PROPERTIES OF PHOSPHOFRUCTOKINASE FROM RAT LIVER AND THEIR RELATION TO THE CONTROL OF GLYCOLYSIS AND GLUCONEOGENESIS

Abstract

Phosphofructokinase from rat liver was partially purified by (NH4)2SO4 precipitation so as to remove enzymes that interfere in 1 assay for phosphofructokinase. The properties of this enzyme were similar to those of the same enzyme from other tissues (e.g. cardiac muscle, skeletal muscle and brain), previosuly investigated by other workers. Low concentrations of adenosine triphosphate (ATP) inhibited phosphofructokinase activity by decreasing the affinity of the enzyme for the other substrate, fructose 6-phosphate. Citrate, and other intermediates of the tricarboxylic acid cycle, also inhibited the activity of phosphofructokinase. This inhibition was relieved by either adenosine monophosphate (AMP) or fructose 1,6-diphosphate; however, higher concentrations of ATP decreased and finally removed the effect of these activators. Ammonium sulfate protected the enzyme from inactivation, and increased the activity by relieving the inhibition due to ATP. The latter effect was similar to that of AMP. Phosphofructokinase was found in the same cellular compartment as fructose 1,6-diphosphatase, namely the soluble cytoplasm. The properties of phosphofructokinase and fructose 1,6-diphosphatase are compared and a theory is proposed that affords dual control of both enzymes in the liver. The relation of this to the control of glycolysis and gluconeogenesis is discussed.