Hepatic microsomal metabolism of isoprene in various rodents

Concentration- and time-dependent genotoxicity profiles of isoprene monoepoxides and diepoxide, and the cross-linking potential of isoprene diepoxide in cells

Isoprene, a possible carcinogen, is a petrochemical and a natural product being primarily produced by plants. It is biotransformed to 2-ethenyl-2-methyloxirane (IP-1,2-O) and 2-(1-methylethenyl)oxirane (IP-3,4-O), both of which can be further metabolized to 2-methyl-2,2'-bioxirane (MBO). MBO is mutagenic, but IP-1,2-O and IP-3,4-O are not. While IP-1,2-O has been reported being genotoxic, the genotoxicity of IP-3,4-O and MBO, and the cross-linking potential of MBO have not been examined. In the present study, we used the comet assay to investigate the concentration- and time-dependent genotoxicity profiles of the three metabolites and the cross-linking potential of MBO in human hepatocyte L02 cells. For the incubation time of 1 h, all metabolites showed positive concentration-dependent profiles with a potency rank order of IP-3,4-O > MBO > IP-1,2-O. In human hepatocellular carcinoma (HepG2) and human leukemia (HL60) cells, IP-3,4-O was still more potent in inducing DNA breaks than MBO at high concentrations (>200 μM), although at low concentrations (≤200 μM) IP-3,4-O exhibited slightly lower or similar potency to MBO. Interestingly, their time-dependent genotoxicity profiles (0.5-4 h) in L02 cells were different from each other: IP-1,2-O and MBO (200 μM) exhibited negative and positive profiles, respectively, with IP-3,4-O lying in between, namely, IP-3,4-O-caused DNA breaks did not change over the exposure time. Further experiments demonstrated that hydrolysis of IP-1,2-O contributed to the negative profile and MBO induced cross-links at high concentrations and long incubation times. Collectively, the results suggested that IP-3,4-O might play a significant role in the toxicity of isoprene.

Hemoglobin adducts and micronuclei in rodents after treatment with isoprene monoxide or butadiene monoxide

1,3-Butadiene and isoprene (2-methyl-1,3-butadiene) are chemically related substances that are carcinogenic to rodents. The overall aim of this work is to elucidate the role of the genotoxic action of diepoxide metabolites in the carcinogenesis of the dialkenes. In vivo doses of the diepoxide metabolites were measured through reaction products with hemoglobin (Hb adducts) in studies of induced micronuclei (MN) in rodents. In the reaction with N-terminal valine in Hb, diepoxybutane and isoprenediepoxide form ring-closed adducts, pyrrolidines [N,N-(2,3-dihydroxy-1,4-butadiyl)valine and N,N-(2,3-dihydroxy-2-methyl-1,4-butadiyl)valine, respectively]. The method applied for Hb-adduct measurement is based on tryptic degradation of the protein and liquid chromatography electrospray ionisation tandem mass spectrometry (LC-ESI-MS/MS) analysis. Mice were given single i.p. injections of the monoepoxides of butadiene and isoprene, 1,2-epoxy-3-butene or 1,2-epoxy-2-methyl-3-butene, respectively. Rats were treated in the same way with 1,2-epoxy-3-butene. In mice pyrrolidine adduct levels increased with increasing administered doses of the monoepoxides. The in vivo dose of diepoxybutane was on average twice as high (0.29+/-0.059 mMh) as the in vivo dose of isoprenediepoxide (0.15+/-0.053 mMh) per administered dose (mmol/kg body weight) of the monoepoxides. In mice the genotoxic effects of the two monoepoxides, measured as the increase in the frequencies of micronuclei (MN), were approximately linearly correlated to the in vivo doses of the diepoxides (except at the highest dose of diepoxybutane). In rats the pyrrolidine-adduct levels from diepoxybutane were below the limit of quantification at all administered doses of 1,2-epoxy-3-butene and no significant increase was observed in the frequency of MN. Measurement of the ring-closed adducts to N-termini in Hb by the applied method permits analysis of in vivo doses of diepoxybutane and isoprenediepoxide, which may be further used for the elucidation of the mechanisms of carcinogenesis of butadiene and isoprene.

