The regulation of 6-phosphofructo-1-kinase by insulin and glucagon in isolated hepatocytes of the American eel

Abstract

Kinetic characteristics of American eel liver 6-phosphofructo-1-kinase (PFK-1) and the effects of porcine insulin, bovine glucagon, and dibutyryl-cAMP were studied. At 0.1 m M ATP, kinetics were sigmoidal with respect to fructose-6-phosphate (F-6-P) concentrations and the S 0.5 (F-6-P) increased with higher ATP concentrations. At 2 m M F-6-P, optimal ATP concentrations were 0.1 m M, with maximal inhibition at 0.5 m M. Fructose 2,6-bisphosphate (Fru-2,6-P 2) offset ATP inhibition and activated the enzyme, changing F-6-P kinetic curves from sigmoidal to hyperbolic. At 2 m M F-6-P and 0.1 m M ATP the Fru-2,6-P 2 activation curve was hyperbolic with a K a of approximately 1 μ M. In isolated hepatocytes, porcine insulin decreased the sensitivity of PFK-1 to ATP, an effect that was offset when bovine glucagon was also present. Insulin, alone and with glucagon, increased the Fru-2,6-P 2 activation ratio. In the presence of glucagon, insulin increased Fru-2,6-P 2 concentrations in hepatocytes. These effects suggest that PFK-1 is a potential regulatory point for hormones in the control of carbohydrate metabolism in the American eel liver.