Pyruvate kinase activity in isolated rat hepatocytes during a feeding cycle and during fasting.

The short-term regulation of pyruvate kinase in rat hepatocytes was studied during a feeding cycle or progressive fasting. In fed and fasted rats, the activity ratio of pyruvate kinase (V0.5-mMPEP/Vmax) in crude extracts was directly correlated with the concentration of glucose 1,6-bisphosphate++ in hepatocytes. Precipitation of the enzyme from homogenates with ammonium sulphate, which removes fructose 1,6-bisphosphate, induced in both groups of animals a low activity ratio of pyruvate kinase which remained unchanged during the whole experiment. These results show that in absence of added glucagon in hepatocytes, the activity of pyruvate kinase is mainly controlled by the intracellular level of fructose 1,6-bisphosphate. Addition of glucagon to hepatocytes from fed or fasted rats inactivated pyruvate kinase and decreased the concentration of fructose 1,6-bisphosphate in cells. However, in crude extracts both the activity ratio of pyruvate kinase and fructose 1,6-biphosphate levels were higher in fed rats than in starved rats. These findings suggest that glucagon-induced inactivation of pyruvate kinase also depends upon the concentration of fructose 1,6-biphosphate in hepatocytes.