Stereochemical and kinetic comparisons of mono- and diepoxide formation in the in vitro metabolism of isoprene by liver microsomes from rats, mice, and humans

Isoprene (2-methylbuta-1,3-diene) is a large scale petrochemical used principally in the manufacture of synthetic rubbers. It is also produced by plants and trees and is formed endogenously in mammals as a major endogenous hydrocarbon. Mammalian metabolism of isoprene involves cytochrome P450-dependent monooxygenases to give the regioisomeric monoepoxides, prop-2-enyloxirane and 2-ethenyl-2-methyloxirane. The isoprene monoepoxides are further oxidized to the mutagenic diepoxides, 2-methyl-2,2'-bioxiranes. The present studies have investigated the stereochemistry and comparative rates of the metabolic epoxidation in vitro of isoprene to mono- and diepoxides by liver microsomes from rat, mouse, and human in order to identify stereochemical and kinetic differences between species in the formation of these epoxide metabolites, which are key to understanding the toxicology of isoprene. The assignments of stereochemistry were based on comparisons with synthetic standards, the syntheses for which are described. Comparative enzyme kinetic parameters (apparent K(m) and apparent V(max) values) for the in vitro formation of all of the monoepoxide and diepoxide stereoisomers have been obtained. The rates of formation of both mono- and diepoxides were greater in the rodent systems as compared with the human in vitro system. The results provide comparative kinetic data that have potential for modeling and assessing the relevance of the animal carcinogenicity data for man. The possibility of human interindividual variation was also investigated with liver preparations from several individual humans, but significant differences between individuals were not observed in the formation of the monoepoxides from isoprene.

Prediction of isoprene diepoxide levels in vivo in mouse, rat and man using enzyme kinetic data in vitro and physiologically-based pharmacokinetic modelling

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.

Metabolism and molecular toxicology of isoprene

Isoprene (2-methylbuta-1,3-diene) is a large-scale petrochemical used principally in the manufacture of synthetic rubbers. It is also produced by plants and trees and is the major endogenous hydrocarbon formed by mammals, probably from mevalonic acid. Isoprene is metabolised by mammals in processes that involve epoxidation by cytochrome P450-dependent monooxygenases to the isomeric mono-epoxides, (1-methylethenyl)-oxirane and 2-ethenyl-2-methyloxirane. Further metabolism of the mono-epoxides to mutagenic isoprene di-epoxides, (2, 2')-2-methylbioxiranes, can also occur. The oxidations to the mono- and di-epoxides occur enantioselectively and diastereoselectively. The mono-epoxides are hydrolysed enantioselectively to vicinal diols under catalysis by epoxide hydrolase. 2-Ethenyl-2-methyloxirane is also readily hydrolysed non-enzymatically. Because of the stereochemical possibilities for metabolites, the metabolism of isoprene is complex. The metabolism of isoprene by liver microsomes in vitro from a range of species including rat, mouse and human shows significant differences between species, strains and gender in respect of the diastereoselectivity and enantioselectivity of the metabolic oxidation and hydrolysis reactions. The impact of the extra methyl in isoprene on di-epoxide reactivity also appears to be critically important for the resulting biological effects. Isoprene di-epoxides may exhibit a lower cross-linking potential in vivo compared to butadiene di-epoxides. Differences in metabolism and reactivity of metabolites may be factors contributing to the significant differences in toxicological response to isoprene observed between species.

Conjugation of isoprene monoepoxides with glutathione, catalyzed by alpha, mu, pi and theta-class glutathione S-transferases of rat and man

In the present study, the enzymatic conjugation of the isoprene monoepoxides 3,4 epoxy-3-methyl-1-butene (EPOX-I) and 3,4-epoxy-2-methyl-1-butene (EPOX-II) with glutathione was investigated, using purified glutathione S-transferases (GSTs) of the alpha, mu, pi and theta-class of rat and man. HPLC analysis of incubations of EPOX-I and EPOX-II with [35S]glutathione (GSH) showed the formation of two radioactive fractions for each isoprene monoepoxide. The structures of the EPOX-I and EPOX-II GSH conjugates were elucidated with 1H-NMR analysis. As expected, two sites of conjugation were found for both isoprene epoxides. EPOX-II was conjugated more efficiently than EPOX-I. In addition, the mu and theta class glutathione S-transferases were much more efficient than the alpha and pi class glutathione S-transferases, both for rat and man. Because the mu- and theta-class glutathione S-transferases are expressed in about 50 and 40-90% of the human population, respectively, this may have significant consequences for the detoxification of isoprene monoepoxides in individuals who lack these enzymes. Rat glutathione S-transferases were more efficient than human glu tathione S-transferases: rat GST T1-1 showed about 2.1-6.5-fold higher activities than human GST T1-1 for the conjugation of both EPOX-I and EPOX-II, while rat GST M1-1 and GST M2-2 showed about 5.2-14-fold higher activities than human GST M1a-1a. Most of the glutathione S-transferases showed first order kinetics at the concentration range used (50-2000 microM). In addition to differences in activities between GST-classes, differences between sites of conjugation were found. EPOX-I was almost exclusively conjugated with glutathione at the C4-position by all glutathione S-transferases, with exception of rat GST M1-1, which also showed significant conjugation at the C3-position. This selectivity was not observed for the conjugation of EPOX-II. Incubations with EPOX-I and EPOX-II and hepatic S9 fractions of mouse, rat and man, showed similar rates of GSH conjugation for mouse and rat. Compared to mouse and rat, human liver S9 showed a 25-50-fold lower rate of GSH conjugation.

