Investigation of species differences in isobutene (2-methylpropene) metabolism between mice and rats

The "Tuebingen desiccator" system, a tool to study oxidative stress in vivo and inhalation toxicokinetics

The "Tuebingen desiccator," a gas-tight all-glass closed chamber system (CCS), has been established in Herbert Remmer's Institute of Toxicology, University of Tuebingen, to investigate the mechanisms underlying the exhalation of endogenous volatile hydrocarbons in rats under oxidative stress. Remmer and associates confirmed the former view that ethane and n-pentane were derived from polyunsaturated fatty acids, and they demonstrated that propane, n-butane and isobutane were released from amino acids. Hydrocarbons exhaled following acute ethanol treatment of rats resulted predominantly from ethanol-dependent inhibition of their metabolism and partly from oxidation of proteins. Exhalation of alkanes in carbon tetrachloride exposed rats did not reflect liver damage, which was, however, directly linked to the amount of carbon tetrachloride metabolized. As has first been shown in Herbert Remmer's institute by investigating the fate of inhaled vinyl chloride in rats, the CSS proved to be also an excellent tool for studying toxicokinetics of inhaled gaseous xenobiotics by means of gas uptake experiments. Based on results gained by such studies, it was recently demonstrated that knowledge of compound-specific physicochemical and species-specific physiological parameters are often sufficient to predict important toxicokinetic properties of inhaled chemicals such as tissue burdens at steady state. By means of the CCS, not only kinetics of a parent gaseous substance but also of gaseous metabolites can be investigated in vivo, as exemplified for ethylene oxide and 1, 2-epoxy-3-butene, metabolites of ethylene and 1,3-butadiene, respectively. Gas uptake studies in closed chamber systems are now worldwide used for determining toxicokinetic parameters relevant for physiological toxicokinetic modeling.

Toxicokinetics of inhaled propylene in mouse, rat, and human

A physiological toxicokinetic (PT) model was developed for inhaled propylene gas (PE) in mouse, rat, and human. Metabolism was simulated to occur in the liver (90%) and in the richly perfused tissue group (10%). The partition coefficients tissue:air were determined in vitro using tissues of mice, rats, and humans. Most of the tissues have partition coefficients of around 0.5. Only adipose tissue displays a 10 times higher value. The partition coefficient blood:air in human is 0.44, about half of that in rodents. PE can accumulate in the organism only barely. For male B6C3F1 mice and male Fischer 344/N rats, parameters of PE metabolism were obtained from gas uptake experiments. Maximum rates of metabolism (V(maxmo)) were 110 micromol/h/kg in mice and 50.4 micromol/h/kg in rats. V(maxmo)/2 was reached in mice at 270 ppm and in rats at 400 ppm of atmospheric PE. Pretreatment of the animals with sodium diethyldithiocarbamate resulted in an almost complete inhibition of PE metabolism in both species. Preliminary toxicokinetic data on PE metabolism in humans were obtained in one volunteer who was exposed up to 4.5 h to constant concentrations of 5 and 25 ppm PE. The PT model was used to calculate PE blood concentrations at steady state. At 25 ppm, the blood values were comparable across species, with 0.19, 0.32, and 0.34 micromol/L for mouse, rat, and human, respectively. However, the corresponding rates of PE metabolism differed dramatically, being 8.3, 2.1, and 0.29 micromol/h/kg in mouse, rat, and human. For a repeated human exposure to 25 ppm PE in air (8 h/day, 5 days/week), PE concentrations in venous blood were simulated. The prediction demonstrates that PE is eliminated so rapidly that it cannot accumulate in the organism. For low exposure concentrations, it became obvious that the rate of uptake into blood by inhalation is limited by the blood flow through the lung and the rate of metabolism is limited by the blood flow through the metabolizing organs.

