Inhibition by AICA riboside of gluconeogenesis in isolated rat hepatocytes

5-Amino-4-imidazolecarboxamide (AICA) riboside, the nucleoside corresponding to AICA ribotide (AICAR or ZMP), an intermediate of the de novo pathway of purine biosynthesis, was found to exert a dose-dependent inhibition on gluconeogenesis in isolated rat hepatocytes. Production of glucose from lactate-pyruvate mixtures was half-maximally inhibited by approximately 100 microM and completely suppressed by 500 microM AICA riboside. AICA riboside also inhibited the production of glucose from all other gluconeogenic precursors investigated, i.e., fructose, dihydroxyacetone, and L-proline. Measurements of intermediates of the glycolytic-gluconeogenic pathway showed that AICA riboside provoked elevations of triose phosphates and fructose-1,6-bisphosphate and decreases in fructose-6-phosphate and glucose-6-phosphate. The effects of AICA riboside persisted when the cells were washed 10 min after its addition but were suppressed by 5-iodotubercidin, an inhibitor of adenosine kinase. AICA riboside provoked a dose-dependent buildup of normally undetectable Z nucleotides. After 20 min of incubation with 500 microM AICA riboside, ZMP, ZTP, and ZDP reached 3, 0.3, and 0.1 mumol/g cells, respectively. Concentrations of ATP were not significantly modified by addition of up to 500 microM AICA riboside when the cells were incubated with lactate-pyruvate but decreased with fructose or dihydroxyacetone. The activity of rat liver fructose-1,6-bisphosphatase was inhibited by ZMP with an apparent Ki of 370 microM. It is concluded that AICA riboside exerts a suppressive effect on gluconeogenesis because it provokes an accumulation of ZMP, which inhibits fructose-1,6-bisphosphatase.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition by substrate of fructose 1,6-bisphosphatase purified from rat kidney cortex. Calculation of the kinetic constants of the enzyme

Fructose 1,6-bisphosphatase is a typical enzyme that is severely inhibited by its own substrate. This makes it difficult to determine all the parameters involved in its kinetics. It has been shown recently that if Vm is satisfactorily estimated the remaining parameters can be determined using the Hill plot (Bounias, M. (1988) Biochem. Int. 17, 147-154). The enzyme has been purified from rat kidney cortex nearly to homogeneity, and its kinetic constants have been calculated using a rigorous algebraic method. The most interesting result is that the substrate is unable to bind to the free enzyme as an inhibitor, which indicates that the enzyme lacks an allosteric site for hexose bisphosphates.

Identification of ATP diphosphohydrolase activity in human term placenta using a novel assay for AMP

Human term placenta contains an ATP diphosphohydrolase activity which hydrolyses ATP to ADP and inorganic phosphate and ADP to AMP and a second mole of inorganic phosphate. The activity has a pH optimum between 8.0 and 8.5. Magnesium or calcium ions are required for maximum activity. Other nucleoside phosphates, p-nitrophenyl phosphate or sodium pyrophosphate, are not hydrolysed. The activity is not due to ATPases, or to myokinase, as determined by the use of inhibitors. NaF and NaN3 were found to inhibit strongly the activity thus identifying it as an ATP diphosphohydrolase. A sensitive enzymatic assay for measurement of AMP, one of the products of the reaction, was established, based on the strong inhibition of muscle fructose 1,6-biphosphatase by AMP. The range of the assay was 0.05-0.8 microM AMP. ATP diphosphohydrolase was found to have a rate of AMP production from ADP twice the rate from ATP. Under the same conditions, the assay for Pi release, on the other hand, gave velocities similar to each other for the two substrates. The activity appears to be identical to the ADP-hydrolysing activity in placenta reported by others.

[The allosteric nature of substrate inhibition of rabbit skeletal muscle fructose-1,6-diphosphatase]

The mechanism of substrate inhibition of rabbit skeletal muscle fructose-1.6-bisphosphatase was examined. Analysis of substrate saturation curves obtained at different concentrations of Mg2+ revealed that the inhibiting effect of the substrate is manifested only within the complex with Mg2+, whereas the free form of the substrate causes no inhibition. Evidence for the allosteric nature of substrate inhibition was obtained by partial desensitization of the enzyme in the presence of salicylates. It was shown that fructose-1.6-bisphosphatase inhibition by the substrate obeys positively cooperative kinetics and is noncompetitive with respect to the substrate involved in the catalytic process. Studies on enzyme modification in the presence of DTNB and pyridoxal-5'-phosphate demonstrated that the inhibiting concentrations of the substrate are bound to the center which differs from the allosteric site for AMP. It is suggested that the antagonism of simultaneous action of AMP and high substrate concentrations may be due to the competition of the phosphate groups of these ligands for binding to the common lysine residue located in the overlapping region of two allosteric sites.

