Heating of food and haemoglobin adducts from carcinogens: possible precursor role of glycidol

A comparison of the exposure system of glycidol-related chemicals on the formation of glycidol-hemoglobin adducts

Glycidol fatty acid esters that are present in foods are degraded in vivo to the animal carcinogen glycidol, which binds to the N-terminal valine of hemoglobin (Hb) to form N-(2,3-dihydroxypropyl)valine (diHOPrVal) adducts. The existence of other chemicals that are converted to glycidol is unknown. To determine the effect of different exposure conditions on the formation of diHOPrVal adducts, several glycidol-related chemicals (3-monochloropropane-1,2-diol; 3-MCPD, epichlorohydrin, glyceraldehyde, acrylic acid, and 1,2-propanediol) were evaluated using in vitro and in vivo (single/repeated dose) methods. In vitro, the reaction of 3-MCPD or epichlorohydrin with human Hb produced 17% and 0.7% of diHOPrVal, as compared to equimolar glycidol, respectively. Following a single administration of glycidol-related compounds to ICR mice, diHOPrVal formation was observed only in the epichlorohydrin-treated group after day 5 of exposure. After 14 days of repeated dosing, the amounts of diHOPrVal produced by epichlorohydrin and 3-MCPD in vivo were <1% of diHOPrVal produced by an equal molar concentration of glycidol. Furthermore, glyceraldehyde group produced 0.2% of diHOPrVal at the same molar concentration of glycidol equivalents, in which diHOPrVal formation could not be confirmed by the in vitro assay. The results indicate the usefulness of diHOPrVal as an exposure marker for glycidol; however, the contribution of its formation in vivo by exposure to various chemicals will be necessary to validate and interpret the results.

Potential Role of Lipase Activity on the Internal Exposure Assessment of Glycidol Released from Its Fatty Acid Esters

Glycidyl fatty acid esters (GEs) can be found in food, and they can be converted into genotoxic animal carcinogen glycidol in vivo by the action of lipase. This study examined whether human ingestion of charbroiled pork containing high levels of GEs (300 µg/day) increased glycidol-hemoglobin adduct (diHOPrVal), a marker of internal exposure to glycidol using LC-MS/MS. Contrary to expectation, the diHOPrVal value before ingesting charbroiled pork was 3.11 ± 1.10 pmol/g globin, which slightly decreased to 2.48 ± 0.47 pmol/g globin after 5 days of consumption. The decrease in lipase activity caused by the continuous consumption of lipid-rich foods such as meat in humans might decrease internal exposure to glycidol released from its esters. Thus, lipase activity was measured in C57/BL6J mice fed a high-fat diet (HFD) for 8 weeks, and diHOPrVal formation was measured after the administration of glycidyl oleate. Lipase activity was significantly lower in the HFD group than in the normal diet group. The amount of diHOPrVal was reduced in the HFD group. Therefore, the lipase activity was reduced by HFD, thereby decreasing the degradation of glycidol from glycidyl oleate. These results indicate that changes in lipase activity depending on the amount of lipids in the diet may affect the assessment of GEs exposure, and monitoring the lipase activity would provide a comprehensive understanding of exposure assessment.

Development of physiologically based toxicokinetic models for 3-monochloropropane-1,2-diol and glycidol

3-Monochloropropane-1,2-diol (3-MCPD), glycidol, together with their fatty acid esters are commonly presented in various food and have shown carcinogenicity in various laboratory animals. Public health risk assessment of 3-MPCD and glycidol exposure relies on quantitative tools that represent their in vivo toxicokinetics. In order to better understand the absorption, distribution, metabolism, and excretion profiles of 3-MCPD and glycidol in male rats, a physiologically based pharmacokinetic (PBTK) model was developed. The model's predictive power was evaluated by comparing in silico simulations to in vivo time course data obtained from experimental studies. Results indicate that our PBTK model successfully captured the toxicokinetics of both free chemicals in key organs, and their metabolites in accessible biological fluids. With the validated PBTK model, we then gave an animal-free example on how to extrapolate the toxicological knowledge acquired from a single gavage to a realistic dietary intake scenario. Three biomarkers, free compound in serum, urinary metabolite DHPMA, and glycidol-hemoglobin adduct (diHOPrVal) were selected for in silico simulation following constant dietary intakes, and their internal levels were correlated with proposed external daily exposure via reverse dosimetry approaches. Taken together, our model provides a computational approach for extrapolating animal toxicokinetic experiments to biomonitoring measurement and risk assessment.

