particularly relevant for risk assessment, with the assumption of the existence of no-effect concentrations (threshold levels) in case of the latter group. In contrast, genotoxic carcinogens, their metabolic precursors and DNA reactive metabolites are considered to represent risk factors at all concentrations since even one or a few DNA lesions may in principle result in mutations and, thus, increase tumour risk. Within the current document, an updated risk evaluation for genotoxic carcinogens is proposed, based on mechanistic knowledge regarding the substance (group) under investigation, and taking into account recent improvements in analytical techniques used to quantify DNA lesions and mutations as well as "omics" approaches. Furthermore, wherever possible and appropriate, special attention is given to the integration of background levels of the same or comparable DNA lesions. Within part A, fundamental considerations highlight the terms hazard and risk with respect to DNA reactivity of genotoxic agents, as compared to non-genotoxic agents. Also, current methodologies used in genetic toxicology as well as in dosimetry of exposure are described. Special focus is given on the elucidation of modes of action (MOA) and on the relation between DNA damage and cancer risk. Part B addresses specific examples of genotoxic carcinogens, including those humans are exposed to exogenously and endogenously, such as formaldehyde, acetaldehyde and the corresponding alcohols as well as some alkylating agents, ethylene oxide, and acrylamide, but also examples resulting from exogenous sources like aflatoxin B1, allylalkoxybenzenes, 2-amino-3,8-dimethylimidazo[4,5-f] quinoxaline (MeIQx), benzo[a]pyrene and pyrrolizidine alkaloids. Additionally, special attention is given to some carcinogenic metal compounds, which are considered indirect genotoxins, by accelerating mutagenicity via interactions with the cellular response to DNA damage even at low exposure conditions. Part C finally encompasses conclusions and perspectives, suggesting a refined strategy for the assessment of the carcinogenic risk associated with an exposure to genotoxic compounds and addressing research needs.

Chemical Aspects of Biological Activity of Isothiocyanates and Indoles, the Products of Glucosinolate Decomposition

There is growing evidence that cancer chemoprevention employing natural, bioactive compounds may halt or at least slow down the different stages of carcinogenesis. A particularly advantageous effect is attributed to derivatives of sulfur-organic phytochemicals, such as glucosinolates (GLs) synthesized mainly in Brassicaceae plant family. GLs are hydrolysed enzymatically to bioactive isothiocyanates (ITC) and indoles, which exhibit strong anti-inflammatory and anti-carcinogenic activity. Highly bioavailable electrophilic ITC are of particular interest, as they can react with nucleophilic groups of important biomolecules to form dithiocarbamates, thiocarbamates and thioureas. These modifications seem responsible for the chemopreventive activity, but also for genotoxicity and mutagenicity. It was documented that ITC can permanently bind to important biomolecules such as glutathione, cytoskeleton proteins, transcription factors NF-κB and Nrf2, thiol-disulfide oxidoreductases, proteasome proteins or heat shock proteins. Furthermore, ITC may also affect epigenetic regulation of gene expression, e.g. by inhibition of histone deacetylases. Some other derivatives of glucosinolates, especially indoles, are able to form covalent bonds with nucleobases in DNA, which may result in genotoxicity and mutagenicity. This article summarizes the current state of knowledge about glucosinolates and their degradation products in terms of possible interactions with reactive groups of cellular molecules.

1-Methoxy-3-indolylmethyl DNA adducts in six tissues, and blood protein adducts, in mice under pak choi diet: time course and persistence

We previously showed that purified 1-methoxy-3-indolylmethyl (1-MIM) glucosinolate, a secondary plant metabolite in Brassica species, is mutagenic in various in vitro systems and forms DNA and protein adducts in mouse models. In the present study, we administered 1-MIM glucosinolate in a natural matrix to mice, by feeding a diet containing pak choi powder and extract. Groups of animals were killed after 1, 2, 4 and 8 days of pak choi diet, directly or, in the case of the 8-day treatment, after 0, 8 and 16 days of recovery with pak choi-free diet. DNA adducts [N²-(1-MIM)-dG, N⁶-(1-MIM)-dA] in six tissues, as well as protein adducts [τN-(1-MIM)-His] in serum albumin (SA) and hemoglobin (Hb) were determined using UPLC-MS/MS with isotopically labeled internal standards. None of the samples from the 12 control animals under standard diet contained any 1-MIM adducts. All groups receiving pak choi diet showed DNA adducts in all six tissues (exception: lung of mice treated for a single day) as well as SA and Hb adducts. During the feeding period, all adduct levels continuously increased until day 8 (in the jejunum until day 4). During the 14-day recovery period, N²-(1-MIM)-dG in liver, kidney, lung, jejunum, cecum and colon decreased to 52, 41, 59, 11, 7 and 2%, respectively, of the peak level. The time course of N⁶-(1-MIM)-dA was similar. Immunohistochemical analyses indicated that cell turnover is a major mechanism of DNA adduct elimination in the intestine. In the same recovery period, protein adducts decreased more rapidly in SA than in Hb, to 0.7 and 37%, respectively, of the peak level, consistent with the differential turnover of these proteins. In conclusion, the pak choi diet lead to the formation of high levels of adducts in mice. Cell and protein turnover was a major mechanism of adduct elimination, at least in gut and blood.

