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

1. [UL- 7,8-ring C-14]- 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 bl ood was sampled at multiple time points. Tissues were removed for combustio n analysis. 2. Each tissue contained < 0.6% of the dose at 96 h. Tissues with highest l evels of 1278-TCDD, as a percentage of administered dose, were the large an d 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 H-1-NM R. 4. Plasma levels of C-14 peaked at 24 h, and decreased to near background a t 96 h. Detectable plasma levels of 1278-TCDD were observed by 2 h. 5. A hydroxylated metabolite of 1278-TCDD was detected in calf plasma, whic h has the potential to interfere with thyroid hormone homeostasis.