Glycidol Fatty Acid Ester and 3-Monochloropropane-1,2-Diol Fatty Acid Ester in Commercially Prepared Foods

Glycidyl fatty acid esters (GEs), which are the main pollutant in processed oils, are potential mutagens or carcinogens. 3-Monochloropropane-1,2-diol fatty acid esters (3-MCPDEs) are also well-known food processing contaminants. 3-MCPDEs are believed to be a precursor to GEs in foodstuffs. In vivo, lipase breaks down the phosphate ester of GEs and 3-MCPDEs to produce glycidol and 3-MCPD, respectively, which are genotoxic carcinogens. Thus, it is important to determine human exposure to GEs and 3-MCPDEs through foodstuffs. There are only reports on the amount of GE and 3-MCPDE in cooking oils and cooked foods. The content in multiple types of foods that are actually on the market was not clarified. In this study, 48 commercially prepared foods were analyzed to identify other sources of exposure to GE and 3-MCPDE. All of them contained relatively high amounts of GEs and 3-MCPDEs. The correlation between GEs and 3-MCPDEs in individual foods was examined. There was a correlation between the amounts of GEs and 3-MCPDEs in the food products (r = 0.422, p < 0.005). This is the first report on the content in multiple types of commercially prepared foods that are actually on the market was clarified.

Does External Exposure of Glycidol-Related Chemicals Influence the Forming of the Hemoglobin Adduct, N-(2,3-dihydroxypropyl)valine, as a Biomarker of Internal Exposure to Glycidol?

Glycidyl fatty acid esters (GE) are constituents of edible oils and fats, and are converted into glycidol, a genotoxic substance, in vivo. N-(2,3-dihydroxypropyl)valine (diHOPrVal), a hemoglobin adduct of glycidol, is used as a biomarker of glycidol and GE exposure. However, high background levels of diHOPrVal are not explained by daily dietary exposure to glycidol and GE. In the present study, several glycidol-related chemicals (glycidol, (±)-3-chloro-1,2-propanediol, glycidyl oleate, epichlorohydrin, propylene oxide, 1-bromopropane, allyl alcohol, fructose, and glyceraldehyde) that might be precursors of diHOPrVal, were administered to mice, and diHOPrVal formation from each substance was examined with LC-MS/MS. DiHOPrVal was detected in animals treated with glycidol and glycidyl oleate but not in mice treated with other chemicals (3-MCPD, epichlorohydrin, propylene oxide, 1-bromopropane, allyl alcohol, fructose, and glyceraldehyde). The amount of diHOPrVal per administered dose produced from other chemicals was negligible compared to the amounts associated with dietary glycidol and GE. The present study provides important knowledge for exploring other sources for internal exposure to glycidol.

Internal Doses of Glycidol in Children and Estimation of Associated Cancer Risk

The general population is exposed to the genotoxic carcinogen glycidol via food containing refined edible oils where glycidol is present in the form of fatty acid esters. In this study, internal (in vivo) doses of glycidol were determined in a cohort of 50 children and in a reference group of 12 adults (non-smokers and smokers). The lifetime in vivo doses and intakes of glycidol were calculated from the levels of the hemoglobin (Hb) adduct N-(2,3-dihydroxypropyl)valine in blood samples from the subjects, demonstrating a fivefold variation between the children. The estimated mean intake (1.4 μg/kg/day) was about two times higher, compared to the estimated intake for children by the European Food Safety Authority. The data from adults indicate that the non-smoking and smoking subjects are exposed to about the same or higher levels compared to the children, respectively. The estimated lifetime cancer risk (200/10⁵) was calculated by a multiplicative risk model from the lifetime in vivo doses of glycidol in the children, and exceeds what is considered to be an acceptable cancer risk. The results emphasize the importance to further clarify exposure to glycidol and other possible precursors that could give a contribution to the observed adduct levels.