Metabolism and toxicity of 2-methylpropene (isobutene)--a review

2-Methylpropene (MP) or isobutene is a gaseous chemical used on a large scale in the synthetic rubber industry. The present review covers the rather scarce literature on MP with respect to its metabolic fate and toxicity in laboratory animals and humans. It has been shown both in vivo and in vitro that MP is metabolized to the primary metabolite 2-methyl-1,2-epoxypropane (MEP) by rodent and human liver tissue. The formation of this reactive epoxide intermediate is catalyzed by CYP2E1, while epoxide hydrolase and glutathione S-transferase appear to be involved in its inactivation. In rats, the capacity to absorb and metabolize MP is saturable. MP is oxidized to compounds that are mainly excreted in urine. Data indicate that rodents can tolerate low levels of MP without apparent toxicity. The primary metabolite MEP, however, is able to produce genetic damage in both prokaryotic and eukaryotic cells in vitro. MP is thus not toxic per se but elicits metabolic activation to become potentially harmful. Consequently, the balance between formation and detoxification of MEP plays a key role in determining the potential toxicity of the parent compound. Obviously, further research, including repeated exposure toxicity studies, is required before an estimation of the risk for man can be made.

In vitro biotransformation of 2-methylpropene (isobutene) in rat lung tissue in comparison with liver tissue

The epoxidation of the gaseous alkene 2-methylpropene or isobutene was studied in vitro in rat lung tissue in comparison with rat liver. Pulmonary tissue appears to be less exposed to the toxic epoxide metabolite than is the case for hepatic tissue. The results are correlated with the low capacity of the mixed function oxidase system, expressed by means of the cytochrome P-450 content and the 7-ethoxycoumarin O-deethylase activity, to form reactive intermediates. The activities of the principal epoxide detoxifying enzymes glutathione S-transferase and epoxide hydrolase represent only 5-10% of the values measured in rat liver.

Metabolic transformation of halogenated and other alkenes--a theoretical approach. Estimation of metabolic reactivities for in vivo conditions

Olefinic hydrocarbons are metabolized in vivo by cytochrome P450-dependent monooxygenases to the corresponding epoxides. The maximum in vivo metabolic rate, which is an important toxicokinetic parameter, has been used to define the apparent rate constant (kapp) describing in vivo metabolic reactivity of alkenes. To derive kapp, the metabolic rate normalized per body weight was divided by the corresponding average alkene concentration in the body at saturation conditions of 90%. Toxicokinetic data obtained in rats for 13 compounds (ethene, 1-fluoroethene, 1,1-difluoroethene, 1-chloroethene, 1,1-dichloroethene, cis-1,2-dichloroethene, trans-1,2-dichloroethene, 1,1,2-trichloroethene, perchloroethene, propene, isoprene, 1,3-butadiene and styrene) have been used to calculate kapp values. A theoretical model, based on the assumption that in vivo epoxidation can be described as a cytochrome P450-mediated electrophilic reaction, has been developed. Using the olefinic hydrocarbons as an example it has been shown that kapp can be explained solely by the following molecular parameters: ionization potential, dipole moment and pi-electron density. These molecular parameters were calculated by a quantum chemical method or were taken from the literature. Furthermore, the model was tested also by predicting kapp for isobutene, an alkene which was not used for the model development. The predicted value of kapp agrees with the one derived experimentally, demonstrating that molecular parameters of halogenated and other alkenes can be used to predict in vivo metabolic reactivity. The model presented here is a first contribution to the ultimate goal to predict toxicokinetic parameters for in vivo conditions based on physicochemical parameters of enzymes and compounds exclusively.

Age- and gender-related changes in the hepatic metabolism of 2-methylpropene and relationship to epoxide metabolizing enzymes

The effect of age and gender on the in vitro biotransformation of 2-methylpropene, an alkene metabolized to 2-methyl-1,2-epoxypropane, was studied. The epoxide concentration and the epoxide metabolizing enzymatic activities were investigated in male and female Brown Norway rats of different ages. Liver tissue of senescent rats was exposed to smaller 2-methyl-1,2-epoxypropane concentrations than that of young animals, although changes during ageing were rather modest. With advancing age a feminization of male glutathione S-transferase and cytosolic epoxide hydrolase activities was found, as well as a significant decline of the female microsomal epoxide hydrolase activity and an increase of the cytochrome P-450 content in the oldest female rats.

