Identification of the glycogenic compound 5-iodotubercidin as a general protein kinase inhibitor

Addition of micromolar concentrations of the adenosine derivative 5-iodotubercidin (Itu) initiates glycogen synthesis in isolated hepatocytes by causing inactivation of phosphorylase and activation of glycogen synthase [Flückiger-Isler and Walter (1993) Biochem. J. 292, 85-91]. We report here that Itu also antagonizes the effects of saturating concentrations of glucagon and vasopressin on these enzymes. The Itu-induced activation of glycogen synthase could not be explained by the removal of phosphorylase a (a potent inhibitor of the glycogen-associated synthase phosphatase). When tested on purified enzymes, Itu did not affect the activities of the major Ser/Thr-specific protein phosphatases (PP-1, PP-2A, PP-2B and PP-2C), but it inhibited various Ser/Thr-specific protein kinases as well as the tyrosine kinase activity of the insulin receptor (IC50 between 0.4 and 28 microM at 10-15 microM ATP). Tubercidin, which did not affect glycogen synthase or phosphorylase in liver cells, was 300 times less potent as a protein kinase inhibitor. Kinetic analysis of the inhibition of casein kinase-1 and protein kinase A showed that Itu acts as a competitive inhibitor with respect to ATP, and as a mixed-type inhibitor with respect to the protein substrate. We propose that Itu inactivates phosphorylase and activates glycogen synthase by inhibiting phosphorylase kinase and various glycogen synthase kinases. Consistent with the broad specificity of Itu in vitro, this compound decreased the phosphorylation level of numerous phosphopolypeptides in intact liver cells. Our data suggest that at least some of the biological effects of Itu can be explained by an inhibition of protein kinases.