Characterization of human fructose-1,6-bisphosphatase in control and deficient tissues

The regulatory properties of human liver and muscle fructose-1,6-bisphosphatases (FBPase) have been studied in control tissues obtained at autopsy and in tissues from a neonate with FBPase deficiency who died as a result of an overwhelming acidosis. Evidence is presented which suggests that the alkaline isoenzyme of FBPase, which is widely regarded as a laboratory artefact, may have an important role in vivo in the regulation and control of glycolysis and gluconeogenesis. FBPase exhibits the hysteretic and dissociative properties associated with regulatory enzymes, and many of the factors which effect FBPase have inverse effects on phosphofructokinase activity, thus providing an integrated regulatory cycle for the control of the direction and rate of flux through the glycolytic pathway.

[The mechanism of rat liver fructose-1,6-bisphosphate inhibition by fructose-2,6-bisphosphate]

It was found that a decrease in the activating cation (Mg2+) concentration below [A]0.5 causes the disappearance of cooperativity of the fructose 1.6-bisphosphatase substrate binding sites induced by high fructose 2.6-bisphosphate concentrations without any significant alteration in the extent of the enzyme inhibition. Under these conditions, a competitive type of inhibition (with respect to the substrate) is transformed into a non-competitive type with an increase in the fructose 2.6-bisphosphate concentration. The data obtained confirm the viewpoint that fructose 2.6-bisphosphate binds to the enzyme at two distinct sites, a catalytic and an allosteric ones, differing in their affinity for the inhibitor. It is supposed that the interaction between the allosteric fructose 2.6-bisphosphate binding site and the activator site occupied by Mg2+ is necessary for the cooperative response of the enzyme to the substrate.

Cyclic AMP, fructose-2,6-bisphosphate and catabolite inactivation of enzymes in the hydrocarbon-assimilating yeast Candida maltosa

The inactivation of fructose-1,6-bisphosphatase, isocitrate lyase and cytoplasmic malate dehydrogenase in Candida maltosa was found to occur after the addition of glucose to starved cells. The concentration of cyclic AMP and fructose-2,6-bisphosphate increased drastically within 30 s when glucose was added to the intact cells of this yeast. From these results it was concluded that catabolite inactivation, with participation of cyclic AMP and fructose-2,6-bisphosphate, is an important control mechanism of the gluconeogenetic sequence in the n-alkane-assimilating yeast Candida maltosa, as described for Saccharomyces cerevisiae.

On the regulatory significance of inhibitors acting on non-equilibrium enzymes in the Calvin photosynthesis cycle

Control analyses and kinetic model studies have been performed in order to obtain quantitative information on the regulatory significance of 12 experimentally well-documented inhibitory interactions of Calvin cycle intermediates with the four non-equilibrium cycle enzymes. Evidence is presented to show that none of these interactions contributes significantly to the cycle flux control over the range of external orthophosphate concentrations where the reaction cycle shows close to optimal activity. Contrary to what has been generally supposed, the examined inhibitions appear to be of little interest for our understanding of the biological regulation of the Calvin photosynthesis cycle under conditions of light and carbon dioxide saturation.

Mouse thymoma cell line expresses a gluconeogenic enzyme, fructose 1,6-bisphosphatase

Fructose 1,6-bisphosphatase was observed in a thymic lymphoma cell line, WEH17.1 (11.5 +/- 0.8 munits/mg cytosol protein). Only a trace amount of the enzyme activity was observed in normal thymus tissue. The WEH17.1 enzyme had a pH optimum at around 7.5. The AMP-concentration giving 50% inhibition of the activity was about 73 microM. That of the crude mouse liver enzyme was 35 microM. The antibodies against the liver and intestinal enzymes cross-reacted with the WEH17.1 enzyme with a lower affinity than the liver enzyme. Immunoblot showed that the subunit molecular weight of the WEH17.1 enzyme was the same as that of the liver enzyme.

