Biotransformation of Perchloroethene: Dose-Dependent Excretion of Trichloroacetic Acid, Dichloroacetic Acid, andN-acetyl-S-(trichlorovinyl)-l-Cysteine in Rats and Humans after Inhalation

Abstract

Chronic exposure of rodents to perchloroethene (PER) increased the incidence of liver tumors in male mice and resulted in a small but significant increase in the incidence of renal tumors in male rats. The tumorigenicity of PER is mediated by metabolic activation reactions. PER is metabolized by cytochrome P450 and by conjugation with glutathione. Cytochrome P450 oxidation of PER results in trichloroacetyl chloride which reacts with water to trichloroacetic acid (TCA) which is excreted. The formation ofS-(trichlorovinyl)glutathione (TCVG) from PER results in nephrotoxic metabolites. TCVG is cleaved toS-(trichlorovinyl)-L-cysteine (TCVC) and acetylated toN-acetyl-S-(trichlorovinyl)-L-cysteine (N-ac-TCVC), which is excreted with urine. TCVC is also cleaved in the kidney by cysteine conjugate β-lyase to dichlorothioketene which may react with water to dichloroacetic acid (DCA) or with cellular macromolecules. The object of this study was to comparatively quantify the dose-dependent excretion of PER metabolites in urine of humans and rats after inhalation exposure. Three female and three male human volunteers and three female and three male rats were exposed to 10, 20, and 40 ppm PER for 6 h, and three female and three male rats to 400 ppm. A dose-dependent increase in the excretion of TCA andN-ac-TCVC after exposure to PER was found both in humans and in rats. A total of 20.4 ± 7.77 μmol of TCA and 0.21 ± 0.05 μmol ofN-ac-TCVC were excreted in urine of humans over 78 h after the start of exposure to 40 ppm PER; only traces of DCA were present. After identical exposure conditions, rats excreted 1.64 ± 0.42 μmol of TCA, 0.006 ± 0.002 μmol ofN-ac-TCVC and 0.18 ± 0.04 μmol of DCA. Excretion ofN-ac-TCVC in male rats exposed to 400 ppm PER (103.7 nmol) was significantly higher, compared to female rats (31.5 nmol) exposed under identical conditions.N-ac-TCVC was rapidly eliminated with urine both in humans (t1/2= 14.1 h) and in rats (t1/2= 7.5 h). When comparing the urinary excretion ofN-ac-TCVC, a potential marker for the formation of reactive intermediates in the kidney, humans received a significantly lower dose (3 nmol/kg at 40 ppm) compared to rats (23.0 nmol/kg) after identical exposure conditions. In addition, rats excreted large amounts of DCA which likely is a product of the β-lyase-dependent metabolism of TCVC in the kidney. The obtained data suggest that glutathione conjugate formation and β-lyase-dependent bioactivation of TCVC in PER metabolism is significantly higher in rats than in humans. Thus, using rat tumorigenicity data for human risk assessment of PER exposure may overestimate human tumor risks.