Journal of Agricultural and Food Chemistry, Jul 1, 2006
A high intake of cruciferous vegetables is associated with a reduced risk of cancer and cardiovas... more A high intake of cruciferous vegetables is associated with a reduced risk of cancer and cardiovascular diseases. This protective effect has been linked to isothiocyanates, enzymatic hydrolysis products of glucosinolates. In this study, the metabolic fate of glucosinolates and isothiocyanates after ingestion of 19 different cruciferous vegetables was studied in three male subjects. After the consumption of 13 cruciferous vegetables (glucosinolate content, 0.01-0.94 mmol/kg) and six condiments (isothiocyanate content, 0.06-49.3 mmol/kg), eight different isothiocyanate mercapturic acids were determined in urine samples. Excretion levels after the consumption of raw vegetables and condiments were higher (bioavailability, 8.2-113%) as compared to cooked vegetables (bioavailability, 1.8-43%), but the excretion rate was similar (t1/2=2.1-3.9 h). Isothiocyanates in urine remain longer at a nonzero level after the consumption of glucosinolates from cooked vegetables, as compared to raw vegetables and condiments, and maximal levels in urine were reached about 4 h later. Isothiocyanate mercapturic acids can be used as a biomarker to reflect the active dose of isothiocyanates absorbed.
On average 80% of muscular physical activity energy has to be released by the body as heat. Obese... more On average 80% of muscular physical activity energy has to be released by the body as heat. Obese people have a thick insulating fat layer. We investigated whether the concomitant thermal isolation...
Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic ... more Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic matter, which is explained by reduction of the freely dissolved aqueous concentration. In the present study, this reduction was measured, using equilibrium partitioning of micropollutants between water and a hydrophobic solid phase (C-18 Empore disk). The solid phase was exposed in prepared aqueous solutions which contained several micropollutants and Aldrich humic acid at different concentrations. The initial uptake rate of the micropollutants on the solid phase was measured and shown to be linear related to the freely dissolved concentration in the solution. Partitioning coefficients of tetra and hexachlorobenzene, pentachloroaniline, iodofenphos and DDT towards Aldrich humic acid were calculated. The relation between uptake rate and freely dissolved concentration was shown to be almost constant for all five pollutants, despite their difference in hydrophobicity and molecular weight.
The present study was designed to explain the differences in isoprene toxicity between mouse and ... more The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters K(m) and V(max) were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice.
Environmental Toxicology and Pharmacology, Jul 1, 2002
A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,... more A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,1,2-trichloroethene, TRI) for rat and humans, based on in vitro metabolic parameters. These were obtained using individual cytochrome P450 and glutathione S-transferase enzymes. The main enzymes involved both for rats and humans are CYP2E1 and the μ- and π-class glutathione S-transferases. Validation experiments were performed in order to test the predictive value of the enzyme kinetic parameters to describe 'whole-body' disposition. Male Wistar rats were dosed orally or intravenously with different doses of trichloroethylene. Obtained exhaled radioactivity, excreted radioactivity in urine, and obtained blood concentration-time curves of trichloroethylene for all dosing groups were compared to predictions from the PBPK model. Subsequently, using the scaling factor derived from the rat experiments predictions were made for the extreme cases to be expected in humans, based on interindividual variations of the key enzymes involved. On comparing these predictions with literature data a very close match was found. This illustrates the potential application of in vitro metabolic parameters in risk assessment, through the use of PBPK modeling as a tool to understand and predict in vivo data. From a hypothetical 8 h exposure scenario to 35 ppm trichloroethylene in rats and humans, and assuming that the glutathione S-transferase pathway is responsible for the toxicity of trichloroethylene, it was concluded that humans are less sensitive for trichloroethylene toxicity than rats.