Allosteric inhibition of Dictyostelium discoideum fructose-1,6-bisphosphatase by fructose 2,6-bisphosphate

It has been found that the inhibition of Dictyostelium discoideum fructose-1,6-bisphosphatase by fructose 2,6-P2 greatly diminished when the pH was raised to the range 8.5-9.5, which resulted in a marked decrease of the affinity for the inhibitor with no change in the Km for the substrate. This provides evidence for the involvement of an allosteric site for fructose 2,6-P2. Moreover, the fact that excess substrate inhibition also decreased at the pH values for minimal fructose 2,6-P2 inhibition, and was essentially abolished in the presence of fructose 2,6-P2, strongly suggests that this inhibition takes place by binding of fructose 1,6-P2 as a weak analogue of the physiological effector fructose 2,6-P2.

Irreversible inactivation of Saccharomyces cerevisiae fructose-1,6-bisphosphatase independent of protein phosphorylation at Ser11

The fructose-1,6-bisphosphatase gene was used with multicopy plasmids to study rapid reversible and irreversible inactivation after addition of glucose to derepressed Saccharomyces cerevisiae cells. Both inactivation systems could inactivate the enzyme, even if 20-fold over-expressed. The putative serine residue, at which fructose-1,6-bisphosphatase is phosphorylated, was changed to an alanine residue without notably affecting the catalytic activity. No rapid reversible inactivation was observed with the mutated enzyme. Nonetheless, the modified enzyme was still irreversibly inactivated, clearly demonstrating that phosphorylation is an independent regulatory circuit that reduces fructose-1,6-bisphosphatase activity within seconds. Furthermore, irreversible glucose inactivation was not triggered by phosphorylation of the enzyme.

Relationship between thiol group modification and the binding site for fructose 2,6-bisphosphate on rabbit liver fructose-1,6-bisphosphatase

A thiol group present in rabbit liver fructose-1,6-bisphosphatase is capable of reacting rapidly with N-ethylmaleimide (NEM) with a stoichiometry of one per monomer. Either fructose 1,6-bisphosphate or fructose 2,6-bisphosphate at 500 microM protected against the loss of fructose 2,6-bisphosphate inhibition potential when fructose-1,6-bisphosphatase was treated with NEM in the presence of AMP for up to 20 min. Fructose 2,6-bisphosphate proved more effective than fructose 1,6-bisphosphate when fructose-1,6-bisphosphatase was treated with NEM for 90-120 min. The NEM-modified enzyme exhibited a significant loss of catalytic activity. Fructose 2,6-bisphosphate was more effective than the substrate in protecting against the thiol group modification when the ligands are present with the enzyme and NEM. 100 microM fructose 2,6-bisphosphate, a level that should almost saturate the inhibitory binding site of the enzyme under our experimental conditions, affords only partial protection against the loss of activity of the enzyme caused by the NEM modification. In addition, the inhibition pattern for fructose 2,6-bisphosphate of the NEM-derivatized enzyme was found to be linear competitive, identical to the type of inhibition observed with the native enzyme. The KD for the modified enzyme was significantly greater than that of untreated fructose-1,6-bisphosphatase. Examination of space-filling models of the two bisphosphates suggest that they are very similar in conformation. On the basis of these observations, we suggest that fructose 1,6-bisphosphate and fructose 2,6-bisphosphate occupy overlapping sites within the active site domain of fructose-1,6-bisphosphatase. Fructose 2,6-bisphosphate affords better shielding against thiol-NEM modification than fructose 1,6-bisphosphate; however, the difference between the two ligands is quantitative rather than qualitative.

The inhibition of fructose 1,6-bisphosphatase by fructose 2,6-bisphosphate is enhanced by EDTA and diminished by zinc(II)

The sensitivity of the Mg(II)-dependent activity of rabbit liver fructose 1,6-bisphosphatase (FBPase, EC 3.1.3.11) to inhibition by fructose 2,6-bisphosphate (Fru-2,6-P2) was enhanced by EDTA and diminished to negligible levels by 0.5-2 microM Zn(II) added as another FBPase inhibitor. Fru-2,6-P2 was more efficient in the presence of the synergistic effector AMP: still, the Fru-2,6-P2 concentration inhibiting 50% changed from 3 microM (with EDTA) to higher than 50 microM (with Zn(II]. On the other hand, the Zn(II)-dependent FBPase activity was inhibited by Fru-2,6-P2 to a much lesser extent than the Mg(II)-dependent activity.

