Fructose 2,6-bisphosphate activates the cAMP-dependent phosphorylation of yeast fructose-1,6-bisphosphatase in vitro

Fructose-1,6-bisphosphatase purified from Saccharomyces cerevisiae is phosphorylated in vitro by a cAMP-dependent protein kinase. The phosphorylation reaction incorporates 1 mol of phosphate/mol of enzyme and is greatly stimulated by fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate acts upon fructose-1,6-bisphosphatase, not on the protein kinase. The phosphorylation of fructose 1,6-bisphosphatase lowers its activity by about 50%. The characteristics of the phosphorylation reaction in vitro show that this modification is responsible for the inactivation of fructose-1,6-bisphosphatase observed in vivo.

Glutathione and ascorbic acid in spinach (Spinacia oleracea) chloroplasts. The effect of hydrogen peroxide and of Paraquat

The stroma of spinach chloroplasts contains ascorbic acid and glutathione at millimolar concentrations. [Reduced glutathione]/[oxidized glutathione] and [ascorbate]/[dehydroascorbate] ratios are high under both light and dark conditions and no evidence for a role of oxidized glutathione or dehydroascorbate in the dark-deactivation of fructose bisphosphatase could be obtained. Addition of H2O2 to chloroplasts in the dark decreases the above ratios, an effect that is reversed on illumination. Addition of Paraquat to illuminated chloroplasts caused a rapid oxidation of reduced glutathione and ascorbate, and apparent loss of dehydroascorbate. Paraquat rapidly inactivated fructose bisphosphatase activity, as assayed under physiological conditions.

Phosphorylation and inactivation of yeast fructose-bisphosphatase in vivo by glucose and by proton ionophores. A possible role for cAMP

Addition of glucose to yeast cells causes a phosphorylation and an inactivation of the gluconeogenic enzyme fructose-bisphosphatase [Mazón, M.J., Gancedo, J.M., and Gancedo, C. (1982) J. Biol. Chem. 257, 1128-1130]. We report here that the addition of the proton ionophores 2,4-dinitrophenol and carbonylcyanide m-chlorophenylhydrazone to yeast cells produces the same effect as that of glucose. Both glucose and ionophores produced: (a) phosphorylation and inactivation of fructose-bisphosphatase, (b) an immediate rise in the intracellular concentration of cAMP, (c) an instant inhibition of the transport of amino acids driven by the membrane potential. It is proposed that the effect of glucose on fructose-bisphosphatase involves as a first step the depolarization of the plasma membrane resulting in an increase of the intracellular concentration of cAMP. This in turn would stimulate phosphorylation of fructose-bisphosphatase.

A method for determining kinetic parameters at high enzyme concentrations

A graphical method is described which allows determination of kinetic parameters when substrate, inhibitor or activator concentrations must be in the vicinity of the enzyme concentration and a significant fraction of ligand is bound. Velocity is measured at several ligand: enzyme ratios at two or more enzyme concentrations. Results are obtained in terms of free and bound ligand corresponding to particular velocities. The relationship between velocity and bound and free ligand may then be analysed by any desired plotting technique. Preknowledge of the reaction mechanism or experimental determination of Vmax. is not required. The relationship between ligand bound and enzyme activity need not be linear and the method is equally suitable for analysing co-operative as well as simple kinetics. Application of the method is demonstrated by analysis of the inhibition of fructose, 1,6-bisphosphatase by AMP.

Inactivation of yeast fructose-1,6-bisphosphatase. In vivo phosphorylation of the enzyme

Incorporation of 32P into yeast fructose-1,6-bisphosphatase (EC 3.1.3.11) was observed after addition of glucose to a cell suspension incubated with (32P)orthophosphoric acid. The 32P counts were coincident with the enzyme band when immunoprecipitates were subjected to sodium dodecyl sulfate disc gel electrophoresis. The incorporation of phosphate was associated with a decrease in enzyme activity. Approximately 1 mol of phosphate was incorporated/mol of enzyme. The phosphate is bound to the enzyme in a phosphoester linkage with a serine residue. Release of 32P accompanying enzyme reactivation was observed both in vivo and in cell-free extracts.

