Thyroid hormone increases type I adenosine 3', 5'-monophosphate-dependent protein kinase and casein kinase activities in rat liver cytosol: analysis of protein kinases by polyacrylamide disc gel electrophoresis

We investigated the action of thyroid hormone on each protein kinase in rat liver cytosol. Kinases were analyzed by polyacrylamide disc gel electrophoresis and isoelectric focusing in polyacrylamide gel. Polyacrylamide disc gel electrophoresis separated cAMP-dependent protein kinase type I (Rf = 0.35), type II (Rf = 0.44), their catalytic subunit (Rf = 0.26), and cAMP-independent protein kinase (Rf = 0.50). Casein kinase was detected at Rf = 0.37. In addition to the catalytic subunit with Rf = 0.26, another catalytic subunit was found at Rf = 0.44 when the cytosol was preincubated with cAMP. The administration of T3 (20 micrograms/100 g BW for 3 days) to hypothyroid rats increased enzyme activities of type I holoenzyme and casein kinase by 48%. Free catalytic subunit, separated from holoenzyme, had the same level of enzyme activity in both groups, suggesting greater endogenous dissociation of type I holoenzyme in hypothyroid rats. When heat-inactivated rat liver cytosol was used as substrate in the assay of protein kinase activity, the peak enzyme active in phosphorylating the cytosol corresponded to the casein kinase peak. Our data indicate that casein kinase is the main enzyme that mediates phosphorylation of endogenous proteins in rat liver cytosol, and that T3 treatment increases the activity of casein kinase and of type I cAMP-dependent protein kinase.