Inactivation of fructose-1,6-bisphosphatase by o-phthalaldehyde

Rabbit liver fructose-1,6-bisphosphatase, a tetramer of identical subunits was rapidly and irreversibly inactivated by o-phthalaldehyde at 25 degrees C (pH 7.3). The second-order rate constant for the inactivation was 30 M-1s-1. Fructose-1,6-bisphosphatase was completely protected from inactivation by the substrate--fructose-1,6-diphosphate but not by the allosteric effector--adenosine monophosphate. The absorption spectrum (lambda max 337 nm) and, fluorescence excitation (lambda max 360 nm) and fluorescence emission spectra (lambda max 405 nm) were consistent with the formation of an isoindole derivative in the subunit between a cysteine and a lysine residue about 3A apart. About 4 isoindole groups per mol of the bisphosphatase were formed following complete loss of the phosphatase activity. This suggests that the amino acid residues of the biphosphatase participating in reaction with o-phthalaldehyde more likely reside at or near the active site instead of allosteric site. The molar transition energy of fructose-1,6-bisphosphatase--o-phthalaldehyde adduct was estimated 121 kJ/mol and compares favorably with 127 kJ/mol for the synthetic isoindole, 1-[(beta-hydroxyethyl)thio]-2-(beta-hydroxyethyl) isoindole in hexane. It is, thus, concluded that the cysteine and lysine residues participating in isoindole formation in reaction between fructose-1,6-bisphosphatase and o-phthalaldehyde are located in a hydrophobic environment.

Purification and characterization of fructose-1,6-bisphosphatase from bovine brain

Fructose-1,6-bisphosphatase from bovine brain tissue has been purified to near homogeneity. This enzyme is similar to other mammalian fructose-1,6-bisphosphatases in many respects, and its properties are distinctly different from those reported for the enzyme from rat brain [A. L. Majumder and F. Eisenberg (1977) Proc. Natl. Acad. Sci. USA 74, 3222-3225; S. Chattoraj and A. L. Majumder (1986) Biochem. Biophys. Res. Commun. 139, 571-580]. The bovine enzyme (sp act 4, pH ratio (7.5/9.6) = 3.6) has a pH optimum of 7.5. The Km is 2 microM. Divalent metal ion is required for activity, and Vmax is obtained at either 4 mM Mg2+ or 0.3 mM Mn2+. Fructose 2,6-bisphosphate is a competitive inhibitor (Ki = 0.07 microM), and AMP a noncompetitive inhibitor (kis = 24 microM, Kii = 10 microM) of bovine brain fructose-1,6-bisphosphatase. The enzyme activity is enhanced by small amounts of EDTA relative to metal, and AMP inhibits fructose-1,6-bisphosphatase in either the presence or absence of the metal chelator; however, AMP is more effective in the absence of EDTA.

Selective thiol group modification renders fructose-1,6-bisphosphatase insensitive to fructose 2,6-bisphosphate inhibition

Limited treatment of native pig kidney fructose-1,6-bisphosphatase (50 microM enzyme subunit) with [14C]N-ethylmaleimide (100 microM) at 30 degrees C, pH 7.5, in the presence of AMP (200 microM) results in the modification of 1 reactive cysteine residue/enzyme subunit. The N-ethylmaleimide-modified fructose-1,6-bisphosphatase has a functional catalytic site but is no longer inhibited by fructose 2,6-bisphosphate. The enzyme derivative also exhibits decreased affinity toward Mg2+. The presence of fructose 2,6-bisphosphate during the modification protects the enzyme against the loss of fructose 2,6-bisphosphate inhibition. Moreover, the modified enzyme is inhibited by monovalent cations, as previously reported (Reyes, A., Hubert, E., and Slebe, J.C. (1985) Biochem. Biophys. Res. Commun. 127, 373-379), and does not show inhibition by high substrate concentrations. A comparison of the kinetic properties of native and N-ethylmaleimide-modified fructose-1,6-bisphosphatase reveals differences in some properties but none is so striking as the complete loss of fructose 2,6-bisphosphate sensitivity. The results demonstrate that fructose 2,6-bisphosphate interacts with a specific allosteric site on fructose-1,6-bisphosphatase, and they also indicate that high levels of fructose 1,6-bisphosphate inhibit the enzyme by binding to this fructose 2,6-bisphosphate allosteric site.

