Liver protein kinase activity and triiodothyronine

Protein kinase activities were determined in liver from normal, thyroidectomized and triiodothyronine (T3)-treated rats. Changes related to thyroid hormone were observed in cytosol and nuclear protein kinase activities. When protamine was used as substrate for phosphorylation, thyroidectomy induced a decrease of protein kinase activity associated with nuclei but an increase of activity was found in the cytosol. Fifteen hours after injection of T3 the levels in nuclei and cytosol were restored to normal. When casein was used as substrate, hypothyroidism led to a lowering of protein kinase activity in both fractions and T3 treatment augmented the activity in both. These studies suggest that thyroid hormones modify hepatic protein kinase activity. Results differ depending upon the substrate used. The hormones also appear to alter the subcellular distribution of some protein kinase activities.