Binding of T3 in rat liver nuclei.

The binding of labeled triiodothyronine has been studied in rat liver nuclei prepared after in vivo administration of hormone. Nuclear pellets prepared from homogenates made in 0.32M sucrose + 1 mm magnesium chloride and centrifuged through 2.3M sucrose density gradients contain, 2 hr after labeling, approximately 9% of liver isotope in normal animals and 18%in surgically thyroidectomized animals. 60% of DNA is recovered in the nuclear pellet so prepared. The fraction of total liver isotope present in the purified nuclear pellets is 6%at 10 min, peaks between 2 and 3.5 hr, and declines gradually over the subsequent 24 hr as total blood, liver, and nuclear isotope rapidly decline. These data suggest relatively free exchange between nucleus and other components of the liver. The nuclear binding of receptor sites in normal and thyroidectomized animals can be saturated by addition of increasing doses of carrier unlabeled triiodothyronine. Scatchard analysis suggests the presence of high affinity binding sites...