Inhibition of protein phosphorylation by chloroquine

The rapid phase of fructose-1,6-bisphosphatase (FBPase) inactivation following glucose addition to starved yeast cells [reported previously] is inhibited on addition of 10 mM chloroquine (CQ) at about pH 8. This inhibition of inactivation was shown to be due to the prevention of phosphorylation of the enzyme. CQ was also found to inhibit general protein phosphorylation in the yeast cells. Glycolysis, as observed by changes in intracellular glucose-6-phosphate and extracellular glucose and ethanol concentrations, was shown to be significantly inhibited in cells treated with CQ. Similarly, a decrease in ATP concentrations was observed. However, during the early stages of phosphorylation of FBPase, levels of ATP were similar in cells containing CQ as in those without CQ. Thus, decrease in ATP levels is not thought to be significantly responsible for the inhibition of protein phosphorylation. However, the phosphorylating activity of cyclic AMP-dependent protein kinases is inhibited in vitro by relatively low concentrations of CQ. Thus, prevention of protein phosphorylation by CQ is believed to be due to inhibition of protein kinases in yeast cells.

Catabolite inactivation of isocitrate lyase from Saccharomyces cerevisiae

A reversible carbon catabolite inactivation step is described for isocitrate lyase from Saccharomyces cerevisiae. This reversible inactivation step of isocitrate lyase is similar to that described for fructose 1,6-bisphosphatase. Addition of 2,4-dinitrophenol, nystatin or glucose to cultures, grown in ethanol as carbon source, caused a rapid loss of the isocitrate lyase and fructose 1,6-bisphosphatase activities at pH 5.5 but not at pH 7.5. These results suggest that intracellular acidification and thus a cAMP increase is involved in the catabolite inactivation mechanism of both enzymes. From results obtained by addition of glucose to yeast cultures at pH 7.5 it was concluded that others factors than cAMP can play a role in the catabolite inactivation mechanism of both enzymes.

[Relation between Mg2+ activation and AMP inhibition of fructose-1,6-diphosphatase from rabbit skeletal muscles]

It was shown that AMP, an allosteric inhibitor of fructose-1.6-bisphosphatase, decreases the apparent affinity of the enzyme for the activating cation, Mg2+, which is accompanied by a decrease of the kinetic cooperativity between the Mg2+-binding sites. In its turn, the Mg2+ increase diminishes the enzyme sensitivity to the inhibiting effect of AMP and decreases the cooperativity of the inhibitor binding. The heterotropic interactions between the allosteric inhibitor and activator binding centers are consistent with the predictions of the Monod-Wyman-Changeux model which involves two conformational states of the enzyme (of which one is catalytically inactive) differing in their affinity for the ligands. An increase in pH from 7.4 to 9.0 increases the enzyme affinity for Mg2+ and causes an equilibrium shift towards the catalytically active state of the enzyme.

Evidence for gluconeogenesis in the amphibian retina

Evidence for the existence of a gluconeogenic pathway was provided in the amphibian retina. It was found that [3H]glutamate was converted to [3H]glucose derived from [3H]glutamate was incorporated into glycogen. The rate for this incorporation was found to be essentially the same in both light- and dark-adapted retinas: 0.147 vs. 0.142 nmol (mg protein X 2 hr)-1, respectively. However, the rate of incorporation was found to decline progressively with time. The rate for the incorporation of label derived from glutamate into glycogen was found to be considerably less than that for [3H]glucose: 10.2 nmol (mg protein X 2 hr)-1. The activity of a key gluconeogenic enzyme, fructose-1,6-bisphosphatase, also was demonstrated in retinal supernatants, approximately 1 nmol (mg X min)-1, and the activity of this enzyme was found to be inhibited both by adenosine monophosphate and by fructose-2,6-bisphosphate.

