Identification of beta-glucuronidase-resistant diastereomeric glucuronides of 3-hydroxy-3-methyloxindole formed during 3-methylindole metabolism in goats.

Goats were jugularly infused with the pneumotoxin 3-methylindole (3MI; 15 mg/kg, 0.5 microCi/kg) dissolved in cremophor-EL to characterize the urinary metabolites of 3MI in a ruminant specie. Urine was collected for 36 hr after the beginning of a 2-hr infusion period, and 3MI metabolites were purified using reversed-phase HPLC. Goats excreted 3MI as at least 11 distinct metabolites. Metabolites were characterized using a combination of UV spectroscopy, 1H- and 13C-NMR spectroscopy, and negative-ion FAB/MS. Two of the metabolites (E1 and E2), representing approximately 30% of the urinary radioactivity, were unambiguously identified as diastereomeric glucuronides of 3-hydroxy-3-methyloxindole [HMOI; 3-(beta-D-glucosiduronic acid)-3-methyloxindole]. Glucuronide conjugates were investigated using enzymatic and chemical hydrolysis. These ethereal glucuronides were unique in that they were not readily hydrolyzable with bovine beta-glucuronidase, although one of the diastereomers was hydrolyzed sparingly by beta-glucuronidase from Helix pomatia. Treatment of the glucuronides with 6 M HCI for a 2-hr period liberated unconjugated HMOI. Treatment of each diastereomer with dilute acid (pH 3) or dilute alkali (pH 10) was ineffective at hydrolyzing the conjugates. Goats form HMOI from 3MI and extensively glucuronidate the metabolite before excreting it, as opposed to mice that do not conjugate HMOI before excretion. These ethereal glucuronic acid conjugates seem to be unique in that they are essentially resistant to beta-glucuronidase-catalyzed hydrolysis.