Bioactivation of tetrachloroethylene. Role of glutathione S-transferase-catalyzed conjugation versus cytochrome P-450-dependent phospholipid alkylation

Abstract

The metabolism of [14C]tetrachloroethylene (Tetra) and its metabolite S-(1,2,2-trichlorovinyl)-L-cysteine (TCVC) was investigated with in vitro systems to substantiate metabolic pathways of Tetra deduced from in vivo experiments. In the presence of NADPH, rat hepatic microsomal fractions metabolized Tetra to soluble metabolites, which were identified as trichloroacetic acid and oxalic acid by gas chromatography/mass spectroscopy and a metabolite largely bound to microsomal macromolecules. The majority of the alkylated macromolecules were identified as N-trichloroacetylated phospholipids by high performance liquid chromatography and GC/MS. When Tetra was incubated with hepatic microsomes and cytosol in the presence of 10 mM glutathione, but in the absence of NADPH, the formation of a polar metabolite other than trichloroacetic acid and oxalic acid was observed. This metabolite was identified, after hydrolysis to the corresponding cysteine conjugate, as S-(1,2,2-trichlorovinyl)-glutathione (TCVG). Microsomal GSH S-transferases catalyzed TCVG formation more efficiently than cytosolic GSH S-transferases; the competitive substrate 1-chloro-2,4-dinitrobenzene inhibited TCVG formation. In the presence of both NADPH and GSH, TCVG formation in microsomes was decreased, indicating that oxidative metabolism and GSH conjugation of Tetra are competitive reactions. The Tetra metabolite TCVC was cleaved by bacterial cysteine conjugate b-lyase to dichloroacetic acid and pyruvate. The obtained results substantiate the postulated pathways of Tetra biotransformation and demonstrate that both oxidative and conjugative reactions occur in hepatic Tetra metabolism. Phospholipid alkylation, which occurs during oxidative metabolism, may be a deactivation reaction, whereas TCVG formation, renal metabolism to TCVC, and cleavage of TCVC by b-lyase under formation of mutagenic intermediates may contribute to the nephrocarcinogenic effect of Tetra.