CHARACTERIZATION OF THE BINDING-SITES FOR [H-3] GLIBENCLAMIDE IN RAT-LIVER MEMBRANES

The specific binding sites for sulfonylureas in the rat liver membrane fraction were demonstrated and characterized. [H-3]Glibenclamide binding to the liver membrane was specific, time-and temperature-dependent, and reversible. Scatchard analysis showed a single class binding site. The dissociation constant (K-d) for glibenclamide was 1.1 mu M and the binding capacity (B-max) was 50 pmol/mg protein. [H-3]Glibenclamide binding could be displaced by other sulfonylureas. Half-maximal inhibition of binding (IC50) for glimepiride, gliclazide, acetohexamide, tolbutamide and chlorpropamide was 4.2 mu M, 74 mu M, 0.33 mM, 0.60 mM, 1.2 mM, respectively. Each value is close to the reported blood concentration when a therapeutic dose of each drug is administered orally. The order of IC50 values is coincident with the order of potency of the clinical hypoglycemic effect of these drugs. We had shown that these concentrations of sulfonylureas stimulate 6-phosphofructo-2-kinase in the liver or hepatocytes and inhibit phosphoenolpyruvate carboxykinase in the hepatoma cells. The specific binding sites demonstrated here may play some roles when sulfonylureas affect carbohydrate metabolism in the liver.