This report describes a physiologically based pharmacokinetic model for cyclohexane and its use i... more This report describes a physiologically based pharmacokinetic model for cyclohexane and its use in comparing internal doses in rats and volunteers following inhalation exposures. Parameters describing saturable metabolism of cyclohexane are measured in rats and used along with experimentally determined partition coefficients. The model is evaluated by comparing predicted blood and brain concentrations to data from studies in rats and then allometrically scaling the results to humans. Levels of cyclohexane in blood and exhaled air are measured in human volunteers and compared with model values. The model predicts that exposure of volunteers to cyclohexane at levels of 4100 mg/m(3) ( approximately 1200 ppm) will result in brain levels similar to those in rats exposed to 8000 mg/m(3) (the no-effect level for acute central nervous system effects). There are no acute central nervous system effects in humans exposed to 860 mg/m(3), consistent with model predictions that current occupational exposure levels for cyclohexane protect against acute central nervous system effects.
Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic ... more Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic matter, which is explained by reduction of the freely dissolved aqueous concentration. The partition coefficient between organic matter and the aqueous phase, KDOC, is used to describe the distribution of neutral hydrophobic micropollutants. In the present study, the freely dissolved concentration was measured using partitioning of micropollutants between water and a hydrophobic solid‐phase filter disk (C‐18 Empore™ disk). Slivers of the disk were exposed in aqueous solutions that contained several micropollutants and Aldrich humic acid at different concentrations. Initial uptake rates of the micropollutants from water to disk (k1) were measured to estimate the freely dissolved concentration in the solution. Partition coefficients, KDOC, of tetra‐ and hexachlorobenzene, pentachloroaniline, iodofenphos, and p,p′ DDT were calculated. These values were comparable to earlier, published KDOCs toward Aldrich humic acid. In a validation experiment, the uptake rates of three chlorobenzenes in guppy (Poecilia reticulata) were measured in the absence and presence of humic acids. The lowering of the uptake rates in guppy was correctly predicted with the KDOC values from the Empore disk experiment. These findings suggest that the proposed method to measure the freely dissolved concentration is a valid tool to predict the bioavailability of hydrophobic compounds in the aqueous environment.
Environmental Toxicology and Pharmacology, Dec 1, 2004
This paper describes the &amp... more This paper describes the "quest" of our institute trying to assess the toxicology of chemical mixtures. In this overview, we will discuss some critical developments in hazard identification and risk assessment of chemical mixtures during these past 15 years. We will stand still at empirical and mechanistic modeling. "Empirical" means that only information on doses or concentrations and effects is available in addition to an often empirically selected quantitative dose-response relationship. Empirical models have played a dominant role in the last decade to identify health and safety characteristics of chemical mixtures. Many of these models are based on the work of pioneers in mixture toxicology who defined three basic types of action for combinations of chemicals: simple similar action, simple dissimilar action and interaction. Nowadays, empirical models are mainly based on response-surface analysis and make use of advanced statistical designs. However, possible interactions between components in a mixture can also be given in terms of mechanistic models. In terms of "mechanistic" (or biological) understanding, interactions between compounds may occur in the kinetic phase (processes of uptake, distribution, metabolism and excretion) or in the dynamic phase (effects of chemicals on the receptor, cellular target or organ). A biological phenomenon such as competitive agonism as described for mixtures of drugs (biotransformation enzymes) or sensory irritants (nerve receptors) can accurately predict the effect of any of these mixtures. Thus, far mechanistic and empirical analyses of interactions are usually unrelated. It is one of the future challenges for mixtures research to combine information from both approaches. Also, our current biology-based models have their limitations, since they cannot integrate every relevant biological mechanism. In this respect, mechanistic modeling of mixtures may benefit from the developments coming from the arena of molecular biology (toxicogenomics) which offers an in-depth analysis of several involved enzymatic pathways in parallel through the use of a systems biology approach. This was illustrated with mixtures of food additives known to affect the liver. Key to further maturation of mixture toxicology is collaboration of experimental toxicologists, biomathematicians, biologists, pharmacologists, model developers, molecular biologists and bioinformaticians to ensure parallel and coordinated research in this challenging area of toxicology. For this reason, the next sequel will be even more challenging and exciting to that first 15 years of empirical testing.
