Inhibition of rabbit liver fructose 1,6-biphosphatase by AMP: effect of temperature and physiological concentrations of cations and anions

Kinetic properties of ovine adipose tissue fructose-1,6-bisphosphatase

The presence of FBPase was confirmed in both human and ovine white adipose tissue in metabolically significant amounts. The partially purified enzyme from ovine adipose tissue exhibited kinetic properties very similar to other mammalian FBPases (pH optimum of 7.5, absolute requirement for divalent metal ions and strong inhibition by both AMP and F-2,6-P2). The micromolar S0.5 value obtained suggests that the enzyme may be of physiological significance.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. II. Effects of adenine nucleotides

1. The native rat-kidney cortex Fructose-1,6-bisphosphatase is differentially regulated by adenine nucleotides in the presence of divalent cations. 2. Binding of AMP and ADP to the enzyme is co-operative. The inhibition by both nucleotides show an uncompetitive mechanism AMP being the most efficient inhibitor. 3. Mg2+ decreases the inhibition produced by AMP and ADP by enhancing their I0.5 and completely annulates the inhibitory effect of ATP. 4. In the presence of Mn2+ ADP behaves as an inhibitor but no inhibition is evident with AMP, suggesting the existence of different allosteric sites for each nucleotide.

Regulation of rat-kidney cortex fructose-1,6-bisphosphatase activity. I. Effects of fructose-2,6-bisphosphate and divalent cations

1. The native rat-kidney cortex Fructose-1,6-BPase is differentially regulated by Mg2+ and Mn2+. 2. Mg2+ binding to the enzyme is hyperbolic and large concentrations of the cation are non-inhibitory. 3. Mn2+ produces a 10-fold rise in Vmax higher than Mg2+. [Mn2+]0.5 is much larger than [Mg2+]0.5. At elevated [Mn2+] inhibition is observed. 4. Mg2+ and Mn2+ produce antagonistic effects on the inhibition of the enzyme by high substrate. 5. Fru-2,6-P2 inhibits the enzyme by rising the S0.5 and favouring a sigmoidal kinetics. 6. The inhibition by Fru-2,6-P2 is released by Mg2+ and more powerfully by Mn2+ increasing the I0.5.

Kinetic parameters of human and rabbit liver D-fructose 1,6-diphosphate 1-phosphohydrolase determined at 25 and 37 degrees C

1. Kinetic parameters of human and rabbit liver D-fructose 1,6-diphosphate 1-phosphohydrolase (EC 3.1.3.11) (FDP-ase) at 25 and 37 degrees C have been determined. 2. Km determined at 25 degrees C were 1.4 microM for human and 1.6 microM for rabbit enzyme; at 37 degrees C, corresponding values were 1.7 and 1.8 microM. 3. Both enzymes are allosterically inhibited by AMP. Respective values of I0.5 were 7.2 microM for human and 13.2 microM for rabbit at 25 degrees C, and 16.6 microM for human and 27.3 microM for rabbit at 37 degrees C. 4. Fructose 2,6-diphosphate, a potent regulator of gluconeogenesis, is more effective at 25 than at 37 degrees C. Ki determined at 25 degrees C was 0.07 microM for human and 0.035 microM for rabbit in comparison with 0.17 microM for human and 0.09 microM for rabbit at 37 degrees C. 5. Affinity of FDP-ase for magnesium is also dependent on temperature. For the human enzyme, Km at 25 degrees C was 226 microM and at 37 degrees C, 176 microM. For the rabbit enzyme, corresponding values were 256 and 240 microM. 6. Both enzymes are activated by KCl. Determined values of A0.5 were 91 mM for human, and 50 mM for rabbit enzyme at 25 degrees C, and 129 mM for human and 100 mM for rabbit enzyme at 37 degrees C.

Evidence for heat-stable liver cytosol substance(s) capable of causing oxidative activation of fructose 1,6-bisphosphatase

The endogenous fructose 1,6-bisphosphatase (FBPase) in chicken liver extract undergoes a drastic increase in activity if the pH of the extract is in the alkaline range. Greater and more consistent activation occurs when purified FBPase, placed inside dialysis sack, is incubated in liver extract. Maximal activation (over 16-fold) is accompanied by the disappearance of 4 highly reactive sulfhydryl groups (SH) per molecule of enzyme. The activating effect of the extract remains essentially unchanged after heating to 100 degrees C. Activation can be reversed by dithiothreitol. These data show the existence in liver cytosol of heat-stable substance(s) capable of activating FBPase presumably by forming disulfide bonds with the enzyme's highly reactive SH groups.

Biochemical observations on a case of hepatic fructose-1,6-diphosphatase deficiency

A case of hepatic fructose-1,6-diphosphatase deficiency is described. She presented with congenital bilateral cataracts, failure to thrive, hypoglycaemia and hyperlactacidaemia. A liver biopsy revealed normal levels of gluconeogenic enzymes except fructose-1,6-diphosphatase which was present at 30% of the level of the lower control values. The residual activity had a normal affinity for fructose-1,6-diphosphate, a decreased sensitivity for inhibition by fructose-2,6-diphosphate and an increased resistance toward conversion to the AMP-insensitive form of the enzyme. As a result of this mutation, the residual FDPase will always be maintained in the AMP-inhibited form.

