Metabolism of [14C]1,2,7,8-tetrachlorodibenzo-p-dioxin in a ruminating Holstein calf.

1. [UL-7,8-ring 14C]-1,2,7,8-tetrachlorodibenzo-p-dioxin (1278-TCDD) was administered orally to a ruminating Holstein bull calf (43.6 kg; 1.2 mg kg(-1) body weight). Urine and faeces were collected daily for 96 h, while blood was sampled at multiple time points. Tissues were removed for combustion analysis. 2. Each tissue contained < 0.65 of the dose at 96h. Tissues with highest levels of 1278-TCDD, as a percentage of administered dose, were the large and small intestine, rumen, liver and carcass. 3. Urinary excretion accounted for 10.6% of the dose, and faecal excretion accounted for 81.6% of the administered dose. The major urinary and faecal metabolites were isolated and characterized by mass spectrometry and 1H-NMR. 4. Plasma levels of 14C peaked at 24h, and decreased to near background at 96 h. Detectable plasmal levels of 1278-TCDD were observed by 2 h. 5. A hydroxylated metabolite of 1278-TCDD was detected in calf plasma, which has the potential to interfere with thyroid hormone homeostasis.