Effect of Tolbutamide and Glyburide on cAMP-Dependent Protein Kinase Activity in Rat Liver Cytosol

Abstract

The effect of sulfonylureas tolbutamide and glyburide on adenylate cyclase- and cAMP-dependent protein kinase (A-kinase) was examined in rat liver cytosol. Both tolbutamide and glyburide inhibited the A-kinase activity in a dose-dependent manner. Half-maximal inhibition was obtained at 10 mM with tolbutamide and at 0.2 mM with glyburide, indicating that glyburide was 50-fold as potent as tolbutamide. Neither tolbutamide nor glyburide affected [3H]cAMP binding to the protein kinase, but both inhibited the activity of catalytic units of the A-kinase. Lineweaver-Burk double-reciprocal plots revealed that the inhibitory effects of these drugs were noncompetitive with respect to the protein substrate histone, as well as to the phosphate-donor substrate ATP. Thus, tolbutamide and glyburide inhibited the A-kinase activity in rat liver cytosol, and it was suggested that, through the inhibition of A-kinase, the sulfonylureas would affect the carbohydrate metabolism in the liver. In fact, the relative potencies of these two drugs on A-kinase activity corresponded well with those of their reported antidiabetic effects.

Tolbutamide Inhibits cAMP-Dependent Phosphorylation of Liver 6-Phosphofructo-2-Kinase/Fructose-2,6-Bisphosphatase

“…Glucagon inhibition of PFK-2 and glucagon stimulation of F-2,6-Pase in rat hepatocytes were re- leased by the addition of tolbutamide (17). The recent report indicated that tolbutamide and glyburide directly inhibited PKA activity in rat liver cytosol (20). An effective concentration of tolbutamide on liver F-2,6-P 2 formation did not alter the intracellular cAMP accumulation (16).…”

“…Several lines of evidence appeared to indicate that inhibition of PKA activity is a common characteristic of sulfonylurea (18)(19)(20). Several lines of evidence appeared to indicate that inhibition of PKA activity is a common characteristic of sulfonylurea (18)(19)(20).…”

“…PKA-dependent phosphorylation of this bifunctional enzyme results in the inactivation of PFK-2, a synthesizing enzyme accompanied with the reciprocal activation of F-2,6-Pase, a degrading enzyme of F-2,6-P 2 . On the other hand, tolbutamide inhibits the activity of PKA in rat adipose tissue (18), parotid gland (19), and liver cytosol (20). On the other hand, tolbutamide inhibits the activity of PKA in rat adipose tissue (18), parotid gland (19), and liver cytosol (20).…”