The hemoglobin adduct N-(2,3-dihydroxypropyl)-valine as biomarker of dietary exposure to glycidyl esters: a controlled exposure study in humans

Fatty acid esters of glycidol (glycidyl esters) are heat-induced food contaminants predominantly formed during industrial deodorization of vegetable oils and fats. After consumption, the esters are digested in the gastrointestinal tract, leading to a systemic exposure to the reactive epoxide glycidol. The compound is carcinogenic, genotoxic and teratogenic in rodents, and rated as probably carcinogenic to humans (IARC group 2A). Assessment of exposure from occurrence and consumption data is difficult, as lots of different foods containing refined oils and fats may contribute to human exposure. Therefore, assessment of the internal exposure using the hemoglobin adduct of glycidol, N-(2,3-dihydroxypropyl)-valine (2,3-diHOPr-Val), may be promising, but a proof-of-principle study is needed to interpret adduct levels with respect to the underlying external exposure. A controlled exposure study was conducted with 11 healthy participants consuming a daily portion of about 36 g commercially available palm fat with a relatively high content of ester-bound glycidol (8.7 mg glycidol/kg) over 4 weeks (total amount 1 kg fat, individual doses between 2.7 and 5.2 µg/kg body weight per day). Frequent blood sampling was performed to monitor the 2,3-diHOPr-Val adduct levels during formation and the following removal over 15 weeks, using a modified Edman degradation and ultrahigh performance liquid chromatography-tandem mass spectrometry (UHPLC-MS/MS). Results demonstrated for the first time that the relatively high exposure during the intervention period was reflected in corresponding distinct increases of 2,3-diHOPr-Val levels in all participants, following the expected slope for hemoglobin adduct formation and removal over time. The mean adduct level increased from 4.0 to 12.2 pmol 2,3-diHOPr-Val/g hemoglobin. By using a nonlinear mixed model, values for the adduct level/dose ratio (k, mean 0.082 pmol 2,3-diHOPr-Val/g hemoglobin per µg glycidol/kg body weight) and the adduct lifetime (τ, mean 104 days, likely the lifetime of the erythrocytes) were determined. Interindividual variability was generally low. 2,3-DiHOPr-Val was therefore proven to be a biomarker of the external dietary exposure to fatty acid esters of glycidol. From the background adduct levels observed in our study, a mean external glycidol exposure of 0.94 µg/kg body weight was estimated. This value is considerably higher than current estimates for adults using occurrence and consumption data of food. Possible reasons for this discrepancy are discussed (other oral or inhalational glycidol sources, endogenous formation, exposure to other chemicals also forming the adduct 2,3-diHOPr-Val). Further research is necessary to clarify the issue.

Anharmonic effect of the unimolecular dissociation of Glycerol to Glycidol

The unimolecular dissociation rate constants of the dehydration of Glycerol to Glycidol were calculated at the MP2/6–311G(d,p) level using the Rice–Ramsperger–Kassel–Marcus (RRKM) theory. The anharmonic effect of the reactions was examined by comparing the rate constants at temperatures (700–3000[Formula: see text]K) of the canonical case and total energies (25654–53089[Formula: see text]cm[Formula: see text]) of the microcanonical system. The calculations showed that high temperatures are required for the reaction to proceed. As the temperatures and total energies increased, the rate of reactions increased. However, the growth rate of the unimolecular dissociation rate constants was high and slower both in the canonical and microcanonical systems. Comparative analysis showed that the anharmonic effect was most significant for the reaction [Formula: see text] and least significant for the reaction [Formula: see text]. The anharmonic effect became more significant as the temperatures and total energies increased. Compared with the microcanonical situation, the anharmonic effect of the canonical system was more pronounced.

Exposure assessment of process-related contaminants in food by biomarker monitoring

Exposure assessment is a fundamental part of the risk assessment paradigm, but can often present a number of challenges and uncertainties. This is especially the case for process contaminants formed during the processing, e.g. heating of food, since they are in part highly reactive and/or volatile, thus making exposure assessment by analysing contents in food unreliable. New approaches are therefore required to accurately assess consumer exposure and thus better inform the risk assessment. Such novel approaches may include the use of biomarkers, physiologically based kinetic (PBK) modelling-facilitated reverse dosimetry, and/or duplicate diet studies. This review focuses on the state of the art with respect to the use of biomarkers of exposure for the process contaminants acrylamide, 3-MCPD esters, glycidyl esters, furan and acrolein. From the overview presented, it becomes clear that the field of assessing human exposure to process-related contaminants in food by biomarker monitoring is promising and strongly developing. The current state of the art as well as the existing data gaps and challenges for the future were defined. They include (1) using PBK modelling and duplicate diet studies to establish, preferably in humans, correlations between external exposure and biomarkers; (2) elucidation of the possible endogenous formation of the process-related contaminants and the resulting biomarker levels; (3) the influence of inter-individual variations and how to include that in the biomarker-based exposure predictions; (4) the correction for confounding factors; (5) the value of the different biomarkers in relation to exposure scenario's and risk assessment, and (6) the possibilities of novel methodologies. In spite of these challenges it can be concluded that biomarker-based exposure assessment provides a unique opportunity to more accurately assess consumer exposure to process-related contaminants in food and thus to better inform risk assessment.

