Enterohepatic circulation of the mercapturic acid and cysteine conjugates of propachlor

Germfree (GF) rats and antibiotic-treated rats with cannulated bile ducts (ABC) were given single oral doses of 2-(S-(N-[²H₃] acetyl)cysteine)-N-isopropyl[1-¹⁴C]acetanilide. The GF rats excreted the dose in about equal quantities in the urine and feces; and the ABC rats excreted the dose in about equal quantities in the urine, feces, and bile. The mercapturate (60-75% of the dose in both cases) was isolated and the amount of exchange of N-acetyl deuterium to N-acetyl hydrogen was determined for all samples by mass spectrometry. The percentages of exchange were: ABC rats, bile 6.5%, urine 13%, feces 0.0%; GF rats, urine 15%, feces 4.7%. The remainder of the doses was present as the mercapturic acid sulfoxide. ABC rats dosed with 2-(S-[3,3-³H]cysteine)-N-isopropyl[1-¹⁴C]acetanilide (¹⁴C/³H = 0.50) excreted 42% of the dose in the urine and bile as the mercapturic acid that had a ¹⁴C/³H ratio the same as the original cysteine conjugate. The results of these studies show that a mercapturic acid and a cysteine conjugate can be absorbed from the gastrointestinal tract and resecreted in the bile or excreted by the kidney without having undergone metabolism other than acetylation of the cysteine nitrogen atom and oxidation of the sulfur to a sulfoxide. ABC rats were also dosed with 2-methylthio-N-isopropyl[1-¹⁴C]acetanilide, a suspected metabolic intermediate in the metabolism of propachlor (2-chloro-N-isopropylacetanilide). The dose was excreted in about equal quantities in the bile (48%) as metabolites with a methylsulfonyl group in the 2-position. All are known metabolites of propachlor in conventional rats.