Comparative analysis of metabolism of trichloroethylene and tetrachloroethylene among mouse tissues and strains

Abstract

Trichloroethylene (TCE) and tetrachloroethylene (PCE) are structurally similar chemicals that are metabolized through oxidation and glutathione conjugation pathways. Both chemicals have been shown to elicit liver and kidney toxicity in rodents and humans; however, TCE has been studied much more extensively in terms of both metabolism and toxicity. Despite their qualitative similarities, quantitative comparison of tissue- and strain-specific metabolism of TCE and PCE has not been performed. To fill this gap, we conducted a comparative toxicokinetic study where equimolar single oral doses of TCE (800 mg/kg) or PCE (1000 mg/kg) were administered to male mice of C57BL/6J, B6C3F1/J, and NZW/LacJ strains. Samples of liver, kidney, serum, brain, and lung were obtained for up to 36 h after dosing. For each tissue, concentrations of parent compounds, as well as their oxidative and glutathione conjugation metabolites were measured and concentration-time profiles constructed. A multi-compartment toxicokinetic model was developed to quantitatively compare TCE and PCE metabolism. As expected, the flux through oxidation metabolism pathway predominated over that through conjugation across all mouse strains examined, it is 1,200-3,800 fold higher for TCE and 26-34 fold higher for PCE. However, the flux through glutathione conjugation, albeit a minor metabolic pathway, was 21-fold higher for PCE as compared to TCE. The degree of inter-strain variability was greatest for oxidative metabolites in TCE-treated and for glutathione conjugation metabolites in PCE-treated mice. This study provides critical data for quantitative comparisons of TCE and PCE metabolism, and may explain the differences in organ-specific toxicity between these structurally similar chemicals.

Keywords: Glutathione conjugation; Oxidation; Tetrachloroethylene; Toxicokinetics; Trichloroethylene.

Figure 1.. Schematic representation of study design.

Male mice from one hybrid strain and two inbred strain (B6C3F1/J, C57BL/6J, and NZW/LacJ) were used in this study. Mice were intragastrically administered with a single dose of trichloroethylene or tetrachloroethylene (6 mmole/kg in 5% Alkamuls EL-620 (5 ml/kg)). Unchanged (TCE or PCE), oxidative (TCA and TCOH), and GSH conjugation metabolites (DCVG or TCVG, DCVC or TCVC, and NAcDCVC or NAcTCVC) were quantified in liver, kidney, brain, lung, and serum at various time points up to 36 hours (n=3 per time point/strain).

Figure 2.. Toxicokinetics analyses of TCE and PCE metabolites.

The model is composed of one parent compartment (TCE or PCE), up to two oxidative compartments (TCA and TCOH), three GSH conjugation compartments (DCVG or TCVG, DCVC or TCVC, and NAcDCVC or NAcTCVC), and one bio-activation compartment (“Unidentified reactive species”). The first order kinetics of formation of TCA, TCOH, D/TCVG, D/TCVC, NAcD/TCVC, and reactive species were described by rate constants K_TCA, K_TCOH, K_GSH, K_Cys, K_NAc, and K_bio accordingly. The first order elimination kinetics of TCE or PCE, TCA, TCOH, and NAcDCVC or NAcTCVC was described by K_eT/PCE, K_eTCA, K_eTCOH, and K_eNAc. Median estimates of selected rate constants are shown. All rate constants are listed in Table 1.

Figure 3.. Global evaluation of toxicokinetic model fit for TCE (A), PCE (B), and their respective metabolites.

Pearson (r) and spearman (ρ) correlation were used to evaluate the model fit. The correlation was considered statistically significant if p−value was lower than 0.05.

Figure 4.. The model-predicted AUCs _(0–36hr) of parent compound (TCE or PCE), oxidative metabolites (TCA and TCOH), and GSH conjugation metabolites (D/TCVG, D/TCVC, and NAcD/TCVC) in liver, kidney, and serum of male B6C3F1/J (Δ), C57BL/6J (○), and NZW/LacJ (□) mice.

Data points are shown as mean ± standard deviation. Open symbols represent TCE, and closed symbols PCE. The grey lines represent the population means of TCE-treated (dashed line) and PCE-treated (solid line) groups.

Figure 5.. Predicted disposition of TCE (A), PCE (B), and their respective metabolites in male B6C3F1/J, C57BL/6J, and NZW/LacJ mice.

Pie charts are used to provide a relative comparison among various metabolites as predicted by the model in each strain. Parent compounds (black) or their oxidative metabolites (TCOH, dark gray; TCE, light gray) are shown on the left with the GSH onjugation fraction magnified to the right of the main pie chart. For GSH conjugate pie charts, the fraction of reactive species formed is represented by a yellow slice.