Species differences in the stereochemistry of the metabolism of isoprene in vitro

1. Comparative studies on the stereochemistry of the metabolism of isoprene in vitro have been carried out using liver microsomes from rats, mice, monkeys, dogs, rabbits and humans. Differences between strains and gender were also investigated. 2. In the production of the isoprene monoepoxides, microsomes from the livers of the male Sprague-Dawley or Wistar rat showed an approximately 2:1 preference for the formation of (S)-2-(1-methylethenyl)oxirane compared with the (R)-enantiomer. No enantioselectivity was observed for mouse or rabbit. In contrast, liver microsomes from dog, monkey or male human preferentially formed (R)-2-(1-methylethenyl)oxirane. There was no enantioselectivity observed with microsomes from female human liver. 3. The significant differences between species in the in vitro metabolism of isoprene indicate that stereochemical and mechanistic data should be taken into account when evaluating the results of animal studies designed to assess the carcinogenic risks to humans that may be associated with exposure to isoprene.

The biotransformation of isoprene and the two isoprene monoepoxides by human cytochrome P450 enzymes, compared to mouse and rat liver microsomes

The metabolism of isoprene was investigated with microsomes derived from cell lines expressing human CYP1A1, CYP1A2, CYP2A6, CYP2B6, CYP2C9, CYP2D6, CYP2E1, or CYP3A4. The formation of epoxide metabolites was determined by gas chromatographic analysis. CYP2E1 showed the highest rates of formation of the isoprene monoepoxides 3,4-epoxy-3-methyl-1-butene (EPOX-I) and 3,4-epoxy-2-methyl-1-butene (EPOX-II), followed by CYP2B6. CYP2E1 was the only enzyme showing detectable formation of the diepoxide of isoprene, 2-methyl-1,2:3,4-diepoxybutane. Both isoprene monoepoxides were oxidized by CYP2E1 to the diepoxide at similar enzymatic rates. In order to determine the relative role of CYP2E1 in hepatic metabolism, isoprene as well as the two monoepoxides were also incubated with a series of ten human liver microsomal preparations in the presence of the epoxide hydrolase inhibitor cyclohexene oxide. The obtained activities were correlated with activities towards specific substrates for CYP1A2, CYP2A6, CYP2C9, CYP2D6, CYP2E1 and CYP3A. The results were supportive for those obtained with single human P450 enzymes. Isoprene (monoepoxide) metabolism sowed a significant correlation with CYP2E1 activity, determined as chlorzoxazone 6-hydroxylation. CYP2E1 is therefore the major enzyme involved in hepatic metabolism of isoprene and the isoprene monoepoxides in vitro. To investigae species differences with regard to the role of epoxide hydrolase in the metabolism of isoprene monoepoxides, the epoxidation of isoprene by human liver microsomes was compared to that of mouse and rate liver microsomes. The amounts of monoepoxides formed as a balance between epoxidation and hydrolysis, was measured in incubations with and without the epoxide hydrolase inhibitor cyclohexene oxide. Inhibition of epoxide hydrolase resulted in similar rates of monoepoxide formation in mouse, rat and man. Without inhibitor, however, the total amount of monoepoxides present at the end of the incubation period was twice as high for mouse liver microsomes than for rat and even 15 times as high as for human liver microsomes. Thus, differences in epoxide hydrolase activity between species may be of crucial importance for the toxicity of isoprene in the various species.

Toxicokinetics of isoprene in rodents and humans

A physiological toxicokinetic model (PT model) was developed for inhaled isoprene in mouse, rat and man. Partition coefficients blood:air and tissue:blood were determined in vitro by a headspace method. Parameters of a saturable isoprene metabolism in B6C3F1 mice, Sprague-Dawley rats and volunteers were obtained from gas uptake experiments in closed systems, analyzed by means of a two-compartment model. Incorporation of these parameters into the PT model revealed that isoprene was metabolized not only in the liver but also in extrahepatic organs. Endogenous production of isoprene in man was quantified from experiments with volunteers breathing into a closed system. The PT model was validated for mice, rats and humans by comparing simulated values with data determined by other authors.