The present research aimed to study the interaction of three chemicals, methyl mercury, benzene a... more The present research aimed to study the interaction of three chemicals, methyl mercury, benzene and trichloroethylene, on mRNA expression alterations in rat liver and kidney measured by microarray analysis. These compounds were selected based on presumed different modes of action. The chemicals were administered daily for 14 days at the Lowest-Observed-Adverse-Effect-Level (LOAEL) or at a two- or threefold lower concentration individually or in binary or ternary mixtures. The compounds had strong antagonistic effects on each other's gene expression changes, which included several genes encoding Phase I and II metabolizing enzymes. On the other hand, the mixtures affected the expression of "novel" genes that were not or little affected by the individual compounds. The three compounds exhibited a synergistic interaction on gene expression changes at the LOAEL in the liver and both at the sub-LOAEL and LOAEL in the kidney. Many of the genes induced by mixtures but not by single compounds, such as Id2, Nr2f6, Tnfrsf1a, Ccng1, Mdm2 and Nfkb1 in the liver, are known to affect cellular proliferation, apoptosis and tissue-specific function. This indicates a shift from compound specific response on exposure to individual compounds to a more generic stress response to mixtures. Most of the effects on cell viability as concluded from transcriptomics were not detected by classical toxicological endpoints illustrating the benefit of increased sensitivity of assessing gene expression profiling. These results emphasize the benefit of applying toxicogenomics in mixture interaction studies, which yields biomarkers for joint toxicity and eventually can result in an interaction model for most known toxicants.
Journal of Agricultural and Food Chemistry, Jul 1, 2006
A high intake of cruciferous vegetables is associated with a reduced risk of cancer and cardiovas... more A high intake of cruciferous vegetables is associated with a reduced risk of cancer and cardiovascular diseases. This protective effect has been linked to isothiocyanates, enzymatic hydrolysis products of glucosinolates. In this study, the metabolic fate of glucosinolates and isothiocyanates after ingestion of 19 different cruciferous vegetables was studied in three male subjects. After the consumption of 13 cruciferous vegetables (glucosinolate content, 0.01-0.94 mmol/kg) and six condiments (isothiocyanate content, 0.06-49.3 mmol/kg), eight different isothiocyanate mercapturic acids were determined in urine samples. Excretion levels after the consumption of raw vegetables and condiments were higher (bioavailability, 8.2-113%) as compared to cooked vegetables (bioavailability, 1.8-43%), but the excretion rate was similar (t1/2=2.1-3.9 h). Isothiocyanates in urine remain longer at a nonzero level after the consumption of glucosinolates from cooked vegetables, as compared to raw vegetables and condiments, and maximal levels in urine were reached about 4 h later. Isothiocyanate mercapturic acids can be used as a biomarker to reflect the active dose of isothiocyanates absorbed.
On average 80% of muscular physical activity energy has to be released by the body as heat. Obese... more On average 80% of muscular physical activity energy has to be released by the body as heat. Obese people have a thick insulating fat layer. We investigated whether the concomitant thermal isolation...
Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic ... more Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic matter, which is explained by reduction of the freely dissolved aqueous concentration. In the present study, this reduction was measured, using equilibrium partitioning of micropollutants between water and a hydrophobic solid phase (C-18 Empore disk). The solid phase was exposed in prepared aqueous solutions which contained several micropollutants and Aldrich humic acid at different concentrations. The initial uptake rate of the micropollutants on the solid phase was measured and shown to be linear related to the freely dissolved concentration in the solution. Partitioning coefficients of tetra and hexachlorobenzene, pentachloroaniline, iodofenphos and DDT towards Aldrich humic acid were calculated. The relation between uptake rate and freely dissolved concentration was shown to be almost constant for all five pollutants, despite their difference in hydrophobicity and molecular weight.