Mutagenicity of 2-methylpropene (isobutene) and its epoxide in a modified Salmonella assay for volatile compounds

The mutagenic properties of 2-methylpropene (MP) and 2-methyl-1,2- epoxypropane (MEP) were investigated in the Salmonella assay. A simple exposure system, consisting of gastight tissue culture flasks, was used. This method has the advantage that the volatile test chemical is present during the entire incubation period and that several concentrations of the investigated compound can be tested on a single day. MP is not mutagenic in strains TA100, TA102 and TA1535, and in the latter strain not even in the presence of metabolizing S9 mix. MEP is mutagenic in all the strains tested, as demonstrated by a clear dose-response relationship. Strain TA1535 seems to be most sensitive to MEP compared with the other bacterial strains studied. For this strain, the mutagenic activity of MEP decreased significantly in the presence of S9 mix, compatible with the epoxide being inactivated by epoxide hydrolase and by glutathione S-transferase, as reported previously. From the present study it can be concluded that the parent compound MP is not mutagenic, but that its primary metabolite MEP is a mutagenic substance. However, very high concentrations are necessary to induce a mutagenic effect and the epoxide is efficiently detoxified by different liver enzymes.

Experimental data from closed chamber gas uptake studies in rodents suggest lower uptake rate of chemical than calculated from literature values on alveolar ventilation

Experimental data obtained in vivo with the closed-chamber gas uptake technique have been reported for a series of volatile chemicals. Pharmacokinetic analyses of these data have been performed either by using a two-compartment model or physiological models. In the former the transfer rate of chemical from ambient air to body is defined by the clearance of uptake. In the latter models the transfer rate depends on alveolar ventilation, cardiac output, and blood: air partition coefficient. In this communication we describe the quantitative relationship between clearance of uptake and alveolar ventilation, cardiac output, and blood: air partition coefficient. Theoretical values of clearance of uptake were calculated for a variety of volatile chemicals using literature data on alveolar ventilation, cardiac output, and blood: air partition coefficient. For most chemicals the experimentally determined values in rats and mice were about 60% of the theoretical values. This suggests that the inhalatory uptake rate of chemical may be overestimated if literature values of alveolar ventilation are used in physiological pharmacokinetic models for rodents.

In vitro biotransformation of 2-methylpropene (isobutene): epoxide formation in mice liver

Until now, no data are available concerning the biotransformation and toxicity of 2-methylpropene (or isobutene), a gaseous alkene widely used in industry (rubber, fuel additives, plastic polymers, adhesives, antioxidants). In this work, the biotransformation of 2-methylpropene (MP) has been studied, using total liver homogenates of mice, supplemented with a NADPH-generating system. In analogy to other olefins, 2-methylpropene is metabolized to its epoxide 2-methyl-1,2-epoxypropane (MEP), as proved by the identification by gas chromatography coupled with mass spectrometry. The epoxidation is cytochrome P-450 dependent, as shown by experiments in the absence of the NADPH-generating system and in the presence of various concentrations of metyrapone and SKF 525-A, two known inhibitors of the mono-oxygenases. A simple gas chromatographic headspace method has been developed for the quantitative determination of the epoxide formed. The formation of MEP is never linear in function of time and it reaches a maximum after 20 min. Thereafter is decreases continuously to undetectable levels. This observation can be explained by the immediate action of epoxide hydrolase and glutathione S-transferase, converting the epoxide to 2-methyl-1,2-propanediol and to the glutathione conjugate respectively. The involvement of both enzymes has been demonstrated by the addition of 3,3,3-trichloropropene oxide and indomethacin. These inhibitors of, respectively, epoxide hydrolase and glutathione S-transferase increase the epoxide formation in a significant way. The actual concentration of MEP is therefore not only dependent on its formation by cytochrome P-450 dependent mono-oxygenases, but also on its conversion by epoxide hydrolase and glutathione S-transferase, both very active in liver tissue.