Partial characterization of specific nuclear triiodothyronine binding sites in two transplantable Morris hepatomas in the rat.

Binding sites for 3,3' -5-triiodo-L-thyronine are shown to be present in nuclei prepared from either the 5123tc or the 7777 minimal-deviation murine hepatomas. Certain of the apparent in vivo characteristics of these binding proteins in the hepatomas were found to differ from those seen in host liver nuclei. The maximal binding capacities of these binding sites in the tumors were found to be 60% of that in host rat liver nuclei, and the percent-occupancy in vivo somewhat elevated in the tumors. On the other hand, intrinsic affinity constants (Ka) were found similar when comparing the liver and hepatoma nuclei. Also, using DEAE-Sephadex column chromatography, the binding sites in the 7777 tumor were found to co-elute with similar proteins derived from host liver nuclei. It is concluded then that any differences noted between the characteristics of these binding sites in the liver and hepatoma nuclei are on a functional rather than on a structural basis. A possible connection between the lowered levels of these binding sites in hepatoma nuclei and the proliferative rates of these tumor cells is suggested.