Triiodothyronine Stimulation of Nuclear Protein Synthesis*

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Abstract
The incorporation of leucine into rat liver protein was studied in vivo in control animals, hypothyroid animals and hypothyroid animals given 0.1-15 .mu.g triiodothyronine (T3/100 g BW [body weight] 3-48 h before sacrifice. Cytosol and nuclei were separated by differential centrifugation, and nuclear globulins, histones, and nonhistone proteins were recovered by extraction procedures. When studied 40 min after i.v. administration of 100 .mu.Ci [3H]leucine, incorporation of isotope was reduced in cytosol, nuclear globulins, histones, and nonhistone proteins by about 50% in hypothyroid animals in comparison to normal controls. Injection of T3 augmented isotope incorporation within 5 h and restored it to normal by 24 h. The response was not related to alteration in specific activity of the leucine pool. Half-maximal response occurred at 0.5-1 .mu.g T3/100 g BW. In hypothyroid animals one nuclear globulin of 100,000 MW was reduced in concentration, and a second of 79,000 MW was increased. These changes were promptly reversed by T3 treatment. These specific changes in nuclear globulin synthesis and content may be related to the reduced polymerase function present in T3-deprived animals.