The present study was designed to explain the differences in isoprene toxicity between mouse and ... more The present study was designed to explain the differences in isoprene toxicity between mouse and rat based on the liver concentrations of the assumed toxic metabolite isoprene diepoxide. In addition, extrapolation to the human situation was attempted. For this purpose, enzyme kinetic parameters K(m) and V(max) were determined in vitro in mouse, rat and human liver microsomes/cytosol for the cytochrome P450-mediated formation of isoprene mono- and diepoxides, epoxide hydrolase mediated hydrolysis of isoprene mono- and diepoxides, and the glutathione S-transferases mediated conjugation of isoprene monoepoxides. Subsequently, the kinetic parameters were incorporated into a physiologically-based pharmacokinetic model, and species differences regarding isoprene diepoxide levels were forecasted. Almost similar isoprene diepoxide liver and lung concentrations were predicted in mouse and rat, while predicted levels in humans were about 20-fold lower. However, when interindividual variation in enzyme activity was introduced in the human model, the levels of isoprene diepoxide changed considerably. It was forecasted that in individuals having both an extensive oxidation by cytochrome P450 and a low detoxification by epoxide hydrolase, isoprene diepoxide concentrations in the liver increased to similar concentrations as predicted for the mouse. However, the interpretation of the latter finding for human risk assessment is ambiguous since species differences between mouse and rat regarding isoprene toxicity could not be explained by the predicted isoprene diepoxide concentrations. We assume that other metabolites than isoprene diepoxide or different carcinogenic response might play a key role in determining the extent of isoprene toxicity. In order to confirm this, in vivo experiments are required in which isoprene epoxide concentrations will be established in rats and mice.
Environmental Toxicology and Pharmacology, Jul 1, 2002
A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,... more A physiologically based pharmacokinetic (PBPK) model has been developed for trichloroethylene (1,1,2-trichloroethene, TRI) for rat and humans, based on in vitro metabolic parameters. These were obtained using individual cytochrome P450 and glutathione S-transferase enzymes. The main enzymes involved both for rats and humans are CYP2E1 and the μ- and π-class glutathione S-transferases. Validation experiments were performed in order to test the predictive value of the enzyme kinetic parameters to describe 'whole-body' disposition. Male Wistar rats were dosed orally or intravenously with different doses of trichloroethylene. Obtained exhaled radioactivity, excreted radioactivity in urine, and obtained blood concentration-time curves of trichloroethylene for all dosing groups were compared to predictions from the PBPK model. Subsequently, using the scaling factor derived from the rat experiments predictions were made for the extreme cases to be expected in humans, based on interindividual variations of the key enzymes involved. On comparing these predictions with literature data a very close match was found. This illustrates the potential application of in vitro metabolic parameters in risk assessment, through the use of PBPK modeling as a tool to understand and predict in vivo data. From a hypothetical 8 h exposure scenario to 35 ppm trichloroethylene in rats and humans, and assuming that the glutathione S-transferase pathway is responsible for the toxicity of trichloroethylene, it was concluded that humans are less sensitive for trichloroethylene toxicity than rats.
This report describes a physiologically based pharmacokinetic model for cyclohexane and its use i... more This report describes a physiologically based pharmacokinetic model for cyclohexane and its use in comparing internal doses in rats and volunteers following inhalation exposures. Parameters describing saturable metabolism of cyclohexane are measured in rats and used along with experimentally determined partition coefficients. The model is evaluated by comparing predicted blood and brain concentrations to data from studies in rats and then allometrically scaling the results to humans. Levels of cyclohexane in blood and exhaled air are measured in human volunteers and compared with model values. The model predicts that exposure of volunteers to cyclohexane at levels of 4100 mg/m(3) ( approximately 1200 ppm) will result in brain levels similar to those in rats exposed to 8000 mg/m(3) (the no-effect level for acute central nervous system effects). There are no acute central nervous system effects in humans exposed to 860 mg/m(3), consistent with model predictions that current occupational exposure levels for cyclohexane protect against acute central nervous system effects.
Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic ... more Many organic micropollutants show a reduced bioavailability in the presence of dissolved organic matter, which is explained by reduction of the freely dissolved aqueous concentration. The partition coefficient between organic matter and the aqueous phase, KDOC, is used to describe the distribution of neutral hydrophobic micropollutants. In the present study, the freely dissolved concentration was measured using partitioning of micropollutants between water and a hydrophobic solid‐phase filter disk (C‐18 Empore™ disk). Slivers of the disk were exposed in aqueous solutions that contained several micropollutants and Aldrich humic acid at different concentrations. Initial uptake rates of the micropollutants from water to disk (k1) were measured to estimate the freely dissolved concentration in the solution. Partition coefficients, KDOC, of tetra‐ and hexachlorobenzene, pentachloroaniline, iodofenphos, and p,p′ DDT were calculated. These values were comparable to earlier, published KDOCs toward Aldrich humic acid. In a validation experiment, the uptake rates of three chlorobenzenes in guppy (Poecilia reticulata) were measured in the absence and presence of humic acids. The lowering of the uptake rates in guppy was correctly predicted with the KDOC values from the Empore disk experiment. These findings suggest that the proposed method to measure the freely dissolved concentration is a valid tool to predict the bioavailability of hydrophobic compounds in the aqueous environment.