Enzyme specific kinetics of 1,2-epoxybutene-3 in microsomes and cytosol from livers of mouse, rat, and man

Kinetics of the metabolism of 1,2-epoxybutene-3 (butadiene monoxide) were investigated in liver fractions of mouse, rat, and man. In these species similar enzyme characteristics were found. In microsomes, no NADPH-dependent metabolism of butadiene monoxide was detectable. Epoxide hydrolase activity was found only in microsomes. The Vmax [nmol butadiene monoxide/(mg protein x min)] was 19 in mouse, 17 in rat, and 14 in man and the apparent Km (mmol butadiene monoxide/l incubate) was 1.5 in mouse. 0.7 in rat, and 0.5 in man. Glutathione S-transferase activity was found in cytosol only, revealing first order kinetics in the measured range. The ratio Vmax/Km [(nmol butadiene monoxide x 1)/(mg protein x min x mmol of butadiene monoxide)] was 15 in mouse, 11 in rat, and 8 in man. The data obtained were used to extrapolate on the total rate of butadiene monoxide metabolism for each species in vivo: it was calculated to be 1.3 times higher in mice and 2.3 times lower in man compared to rats, when corrected for body weight.

Environmental tobacco smoke: overview of chemical composition and genotoxic components

Tobacco smoke contains numerous compounds emitted as gases and condensed tar particles. The sidestream smoke emissions, which constitute the major part of environmental tobacco smoke (ETS), are generally larger than the mainstream smoke emissions. Many of the organic compounds, belonging to a variety of chemical classes, are known to be genotoxic and carcinogenic. These include the known constituents, alkenes, nitrosamines, aromatic and heterocyclic hydrocarbons and amines. Emission of sidestream smoke in indoor environments with relatively low ventilation rates can result in pollutant concentrations above those generally encountered in ambient air in urban areas. The chemical characteristics of ETS thus support the indications that exposure to ETS can be causally associated with the induction of several types of cancer.

Public exposure to environmental tobacco smoke

Airborne particulate matter has been collected by personal samplers in public indoor areas and travel situations with environmental tobacco smoke pollution. Following extraction, the samples were assayed for mutagenicity in the presence of S9 with a sensitive microsuspension test using Salmonella TA98. The mutagenic responses of indoor air from public areas were much higher than those of ambient outdoor air. Depending on the circumstances, the mutagenic response varied in trains and airplanes but the results show that physical separation of non-smoking sections from smoking sections is necessary in order to achieve genuine non-smoking areas. Chemical fractionation and mutagenicity assay of the basic fraction show that Salmonella mutagenicity of airborne particulate matter might be used as a tobacco smoke-specific indicator, as the basic fraction of environmental tobacco smoke contains a large part of the mutagenic activity whereas this is not the case for outdoor ambient airborne particulate matter.

Pharmacokinetics of isoprene in mice and rats

Pharmacokinetic analysis of isoprene inhaled by male Wistar rats and male B6C3F1 mice showed saturation kinetics in both species. Below atmospheric concentrations of 300 ppm in rats and in mice the rate of metabolism is directly proportional to the concentration. The low accumulation of isoprene in the body at low atmospheric concentrations suggests transport limitation of the metabolism. Only small amounts of isoprene taken up are exhaled as unchanged substance (15% in rats and 25% in mice). Its half life in rats is 6.8 min and in mice 4.4 min. At concentrations above 300 ppm the rate of metabolism does not increase further in proportion to the atmospheric concentration. It finally approaches maximal values of 130 mumol/(h X kg) body weight at atmospheric concentrations above 1500 ppm in rats, and 400 mumol/(h X kg) body weight at concentrations above 2000 ppm in mice. This indicates limited production of the two possible mono-epoxides of isoprene at high concentrations. Isoprene is endogenously produced and is systemically available. Its production rate is 1.9 mumol/(h X kg) in rats, and 0.4 mumol/(h X kg) in mice, respectively. Part of the endogenous isoprene is exhaled by the animals but it is metabolized to a greater extent: the rate of metabolism of endogenously produced and systemically available isoprene is 1.6 mumol/(h X kg) (rats) and 0.3 mumol/(h X kg) (mice).