Effect of Mn2+ on fructose 2,6-bisphosphate inhibition of mouse liver, intestinal, and muscle fructose-1,6-bisphosphatases

Fructose 2,6-bisphosphate inhibited all three fructose-1,6-bisphosphatases from the liver, intestine, and muscle of the mouse. The sensitivity of the liver enzyme to the inhibitor was significantly diminished when Mg2+ was replaced by Mn2+ as the activating cation. Inhibition of the liver enzyme by fructose 2,6-bisphosphate decreased as the concentration of the metal activator, Mn2+ or Mg2+, increased. The respective I50 values obtained by extrapolation of metal ion concentrations to zero were 40 microM with Mn2+ and 0.25 microM with Mg2+. The extent of desensitization to either fructose 2,6-bisphosphate or AMP inhibition by Mn2+ decreased in the order of the liver, intestine, and muscle enzyme. Only in the case of the liver enzyme was the substrate cooperativity induced by fructose 2,6-bisphosphate in the presence of Mg2+. In all three isoenzymes from the mouse, fructose 2,6-bisphosphate greatly potentiated the AMP inhibition of the enzyme in the presence of either Mg2+ or Mn2+. The liver enzyme with Mn2+ in addition to Mg2+ was still active in the presence of less than 1 microM fructose 2,6-bisphosphate, even though AMP was present at 100-200 microM.

Fructose 1,6-bisphosphatase in rat liver cytosol: interactions between the effects of K+, Zn2+, Mn2+, and fructose 2,6-bisphosphate as measured in a steady-state assay

Fructose 1,6-bisphosphatase activity was determined in rat liver cytosols using glyceraldehyde 3-phosphate as primary substrate. Fructose 1,6-bisphosphate was formed in situ and steady-state concentrations ranging from 1 to 30 microM were observed depending on the activity of fructose 1,6-bisphosphatase and the concentration of added glyceraldehyde 3-phosphate. The system was free of contaminating low-molecular-weight compounds, divalent cations, and chelators. Under these conditions, fructose 1,6-bisphosphatase was inhibited by K+ (less than or equal to 200 mM). This inhibition was due to a reduction of V and was observed in presence of low (0.4 mM) and high (5 mM) concentrations of Mg2+. In presence of 0.4 mM Mg2+, 1 microM Zn2+ inhibited fructose 1,6-bisphosphatase by 50%; the same effect was obtained with 0.3 microM Zn2+ when the system was supplemented with 100 mM KCl. On the other hand, 0.2 microM Zn2+ enhanced the inhibitory effect of K+ and decreased the concentration of K+ yielding half-maximal inhibition from 175 to 100 mM when measured at 0.4 mM Mg2+. The effect of Zn2+ on the inhibition by K+ could be abolished by Mn2+ (less than 5 microM) or by 5 mM Mg2+. One hundred millimolar K+ enhanced the inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate and changed the type of inhibition from mainly competitive to a mixed-type inhibition (increase of Km, decrease of V). Mn2+ (less than 10 microM) reduced the effect of fructose 2,6-bisphosphate, especially in the presence of K+. It is proposed that K+ and Mn2+ may play a role in the regulation of gluconeogenesis.

Regulatory properties of a fructose 1,6-bisphosphatase from the cyanobacterium Anacystis nidulans

A fructose 1,6-bisphosphatase (EC 3.1.3.11) (FBPase) was purified over 100-fold from Anacystis nidulans. At variance with a previous report (R. H. Bishop, Arch. Biochem. Biophys. 196:295-300, 1979), the regulatory properties of the enzyme were found to be like those of chloroplast enzymes rather than intermediate between chloroplast (photosynthetic) and heterotrophic FBPases. The pH optimum of Anacystis FBPase was between 8.0 and 8.5 and shifted to lower values with increasing Mg2+ concentration. Under the experimental conditions used by Bishop, we found the saturation curve of the enzyme to be sigmoidal for Mg2+ ions and hyperbolic for fructose 1,6-bisphosphate. The half-maximal velocity of the Anacystis FBPase was reached at concentrations of 5 mM MgCl2 and 0.06 mM fructose 1,6-bisphosphate. AMP did not inhibit the enzyme. The activity of the FBPase was found to be under a delicate control of oxidizing and reducing conditions. Oxidants like O2, H2O2, oxidized glutathione, and dehydroascorbic acid decreased the enzyme activity, whereas reductants like dithiothreitol and reduced glutathione increased it. The oxido-reductive modulation of FBPase proved to be reversible. Reduced glutathione stimulated the enzyme activity at physiological concentrations (1 to 10 mM).l The reduced glutathione-induced activation was higher at pH 8.0 than at pH 7.0.