SOME PROPERTIES OF FRUCTOSE 1,6-DIPHOSPHATASE OF RAT LIVER AND THEIR RELATION TO THE CONTROL OF GLUCONEOGENESIS

Abstract

Fructose 1, 6-diphosphatase has been purified tenfold from rat liver. The final preparation was not contaminated by either glucose 6-phosphatase or phosphofructokinase. The properties of the enzyme have been investigated in an attempt to define factors that could be of revelance to metabolic control of fructose 1,6-diphosphatase activity. The metal ions Fe2+, Fe3+ and Zn2+ inhibited the activity of fructose 1, 6-diphosphatase even in the presence of an excess of mercaptoethanol; other metal ions tested had no effect. The inhibition produced by Zn2+ was reversed by EDTA, but that produced by either Fe2+ or Fe3+ was not reversible. The enzyme has a very low Km for fructose 1, 6-diphos-phate (2.0 [mu]M). Concentrations of fructose 1, 6-diphosphate above 75 [mu]M inhibited the activity; however, even at very high fructose 1, 6-diphosphate concentrations only 70% inhibition was obtained. The activity was also inhibited by low concentrations of AMP, which lowered Vmax. and increased Km for fructose 1, 6-diphosphate. Evidence is presented that suggests that AMP can be defined as an allosteric inhibitor of fructose 1,6-diphosphatase. The inhibitions by both fructose 1,6-diphosphate and AMP were extremely specific. Also, the degree of inhibition was not affected by the presence of intermediates of glycolysis, of the tricarboxylic acid cycle, of animo acid metabolism or of fatty acid metabolism. It is suggested that the intracellular concentrations of AMP and fructose 1, 6-diphosphate could be of significance in controlling the activity of fructose 1, 6-diphosphatase in the liver cell. The possible relationship between these intermediates and the control of gluconeogenesis is discussed.