Environmental Toxicology and Pharmacology, Dec 1, 2004
This paper describes the &amp... more This paper describes the "quest" of our institute trying to assess the toxicology of chemical mixtures. In this overview, we will discuss some critical developments in hazard identification and risk assessment of chemical mixtures during these past 15 years. We will stand still at empirical and mechanistic modeling. "Empirical" means that only information on doses or concentrations and effects is available in addition to an often empirically selected quantitative dose-response relationship. Empirical models have played a dominant role in the last decade to identify health and safety characteristics of chemical mixtures. Many of these models are based on the work of pioneers in mixture toxicology who defined three basic types of action for combinations of chemicals: simple similar action, simple dissimilar action and interaction. Nowadays, empirical models are mainly based on response-surface analysis and make use of advanced statistical designs. However, possible interactions between components in a mixture can also be given in terms of mechanistic models. In terms of "mechanistic" (or biological) understanding, interactions between compounds may occur in the kinetic phase (processes of uptake, distribution, metabolism and excretion) or in the dynamic phase (effects of chemicals on the receptor, cellular target or organ). A biological phenomenon such as competitive agonism as described for mixtures of drugs (biotransformation enzymes) or sensory irritants (nerve receptors) can accurately predict the effect of any of these mixtures. Thus, far mechanistic and empirical analyses of interactions are usually unrelated. It is one of the future challenges for mixtures research to combine information from both approaches. Also, our current biology-based models have their limitations, since they cannot integrate every relevant biological mechanism. In this respect, mechanistic modeling of mixtures may benefit from the developments coming from the arena of molecular biology (toxicogenomics) which offers an in-depth analysis of several involved enzymatic pathways in parallel through the use of a systems biology approach. This was illustrated with mixtures of food additives known to affect the liver. Key to further maturation of mixture toxicology is collaboration of experimental toxicologists, biomathematicians, biologists, pharmacologists, model developers, molecular biologists and bioinformaticians to ensure parallel and coordinated research in this challenging area of toxicology. For this reason, the next sequel will be even more challenging and exciting to that first 15 years of empirical testing.
The present research aimed to study the interaction of three chemicals, methyl mercury, benzene a... more The present research aimed to study the interaction of three chemicals, methyl mercury, benzene and trichloroethylene, on mRNA expression alterations in rat liver and kidney measured by microarray analysis. These compounds were selected based on presumed different modes of action. The chemicals were administered daily for 14 days at the Lowest-Observed-Adverse-Effect-Level (LOAEL) or at a two- or threefold lower concentration individually or in binary or ternary mixtures. The compounds had strong antagonistic effects on each other's gene expression changes, which included several genes encoding Phase I and II metabolizing enzymes. On the other hand, the mixtures affected the expression of "novel" genes that were not or little affected by the individual compounds. The three compounds exhibited a synergistic interaction on gene expression changes at the LOAEL in the liver and both at the sub-LOAEL and LOAEL in the kidney. Many of the genes induced by mixtures but not by single compounds, such as Id2, Nr2f6, Tnfrsf1a, Ccng1, Mdm2 and Nfkb1 in the liver, are known to affect cellular proliferation, apoptosis and tissue-specific function. This indicates a shift from compound specific response on exposure to individual compounds to a more generic stress response to mixtures. Most of the effects on cell viability as concluded from transcriptomics were not detected by classical toxicological endpoints illustrating the benefit of increased sensitivity of assessing gene expression profiling. These results emphasize the benefit of applying toxicogenomics in mixture interaction studies, which yields biomarkers for joint toxicity and eventually can result in an interaction model for most known toxicants.
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