Gut Glucosinolate Metabolism and Isothiocyanate Production

The glucosinolate-myrosinase system in plants has been well studied over the years while relatively little research has been undertaken on the bacterial metabolism of glucosinolates. The products of myrosinase-based glucosinolate hydrolysis in the human gut are important to health, particularly the isothiocyanates, as they are shown to have anticancer properties as well as other beneficial roles in human health. This review is concerned with the bacterial metabolism of glucosinolates but is not restricted to the human gut. Isothiocyanate production and nitrile formation are discussed together with the mechanisms of the formation of these compounds. Side chain modification of the methylsulfinylalkyl glucosinolates is reviewed and the implications for bioactivity of the resultant products are also discussed.

In Vitro Determination of Protein Conjugates in Human Cells by LC-ESI-MS/MS after Benzyl Isothiocyanate Exposure

Glucosinolates and their breakdown products, especially isothiocyanates (ITCs), are hypothesized to exert a broad range of bioactivities. However, physiological mechanisms are not yet completely understood. In this study, formation of protein conjugates after incubation with benzyl isothiocyanate (BITC) was investigated in vitro. A survey of protein conjugates was done by determining BITC cysteine and lysine amino acid conjugates after protein digestion. Therefore, a liquid chromatography-tandem mass spectrometry (LC-ESI-MS/MS) method was developed and validated. Stability studies showed that cysteine conjugates are not stable under alkaline conditions, and lysine conjugates did not show any correlation to pH values, although stability increased at low temperatures. Lysine conjugates were the preferred form of protein conjugates, and longer BITC exposure times led to higher amounts. Knowledge about the reaction sites of ITCs in eukaryotic cells may help to understand the mode of action of ITCs leading to health promoting as well as toxicological effects in humans.

A hemoglobin adduct as a biomarker for the internal exposure to the rodent carcinogen furfuryl alcohol

Furfuryl alcohol is a common food contaminant, which is formed by acid- and heat-catalyzed degradation of fructose and glucose. Its carcinogenic effect in rodents originates most likely from sulfotransferase (SULT)-catalyzed conversion into the mutagenic sulfate ester 2-sulfoxymethylfuran. In this study, a protein adduct biomarker was sought for the medium-term internal exposure to furfuryl alcohol. A UPLC-MS/MS screening showed that the adduct N-((furan-2-yl)methyl)-Val (FFA-Val) at the N-terminus of hemoglobin is a valid target analyte. The Val cleavage by fluorescein isothiocyanate-mediated Edman degradation yielded 3-fluorescein-1-(furan-2-ylmethyl)-5-(propan-2-yl)-2-thioxoimidazolidin-4-one (FFA-Val-FTH), which was characterized by ¹H and ¹³C NMR spectroscopy. An isotope-dilution method for the quantification of FFA-Val-FTH by UPLC-MS/MS was developed. It was used to study the adduct formation in furfuryl alcohol-treated FVB/N mice and the influence of ethanol and the alcohol dehydrogenase (ADH) inhibitor 4-methylpyrazole on the adduct levels. The administration of 400 mg/kg body weight furfuryl alcohol alone led to 12.5 and 36.7 pmol FFA-Val/g Hb in blood samples of male and female animals, respectively. The co-administration of 1.6 g ethanol/kg body weight increased FFA-Val levels by 1.4-fold in males and by 1.5-fold in females. The co-administration of 100 mg 4-methylpyrazole/kg body weight had a similar effect on the adduct levels. A high correlation was observed between adduct levels in hemoglobin and in hepatic DNA samples determined in the same animal experiment. This indicated that FFA-Val is a valid biomarker for the internal exposure to 2-sulfoxymethylfuran, which may be suitable to monitor furfuryl alcohol exposure also in humans.

The sphingosine 1-phosphate breakdown product, (2E)-hexadecenal, forms protein adducts and glutathione conjugates in vitro

Sphingosine 1-phosphate (S1P), a bioactive lipid involved in various physiological processes such as cell proliferation and apoptosis, can be irreversibly cleaved by S1P lyase, yielding phosphoethanolamine and (2E)-hexadecenal (2EHD). The latter metabolite, an α,β-unsaturated fatty aldehyde, may be susceptible to nucleophilic attack by cellular biomolecules. Hence, we studied whether 2EHD forms reaction products with GSH and proteins in vitro. Using LC-MS/MS and stable isotopically labeled reference material, we identified a total of nine novel reaction products of 2EHD in a cell-free approach: two GSH conjugates and seven l-amino acid adducts. Both GSH conjugates were also found in HepG2 cell lysates incubated with 2EHD. Likewise, we detected four out of seven amino acid adducts released from the model protein, BSA, and proteins extracted from HepG2 cells. On this occasion, the 2EHD Michael adduct with l-histidine proved to be the most prominent adduct. Most interestingly, inhibition of the enzymatically driven oxidative degradation of 2EHD resulted in increased levels of both GSH conjugates and protein adducts in HepG2 cell lysates. Hence, our data provide new insights into sphingolipid metabolism and will be useful to investigate certain disorders linked to an impaired fatty aldehyde metabolism in more detail.