Inhibition of fructose-1,6-bisphosphatase by fructose 2,6-biphosphate

Fructose 2,6-bisphosphate, a known powerful stimulator of phosphofructokinase [Van Schaftingen, E., Hue, L. & Hers, H.-G. (1980) Biochem. J. 192, 897-901] was found to inhibit, at micromolar concentrations, liver and muscle fructose-1,6-biphosphate (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11). The main characteristics of this inhibition are that (i) it is much stronger at low than at high substrate concentrations, (ii) it changes the substrate saturation curve from almost hyperbolic to sigmoidal, and (iii) it is synergistic with the inhibition by AMP. This inhibition may play an important role in the stimulation of gluconeogenesis by glucagon, because this hormone is known to decrease the concentration of fructose 2,6-bisphosphate in the liver [Van Schaftingen, E., Hue, L. & Hers, H.-G. (1980) Biochem. J. 192, 887-895].

Inhibition of fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate

Rat liver fructose-1,6-bisphosphatase, which was assayed by measuring the release of 32P from fructose 1,6-[1-32P]bisphosphate at pH 7.5, exhibited hyperbolic kinetics with regard to its substrate. beta-D-Fructose 2,6-bisphosphate, an activator of hepatic phosphofructokinase, was found to be a potent inhibitor of the enzyme. The inhibition was competitive in nature and the Ki was estimated to be 0.5 microM. The Hill coefficient for the reaction was 1.0 in the presence and absence of fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate also enhanced inhibition of the enzyme by the allosteric inhibitor AMP. The possible role of fructose 2,6-bisphosphate in the regulation of substrate cycling at the fructose-1,6-bisphosphatase step is discussed.

Metabolic regulation by pH gradients. Inhibition of photosynthesis by indirect proton transfer across the chloroplast envelope

Anions of several weak acids inhibited photosynthesis in isolated spinach chloroplasts. Inhibition was drastic at low pH and weak or absent at high pH. Glyoxylate was particularly effective and inhibition decreased in the order: glyoxylate, nitrite, glycerate, formate, hydroxypyruvate, glycolate, propionate, acetate, pyruvate. These anions operated as indirect proton shuttles across the chloroplast envelope. They compensated active proton fluxes into the medium, minimized gradients in proton activity across the chloroplast envelope, and so prevented light-dependent stroma alkalization. This caused inhibition of sugar bisphosphatases which are known to be pH-regulated. At concentrations that caused potosynthesis inhibition, the proton shuttles were not effective in decreasing the proton gradient across the thylakoids. Some anions also inhibited fructose-bisphosphatase directly, when present at concentratins higher than needed for photosynthesis inhibition.

Purification of human liver fructose-1,6-bisphosphatase

Human liver fructose-1,6-bisphosphatase (D-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) has been purified 1200-fold using a heat treatment step followed by absorption on phosphocellulose at pH 8 and specific elution with buffer containing the substrate (fructose 1,6-bisphosphate) and allosteric effector (AMP). The enzyme is homogeneous in electrophoresis in polyacrylamide gel, in the presence and absence of denaturing agent. It has a molecular weight of 144 000 and is composed of four identical or nearly identical subunits. Fluorescence spectra indicate that the enzyme does not contain tryptophan residues. The pH optimum is 7.5 and the Km is determined as 0.8 microM. The enzyme is inhibited by AMP in cooperative manner with a K0 x 5 of 6 microM.

Effect of hydrogen peroxide on spinach (Spinacia oleracea) chloroplast fructose bisphosphatase

Thiol-treated spinach (Spinacia oleracea) chloroplast fructose bisphosphatase (EC 3.1.3.11) is severely inhibited by H2O2, whereas the freshly purified enzyme is little affected. Dithiothreitol reverses inhibition by H2O2, indicating that essential thiol groups are oxidized during H2O2 inactivation. A new role for the dithiol and thioredoxin systems that are operative in illuminated chloroplasts is proposed.

Action of calcium ions on spinach (Spinacia oleracea) chloroplast fructose bisphosphatase and other enzymes of the Calvin cycle

Thiol-treated spinach (Spinacia oleracea) chloroplast fructose bisphosphatase is powerfully inhibited by Ca2+ non-competitively with respect to its substrate, fructose 1,6-bisphosphate. 500 microM-Ca2+ causes virtually complete inhibition and the Ki is 40 microM. Severe inhibition of sedoheptulose bisphosphatase is also caused by Ca2+. A role for Ca2+ in regulation of the Calvin cycle in spinach chloroplasts is proposed.