Some characteristics and hormonal control of thyroidal cAMP-independent protein kinases

Cyclic AMP-independent protein kinases in cytosol from rat thyroid glands were evaluated using histone and casein as exogenous substrates. Compared with other rat tissues, thyroid gland is rich in histone kinases, while its casein kinase activity is lower than that of liver and brain. Thyroidal cAMP-independent protein kinases can be resolved by sucrose gradient ultracentrifugation into two distinct peaks of histone (HKi-1 and HKi-2) and two peaks of casein (CKi-1 and CKi-2) kinases. An intermediate peak of histone kinase activity is only occasionally present. The four main kinase peaks are distinct with respect to several properties: their sedimentation coefficients are significantly different; only one out of the four peaks (CKi-2) can use GTP as substrate; monovalent ions strongly inhibit (50%) light peaks (HKi-1 and CKi-1), while heavy peaks (HKi-2 and CKi-2) are slightly but significantly stimulated (30%); light peaks are very sensitive to thermal inactivation, while heavy peaks are much more resistant. Their reactivity to hormonal action is different: in chronically stimulated glands HKi-2 is selectively and strongly stimulated (240%) while CKi-1 is not changed. In human pathological tissues independent alterations in different kinase entities were observed compared with healthy tissue. In conclusion, the four thyroidal c-AMP-independent protein kinases resolved on sucrose gradient seem to represent distinct entities which are independently and selectively controlled by hormones, and specifically altered in human pathological tissues.