Biomonitoring Human Albumin Adducts: The Past, the Present, and the Future

Serum albumin (Alb) is the most abundant protein in blood plasma. Alb reacts with many carcinogens and/or their electrophilic metabolites. Studies conducted over 20 years ago showed that Alb forms adducts with the human carcinogens aflatoxin B1 and benzene, which were successfully used as biomarkers in molecular epidemiology studies designed to address the role of these chemicals in cancer risk. Alb forms adducts with many therapeutic drugs or their reactive metabolites such as β-lactam antibiotics, acetylsalicylic acid, acetaminophen, nonsteroidal anti-inflammatory drugs, chemotherapeutic agents, and antiretroviral therapy drugs. The identification and characterization of the adduct structures formed with Alb have served to understand the generation of reactive metabolites and to predict idiosyncratic drug reactions and toxicities. The reaction of candidate drugs with Alb is now exploited as part of the battery of screening tools to assess the potential toxicities of drugs. The use of gas chromatography-mass spectrometry, liquid chromatography, or liquid chromatography-mass spectrometry (LC-MS) enabled the identification and quantification of multiple types of Alb xenobiotic adducts in animals and humans during the past three decades. In this perspective, we highlight the history of Alb as a target protein for adduction to environmental and dietary genotoxicants, pesticides, and herbicides, common classes of medicinal drugs, and endogenous electrophiles, and the emerging analytical mass spectrometry technologies to identify Alb-toxicant adducts in humans.

Covalent adducts of melphalan with free amino acids and a model peptide studied by liquid chromatography/tandem mass spectrometry

RATIONALE

Melphalan is a frequently used chemotherapeutical agent for the treatment of myeloma, breast cancer, ovarian cancer and sarcoma of soft tissue. A good knowledge of the reactivity of the drug toward the different amino acids, e.g. covalent adduct formation, is crucial for the understanding of its activity and side effects during cancer treatment.

METHODS

The reactivity of melphalan and sites of adduct formation were studied by in vitro incubation of melphalan with free amino acids and glutathione as a model peptide. The formed covalent adducts were investigated using ultra-performance liquid chromatography tandem mass spectrometry (UPLC/MS/MS) using a triple-quadrupole instrument. Accurate mass measurements for the confirmation of characteristic product ions were performed on a quadrupole time-of-flight (QTOF) mass spectrometer.

RESULTS

The incubation of melphalan with different classes of amino acids resulted in the formation of adducts on the amino and carboxyl termini, as well as adduct formation in the reactive side chains of Cys, Met, Tyr, His, Lys, Asp and Glu. All these melphalan adducts could be identified by their characteristic collision-induced dissociation (CID) product ion patterns.

CONCLUSIONS

The present study demonstrates the reactivity of melphalan towards the functional groups of amino acids. The different alkylation site products show distinctive fragmentation patterns, which enable a fast identification of the different melphalan adducts. This study is a first important step towards a better understanding of the adduct formation in more complex molecules, e.g. peptides and proteins.

High mutagenic activity of juice from pak choi (Brassica rapa ssp. chinensis) sprouts due to its content of 1-methoxy-3-indolylmethyl glucosinolate, and its enhancement by elicitation with methyl jasmonate

Cruciferous vegetables have the reputation to protect against cancer, an effect attributed to glucosinolates (GLS) and their breakdown products. However, some GLS are mutagenic, an activity associated with cancer initiation rather than chemoprevention. We show that juices from steamed pak choi sprouts are strongly mutagenic in Salmonella typhimurium TA100 upon addition of fresh myrosinase. Growth of the plants in the presence of methyl jasmonate, a hormone eliciting defence factors, led to 20-fold enhanced mutagenic activity. The level of 1-methoxy-3-indolylmethyl (1-MIM)-GLS was similarly increased, whereas those of other GLS were only elevated 0.8- to 3.2-fold. 1-MIM-GLS is a potent mutagen, whose activity is further enhanced by human sulphotransferase 1A1 (hSULT1A1), an activation not observed with other GLS. The mutagenicity of the pak choi juices was increased 20-fold in bacteria expressing hSULT1A1. A tiny level of juice from elicitated sprouts, 0.04% in the mutagenicity assay, was sufficient to double the number of revertants above the spontaneous level. We conclude that pak choi juice is mutagenic, an activity that can be strongly affected by the growth conditions. It is owed essentially to a single component, 1-MIM-GLS. We recommend using cultivars, growth conditions and/or food preparations that keep the level of this GLS congener low.

Glucosinolates from pak choi and broccoli induce enzymes and inhibit inflammation and colon cancer differently

Feeding a glucosinolate-enriched pak choi diet reduced colitis and tumor numbers. No effects were observed by a glucosinolate-enriched broccoli diet.