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.

Toward an "omic" physiopathology of reactive chemicals: thirty years of mass spectrometric study of the protein adducts with endogenous and xenobiotic compounds

Cancer and degenerative diseases are major causes of morbidity and death, derived from the permanent modification of key biopolymers such as DNA and regulatory proteins by usually smaller, reactive molecules, present in the environment or generated from endogenous and xenobiotic components by the body's own biochemical mechanisms (molecular adducts). In particular, protein adducts with organic electrophiles have been studied for more than 30 [see, e.g., Calleman et al., 1978] years essentially for three purposes: (a) as passive monitors of the mean level of individual exposure to specific chemicals, either endogenously present in the human body or to which the subject is exposed through food or environmental contamination; (b) as quantitative indicators of the mean extent of the individual metabolic processing which converts a non-reactive chemical substance into its toxic products able to damage DNA (en route to cancer induction through genotoxic mechanisms) or key proteins (as in the case of several drugs, pesticides or otherwise biologically active substances); (c) to relate the extent of protein modification to that of biological function impairment (such as enzyme inhibition) finally causing the specific health damage. This review describes the role that contemporary mass spectrometry-based approaches employed in the qualitative and quantitative study of protein-electrophile adducts play in the discovery of the (bio)chemical mechanisms of toxic substances and highlights the future directions of research in this field. A particular emphasis is given to the measurement of often high levels of the protein adducts of several industrial and environmental pollutants in unexposed human populations, a phenomenon which highlights the possibility that a number of small organic molecules are generated in the human organism through minor metabolic processes, the imbalance of which may be the cause of "spontaneous" cases of cancer and of other degenerative diseases of still uncharacterized etiology. With all this in mind, it is foreseen that a holistic description of cellular functions will take advantage of new analytical methods based on time-integrated metabolomic measurements of a new biological compartment, the "adductome," aimed at better understanding integrated organism response to environmental and endogenous stressors.

Use of haemoglobin adducts in exposure monitoring and risk assessment

Many industrial bulk chemicals are oxiranes or alkenes that are easily metabolised to oxiranes in mammalian systems. Many oxiranes may react with DNA and are therefore mutagenic in vitro. Some oxiranes have been shown to be carcinogenic in rodents in vivo as well. Despite the very limited evidence of the carcinogenicity of oxiranes in humans, they should be considered potential human carcinogens. As a consequence, exposure to these compounds should be minimised and controlled. Twenty-five years ago, Ehrenberg and co-workers suggested that exposure to oxiranes might be determined through the measurement of the adducts they form with haemoglobin (Hb). Ten years later, a modification of the Edman degradation was developed at Stockholm University that allowed determination of adducts with the N-terminal valine of Hb by GC-MS. In our laboratory, this methodology was modified and adapted for analysis on an industrial scale. Since 1987, exposure of operators in our facilities to ethylene oxide (EO) has been routinely monitored by determination of N-(2-hydroxyethyl)valine in Hb. Biological monitoring programmes for propylene oxide (PO) and 1,3-butadiene (BD) were developed later. In this review, the methodology and its results are discussed as a tool in human risk assessment of industrial chemicals. Two major advantages of Hb adduct determinations in risk assessment are (1) the qualitative information on the structure of reactive intermediates that may be obtained through the mass spectrometry, which may provide insight in the molecular toxicology of compounds such as BD, and (2) the possibility of reliable determination of exposure over periods of several months with limited number of samples for compounds such as ethylene oxide (EO), propylene oxide (PO) and BD which form stable adducts with Hb. Since good correlations between the airborne concentrations of these chemicals with their respective adducts have been established, Hb adducts can also be used to quantitate airborne exposure which is of paramount importance as exposure assessment is usually one of the weaker parameters in risk assessment.