Fructose 2,6-bisphosphate in Dictyostelium discoideum. Independence of cyclic AMP production and inhibition of fructose-1,6-bisphosphatase

The occurrence of fructose 2,6-bisphosphate was detected in Dictyostelium discoideum. The levels of this compound were compared with those of cyclic AMP and several glycolytic intermediates during the early stages of development. Removal of the growth medium and resuspension of the organism in the differentiation medium decreased the content of fructose 2,6-bisphosphate to about 20% within 1 h, remaining low when starvation-induced development was followed for 8 h. The content of cyclic AMP exhibited a transient increase that did not correlate with the change in fructose 2,6-bisphosphate. If after 1 h of development 2% glucose was added to the differentiation medium, fructose 2,6-bisphosphate rapidly rose to similar levels to those found in the vegetative state, while the increase in cyclic AMP was prevented. The contents of hexose 6-phosphates, fructose 1,6-bisphosphate and triose phosphates changed in a way that was parallel to that of fructose 2,6-bisphosphate, and addition of sugar resulted in a large increase in the levels of these metabolites. The content of fructose 2,6-bisphosphate was not significantly modified by the addition of the 8-bromo or dibutyryl derivatives of cyclic AMP to the differentiation medium. These results provide evidence that the changes in fructose 2,6-bisphosphate levels in D. discoideum development are not related to a cyclic-AMP-dependent mechanism but to the availability of substrate. Fructose 2,6-bisphosphate was found to inhibit fructose-1,6-bisphosphatase activity of this organism at nanomolar concentrations, while it does not affect the activity of phosphofructokinase in the micromolar range. The possible physiological implications of these phenomena are discussed.

Potassium activation and its relationship to a highly reactive cysteine residue in fructose 1,6-bisphosphatase

The specific chemical modification by sodium cyanate of highly reactive cysteine residues at pH 7.5 in pig kidney fructose 1,6-bisphosphatase results in the reversible loss of activation of the enzyme by monovalent cations. No loss of activation by potassium ions occurs when modification is carried out in the presence of fructose 2,6-bisphosphate. The effect of Mg2+ on native and cyanate-modified enzyme activities implicates the above cysteine residue as being directly linked to the inhibition by both the divalent cation and fructose 2,6-bisphosphate. Incorporation of [14C]cyanate to the enzyme shows that the blockage of two reactive residues per tetramer is sufficient to eliminate the activation of the enzyme by K+.

Modification of brain fructose-1,6-bisphosphatase activity by chelators: "induction" of 5'-AMP sensitivity

Brain fructose-1,6-bisphosphatase (EC 3.1.3.11) from various sources are ordinarily insensitive to 5'-AMP. In addition to stimulation and conferring a "neutral" behaviour, prior treatment with histidine, EDTA or imidazole renders the brain enzyme sensitive to 5'-AMP. The histidine treated enzyme(s) bind to Blue-Sepharose CL-6B column and are specifically eluted by 5'-AMP in contrast to the untreated enzyme(s) which do not bind to the affinity column at all. The histidine effect in inducing 5'-AMP sensitivity was abolished by treatment of the native enzyme by subtilisin or by a number of divalent cations including Zn++.

Characterization of rat muscle fructose 1,6-bisphosphatase

Fructose 1,6-bisphosphatase has been purified from rat muscle. Although the specific activity of the enzyme in the crude extract of rat muscle was extremely low, purification by the present procedure is highly reproducible. The purified enzyme showed a single band in SDS-polyacrylamide gel electrophoresis. The subunit molecular weight of the muscle enzyme was 37,500 in contrast to 43,000 in the case of the liver enzyme. Immunoreactivity of the muscle enzyme to anti-muscle and anti-liver fructose 1,6-bisphosphatase sera was clearly distinct from that of the liver enzyme. All one-dimensional peptide mappings of the muscle enzyme with staphylococcal V8 protease, chymotrypsin, and papain showed different patterns from those of the liver enzyme. When incubated with subtilisin, the extent of activation of muscle fructose 1,6-bisphosphatase at pH 9.1 was smaller than that of the liver enzyme. The subtilisin digestion pattern of the muscle enzyme on SDS-polyacrylamide gel electrophoresis was distinct from that of the liver enzyme. The AMP-concentration giving 50% inhibition of the muscle enzyme was 0.54 microM, whereas that of the liver enzyme was 85 microM. The concentrations of fructose 2,6-bisphosphate that gave 50% inhibition of rat muscle and liver enzymes were 6.3 and 1.5 microM, respectively. Fructose 1,6-bisphosphatase protein was not detected in soleus muscle by immunoelectroblotting with anti-muscle fructose 1,6-bisphosphatase serum.