The existence of a high-affinity, low-capacity 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD)-binding species was demonstrated in cytosol from rat thymus. It was sensitive to heat and to pronase, trypsin or chymotrypsin but not to DNAase or RNAase, indicating that it was a protein. An excess of unlabelled 2,3,7,8-tetrachlorodibenzofuran or β-naphthoflavone displaced [3H]TCDD from the binder whereas phenobarbital, pregnenolone-16-α-carbonitrile or dexamethasone did not compete. Using a dextra-coated charcoal assay, the apparent dissociation constant (Kd) of the [3H]TCDD-binder complex was determined to 0.36 nM and the apparent maximum amount of binding sites (Bmax) to 68 fmol/mg of cytosolic protein. When analyzed by sucrose density-gradient centrifugation at high ionic strength, the [3H]TCDD-binder complex sedimented at 4-5 S; at low ionic strength the complex sedimented more rapidly, probably due to aggregation. All these data support the interpretation that the demonstrated cytosolic TCDD-binder represents the receptor protein for TCDD, as previously described for rat and mouse liver. Following intravenous administration of [3H]TCDD, a low-capacity [3H]TCDD-macromolecule complex was extractable from thymic cell neuclei; this complex behaved identically to the cytosolic [3H]TCDD-receptor complex when exposed to heat or to hydrolytic enzymes and was therefore alos identified as a protein. The nuclear [3H]TCDD-protein complex sedimented at 4-5 S at high ionic strength. Furthermore, a maximum uptake of [3H]TCDD in thymic nuclei was observed simultaneously with a decline in cytosolic radioactivity (at 3 h post-injection). These findings suggest that the nuclear [3H]TCDD-protein complex represented [3H]TCDD-receptor complex translocated from the cytoplasm. In conclusion, the rat thymus contains a cytosolic TCDD receptor at a concentration similar to that of the rat hepatic receptor. However, in vivo experiments showed that the nuclear uptake of [3H]TCDD (expressed as dpm/mg GNA) in the thymus was only about 6% of that in liver. Further studies are needed for an understanding of the mechanism behind this discrepancy.
- Dextran-coated charcoal
- Tetrachlorodibenzodioxin receptor
- Thymic atrophy
ASJC Scopus subject areas
- Molecular Biology